An-Najah National University 

Faculty of Graduate Studies 

 

 

 

 

 

Investigating the Possibility of Trivalent 

Chromium Oxidation to Hexavalent Chromium 

of Tanning Wastewater 

 

By 

Bayan Bsharat 

 
Supervisors 

Prof. Amer El-Hamouz 

Dr. Abdelrahim Abu Safa 

 

 

This Thesis is Submitted in Partial Fulfillment of the Requirements for 

the Degree of Master of Water and Environmental Engineering, 

Faculty of Graduate Studies, An-Najah National University, Nablus, 

Palestine. 

2021 





III 
 

 
 

 

Dedication 

To my mother and father. The words are silent in front of their bounty.  

To the one who taught me how to live life with passion and supports me in 

every moment of grief or joy, who told me don't accept the imposed reality, 

and don't accept less than what I deserve. My dear husband "Mamoun". 

To the little boy who enjoyed tearing papers and playing with the keyboard 

and waited for my arrival from university for a long time. My son "Odai". 

To my dear daughter "Diala" that God gave me her laughter to comfort my 

heart. 

To my sisters, brothers, and everyone who sincerely wish good things to 

me. 

To the land that was created for peace and has never seen peace, 

''Palestine'', to everyone believes that there is something worth living on it. 

 

 

 

 

 

 

 

 



IV 
 

 
 

 

Acknowledgments 

Foremost, I'm grateful to Almighty God for helping me. "Who does not 

thank people does not thank God". I would like to thank and appreciate 

everyone who helped me to complete this research. 

First of all, I would like to thank my dear supervisors, Prof. Amer El- 

Hamouz and Dr. Abdelrahim Abu Safa for their follow-up and assistance to 

me throughout the years of study, their guidance and constructive criticism. 

May God reward them all the best and benefit with their knowledge. Many 

thanks to the Palestinian Dutch Academic Cooperation Program on Water 

(PADUCO) for their financial support. 

Special thanks to Dr. HamdallahBairat, who played a major role in the last 

part of the research and helped me with examine the samples at his 

workplace in the United States. I would like to say: 'Thank you very much' 

to the An-Najah National University team represented by doctors and 

technical laboratory assistants. Many thanks to the Palestine Polytechnic 

University team represented by Dr. Hassan Sawalha and Dr. Maher Al-

Jabari for the good coordination and reception during field visits to 

tanneries in Hebron. Finally, thanks to my friends and workers in tanneries.  

 

 

 

 

 





VI 
 

 
 

 

List of Contents 

Dedication ................................................................................................... III 

Acknowledgments ....................................................................................... IV 

Declaration ................................................................................................... V 

Table of Contents ........................................................................................ VI 

List of Figures ............................................................................................. XI 

List of Tables ............................................................................................... IX 

List of Abbreviations ................................................................................ XIV 

Abstract .................................................................................................... XVI 

Chapter One: Introduction ............................................................................. 1 

1.1 General Introduction ............................................................................... 1 

1.2 Statement of The Problem ....................................................................... 2 

1.3 Significance of Work .............................................................................. 3 

1.4 Research Objective .................................................................................. 4 

1.5 Research Questions ................................................................................. 4 

Chapter Two: Literature Review ................................................................... 5 

2.1 Chromium Element ................................................................................. 5 

2.2 Chemistry Of Cr (Iii) ............................................................................... 5 

2.3 Chemistry Of Cr (Vi) .............................................................................. 6 

2.4 Oxidation Of Cr(Iii) To Cr(Vi) ............................................................... 7 

2.4.1 Oxidation Of Cr3+To Cr6+Under Various Conditions ....................... 8 



VII 
 

 
 

 

2.5 Classification Of Tanning Wastewater As Non-Hazardous Versus 

Hazardous Waste. ................................................................................ 8 

2.6 Toxicity Of Cr ....................................................................................... 10 

2.7 Chromium Cycle In Soil And Water..................................................... 12 

2.8 Chromium Behavior In Soil .................................................................. 13 

2.9 Chemistry Of Soil Cr ............................................................................ 14 

2.9.1 Solid-Phase Speciation Of Cr In Industrial Contaminated Soils ....... 14 

2.10 Solubility Of Cr(Iii) And Cr(Vi) ......................................................... 16 

2.11 Partitioning And Mobility Of Cr ......................................................... 17 

2.12 Adsorption And Desorption Of Cr(Iii) Onto Soil Particles ................ 18 

2.13 Soil Texture ......................................................................................... 19 

2.14 Analyticalmethods For Determining Chromium Concentration In 

Various Sample Matrix. .................................................................... 22 

Chapter Three: Methodology ...................................................................... 26 

3.1 Experimental Setup And Design Of Experiments ................................ 26 

3.1.1 Soil Textural Analysis ........................................................................ 27 

3.1.2 Soil Sample Preparation ..................................................................... 31 

3.1.3 Soil Type Ability To Adsorb Chromium. .......................................... 35 

3.1.4 Oxidation Of  Cr3+From Different Concentrations And Volume With 

Time But Without Soils. ................................................................... 37 

Chapter Four: Results And Discussions...................................................... 39 

4.1 Results Of Hydrometer Analysis .......................................................... 39 

4.2 Xrf Oxide Data And Summery Of Calculations. .................................. 44 

4.3 Xrd Data Analysis ................................................................................. 46 



VIII 
 

 
 

 

4.4 Soil Type Ability To Adsorb Chromium .............................................. 49 

4.5 Oxidation Ofcr3+ From Different Concentrations And Volume (Total 

Amount) With Time Without Soils. ................................................. 59 

4.6 Adsorption Kinetic Models For Removal Of Chromium Onto Soil. .... 61 

Chapter Five: Conclusions And Recommendations ................................... 67 

5.1 Conclusions ........................................................................................... 67 

5.2 Recommendations ................................................................................. 68 

References ................................................................................................... 70 

Appendices .................................................................................................. 78 

 ب ........................................................................................................... الملخص 

 

 

 

 

 

 

 

 

 



IX 
 

 
 

 

List of Tables 

Table 2.1: Analytical methods for determining chromium concentration in 

various samples…………………………………………….…23 

Table 3.1: Effective length corresponding to hydrometer reading…......…30  

Table 3.2: Variation of A with Gs at different temperatures…...…………31 

Table 4.1: Hydrometer reading for red soil…………………………….…42 

Table 4.2: Information to determine the specific gravity of red soil, and 

some properties………………………………………….……42 

Table 4.3: USDA classification of soil particle size………...……………43 

Table 4.4: Hydrometer Reading for black soil............................................45 

Table 4.5: Information to determine the specific gravity of black soil and 

some properties……….……………………………………45 

Table 4.6: Loss of ignition for soil samples………………………………48 

Table 4.7: Sodium concentration in soil samples…………………............48 

Table 4.8: Summery of XRF oxide data and calculations………..………49 

Table 4.9: Hexavalent chromium concentration (mg/l) for the 1061 ppm of 

total chromium without soil……………………………..…65 

Table 4.10:  Hexavalent chromium concentration (mg/l) for the 2653 ppm 

of total chromium without soil………………………..……65 

Table 4.11: Hexavalent chromium concentration (mg/l) for the 5306 ppm 

of total chromium without soil………………………..……66 

Table 4.12: Fitting parameters with pseudo second order model for 

different concentrations onto different soils.........................72 

Table A1: XRF result and oxides percentage in fresh red soil sample 

( B1i)………………………………………………………………………85 

Table A2: XRF result and oxides percentage in red soil sample after treated 

with 2653 ppm chromium solution (B1f)……………….…86 

Table A3: XRF result and oxides percentage in fresh black soil sample 



X 
 

 
 

 

( B2i)………………………………………………………………………87  

Table A4: XRF result and oxides percentage in black soil sample after 

treated with 2653 ppm chromium solution ( B2f)……………88 

 

 

 

 

 

 

 

 

 

 

 

 

 



XI 
 

 
 

 

List of Figures 

Figure 2.1: Soil texture triangle showing soil textures as determined by the 

proportion of sand, silt and clay. ……………………………22 

Figure 3.3: Chromium sulfate solution with different concentration in 

different soils………………………………………………...38 

Figure 3.4: Chromium sulfate solutions with different concentration in 

different volumes without soils……………………………...39 

Figure 4.1: Red soil name from textural triangle as function of soil 

components percentage……………………………….……44 

Figure 4.2: Black soil name from textural triangle as function of soil 

components percentage……………………………...………47   

Figure 4.3: Phase identification for soil sample (B1i) from XRD data…...51 

Figure 4.4: Phase identification for soil sample (B1f) from XRD data…...51 

Figure 4.5: Phase identification for soil sample (B2i) from XRD data…...52 

Figure 4.6: Phase identification for soil sample (B2f) from XRD data…...52 

Figure 4.7: Samples from chromium sulfate solutions after 48 hour……..53 

Figure 4.8: Samples from chromium sulfate solutions after 168 hour……53 

Figure 4.9: Samples from chromium sulfate solutions after 357 hour..…..53 

Figure 4.10: Samples from chromium sulfate solutions after 504 hour…..54 

Figure 4.11: Total chromium concentrations for red and black soil with 

initial total chromium concentration equal to 5306 ppm and 

0.5 l volume…………………………………………...……54 



XII 
 

 
 

 

Figure 4.12: Total chromium concentrations for red and black soil with 

initial total chromium concentration equal to 2653 ppm and 

0.5 l volume………………………………………...………55 

Figure 4.13: Total chromium concentrations for red and black soil with 

initial total chromium concentration equal to 1061 ppm and 

0.5 l volume………………………………………..………56 

Figure 4.14: Total chromium concentrations for red soil with initial total 

chromium concentration equal to 2653 ppm and 1 L 

volume…………………………………………………….57 

Figure 4.15: Total chromium concentrations for red soil with initial total 

chromium concentration equal to 2653 ppm and 1, 0.5 L 

volume…………………………………………………….58 

Figure 4.16: Chromium concentrations for red soil with initial total 

chromium concentration equal to 5306, 2653 and 1061 

ppm………………………………………………………..59 

Figure 4.17: Chromium concentrations for black soil with initial total 

chromium concentration equal to 5306, 2653 and 1061 

ppm……………………………………………………..…..59 

Figure 4.18: Samples with high concentrations of hexavalent chromium..60 

Figure 4.19: Samples with less concentrations of hexavalent chromium...60 

Figure 4.20: Hexavalent chromium concentrations for red and black soil 

with initial total chromium concentration equal to 5306 ppm 

and 0.5 l volume……………………………………..……..61 



XIII 
 

 
 

 

Figure 4.21: Hexavalent chromium concentrations for red and black soil 

with initial total chromium concentration equal to 2653 ppm 

and 0.5 l volume………………………………………..…..62 

Figure 4.22: Hexavalent chromium concentrations for red and black soil 

with initial total chromium concentration equal to 1061 ppm 

and 0.5 l volume……………………………………..…….59 

Figure 4.23: Adsorption capacity (qt) as a function of time for chromium 

solution with initial concentration equal to 1061 ppm in 

different soil types…………………………………..……..69 

Figure 4.24: Adsorption capacity (qt) as a function of time for chromium 

solution with initial concentration equal to 2653 ppm in 

different soil types………………………………..………...69 

Figure 4.25: Adsorption capacity (qt) as a function of time for chromium 

solution with initial concentration equal to 5306 ppm in 

different soil types…………………………………..……...70 

Figure 4.26: Second order kinetic model for the adsorption of total 

chromium onto soil particles for initial concentration equal to 

5306 ppm in different soil………………………….……...70 

Figure 4.27: Second order kinetic model for the adsorption of total 

chromium onto soil particles for initial concentration equal to 

2653 ppm in different soil ………………………….…….71 

Figure 4.28: Second order kinetic model for the adsorption of total 

chromium onto soil particles for initial concentration equal to 

1061 ppm in different soil …………………………….…..71 

  



XIV 
 

 
 

 

List of Abbreviations 

Abbreviation Meaning 

AICI3 Aluminum Chloride 

Al2O3 Aluminum Oxide 

NH3 Ammonia 

APDC Ammonium Pyrrolidine Dithiocarbamate 

(NH4)2SO4 Ammonium Sulfate 

Az Arizona State 

AAS Atomic Absorption Spectrophotometry 

BSE Back Scattered Electron 

B2f Black Soil Sample after Chromium Adsorption 

℃ Celsius Degree 

CHCI3 Chloroform 

CrO4
2− Chromate 

Cr (OH)
3
 Chromium Hydroxide 

Cr(NO3)3 Chromium Nitrate 

Cr2O
3
 Chromium Oxide 

Cr2(So4)
3
 Chromium Sulfate 

DNA Deoxyribonucleic Acid 

(Cr2O7)2− Dichromate 

DPPA Differential Pulse Polarographic Analysis 

EAAS Electrothermal Atomic Absorption Spectrometry 

EDX Energy Dispersive X-ray 

ESEM-FEG 
Environmental-Cell Scanning Electron Microscope 

with Field Emission Gun 

EP Extraction Procedure 

FeCl₃ Ferric chloride 

W2 Final Weight 

FIA/uv/vis 
Flow Injection Analysis-Ultraviolet/Visible 

Spectroscopy 

B2i Fresh Black Soil Sample 

B1i Fresh Red Soil Sample 

GMSF Goldwater Materials Science Facility 

GFAAS Graphite Furnace Atomic Absorption Spectrometry 

Cr6+, Cr (VI) Hexavalent chromium 

HPLC High Pressure Liquid Chromatography 

hr Hour 

HCI Hydrochloric Acid 

HF Hydrofluoric Acid 



XV 
 

 
 

 

HCrO4- Hydrogen chromate 

ICP-AES 
Inductively Coupled Plasma-Atomic Emission 

Spectrometry 

W1 Initial Weight 

LD50 Lethal Dose 50% 

LOI Loss of Ignition 

MnO4 Manganese Oxide 

MIBK Methyl isobutyl Ketone 

µg/L Micro gram per liter 

NAA Neutron Activation Analysis 

HNO3 Nitric Acid 

PWA Palestinian Water Authority 

PVD Physical Vapor Deposition 

PIXE Proton-Induced X-ray Emission Spectrometry 

B1f Red Soil Sample after Chromium Adsorption 

SEM Scanning Electron Microscopy 

SEI Secondary Electron Images 

Na2CO3 Sodium Carbonate 

Na6(PO3)
6
 Sodium hexa meta phosphate 

NaOH Sodium Hydroxide 

Na2O2 Sodium Peroxide 

Cr3+, Cr (III) Trivalent chromium 

UV Ultraviolet 

XRD X-ray Diffraction 

XRF X-ray fluorescence 
 

  

http://www.chemspider.com/Chemical-Structure.96962.html


XVI 
 

 
 

 

Investigating the Possibility of Trivalent Chromium 

Oxidation to Hexavalent Chromium of Tanning Wastewater 

By 

Bayan Bsharat 

Supervisors 

 Prof. Amer El-Hamouz 

Dr. Abdelrahim Abu Safa 

Abstract 

Leather tanning is one of the most important Palestinian industries that 

depend on the use of chemical compounds. The most important hazardous 

substance is chromium, due to the possibility of converting trivalent 

chromium to toxic Hexavalent chromium, which negatively affects the 

environment. 

There is a controversy over this transformation process; therefore, this 

research aims to study the possibility of oxidation the trivalent chromium to 

hexavalent chromium in the soil under natural conditions. The research was 

divided into three parts by using red and black soil and chromium sulfate 

solutions with different concentrations; 5306, 2653 and 1061 ppm. 

 The first part was done to analyze the original and chromium-saturated soil 

samples using X-ray Fluorescence (XRF) and X-ray Diffraction (XRD) 

techniques to examine the concentrations of the chemical components 

especially total chromium and elements or oxides that affect the adsorption 



XVII 
 

 
 

 

process of chromium onto the soil, such as manganese oxide, ferrous, and 

sulfur. XRF results have shown a high concentration of total chromium in 

both soils. Then the name of soil was known by the USDA soil triangle 

based on the particles size presented by the percentage of sand, clay, and 

silt in a soil sample that was calculated by hydrometer analysis.  

The second part was prepared chromium sulfate solutions of different 

concentrations that were exposed to two types of soil; red (silty clay) and 

black (silty loam). Effluent water was tested for the concentration of 

chromium as an indication of the adsorption capacity of the soil. Results 

have shown that black soil has a higher ability to adsorb chromium than red 

soil. The maximum adsorption capacity (qt) of black soil for chromium 

sulfate solutions as a function of time is found to vary with initial 

concentration. For an initial concentration of 5306, 2563, 1061 ppm, 

adsorption capacity was found to be 14.2, 7, 2.8 mg/g respectively. While 

for red soil, maximum adsorption capacity was found to be 12.1, 6.6, 2.7 

mg/g respectively. 

The third part was examined the presence of hexavalent chromium in 

water. Results have shown that theCr6+ concentrations were increased in 

red soil. While in black soil, the Cr6+ concentrations were decreased, but 

the results haven't a clear trend, the concentration was flocculated. Its 

depends on several parameter as pH, chemical elements in soil and 

presence of other compounds from nature or industry effluents. To support 

findings of this research, it is recommended that effect of pH on adsorption 

capacity should be tested on same type of soils.  



1 

 
 

 

Chapter One 

Introduction 

1.1 General Introduction 

Tanning wastewater is characterized by one of the highest toxicity 

intensities per unit of effluent; including high concentrations of organic 

compounds, dissolved solids, and heavy metals like chrome (Affiang et al., 

2018). Heavy metals affect human health, plant and animal life, as well as 

the quality of the limited water resources. When it is released into the 

environment, it may percolate through the soil and contaminate the 

groundwater. For instance, Cr3+ can undergo oxidation into hazardous 

Cr6+(WHO, 2016). 

Chromium is one of the most recognized pollutants in leather industry. 

Tanning process using chromium compounds for processing of hides. In 

this process about 60% - 70% of chromium reacts with the hides. In other 

words, about 30%- 40% of the chromium amount remains in the solid and 

liquid wastes (especially spent tanning solutions). The wastewater of 

tanning process is usually discharged, without proper treatment, into the 

sewerage system causing serious environmental impact(Abdulla et al., 

2010).  

In nature, chromium occurs in two major states Cr(VI) and Cr(III). Even 

when the tanning wastewater has chromium only in trivalent form, since the 

tanning process does not generate chromium (VI), some countries fixed 



2 

 
 

 

regulatory limits for the two species. This criterion appears from the 

assumption that the oxidation would be produced during storage and 

sometimes through the tanning process. Although chromium (III) oxidation 

to chromium (VI) occurs under specific environmental conditions, special 

attention is devoted to this transformation because chromium (VI) causes 

adverse effects for the human health, where Cr (VI) induced acute and chronic 

toxicity, neurotoxicity, dermatotoxicity, genotoxicity, carcinogenicity, 

immunotoxicity. Cr (III) has relatively low toxicity, when soluble Cr (III) is 

added to soil, manganese oxides present in the soil may cause oxidation to Cr 

(VI). When not oxidized to Cr (VI) form, Cr (III) may remain immobilized in 

the soil (Abdulla et al., 2010). 

1.2 Statement of the Problem 

In the West Bank, more than 15 tanneries discharge their wastewater 

contaminated with heavy metals into sewer system or to environment 

without treatment. Most of these tanneries are located in Hebron. Until 

now, wastewater treatment plants in Palestine cannot receive wastewater 

from leather plants due to the high amount of chemicals including heavy 

metals in wastewater that cannot be easily treated (Al-Jabari et al., 2017a).  

In Hebron tanneries, recycling Cr is currently based on the precipitation of 

Cr using alkaline material (lime). After separating the precipitated solids, 

sulphuric acid is added to dissolve Cr for its reuse. Sulphuric acid is a 

banned chemical in Palestine due to security measures; the existing 



3 

 
 

 

techniques are used for precipitating the Cr. Thereafter, the precipitate is 

sent to an ''Israeli'' company. Large waste disposal charges are being paid 

by the companies. This affects the economic development (Al-Jabari et al., 

2017a). 

For Nablus tannery, all effluents from all production process are mixed in 

open pool, they precipitate chrome by add ferric chloride (FeCl₃) with dose 

75 mg/L and let it precipitates for 24 hours, where sludge is heavily 

chromium contaminated. Nablus tannery is not allowed to send sludge to 

Zahrat Al-Fingan (Al-Jabari et al., 2017a), workers in that tannery said that 

they dispose it to the surrounding area, and supernatant is sent by tanks to 

open environment. Therefore, there is a need to track how soil adsorb 

chromium and study the evolution of chromium in different soil types, one 

from near area to Nablus tannery and another different one from another far 

area which is soil from Tammon town, Tubas. 

1.3 Significance of Work 

The idea of the project goes back to the controversy about the danger of 

wastewater resulting from leather tanning in terms of chrome in particular. 

This study aims to know the extent of the oxidation of non-toxic Cr(III) to 

toxic Cr(VI) in the tanning wastewater that used in Palestinian tanneries 

when add it to the soil by studying the effect of two variable parameters on 

chromium adsorption onto soil: soil type and chromium solution 

concentration. After obtaining the results, chromium  negative impact on 



4 

 
 

 

the environment is evaluated, and whether this effect is reduced through 

soil adsorption. Then, leather tanning can be considered a non-hazardous 

industry, and this encourages its continuity and supports it economically. 

1.4 Research Objective 

The main goal of this study is: first to investigate the possibility of 

oxidizing Cr3+present in the tanning effluents to Cr6+; second, measuring 

the ability of two soil types to absorb chromium from aqueous solutions of 

different concentrations. 

1.5 Research Questions 

• DoesCr3+ convert to Cr6+on different surface soil type? 

• To what extent can local soil effectively adsorb and store chromium? 

• How surface soil type and composition affect the adsorption capacity? 



5 

 
 

 

Chapter Two 

Literature Review 

2.1 Chromium Element 

Chromium is a transition metal; it has an atomic number of 24 and mass 

number of 51.9961. Its oxidation states range between −𝟐 and +6, but only 

the +3 and +6 states are the most stable ones under environmental 

conditions. These two oxidation states have different toxicity and mobility. 

Cr6+ is carcinogen and mobile, whereas Cr3+ is none toxic and immobile 

(Pass et al., 1974). 

A soluble Cr3+is used in the leather tanning industry that penetrates the 

hide and forms cross-links between the collagen fibers to give leather its 

durable finish. Although Cr3+is predominant in tanning solutions, the 

presence of Cr6+ raised critical questions about the thermodynamic stability 

of Cr3+. However, in natural systems, manganese oxides can oxidize Cr3+to 

Cr6+(Bartlett, 1991). 

2.2 Chemistry of Cr (III)  

Trivalent Cr, Cr (III), species are generally considered to be nonlabile 

because ligand displacement is slow (hours to days at room temperature) 

compared to most other metal ions (10−9- 10−3sec at room temperature) 

(Cotton and Wilkinson, 1980). Many Cr(III) complex species that are stable in 



6 

 
 

 

solution can be separated due to this kinetic inertness. As other trivalent 

metal ions, namely, Fe(III) and Al(III), the hydrated Cr(III) ion, 

Cr(OH2)6
3+, has a tendency to hydrolyze and this step is often 

accompanied by polymerization. Hydrolysis involves the conversion of a 

bound water molecule to the hydroxide ion and results in the release of a 

proton. Equilibrium measurements have identified the existence of the 

following species in solution:  

Cr(OH)2+, Cr(OH)2
+, Cr(OH)3, Cr2(OH)2

4+, Cr3(OH)4
5+and Cr4(OH)6

6+. 

(Smith and Martell, 1976) 

2.3 Chemistry of Cr (VI) 

Because hexavalent Cr is a strong oxidant, Cr(VI) varies with pH values, 

i.e pH dependent. Therefore, it is considered as soluble oxygenated species 

that are regulated by the equilibria below(Nieboer and Jusys, 1988). 

H2CrO4⇔H+ + HCrO4
−log (Ka1) = 0.6 …………….…(2.1) 

HCrO4
− ⇔H++ CrO4

2−log (Ka2) = −5.9 ……………… (2.2) 

Because the pH of environmental matrices only fluctuates from 3 to 10,

HCrO4
− and CrO4

2− are the dominant species. In addition, at concentrations of 

Cr(VI) greater than 0.01 M (molar), dimerization of the chromate ion 

occurs, yielding the dichromate ion. 

Cr2O7
2−+ H2O⇔2HCrO 4

−log (K) =−2.2 ………………..…. (2.3) 

when the chromate concentrations are below 0.01 M the existence of 



7 

 
 

 

dichromate is not expected to be significant, especially at physiological pH 

values of 7 to 8 (Subramanian et al., 2014). 

2.4 Oxidation of Cr(III) to Cr(VI)  

Oxidation of Cr(III) to Cr(VI) represents a significant environmental 

hazard because a relatively nontoxic species is transformed into a more 

toxic one. Manganese oxides are the only naturally occurring oxidant of 

Cr(III) and oxidation of Cr(III), in the presence of MnO2, was first 

observed by (Bartlett and James. 1979). They noted that Cr(VI) was present 

in the effluents of most soils reacted with Cr(III). Even the manganese 

oxide with the highest zero point charge and the most crystalline structure, 

pyrolusite, is an effective oxidant of Cr(III) (Earyand Rai, 1987; Saleh et 

al., 1989). Adsorption of Cr(III) by Mn oxides is possibly the first step in 

its oxidation by Mn. In soils, manganese oxides typically accumulate on the 

surface of clay and iron oxides at relatively high redox potentials.It was 

noted by (McKenzie, 1977) that Mn minerals tend to have large surface 

areas and high negative charge at all but extremely acidic pH. These 

properties are associated with high adsorptive capacities, particularly for 

heavy metals. Cr(III) can be oxidized to Cr(VI) in the presence of Mn4+ 

where Mn4+acts as the oxidizing agent and is reduced to Mn2+, as shown 

by the equation:  

2Cr3++ 3MnO2+ 2H2O⇔ 2CrO4
2− +3Mn2+ +4H+… (2.4) 

 



8 

 
 

 

2.4.1 Oxidation of Cr
3+

to Cr
6+

Under various Conditions 

Cr3+can be oxidized to Cr6+ under the following conditions 

• Presence of Oxygen: The unreduced chromium in the basic chromium 

sulfate is one of the sources of Cr6+. Oxidation of Cr3+to Cr6+by 

oxygen in air during the processes carried out at higher pH in leather 

manufacturing process. 

• Moderate high temperature: Cr3+ could be oxidized by oxygen at high 

temperature of 200–300 ℃. (Apteet al., 2006), burning of tanning 

sludge showed evidence of enhancement ofCr6+ concentration.  

• Dissolution in Water: Cr(OH)
3
 and MnO2suspension in water, 

Cr(OH)
3
slowly converted to dissolvedCr6+. 

2.5 Classification of Tanning Wastewater as Non-Hazardous versus 

Hazardous Waste. 

Based on Article 1 of the Environmental law (1999), hazardous substance 

defined as: “Hazardous Substance: Any substance or compound, which 

because of its hazardous characteristics poses a danger on the environment 

as toxic, radioactive, biologically infectious, explosive or flammable 

substances”(Elhamouz, 2011). 

Tanning of a 1000 kg of leather resulted a 600 to 700 kg of solid waste and 

40–50 m3of wastewater. Because of the included compounds, primarily 

heavy metals, processed leather waste has a significant environmental 

impact due to the primary consequences and risks: changes in landscape 



9 

 
 

 

and aesthetic discomfort, air pollution, surface water pollution, and changes 

in soil fertility, it is recommended to manage it effectively by recycling and 

recovery or storage in compliance landfills. As a result, tannery waste must 

be handled and kept properly to avoid leakage, odor issues, and air 

emissions (Rosu et al., 2015). 

In the tannery process, generally, tanning agents are used trivalent 

chromium (III) compounds. But, some leather products may contain traces 

of hexavalent chromium, which is considered a hazardous substance, and it 

may appear as a contaminant in the following situations: after UV exposure 

(at over 80°C) the fat-liquoring acids is possibly to lead to the oxidation of 

Cr(III); the formation of Cr(VI) may result in the process of the storage of 

fat liquored leather at 35% humidity. Also, in shoe production, the use of 

alkaline glues may contribute to the formation of Cr(VI) (Kolomaznik et al., 

2008).Cr(VI) may be formed in the leather by Cr(III) oxidation. The 

European Commission considered there was an unacceptable risk to human 

health in case of Cr(VI) presence in the leather goods and articles 

containing parts of leather that comes into contact with the skin (Regulation 

301, 2014).Cr(VI) usually exists in the form of H2Cr2O 7 and its salts and 

in the form of (Cr2O7)2−. Both anions (CrO4)2−and (Cr2O7)2−are water 

soluble and their formation are pH dependent. Above pH 7 predominates 

Cr(III) and below pH 6 predominates Cr(VI) (Fery, 2004). In the European 

legislation, it must be noted that the leather waste, containing chromium 

salts isn’t framed as hazardous waste; only the codes marked with an 



10 

 
 

 

asterisk (*) are considered as a hazardous waste (EC Decision, 2000, 

Government Decision, 2002). 

Palestinian standards comply with international standards. Various 

international systems are available for waste classifications and hazardous 

waste listing. These include: Basel convention for the control of trans-

boundary movement of hazardous waste and their disposal, European waste 

catalogue and hazardous waste list, and EPA-Hazardous Waste Listings in 

USA.Article 11 of the Palestinian Environmental Law No.(7) 1999. The 

"Palestinian National Strategy for Solid Waste Management in the 

Palestinian Territory 2010-2014" (NHWMP) constitutes the framework for 

all decisions, programs, and plans aiming at developing the solid waste 

sector in the West Bank and Gaza Strip. It aims to preparing and publishing 

a list of categories of hazardous waste. International development agencies 

have motivated activities and funded projects (Al-Jabari, 2014). 

Leather industry is believed by community and by some officials to be as 

one of a producer of hazardous waste (in its solid waste containing residues 

of chromium). However, such waste is not hazardous since the usedCr3+ in 

leather tanning is not toxic and thus non-hazardous, although still 

considered to be a pollutant (Al-Jabari, 2014). 

2.6 Toxicity of Cr  

Cr (VI) is more toxic and soluble whereas Cr (III) is relatively nontoxic 

and insoluble.  

Chromate is toxic because it is a strong oxidizing agent, corrosive, and a 



11 

 
 

 

potential carcinogen (National Research Council, 1974). The chromate ion 

is a class human carcinogen by inhalation and an acute irritant to living 

cells, and of all the metal carcinogens Cr exhibits properties most nearly 

consistent with a mutagenic initiation model (Subramanian et al., 2014).  

Systemic toxicity may occur in both the oxidation states, mainly because of 

increased absorption of Cr through broken skin that results in renal 

chromate toxicosis, liver failure, and eventually death (Lippmann, 2000). 

Acute exposure of rats to Cr (VI) by various routes of administration 

affected mainly the liver and kidneys (USEPA, 1980). Soluble salts of 

chromates are also highly toxic when administered parenterally, with an 

LD50 of 10-50 mg/kg, compared to LD50values of 200-350 and 1500 

mg/kg obtained from dermal or oral exposure, respectively. Conversely, 

oral administration of Cr (III) compounds is relatively nontoxic. Other 

effects of Cr (VI) poisoning include gastric distress, olfactory impairment, 

nosebleeds, liver damage, and yellowing of the tongue and teeth 

(Subramanian et al., 2014). 

The Cr (VI) ion is readily taken up into eukaryotic cells by anion-carrying 

proteins, where it is reduced to Cr (III) by a number of cytoplasmic 

reducing agents. The reduction of Cr (VI) to Cr (III) causes the generation 

of oxygen radicals in cells that can produce DNA damage. Additionally, 

the Cr (III) formed can become adducted to the DNA. Recent studies have 

shown that Cr (VI) is very potent in forming DNA protein cross links. This 

complex typically involves the binding of Cr (III) to the phosphate 

backbone of DNA and cross-linking to a protein (Lippmann, 2000). This 



12 

 
 

 

cross-linking may lead to increased mutagenicity and is probably more 

significant in determining the mutagenicity of Cr than the oxidative DNA 

damage produced by oxygen (Lippmann, 2000).  

2.7 Chromium Cycle in Soil and Water 

The starting point for the Cr cycle is Cr6+and in most soil conditions, 

reduction reactions are more preferred. In soil solution, chromate formation 

is pH dependent and dominated by HCrO4- or CrO4
2−through 

adsorption/precipitation reactions, absorption by plants, or leaching from 

the subsurface layers, Chromium may be extracted from the soil. Some of 

theCr6+ are also reduced by carbon to Cr3+also there is electron donors as 

Fe2+or S2-.This process, called dechromification, which reduced Cr6+ 

toCr3+.(Subramanian et al., 2014). 

Reduced Cr3+is bound by ligands such as citrate in soil solution that deliver 

Cr3+to MnO2surfaces where both the organic ligand and Cr3+are oxidized. 

The organic ligand is often recycled because Mn3+created by reverse 

dismutation accepts electrons from the Cr3+in preference to those from the 

organic ligand and thus oxidizes only Cr. When organic ligands are in 

extreme concentrations, they appear to induce reverse dismutation ofMnO2 

by linking the Mn3+, and this Mn3+ organic complex can prevent or 

decrease the formation of Cr6+. (Subramanian et al., 2014). 

 



13 

 
 

 

2.8 Chromium Behavior in Soil 

The contamination of chromium in soil and groundwater due to tannery 

waste has been investigated. Surface soil and water samples were obtained 

from several locations near tanneries, and then analyzed for total Cr. The 

concentration of soil chromium was reduced due to leaching and chromium 

in groundwater was increased. Results showed that the soil near tannery 

industries is polluted, but there is no determination of the existence of Cr 

(Rangasamy et al., 2015). 

Bartlett analyzed soil and water that contain Cr naturally or from any 

contamination source not especially from tannery wastes. The cycling of 

chromium in soils and in waters are between  and  reduction of Cr6+, but there 

are gaps in oxidation of Cr3+understanding factors controlling oxidation-

reduction processes. If solubleCr3+ is added to soil, it will oxidize by 

manganese oxides toCr6+.(Bartlett, 1991). 

The oxidation of Cr3+to Cr6+was examined under three different 

conditions: (1) Cr2O
3
 was heated in the presence of oxygen; (2)Cr (OH) 

3
 

and MnO2 mixtures were suspended at different pH values in aerobic or 

anoxic aqueous media, and (3) Cr (OH) 
3
-MnO2 mixtures interacted in wet 

aerobic conditions (Apte et al., 2006). 

Results indicate that Cr3+in Cr2O
3
 could be converted to Cr6+at a 

temperature range of 200–300C, with conversion rates of up to 50% in 

12 h. Cr (OH) 
3
was slowly converted to dissolved Cr6+ in the presence of 

MnO2 , both in aerobic and anoxic conditions, with conversion rates of up 



14 

 
 

 

to 1% in 60 days. In moist aerobic conditions with rates up to 0.05% in 90 

days. Chromium oxidation also occurred in sludge samples, especially 

under aerobic conditions up to 17% conversion in 30 days (Apte et al., 

2006). 

The reduction kinetics ofCr6+ in soils and its correlation with soil 

properties was studied by (Xiao et al., 2012). The reduction of Cr6+in soils 

was positively related to organic matter content, dissolved organic matter 

content, Fe2+content and clay fraction, but negatively correlated with Mn 

content. In natural soils, the reduction process ofCr6+is not regulated by a 

single soil property, but by the combined effects of dissolved organic 

matter,Fe2+, pH, and distribution of soil particle size (Xiao et al., 2012). 

2.9 Chemistry of Soil Cr 

The concentration of Cr in soil equals to (the amount present in the  

parent material from which the soil was formed plus the amounts added 

through wind, water, and human activities minus the amounts removed 

through leaching, surface runoff, volatilization, and phyto uptake).  

Cary, 1982; Bartlett and James, 1988; Fendorf 1995; Proctor et al., 1997 

reviews factors influence the transformations between Cr(VI) and Cr(III) in 

soils. 

2.9.1 Solid-Phase Speciation of Cr in Industrial Contaminated Soils  

Up to the researcher’s knowledge, few studies to date have examined the 



15 

 
 

 

fractionation of total Cr in industrial contaminated soils. Of the clay loam, 

loam, and sandy clay loam soils collected from a heavy metal contaminated 

site, (Wasay et al., 1998) studied the fractionation of total Cr only in the 

clay loam soil. (Fiedler et al.,1994), propose and used these quential 

extraction scheme and found that of the total 832 mg/kg of total Cr, only 

486.9 mg/ kg (58.5%) was bounded to the organic soil fraction.  

(Phillips and Chapple., 1995) used the sequential extraction scheme of 

(Tessier et al., 1979) to fractionate total Cr along with other metals from a 

soil collected from a former industrial site. Chromium concentrations in all 

soil fractions were low, and approximately 80% of the total Cr were 

associated with organic and oxide fractions with negligible concentrations 

detected in the exchangeable and carbonate fractions.  

In another study by (Maiz et al., 1997), soil samples collected from a 

polluted mine works, steel factory and highway emissions were 

sequentially extracted to find the partitioning of total Cr and other metal 

fractions. A short three step sequential extraction scheme was compared 

with other modified extraction scheme (Tessier et al., 1979) and (Ure et al., 

1993). However, total Cr was found to be predominantly partitioned in the 

residual fraction of the soils using all three extraction schemes.  

In all the aforementioned studies, Cr fractionation was investigated 

simultaneously with several other metals and the concentration of Cr 

present in the soils was not very high. In addition, no attempt was made to 

distinguish between Cr(III) and Cr(VI). 



16 

 
 

 

2.10 Solubility of Cr(III) and Cr(VI)  

Solubility and availability of Cr(III) in soil solution are critical for the 

oxidation of Cr(III) to Cr(VI) in soils. At soil pH value of greater than 5.5, 

the solubility of Cr(III) decreases due to its precipitation as Cr(OH)3. 

Complexation of Cr(III) with some of the low molecular weight organic 

acids such as citrate and gallic acid increases its solubility and mobility 

even at higher pH, there by facilitating its oxidation. 

Bartlett and James(1983) compared the oxidizing tendencies of four forms 

of Cr(III) added to a field moist soil incubated for 15 day. The four forms 

were freshly precipitated Cr(OH)3, Cr citrate, aged Cr(OH)3, and aged 

Cr(OH)3with citrate. The maximum Cr(VI) levels observed decreased in 

the order freshly precipitated Cr(OH)3> Cr citrate > aged Cr(OH)3in citrate 

> agedCr(OH)3 (Bartlettand James, 1983). 

The oxidation of Cr(III) in tannery waste amended to three soil types was 

studied by Milacic and Stupar (1995). Their fractionation study showed 

that after 5 months, 1.1% of the total Cr added was oxidized in clay, 0.45% 

in sand, and only 0.03% in peat soil. The degree of Cr(III) oxidation was 

found to be proportional to the concentration of manganese (IV) oxides and 

water-soluble Cr(III) in the soils. They also observed a decrease in the 

concentration of water soluble Cr and Cr(VI) on continuance of the 



17 

 
 

 

experiment because Cr was redistributed to more sparingly soluble 

fractions (Milacic and Stupar, 1995). 

2.11 Partitioning and Mobility of Cr  

The solid phase speciation studies clearly indicate that at low 

concentrations of total Cr either in natural or contaminated soils, most of 

the total Cr is partitioned in the residual fraction. In highly organic soils a 

significant portion of total cris partitioned in the organic fraction and 

equally partitioned either in the Fe oxide or residual fractions. The 

concentration of Cr in the water soluble and exchangeable fractions is very 

low and indicates low mobility of Cr from these soils(Subramanian et al., 

2014). 

Although in highly contaminated soils Cr is partitioned predominantly in 

the organic and Fe oxide fractions, a significant amount of Cr existed in 

water soluble and exchangeable fractions. The determination of 

exchangeable Cr(III) is necessary because if soil pH conditions are 

favorable, and in the presence of MnO2, this fraction could become 

available for oxidation to toxic Cr (VI). (Milacic and Stupar., 1995) used 

1 M NH4Cl for the exchangeable fraction and 0.015 M K2HPO4for the 

water-soluble fraction of Cr in soils. However, information on the labile 

and exchangeable pools of Cr(VI) in contaminated soils is lacking, and 

such information may be useful in understanding the desorption chemistry 

of Cr(VI) and is essential for the development of a suitable remediation 

strategy for contaminated soils.  



18 

 
 

 

2.12 Adsorption and Desorption of Cr(III) Onto Soil Particles 

Cr(III) has been shown to be sorbed strongly onto soil minerals, to be 

bound to soil organic matter, and to form mineral precipitates (Bartlett and 

Kimble 1976; Cary et.al, 1977; Rai et.al, 1987, 1989; Bartlett and James, 

1988; Palmer and Wittbrodt, 1991). Sorption of Cr(III) decreases when 

other inorganic cations or dissolved organic ligands are present in solution.  

Fendorf et al., (1994) and Fendorf and Sparks., (1994) have studied the 

mechanism of Cr(III) sorption on silica using extended absorption fine 

structure spectroscopy and found that Cr(III) formed a monodentate surface 

complex on silica. Arnfalk et al., (1996) studied Cr(III), and Cr(VI) 

retention on 14 different types of minerals and soil materials considering 

both pH dependency and other soil physicochemical parameters. The 

results verified the importance of geochemical parameters of soils such as 

organic content, type of clay mineral, presence of complexing ions, and 

redox potential for controlling metal uptake. Montmorillonite (in bentonite 

and smectite) showed the highest retention of Cr(III) among all minerals 

and soil materials, whereas illite and kaolinite showed lower retention than 

the soils. The clay mineral montmorillonite showed highest retention 

because it had the highest surface activity (Kashef, 1986).  

The difficulty in displacing Cr from smectite (caly mineral) indicates that the 

Cr is bonded specifically because if Cr was held through outer sphere 

complexes, the smallest hydroxy polymers would be readily displaced by 

Ca2+(Dubbin and Goh, 1995). Drljaca et al.(1992) found that, while the 



19 

 
 

 

montmorillonite was still wet, the adsorbed Cr could be easily exchanged with 

other cations but, upon drying, Cr becomes virtually nonexchangeable. These 

authors suggested that as the inter layer region collapsed due to loss of water, 

Cr came into close contact with the siloxane surface, allowing inner sphere 

complexes to form. Cr(III) is held strongly, likely through covalent bonds, 

and its displacement is extremely difficult through simple exchange 

reactions. However, the potential for Cr(III) to be oxidized to the more 

toxic Cr(VI) form is of some concern because of the instability of bonding 

under strong oxidizing conditions.  

Both adsorption and precipitation reactions and both specific and 

nonspecific reactions are possible for the retention of Cr(III) in soils. 

However, organically complexed Cr (III) could be available in soil solution 

even at high soil pH for oxidation to toxic Cr (VI) in soils.  

Cr (VI) is immobilized in soils and mechanisms of Cr (VI) immobilization 

are CaCrO4 precipitation and recrystallization with Fe hydroxides (Shi et 

al., 2020). 

2.13 Soil texture  

Soil texture can be determined by using quantitative methods such as the 

hydrometer method based on Stokes' law. Soil texture focuses on the 

particles that are less than two millimeters in diameter which 

include sand, silt, and clay.  

Twelve major soil texture classifications as shown in Figure 2.1 are defined 

by the United States Department of Agriculture (USDA)(United States 

https://en.wikipedia.org/wiki/Stokes%27_law
https://en.wikipedia.org/wiki/Sand
https://en.wikipedia.org/wiki/Silt
https://en.wikipedia.org/wiki/Clay


20 

 
 

 

Department of Agriculture, 1987). The twelve classifications are sand, 

loamy sand, sandy loam, loam, silt loam, silt, sandy clay loam, clay loam, 

silty clay loam, sandy clay, silty clay, and clay. Soil textures are classified 

by the fractions of each soil separate (sand, silt, and clay) present in a soil. 

Classifications are typically named for the primary constituent particle size 

or a combination of the most abundant particles sizes, e.g. "sandy clay" or 

"silty clay". A fourth term, loam, is used to describe equal properties of 

sand, silt, and clay in a soil sample, and lends to the naming of even more 

classifications, e.g. "clay loam" or "silt loam"(Soil Survey Division Staff , 

1993). 

Determining soil texture is often aided with the use of a soil texture triangle 

plot. One side of the triangle represents percent sand, the second side 

represents percent clay, and the third side represents percent silt. If the 

percentages of sand, clay, and silt in the soil sample are known, then the 

triangle can be used to determine the soil texture classification.  

Chemical and physical properties of a soil are related to texture. Particle 

size and distribution will affect a soil's capacity for holding water and 

nutrients. Fine textured soils generally have a higher capacity for water 

retention, whereas sandy soils contain large pore spaces that allow 

leaching.  

 

 

 

https://en.wikipedia.org/wiki/Loam
https://en.wikipedia.org/wiki/Clay
https://en.wikipedia.org/wiki/Loam
https://en.wikipedia.org/wiki/Ternary_plot
https://en.wikipedia.org/wiki/Ternary_plot


21 

 
 

 

 

Figure 2.1: Soil texture triangle showing soil textures as determined by the proportion of 

sand, silt and clay. Source: (United States Department of Agriculture, 1987). 

 

 

 



22 

 
 

 

2.14 AnalyticalMethods for Determining Chromium Concentration in 

Various Sample Matrix. 

Table 2.1 shows summery for analytical methods for determining 

chromium in environmental sample matrices (Services, H, 2002).



23 

 
 

 

Table 2.1: Analytical methods for determining chromium in environmental samples. 

Sample matrix Preparation method Analytical method Sample detection limit Percent recovery 

Drinking water, surface 

water, and certain domestic 

and industrial effluents 

(dissolved chromium(VI)) 

Complex chromium(VI) in water with 

APDC at pH 2.4 and extracted with 

MIBK 

AAS 2.3 µg/L No data 

Drinking water, groundwater 

and water effluents 

(chromium(VI)) 

Buffer solution introduced into ion 

chromatograph. Derivitized with 

diphenylcarbazide 

Ion chromatography 

with post-column 

derivatization and UV-

VIS detection 

0.3 µg/L 100% at 100 µg/L 

Waste water and industrial 

effluent for chromium(VI) 

only 

Buffered sample mixed with AICI3 and 

the precipitate separated by 

centrifugation or filtration 

DPPA at pH 10–12 30 µg/L 90% at 0.2 mg/L 

Waste water 1986 

(chromium(VI)) 

Derivatization with o-

nitrophenylfluorone 

UV-VIS spectrometry 

at 582 nm 

Lower than 

diphenylcarbazone method 
No data 

Water (total chromium) 

Calcium nitrate added to water and 

chromium is converted to 

chromium(III) by acidified H2O2 

GFAAS or ICP/AES 
1.0 µg/L (GFAAS) 

7.0 µg/L 

97–101% at 19–77 

µg/L 

Water (chromium(III) and 

chromium (VI)) 

Solid-phase extraction using anion 

exchange resins for Cr(VI) adsorption 

and chelating resins for Cr(III) 

adsorption 

ICP-MS 

0.009 µg/L (chromium 

VI); 

0.03 µg/L (chromium III) 

86–113% 

Industrial wastes, soils, 

sludges, sediments, and other 

solid wastes (total 

chromium) 

Digest with nitric acid/hydrogen 

peroxide 
ICP-AES 4.7 µg/L 

101% at 3.75 

mg/L 

Oil wastes, oils, greases, 

waxes, crude oil (soluble 

Dissolve in xylene or methyl isobutyl 

ketone 
AAS or GFAAS 0.05 mg/L 107% at 15 µg/L 



24 

 
 

 

chromium) 

Groundwater, domestic and 

industrial waste 

(chromium[VI]) 

Chromium(VI) is coprecipitated with 

lead sulfate, reduced, and resolubilized 

in nitric acid 

AAS or GFAAS 
0.05 mg/L (AAS) 2.3 µg/L 

(GFAAS) 

93–96% at 40 

µg/L 

Groundwater-EP extract, 

domestic, and industrial 

waste (chromium[VI]) 

Chelation with ammonium pyrrolidine 

dithiocarbamate and extraction with 

methyl isobutyl ketone 

AAS No data 96% at 50 µg/L 

Water, waste water, and EP 

extracts (chromium(VI)) 
Direct DPPA 10 µg/L 93% at 5 mg/L 

Soil, sediment and sludges 

(chromium(VI)) 

Acid digestion extraction using hot 

HNO3 
GFAAS No data No data 

Sediment (total chromium) 
Samples digested with HNO3 and HF 

and dried 
XRF No data No data 

Sediment 
Acid digestion using 0.5N HCI 

followed by filtration 
AAS No data 94.88% 



25 

 
 

 

In this study the total chromium concentrations were determined by flame 

atomic absorption spectrophotometry (iCE 3000, wavelength 357.9 nm)at 

the chemistry department lab in An-Najah University using Analytical 

Methods for Atomic Absorption Spectroscopy (Perkin Elmer Coorporation, 

1996). This method is simple, rapid, and applicable to a large number of 

environmental samples including, but not limited to, ground water, aqueous 

samples, extracts, industrial wastes, soils, sledges, sediments, and similar 

wastes. Analysis for dissolved elements does not require digestion if the 

sample has been filtered and then acidified. There are many obstacles and 

difficulties during the study that can be summarized as follows: 

1. The chemical analysis of soil elements using the ICP_MS device did 

not give the required result because it checks the total chromium 

concentration to a certain limits. In addition, it does not show the 

result of basic soil components such as silica, which affects the 

calculations. It's also needs a long time for the examination process. 

2. Soil analysis by using XRF and XRD techniques is expensive and 

time consuming due to the use of external laboratories. It also does not 

find Na concentration and the LOI of the soil sample, so it must be 

determined to complete the calculations. 

 

 

 

 



26 

 
 

 

Chapter Three 

Methodology 

The methodology adopted in this study consists of three stages:  

First stage: determine the structural properties of soils by hydrometer 

analysis to get the particle sizes and use it to know the type of soil, and 

chemical properties by XRF that find the concentrations of elements oxides 

and XRD techniques to get the crystallization form.  

Second stage: determination the ability of two different soil types to uptake 

total chromium from different chromium sulfate solutions (Cr2(SO4)3) 

concentration with time. This includes the possibility of the formation 

of Cr6+ in two different soils types with time as a result of chromium 

oxidation. Also, study the adsorption kinetics for chromium solutions in 

soil. 

Third stage: study the formation ofCr6+ from different initial Cr3+ liquid 

solutions of different concentrations without soil.  

3.1 Experimental Setup and Design of Experiments 

To reach the main goal of this study, laboratory work was done to examine 

soil samples, prepare chromium sulfate solutions, and conduct the 

necessary tests for water and soil samples as detailed in the subsequent 

parts. 

 

 



27 

 
 

 

3.1.1 Soil Textural Analysis 

The percentage of clay, silt and sand were calculated by Hydrometer 

Analysis test to know the particle size and predict the soil name, then use 

the result to calculate permeability and porosity.This method has a 

detection limit of 2.0 % for sand, silt, and clay. 

Hydrometer Analysis Test 

This method was used to estimate the distribution of soil particle sizes from 

0.075 mm sieve to 0.001 mm. This analysis is based on Stoke’s law 

governing the rate of sedimentation of particles suspended in water. 

Equipments: Hydrometer (ASTM H-152), 1000 mL cylinder hydrometer 

jar, mixer, dispersion agent (Sodium hexa meta phosphate, Na6(PO3)6 ), 

and Thermometer. 

Procedure:  

Control jar was prepared by adding 125 mL of 4% Sodium hexa meta 

phosphate solution with distilled water to produce 1000 mL. The 

hydrometer was then inserted and adjusted tozero. Then 50 gm of soil 

(passing sieve No.200)were mixed with 125 mL of 4% Sodium hexa meta 

phosphate solution and were allowed to stand for 12 hours. Themixture was 

then transferred to a dispersion cup and water was added until two-third 

full, the solution was transferred to the sedimentation cylinder and water 

was added to 1000 mL. The cylinder was capped with rubber stopper and 

was agitated for 1 minute. 



28 

 
 

 

Sedimentation and control cylinder were put beside each other and 

stopwatch (cumulative time t=0) was started. Hydrometer readings were 

taken at cumulative time t=0.25, 0.5, 1 and 2 minutes, then the hydrometer 

was placed in the control jar. Readings were continued at 5, 15, 30 and 60 

minutes then at 2, 4, 8, 24 and 48 hours. 

The following equations were used to find the soil’s structural 

properties:Das, B. (2002) 

𝑅𝑐𝑝 = 𝑅 + 𝐹𝑡 − 𝐹𝑧 ………………...….. (3.1) 

where, 

R: Hydrometer reading. 

Fz: Zero correction, if the zero reading in hydrometer (in control cylinder) 

is below the water meniscus, its (+), if above its (-) and if at the meniscus 

its zero. 

Ft: Temperature correction which approximated as 

𝐹𝑡 = −4.85 +  0.25 𝑇   ………………….. (3.2) 

For (T between 15-28 C) 

𝑃𝑒𝑟𝑐𝑒𝑛𝑡 𝐹𝑖𝑛𝑒𝑟 =
𝑎∗𝑅𝑐𝑝∗100

𝑊𝑠
  ………………..(3.3) 

where, 

Ws= dry weight of soil used for hydrometer analysis. 

a= correction for specific gravity given by  

𝐆𝐬=(
Wtofdrysoil

Wtofequalvolumeofwater
)…………… …………(3.4) 



29 

 
 

 

𝒂 =
(𝟏.𝟔𝟓∗𝑮𝒔)

(𝑮𝒔−𝟏)(𝟐.𝟔𝟓)
………………………..……..(3.5) 

𝑅𝑐𝑙 =  𝑅 +  𝐹𝑚…………………….……(3.6) 

Rcl: corrected hydrometer reading for determination of effective length. 

Fm: Difference between the upper level of meniscus and water level of 

control cylinder. 

Effective length (L (cm)) corresponding to Rcl given in Table 3.1 then 

determine A from Table 3.2 at different temperatures and Gs, where A is 

varying with Gs. 

Table 3.1: Effective length corresponding to hydrometer reading.  

Source: https://www.labguider.com/hydrometer-analysis/ 

Hydrometer 

reading 

 

L 

(cm) 

Hydrometer 

reading 

 

L 

(cm) 

Hydrometer 

reading 

 

L 

(cm) 

Hydrometer 

reading 

 

L 

(cm) 

0 16.3 13 14.2 26 12 39 9.9 

1 16.1 14 14 27 11.9 40 9.7 

2 16 15 13.8 28 11.7 41 9.6 

3 15.8 16 13.7 29 11.5 42 9.4 

4 15.6 17 13.5 30 11.4 43 9.2 

5 15.5 18 13.6 31 11.2 44 9.1 

6 15.3 19 13.2 32 11.1 45 839 

7 15.2 20 13 33 1.09 46 8.8 

8 15 21 12.9 34 10.7 47 8.6 

9 14.8 22 12.7 35 10.6 48 8.4 

10 14.7 23 12.5 36 10.4 49 8.3 

11 14.5 24 12.4 37 10.2 50 8.1 

12 14.3 25 12.2 38 10.1 51 7.9 

 
 

 

 

 

 

https://www.labguider.com/hydrometer-analysis/


30 

 
 

 

Table 3.2: Variation of A with Gs at different temperatures 

Gs Temperature(C) 

 17 18 19 20 21 22 23 

2.5 0.0149 0.0147 0.0145 0.0143 0.0141 0.0140 0.0138 

2.55 0.0146 0.0144 0.0143 0.0141 0.0139 0.0137 0.0136 

2.6 0.0144 0.0142 0.0140 0.0139 0.0137 0.0135 0.0134 

2.65 0.0142 0.0140 0.0138 0.0137 0.0135 0.0133 0.0132 

2.7 0.0142 0.0138 0.0136 0.0134 0.0133 0.0131 0.0130 

2.75 0.0138 0.0136 0.0136 .0133 0.0131 0.0129 0.0128 

2.8 0.0136 0.0134 0.0134 0.0131 0.0129 0.0128 0.0126 

Gs Temperature(C) 

 24 25 26 27 28 29 30 

2.5 0.0137 0.0135 0.0133 0.0132 0.0130 0.0129 0.0128 

2.55 0.0134 0.0133 0.0131 0.0130 0.0128 0.0127 0.0126 

2.6 0.0132 0.0131 0.0129 0.0128 0.0126 0.0125 0.0124 

2.65 0.0130 0.0129 0.0127 0.0126 0.0124 0.0123 0.0122 

2.7 0.0128 0.0127 0.0125 0.0124 0.0123 .0121 0.0120 

2.75 .0126 0.0125 0.0124 0.0122 0.0121 0.0120 0.0118 

2.8 0.0125 0.0123 0.0122 0.0120 0.0119 0.0118 0.0117 
Source: https://www.labguider.com/hydrometer-analysis/ 

 

𝐷 =  𝐴√(
𝐿

𝑡
)  ………………………..……... (3.7)  

where, 

D: diameter of particle (mm) 

L: Effective length (cm) 

t: Time (min) 

Plot a grain-size distribution graph on semi-log graph paper with percent 

finer on the natural scale and D on log scale. 

Equations for porosity and void ratio calculations 

𝛾𝑑 =
(𝐺𝑠∗𝛾𝑤)

(1+𝑒)
…………………………..…. (3.8) 

e=((𝐺𝑠 *γw)/(γd))-γd …………..………….….(3.9) 

 

https://www.labguider.com/hydrometer-analysis/


31 

 
 

 

where, 

γd = Dry unit weight (kN/m3) 

γw = Unit weight of water (kN/m3) 

e= void ratio  

𝑷𝒐𝒓𝒐𝒔𝒊𝒕𝒚 (ƞ) =  
𝒆

(𝟏+𝒆)
………….....……..(3.10) 

3.1.2 Soil Sample Preparation 

3.1.2.1 Soil Samples for XRF 

Two original soil samples were prepared, the first one was red soil and the 

second one was black, each soil type was ground manually to a fine powder 

using alumina mortar and pestle to get 5 gm for each. Part of the soil that 

adsorbed chrome was taken and dried in the oven at a temperature of 40-60 

℃ for one hour, then ground to get another 5 gm from red and black soil. 

3.1.2.2 Epoxy 

Monomer and initiator were mixed with ratio 2:1 for 5 minutes, some soil 

that was dried in the oven were put in a special plastic cup, then the 

mixture was poured over soil and was dried with cool air for 15 minutes to 

get out the air bubble, after that the sample were dried overnight. 

Grinding and polishing were done for both soil samples. Grinding was 

done by using grinding paper with 320,600 and 1200 P respectively. 



32 

 
 

 

However, the polishing was done by using 0.1 μm Al2O3 and 0.04 

μm Al2O3. 

3.1.2.3 Sodium Element Analysis in Soil Samples 

The Flame Photometric method was used in sample preparation as follow:  

Fresh and treated samples from both red and black soil were grounded, and 

then burned in the oven at a temperature of 500℃ for one hour. Nitric acid 

and water were added to the soil then filtered and diluted to get asolution 

for the test with Flame Photometer apparatus.  

3.1.2.4 Loss of Ignition (LOI) Test 

Fresh and treated samples from both red and black soil were grounded. 

Four empty crucibles were put in the oven at 1000℃ for 1 hour, then 

cooled before weight to record theinitial weight (W1). Soil samples were 

put in crucibles and reheat at 1000 oC for 1 hour and cooled in desiccator, 

finally weight the crucibles and record (W2). Then the LOI was calculated 

as: 

LOI =
W1−W2

W1
∗  100……………………. (3.11) 

3.1.2.5 Instrumental Analysis of Soil Samples 

Two types of soils were analyzed before and after contaminated with 

chromium sulfate solution. The techniques used were: (1) X-ray powder 

diffraction for phase composition; (2) X-ray fluorescence (XRF) for their 

elemental composition; (3) scanning electron microscopy (SEM) 

complemented with energy dispersive X-ray spectrometry (EDX) for 



33 

 
 

 

imaging of grain morphology and elemental microanalysis. Samples were 

prepared adequately for each instrumental technique. Sample preparation, 

type of machine, analytical procedure, and measurement conditions are 

described below. 

1. X-ray powder diffraction (XRD): A small portion of each soil sample 

was ground manually to a fine powder using alumina mortar and pestle, 

transferred to a plastic vial and labeled. A small amount of this powder 

is spread on a sample holder made of a silicon slice. This silicon slide is 

cut off axis to avoid parasite XRD diffraction peaks. The soil powder 

samples were analyzed using a Malvern PA NalyticalAerisdiffracto 

meter with a copper target.It is operated at a voltage of 40kV and a 

current of 15mA. The measurements were carried out for a 2 theta 

range of 5-65º, with a continuous scan and a step size of 0.0109º. The 

raw binary file was then processed and interpreted using Jade10 

software available in the XRD lab at Arizona State University (ASU) 

Goldwater Materials Science Facility (GMSF), Tempe, Arizona (AZ), 

USA. 

2. X-ray fluorescence (XRF): The same aforementioned powdered sample 

was used for XRF. Analyses were done using a Bruker S2 

Pumamachine. This machine has a silver cathode (target) to generate 

X-ray and is used in energy dispersive mode (EDX) with energy 

resolution of 0.139 keV. However, it has a limitation on light elements 

(< Na) analysis. Spectral results are converted to normalized elemental 

wt. % and light elements are included in the sum. As elemental 



34 

 
 

 

compositions of ceramics, rock, minerals and soils are conventionally 

reported as oxide percentages, the elemental concentrations of the 

samples are recalculated to express them as oxides. This machine is 

available at ASU’s Goldwater Materials Science Facility (GMSF), 

Tempe, AZ, USA.   

3. Scanning electron microscopy (SEM) complemented with energy 

dispersive X-ray spectrometry (EDX): Two samples (𝐵1i and B1f
) that 

were embedded in epoxy resin, ground, polished well were observed 

with a FEI XL-30 Environmental-Cell Scanning Electron Microscope 

with Field Emission Gun (ESEM-FEG) at LeRoy Eyring Center for 

Solid State Science, Arizona State University. This microscope offers 

high resolution secondary electron imaging. As the samples are non-

conductive both were coated with a thin film of gold using sputtering 

(physical vapor deposition or PVD). It is also equipped with secondary 

electrons as well a back scattered electron (BSE) detector and X-ray 

energy dispersive detector (EDX) for imaging in BSE mode and for 

elemental analysis, respectively, when needed. This scope has a 

spacious vacuum chamber for large specimens and also a large stage to 

hold multiple samples. Samples were observed using an accelerating 

voltage of 30kV at a working distance of 10-11mm. The secondary 

electron images (SEI) of the surface features of the samples presented 

here were obtained at a varying magnification as low as 36 x and up to 

3500 x. Point and area quantitative microanalyses were conducted on 

different locations (grains), as needed, by switching to EDX mode.  



35 

 
 

 

The following equations were needed to determine oxides percentage in 

soil samples to get the results presented in Table (4.8), see Appendix A. 

Oxidewt =  (Atomic wt ∗ element atom) + (oxygen atom ∗

Atomic wt ofoxygen)…………..… (3.12) 

 Mole of  Element =  
Wt.% Normalized

Atomic wt
………     .………..(3.13) 

 Mole of Oxide =  
 Mole of  Element

 Oxygen atom in oxide formula
……         ……...(3.14) 

Wt. % Oxide =  
Mole of Oxide

Oxide wt
……………     …….....(3.15) 

Normalized Oxide =  (
Wt.% Oxide

∑ Wt.% Oxide 
) ∗ 100……………….(3.16) 

Oxide Wt% =  Normalized Oxide − (1 − LOI)………..(3.17) 

3.1.3 Soil type Ability to Adsorb Chromium. 

In this stage two types of soils and three concentrations of chromium 

sulfate solutions were used for each soil type. 

Chromium sulfate solution of concentration 5306 ppm was prepared by 

adding 10 gm of chromium sulfate to 0.5 L of water. For the 2653 ppm 

concentration, 5 gm of chromium sulfate was added to 0.5 L. Same 

concentration was prepared by adding 10 gm to 1L. Finally, 2 gm were 

added to 0.5L to have 1061 ppm. A170 gm of red soil from Bait Leed, 

Tulkarm and 170gm of black soil from Tammon, Tubas were put in conical 

flask which was 0.5L volume. This was done with 6 flasks, 3 for red soil 

and another 3 for black soil. Also 170 gm of redsoil were added to flask of 



36 

 
 

 

1L volume. 0.5L solutions for each concentration were put in soil type, 3 

different concentrations for red and another 3 for black. Finally, the 

prepared 2653 ppm solution in 1L was added to red soil in flask of 1L 

volume. All flasks were opened to the atmosphere as shown in Figure (3.3). 

A 20 ml samples from each flask were taken each time for sampling. Initial 

pH values were recorded for each sample and found to be 4.13.  

 

Figure 3.3: Chromium sulfate solution with different concentration in different soils. 

3.1.3.1 Sample Preparation and Method of Total Chromium Test 

Each Sample taken from flasks was diluted three times with a dilution 

ratio; 1:100, 2:100 and 3: 100. Standard chromium sulfate solutions were 

prepared with concentrations; 5, 10, 20, 30, 40 and 50 ppm. 

Total chromium concentrations were tested by Flame Atomic Absorption 

Spectrometer, iCE 3000, wavelength 357.9 nm) in post graduate research 

laboratory at the Faculty of Science. This was followed by diluting samples 

to get the required concentration. The chromium concentrations for three 

diluted samples from each sample were drawn vs. dilution factor; intercept 

Black soil 
2653 ppm/0.5 L 

 

 

Black soil 
5306 ppm/0.5 L 

 

 

Red soil 
5306 ppm/0.5 L 

 

 

Red soil 
2653 ppm/0.5 L 

 

 

Red soil 
2653 ppm/1 L 

 

 



37 

 
 

 

from trend line represents the total chromium concentration for the sample. 

This method was repeated for all samples to get the final curve which 

shows the chromium concentration with time. See Appendix B 

3.1.3.2 Hexavalent Chromium Concentrations Test Method 

Hexavalent chromium concentrations were tested by 1, 5 Diphenyl 

carbohydrazide method using a single dry powder formulation called 

Chroma Ver 3 Chromium Reagent. The colored and turbid sample was 

diluted to have a clear sample, 10 ml of sample were put in cell to zero DR 

900 Colorimeter, then the powder was pillowed in cell and re put the cell in 

colorimeter to press time which is 5 min. Results obtained are Cr6+ (mg/L) 

3.1.4 Oxidation of  Cr
3+

from Different Concentrations and Volume 

with Time but without Soils. 

Chromium sulfate solutions were prepared with concentrations 5306, 2653 

and 1061 ppm. Each concentration was put in beakers with different 

volumes which were 100, 250 and 500 ml as shown in Figure 3.11 without 

soil. 

 

Figure 3.4: Chromium sulfate solutions with different concentration in different volumes 

without soils. 

1061ppm 

100ml 

2653ppm 

100ml 

5306ppm 

100ml 

1061ppm 

250ml 

1061ppm 

500ml 

2653ppm 

250ml 

2653ppm 

500ml 

5306ppm 

250ml 

5306ppm 

500ml 



38 

 
 

 

Samples were taken each time from all beakers, then hexavalent chromium 

concentrations were tested by 1, 5 Diphenylcarbohydrazide Method as 

mentioned in previous section. 

 

 

 

 

 

 

 

 

 

 

 

 

 



39 

 
 

 

Chapter Four 

Results and Discussions 

4.1 Results of Hydrometer Analysis 

Soil analysis using a hydrometer is carried out to find the diameter of fine 

soil particles. Sieve analysis is a method that is used to determine the grain 

size distribution of soils that are greater than 0.075 mm in diameter. It is 

usually performed for sand and gravel. The percentage of the fine particles 

passes sieves were recorded and hence the particle size distribution is 

recorded. Gravel percent determined in laboratory which is the particles 

more than 2 mm diameter and still in 2 mm sieve. Then from the Tables 4.1 

and 4.2, seek for clay which diameter less than 0.002 mm, the percentage 

of clay in the sample is represented by % finer. The diameter of sand 

particle is (2-0.05) mm and the percentage represented by % finer greater 

than 0.05 minus % clay that calculated before. Finally, the silt particles 

(0.05-0.002) mm are equal to 100 minus the sum of sand and clay 

percentages minus the percentage of gravel components. 

By using percentages of soil components on the soil triangle, the intercept 

area represents the name of soil.  

Red soil: Wt of soil= 50 gm, T= 25 C, Fm=+1, Fz=+3 

Table 4.1 shows the hydrometer analysis results for red soil. 

 

 



40 

 
 

 

Table 4.1: Hydrometer reading for red soil 

D (mm) A 
depth L 

(cm) 
Rcl % finer Rcp Reading 

Time 

(min) 

0.0703 0.0125 7.9 51 94.52 48.4 50 0.25 

0.0503 0.0125 8.1 50 92.56 47.4 49 0.5 

0.0362 0.0125 8.4 48 88.66 45.4 47 1 

0.0259 0.0125 8.6 47 86.71 44.4 46 2 

0.0186 0.0125 8.9 45 82.8 42.4 44 4 

0.0134 0.0125 9.2 43 78.89 40.4 42 8 

0.01 0.0125 9.6 41 74.99 38.4 40 15 

0.0073 0.0125 10.1 38 69.13 35.4 37 30 

0.0053 0.0125 10.6 35 63.27 32.4 34 60 

0.0038 0.0125 10.9 33 59.37 30.4 32 120 

0.0027 0.0125 11.2 31 55.46 28.4 30 240 

0.0019 0.0125 11.5 29 51.55 26.4 28 480 

0.0011 0.0125 11.7 28 49.6 25.4 27 1440 

 

Table 4.2: Information to determine the specific gravity of red soil, and 

some properties. 

684.66 Wt of flask+soil+water,W2(g) 

669.3 Wt of flask+water,W1(g) 

24.1 Wt of dry soil,W3(g) 

8.74 Wt of equal volume of water,W4(g) 

2.757437 Gs @ T=18 ℃= W3/W4 

1.0006 A 

2.759092 Gs @ T= 20℃= (W3/W4)*𝑎 

17.5 Dry unit weight (kN/m3) 

9.81 Unit weight of water (kN/m3) 

0.54 Void ratio (e) 

0.35 Porosity (n) 

1.02*10-6 Permeability (k) (m/s) 

Table 4.3 shows the USDA classification of soil particle size. 

Table 4.3: The USDA classification of soil particle size. Source: United 

States Department of Agriculture. (1987). 

Type Diameter (mm) 

Sand 2 - 0.05 

Silt 0.05 – 0.002 

Clay < 0.002 



41 

 
 

 

Calculations for Red soil sample: 

From laboratory work, gravel % =4.45% 

From Table 4.1 

Clay (D <0.002) which here D= 0.0019 mm, then  

% clay= % finer (passing) = 51.55 %,  

Particles which have diameter (0.05) =92.56%  

But the silt equal this percent – percent of clay  

% silt= 92.56-51.55=40.95,  

% sand=(100-(51.55+40.95)-4.45)= 3.05%. All these component 

percentage are used in Figure 2.1 to find the clay’s type. For the Red soil, 

Figure 4.1 shows its texture class. 

 
Figure 4.1: Red soil name from textural triangle as a function of soil components 

percentage 

Black soil: Wt of soil= 50 gm, T= 25 C, Fm=+1, Fz=+3 

Table 4.4 shows the hydrometer analysis results for black soil. 



42 

 
 

 

Table 4.4: Hydrometer Reading for black soil. 

D (mm) A 

Effective 

depth L 

(cm) 

Rcl 
% 

finer 
Rcp 

Hydrometer 

Reading R 

Time 

(min) 

0.0753 0.0127 8.8 46 86.02 43.4 45 0.25 

0.0542 0.0127 9.1 44 82.06 41.4 43 0.5 

0.0393 0.0127 9.6 41 76.11 38.4 40 1 

0.0292 0.0127 10.6 35 64.22 32.4 34 2 

0.0214 0.0127 11.4 30 54.31 27.4 29 4 

0.0155 0.0127 12 26 46.38 23.4 25 8 

0.0115 0.0127 12.4 24 42.42 21.4 23 15 

0.0083 0.0127 12.9 21 36.47 18.4 20 30 

0.006 0.0127 13.2 19 32.51 16.4 18 60 

0.0043 0.0127 13.8 15 24.58 12.4 14 120 

0.0031 0.0127 14.2 13 20.61 10.4 12 240 

0.0022 0.0127 14.5 11 16.65 8.4 10 480 

0.0013 0.0127 14.7 10 14.67 7.4 9 1440 

 

Table 4.5: Information to determine the specific gravity of black soil 

and some properties. 

684.64 Wt of flask+soil+water,W2(g) 

669.3 Wt of flask+water,W1(g) 

24.4 Wt of dry soil,W3(g) 

9.06 Wt of equal volume of water,W4(g) 

2.693157 Gs @ T1=18℃,= W3/W4 

1.0006 A 

2.694773 Gs @ T=20 ℃,= (W3/W4)*𝑎 

16 Dry unit weight (kN/m3) 

9.81 Unit weight of water (kN/m3) 

0.65 Void ratio (e) 

0.39 Porosity (n) 

7.19*10-6 Permeability (k) (m/s) 

Calculations for Black soil sample: 

From laboratory work, gravel % =2% 

From Table 4.4 

Clay (D <0.002) which here D= 0.0022 mm, then  

% clay= % finer (passing) = 16.65 %,  



43 

 
 

 

Particles which have diameter (0.05) =82.06% but the silt equal this percent 

– percent of clay  

% silt= 82.06-16.65=65.35%,  

% sand= (100-(16.65+65.35)-2) = 16% Using these percentages, then the 

name of soil can be read from the USDA textural triangle plotted for 12 

basic texture classes as function of components percentage, then it can be 

seen that is Silty Loam as shown in Figure 4.2 

 

Figure 4.2: Black soil name from textural triangle as a function of soil components 

percentage. 

Comparing the Red & Black soils samples, it was found that the porosity, 

void ratio and permeability of the black soil were higher than that for the 

red soil. However, the particle size for the black soil is higher than that of 

the red soil, because the predominant component in the red one is clay 

while in the black one is silt. 



44 

 
 

 

4.2 XRF Oxide Data and Summery of Calculations. 

Since XRF technique does not find Na concentration and the LOI of the 

soil sample, so it must be determined to complete the calculations to get 

results in Table 4.8. 

Loss of ignition and sodium concentration of the soil samples are shown in 

Table 4.6 and Table 4.7 respectively.  

Table 4.6: Loss of ignition for soil samples. 

Soil sample W1(gm) W2(gm) W1-W2 

B1i 24.04 23.81 0.23 

B1f 24.78 24.56 0.22 

B2i 23.54 23.36 0.18 

B2f 24.35 24.17 0.18 

Table 4.7: Sodium concentration in soil samples. 

Soil sample ppm 

B1i 4.5 

B1f 47 

B2i 16.8 

B2f 85 

Table 4.8 represents the Wt% of elements oxides and traces in initial 

original red and black soil (B1i and B2i) , final chromium- saturated red and 

black soil (B1f and B2f). The results in Table 4.8was determined by using 

equations (3.12-3.17).  

For more details of XRF results see Appendix C 

 

 

 



45 

 
 

 

Table 4.8: Summery of XRF oxide data and calculations 

 

When Wt% change of each element and calculated the mass balance for the 

B1f and B2f (soils after treatment with chromium sulfate solution), in the 

results of B2f the total wt% in fresh and chromium saturated samples are 

equal. However, there is 1% extra gain in B1f, which should not be there if 

there is no substantial amounts of other ions in solution. Therefore, the 

possible reason is human error in LOI calculation when reading weight of 

sample, although it's repeated two times. Also, calcium is inexplicably high 

Oxide B1i B1f Delta M Total Loss Total Gain  Balance B2i B2f Delta M Total Loss Total Gain Balance

Formula Wt% Wt% % % % Wt% Wt% % % %

SiO2 35.5729 33.073 -2.500 -9.309 10.305 0.996 46.583 43.742 -2.841 -3.632 3.633 0.001

TiO2 1.6358 1.359 -0.277 1.45 1.358 -0.092

Al2O3 14.415 13.369 -1.046 11.184 11.585 0.401

Fe2O3 17.5031 12.52 -4.983 9.72 9.275 -0.445

MnO 0.23915 0.208 -0.031 0.215 0.21 -0.005

MgO 1.6208 1.385 -0.236 2.736 2.714 -0.022

CaO 3.9844 4.929 0.945 7.051 7.086 0.035

Na2O 0.0002 0.002 0.002 0.001 0.005 0.004

K2O 0.864 0.778 -0.086 1.952 1.816 -0.136

P2O5 0.4912 0.491 0.000 0.475 0.515 0.040

Cr2O3 0.0503 2.825 2.775 0.027 1.62 1.593

SO3 0.22281 6.807 6.584 0.238 1.798 1.560

Trace 0.39834 0.248 -0.150 0.369 0.278 -0.091

Total 77 78 81.990 81.87

LOI 23 22 18.000 18

Trace (ppm)

Br 25 13 22 0

Cl 2459 1700 4205 2529

Nb 235 0 133 0

Ni 681 460 0 0

Pb 56 0 0 0

Rb 498 285 190 169

Sc 13 19 170 135

Sr 509 296 646 585

Zn 1052 658 431 390

Zr 2819 1640 2023 1900

V 671 632 370 392

Y 258 161 108 125

Sum 9276 5864 8298 6225



46 

 
 

 

in that sample comparing with other sample although the same solution was 

used.  

In B1f  the concentration of total Cr increasing with adorable amount, this 

increase is accompanied by a decrease in the amount of iron. The 

concentration ofCr6+increasing due to the amount of Fe and Mn elements.  

4.3 XRD Data Analysis 

Soils phase identifications were carried out using Jade 10 software at 

Arizona State University(ASU) for four samples (initial fresh red soil(B1i), 

 final chromium saturated red soil (B1f),initial fresh black soil(B
2i

),

final chromium saturated black soil(B2f)) and are shown in Figures 4.3 to 

4.6. The major phases in each sample can be pinpointed using its main 

reflection (diffraction) line with the highest intensities for each. This is a 

qualitative phase analysis but can be taken as semi-quantitative analysis. 

The plots show that, the clay component (one or more of the clay minerals 

like kaolinite, illite, montmorillonite); quartz (SiO2) is a major inert 

component; calcite (CaCO3) is present initially in both samples and still 

present after treatment; another component is the feldspar, which is inert 

too. As a product, we can see gypsum (CaSO4.2H2O) in both clays after 

treatment. 



47 

 
 

 

 

Figure 4.3: Phase identification for soil sample (B1i) from XRD data. 

 

 

Figure 4.4: Phase identification for soil sample (B1f) from XRD data. 



48 

 
 

 

 

Figure 4.5: Phase identification for soil sample (B2i) from XRD data. 

 

Figure 4.6: Phase identification for soil sample (B2f) from XRD data. 



49 

 
 

 

4.4 Soil Type Ability to Adsorb Chromium 

Figures 4.7 to 4.10 show samples were taken at various times. Chromium 

concentration of several samples were observed at various times and are 

shown in Figures 4.7 to 4.10. In the figures, R and B stand for Red and 

Black soil samples. The volume of initial solution is 0.5 L and the volume 

of each sample is 20 ml. 

 
Figure 4.7: Samples from chromium sulfate solutions after 48 hours. 

 

 
Figure 4.8: Samples from chromium sulfate solutions after 168 hours. 

 

 

Figure 4.9: Samples from chromium sulfate solutions after 357 hours. 



50 

 
 

 

 
Figure 4.10: Samples from chromium sulfate solutions after 504 hours. 

As time elapse, the intensity of the sample’s color decreases due to the 

adsorption of chromium on the soil samples.  

Figure 4.11shows a quantitative reduction of total chromium with time 

when it was soaked into a 170 g of the red and black soils (separately). The 

initial total chromium concentration (in 0.5l volume) is 5306 ppm and the 

change of total chromium concentration with time is shown. The total 

concentration drops to around zero after around 900 hours. 

 

Figure 4.11: Total chromium concentrations for red and black soil with initial total 

chromium concentration equal to 5306 ppm and 0.5 L volume. 

For red soil the total chromium concentrations were decreased gradually in 

comparison to the sharp decrease of the concentration in the Black sample.  

0

1000

2000

3000

4000

5000

6000

0 500 1000 1500 2000 2500

Total chromium 
concentration 

(mg/l)

Time (hr)

Red (5306 ppm)

Black (5306 ppm)



51 

 
 

 

When comparing the two soil types, the initial uptake amount for red and 

black soils are1994 mg /l, 3312 mg /l, respectively, and the remaining 

concentrations in red and black soil are0.74 mg /l, 0.52 mg /l, respectively. 

So, the black soil is better in adsorption of total chromium than red soil. 

Figure 4.12 represents the total chromium concentrations with different 

times for red and black soil and initial total chromium concentration equal 

to 2653 ppm. 

 
Figure 4.12: Total chromium concentrations for red and black soil with initial total 

chromium concentration equal to 2653 ppm and 0.5 L volume. 

Results shown in Figure 4.12 for a 0.5 l chromium concentration of 

2653 ppm added to 170 g soil (Red and Black soil separately). For red soil 

the total chromium concentrations were decreased gradually in comparison 

to the sharp decrease of the concentration in the Black sample.  

When comparing the two soil types, the initial uptake amount in red and 

black soils is 1376 mg /l, 1929 mg /l, respectively, and the remaining 

0

500

1000

1500

2000

2500

3000

0 500 1000 1500 2000 2500

Total chromium 
concentration 

(mg/l)

Time (hr)

Red (2653 ppm)

Black (2653 ppm)



52 

 
 

 

concentrations in red and black soil are 0.92 mg /l and 0.65 mg /l, 

respectively. So, the black soil is better in total chromium adsorption than 

red soil.  

Figure 4.13 represents the total chromium concentrations with different 

times for red and black soil and initial total chromium concentration equal 

to 1061 ppm. 

 

Figure 4.13: Total chromium concentrations for red and black soil with initial total 

chromium concentration equal to 1061 ppm and 0.5 L volume. 

Results shown in Figure 4.13 for a 0.5 l chromium concentration of 

1061 ppm added to 170 g soil (Red and Black soil separately). For both red 

and black soil the total chromium concentrations were decreased sharply. 

When comparing the two soil types, the initial uptake amount in red and 

black soils is 1009.6 mg /l, 1006 mg /l, respectively, and the remaining 

concentrations in red and black soil is 21 mg /l, 3.7 mg /l, respectively. So, 

the black soil is better in total chromium adsorption than red soil. 

0

200

400

600

800

1000

1200

0 500 1000 1500 2000 2500

Total chromium 
concentration 

(mg/l)

Time( hr)

Red (1061 ppm)

Black (1061 ppm)



53 

 
 

 

Figure 4.14 represents the total chromium concentrations with different 

times for red soil and initial total chromium concentration equal to 2653 

ppm, volume equal 1 L where weight of soil 170 gm. 

 

Figure 4.14: Total chromium concentrations for red soil with initial total chromium 

concentration equal to 2653 ppm and 1 L volume. 

Figure 4.15 shows the total chromium concentrations in 1L volume were 

decreased gradually. While in 0.5 L the total chromium concentrations 

were decreased in two steps not gradually. 

 
Figure 4.15: Total chromium concentrations for red soil with initial total chromium 

concentration equal to 2653 ppm and 1, 0.5 L volume. 

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

Total chromium 
concentration 

(mg/L)

Time (houre)

0

500

1000

1500

2000

2500

3000

0 500 1000 1500 2000 2500

Total chromium 
concentration 

(mg/l)

Time (hr)

Red (2653 ppm, 1 L)

Red (2653 ppm, 0.5 L)



54 

 
 

 

When comparing the 0.5L and 1L of total chromium concentration equal to 

2653ppm and same soil type, the initial uptake amount in 0.5L and 1L 

are1376 mg / l, 1216 mg / l, respectively, and the remaining concentrations 

are0.92 mg / l, 2.4 mg / l, respectively. So, the less volume of same soil and 

concentration is better in total chromium uptake than high one. By 

comparing between 0.5 L and 1L and same amount of soil, when the 

volume is 0.5L then the soil was saturated with total chromium faster.  

The effect of initial chromium concentration on the adsorption capacity for 

Red and Black soil samples is shown in Figures 4.16 and 4.17 respectively. 

At higher initial chromium concentration, the concentration decrease is 

lower than that of lower initial chromium concentration.  

 
Figure 4.16: Chromium concentrations for red soil with initial total chromium 

concentration equal to 5306, 2653 and 1061 ppm. 

 

0

1000

2000

3000

4000

5000

6000

0 100 200 300 400 500 600

Chromium 
concentration 

(mg/l)

Time (hr)

5306

2653

1061



55 

 
 

 

 
Figure 4.17: Chromium concentrations for black soil with initial total chromium 

concentration equal to 5306, 2653 and 1061 ppm. 

For the solutions with initial concentration equal to 5306 and 2653 ppm of 

total chromium, in red soil, the total Cr concentrations were decreased 

gradually. While in black were decreased faster. The black soil is better in 

total chromium adsorption than red soil. When the initial concentration 

equal to 1061 ppm, in both soils, the total Cr concentrations were decreased 

in two steps, but the black soil is better in total chromium adsorption than 

red soil. 

Hexavalent chromium concentrations were tested by 1, 5 

Diphenylcarbohydrazide method for all water samples that taken from 

different soil, total chromium concentration and time.  

Figures 4.18 and 4.19 represent some samples afterCr6+ test, the darkness 

of the purple color represent the concentration strength. 

 

0

1000

2000

3000

4000

5000

6000

0 100 200 300 400 500 600

Chromium 
concentration 

(mg/l)

Time(hr)

5306

2653

1061



56 

 
 

 

 

Figure 4.18: Samples with high concentrations of hexavalent chromium. 
 

 

Figure 4.19: Samples with less concentrations of hexavalent chromium 

Figure 4.20 represents the Cr6+concentrations with different times for red 

and black soil samples at an initial total chromium concentration of 5306 

ppm. 

 

Figure 4.20: Hexavalent chromium concentrations for red and black soil samples with 

initial total chromium concentration of 5306 ppm and 0.5 L volume. 

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 200 400 600 800 1000

Cr6+ concentration 
(mg/l)

Time (hr)

Red (5306 ppm)

Black (5306 ppm)



57 

 
 

 

As shown in Figure 4.20 the Cr6+concentrations fluctuate with time. The 

change behavior of the Cr6+ concentration of the red and black soil samples 

are not same and can be related to the quantity of manganese oxide in the 

sample. This evident from the XRF test results which show that red soil 

sample contains more manganese oxide than the black soil sample. Cr3+ has 

the tendency to be adsorbed by the manganese oxide. In soils, manganese 

oxides (MnO) typically accumulate on the surface of the clay. Hence, 

Cr3+can be oxidized to Cr6+ as shown by equation 4.1:(Subramanian et al., 

2014). 

2Cr3+ + 3MnO2 + 2H2O ⇔2CrO2-
4 + 3Mn2+ + 4H …..………………(4.1) 

Therefore, due to the higher manganese oxide concentration in the red soil 

sample, the more tendency of Cr3+ to be oxidized to Cr6+ in the red soil 

sample than the black soil sample as shown in Figure 4.20. 

Figure 4.21 represents theCr6+ concentrations with different times for red 

soil and initial total chromium concentration equal to 2653 ppm. 

 

Figure 4.21: Hexavalent chromium concentrations for red and black soil with initial total 

chromium concentration equal to 2653 ppm and 0.5 l volume. 

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0 200 400 600 800 1000

Cr6+ concentration 
(mg/l)

Time (hr)

Red (2653 ppm)

Black (2653 ppm)



58 

 
 

 

Figure 4.22 represents theCr6+concentrations with different times for red 

soil and initial total chromium concentration equal to 2653 ppm, volume 

equal 1 L where weight of soil 170 gm. 

 

Figure 4.22: Hexavalent chromium concentrations for red soil with initial total chromium 

concentration equal to 2653 ppm and 1, 0.5 L volume. 

As shown in Figure 4.22the larger the volume of the solution, the more 

oxidation of Cr3+ toCr6+ due the more adsorption of Cr3+ on the manganese 

oxide. The equilibrium concentration of Cr6+ is higher than the allowable 

level 0.1 mg/L stated by PWA. 

In soil, the process of converting Cr3+ toCr6+ is a complex process that 

relies on many variables, such as pH, soil content, and soil components that 

play a role in reactions of oxidation and reduction. The values of the 

concentration of Cr6+are fluctuating, and no clear relationship has been 

achieved. This could be explained by the method of Redox response and 

dechromification(Subramanian et al., 2014) and the presence of ferrous and 

sulfides in the soil samples.  

Manganese and iron oxides that settle on the surface of the soil have a wide 

surface area, a high adsorption potential for heavy metals and a negative 

0

0.2

0.4

0.6

0.8

0 200 400 600 800 1000

Cr6+

concentration 
(mg/l)

Time (hr)

Red (2653 ppm,0.5 L)

Red (2653 ppm, 1 L)



59 

 
 

 

charge under acidic conditions can led to the oxidation of Cr3+ toCr6+. 

Therefore, it is very essential to test the effect of pH on the oxidation of 

Cr3+ to Cr6+ in future work. 

Ferrous and Sulfide as electron doner can reduced some of Cr6+ toCr3+, 

called this dechromification (Subramanian et al., 2014). This can be a 

simple explanation of the fluctuation of the Cr6+with time.  

In red soil, Manganese, Ferrous and Sulfide oxides concentrations are 

higher than in black soil as obtained from XRF data but due to also higher 

concentration of manganese oxide, the tendency of oxidizing Cr3+ to Cr6+ is 

higher than the reduction of Cr6+ to Cr3+, therefore,  the concentration of 

Cr6+ is higher Additionally, from the red soil structure, the lower 

permeability, porosity and void ratio resulted in more Cr3+ in the red soil 

than the black soil and hence more oxidation to Cr6+.  

BecauseCr6+ is anionic, it's attracted to positively charged surfaces. 

Therefore, binding of chromium to soil depends on soil mineralogy 

(Subramanian et al., 2014). So, this binding reduces the Cr6+concentration 

in water that were tested. 

4.5 Oxidation ofCr
3+

 from Different Concentrations and Volume 

(Total Amount) with Time without Soils. 

After chromium sulfate solutions with concentrations 5306, 2653 and 1061 

ppm were put in beakers with different volumes which were 100, 250 and 

500 ml as shown previously in Figure 3.11 without soil. 



60 

 
 

 

Samples were taken each time from all beakers, Then, hexavalent 

chromium concentrations were tested by 1, 5 Diphenylcarbohydrazide 

Method. 

Table 4.9, 4.10 and 4.11 represent the Cr6+concentration over time. 

Table 4.9: Hexavalent chromium concentration (mg/l) for the 1061 

ppm of total chromium without soil. 

Volume of solution =volume of beaker  Concentration in ppm after  

500 mL 250 mL 100 mL  
1.4 1.1 1 7 days (168 hr) 

2 2 1.5 11 days (264 hr) 

2.5 2.5 2 18 days (432 hr) 

 

Table 4.10:  Hexavalent chromium concentration (mg/l) for the 2653 

ppm of total chromium without soil. 

Volume of solution =volume of beaker concentration in ppm after  

500 mL 250 mL 100 mL  
0.9 0.8 1 7 days  (168 hr) 

2 2 1.5 11 days (264 hr) 

2.5 2.5 2 18 days (432 hr) 

Table 4.11: Hexavalent chromium concentration (mg/l) for the 5306 

ppm of total chromium without soil. 

Volume of solution =volume of beaker  concentration in ppm after 

500 mL 250 mL 100 mL  
0.7 0.6 0.3 7 days (168 hr) 

1.5 1.5 1.5 11 days (264 hr) 

2 1.5 1.5 18 days (432 hr) 

 

As shown in tables 4.9, 4.10 and 4.11, when the volume of the same total 

chromium concentration was increased, theCr6+ concentration was slightly 

increased. Also, over times the concentration of Cr6+was increased. With a 

solution of chromium sulfate at initial pH = 4.13, which isCr3+, when 



61 

 
 

 

placing the solution in beakers of different sizes, the opening of the beaker 

increase with increasing volume, therefore the chromium solution was 

exposed to more oxygen concentration. Then Cr3+is oxidized toCr6+. As 

the concentration of Cr3+increases, the oxidation increases and the 

Cr6+increases. 

When comparing the formation of Cr6+between the presence of the soil and 

its absence in the same volume (500 ml), it was shown that the general 

trend ofCr6+formation increased over time without soil more than the 

presence of it. This is due to the effect of soil composition of manganese 

oxide, ferrous and sulfides. Due to the low concentration, more accurate 

experimental testing of Cr6+ is needed.  

4.6 Adsorption Kinetic Models for Removal of Chromium onto Soil. 

It is well known that the mass transfer coefficient towards particle surface 

increases with increasing bulk motion. Therefore, increasing the mass 

transfer coefficient decreases the characteristic time needed to approach 

equilibrium. This means that when wastewater peculates through soil (i.e., 

with the mechanism of flow through porous media), the contamination of 

soil will be larger than for the case with stagnant wastewater (as the case of 

this study). This is because the flow of the wastewater past the soil particles 

increases the mass transfer coefficient. It is obvious that mass transfer 

coefficient in forced convection is larger than that for stagnant particle 

(natural convection) (Al-Jabari et al., 2017b). 



62 

 
 

 

Experimental data of the chromium solution concentration were tested by 

the adsorption kinetic models using pseudo first order and second order rate 

equations, given in equations (4.5) and (4.6), presented in linear forms. 

(Jean Simonina, 2016). 

The amount of adsorption (mg/g) at time t is calculated using the following 

equations: 

( 𝑞𝑡) = 
(Co−Ct)∗V

w
 ………………………… (4.2) 

( 𝑞𝑒) = 
(Co−Ce)∗V

w
 ……………………..…. (4.3) 

% adsorption = 
(Co−Ca)

w
∗ 100 …………..… (4.4) 

where,  

C0: initial concentration of solution (mg/l). 

C𝑒: concentration at equilbrium (mg/l). 

C𝑎: concentration after adsorption (mg/l). 

W: mass of adsorbent (g). 

V: volume of solution (L). 

Pseudo first order: 

Log (𝑞𝑒-𝑞𝑡) = Log (𝑞𝑒) – 
k1

2.303
 t ………………….……..(4.5) 

Where, 𝑞𝑒is the intercept and k1 is slope (min−1). 



63 

 
 

 

Pseudo second order: 

𝑡

𝑞𝑡
= 

1

k12𝑞𝑒
2 + 

1

𝑞𝑒
 t …………………………….… (4.6) 

Where, 𝑞𝑒is the slope and k2 is intercept (g mg−1 min−1). 

Figures 4.23-4.25show the adsorption capacity ( 𝑞𝑡) as a function of time 

for chromium solution with initial concentrations equal to 1061, 2653, 

5306 ppm respectively in different soil types. 

Initially, the adsorption capacity is slightly higher in black soil (Silty 

Loam) than in red soil sample and increases with increasing of chromium 

concentration in solution for the same solution volume (0.5 L).  

 
Figure 4.23: Adsorption capacity (qt) as a function of time for chromium solution with 

initial concentration equal to 1061 ppm in different soil types. 

0

0.5

1

1.5

2

2.5

3

0 0.5 1 1.5 2 2.5

qt (mg/g)

t*103 (hr)

Red (1061 ppm)

Black (1061 ppm)



64 

 
 

 

 
Figure 4.24: Adsorption capacity (qt) as a function of time for chromium solution with 

initial concentration equal to 2653 ppm in different soil types. 

Figure 4.25: Adsorption capacity (qt) as a function of time for chromium solution with 

initial concentration equal to 5306 ppm in different soil types. 

With the second order model, the plot of t/ 𝑞𝑡 versus time is linear, with a 

positive slope of
1

𝑞𝑒
. The resulting lines for the second order model 

areplotted in Figures 4.26-4.28. From data analyses that have done, the 

pseudo second order model gives better fitting than the first order model 

i.e., higher R2 values. 

0
1
2
3
4
5
6
7
8

0 0.5 1 1.5 2 2.5

qt (mg/g)

t*103 (hr)

Red (2653 ppm)

Black (2653 ppm)

0

2

4

6

8

10

12

14

16

0 0.5 1 1.5 2 2.5

qt (mg/g)

t*103 (hr)

Red (5306 ppm)

Black (5306 ppm)



65 

 
 

 

 

Figure 4.26: Second order kinetic model for the adsorption of total chromium onto 

soil particles for initial concentration equal to 5306 ppm in different soil. 

 

Figure 4.27: Second order kinetic model for the adsorption of total chromium onto 

soil particles for initial concentration equal to 2653 ppm in different soil. 

 

Figure 4.28: Second order kinetic model for the adsorption of total chromium onto 

soil particles for initial concentration equal to 1061 ppm in different soil. 

y = 0.0876x - 13.975
R² = 0.9972

y = 0.0892x - 184.96
R² = 0.9974

-2

0

2

4

6

8

10

12

0 50 100 150

(t/qt)*103

t *103 (min)

Red (5306 ppm)

Black (5306 ppm)

y = 0.1787x - 365.59
R² = 0.9979

-5

0

5

10

15

20

25

0 50 100 150

(t/qt)*103

t*103 (min)

Red (2653 ppm)

Black (2653 ppm)

y = 0.4556x - 1074.2
R² = 0.9979

y = 0.4475x - 997.36
R² = 0.9981

-10

0

10

20

30

40

50

60

0 50 100 150

(t/qt)*103

t*103 (min)

Red (1061 ppm)

Black (1061 ppm)



66 

 
 

 

Table 4.12 shows fitting parameters with pseudo second order model for 

adsorption of total chromium from different concentrations (5306, 2653 

and 1061 ppm) onto different soil types (silty clay and silty loam). 

Table 4.12: Fitting parameters with pseudo second order model for 

different concentrations onto different soils. 

Initial 

chromium 

concentration 

(mg/l) 

Red soil (Silty clay) Black soil (Silty loam) 

𝑞𝑒(mg/g) 𝐾2(g mg−1 min−1) R2 𝑞𝑒(mg/g) 𝐾2(g mg−1 min−1) R2 

5306 11.49 0.00054 0.997 11.24 4.28*10−5 0.997 

2653 5.62 8.66*10−5 0.997 5.62 8.66*10−5 0.997 

1061 2.24 0.00019 .0998 2.19 0.00019 0.997 

As shown in table 4.12, The 𝑅2 value for all concentrations in both soil 

types are equal. The high chromium concentration whichis 5306 ppm in 

Red soil has the higher 𝑞𝑒 than the same concentration in black soil which 

is faster in chromium adsorption. However, the values are close. But, for 

the rest concentrations in both soil, the values are similar. 

 

 

 

 

 

 



67 

 
 

 

Chapter Five 

Conclusions and Recommendations 

5.1 Conclusions 

From this work, the following conclusions can be drawn: 

A Silty clay red soil and a Silty Loam black soil were analyzed and used 

successfully to adsorb chromium from Chromium sulfide solution. It was 

found that these soils can adsorb total chromium from solutions 

successfully. The adsorption process fitting shows pseudo second order 

behavior. 

For the solutions with initial concentration equal to 5306 and 2653 ppm of 

total chromium, volume equal 0.5 L and weight of both soil is 170 gm. In 

red soil, the total Cr concentrations were decreased gradually. While in 

black were decreased but in two steps. The black soil is better in total 

chromium adsorption than red soil, when the initial concentration equal to 

1061 ppm, in both soils, the total Cr concentrations were decreased in two 

steps, but the black soil is better in total chromium adsorption than red soil. 

There is a possibility of formationCr6+ from chromium sulfate solutions in 

different soil types. The Cr6+concentrations were increased in red soil, 

wh