THERMALLY REGENERABLE MODIFIED NATURAL CLAY FOR SUSTAINABLE REMOVAL OF TETRACYCLINE AND PHENAZOPYRIDINE FROM WATER

Abstract

Abstract This study introduces a sustainable and cost-effective water treatment method that uses thermally modified natural clay (MC) to remove pharmaceutical contaminants, specifically tetracycline (TC) and phenazopyridine hydrochloride (PHY), from water. The clay underwent thermal and chemical modifications to improve its ability to adsorb, its surface reactivity, and thermal stability. Researchers thoroughly evaluated the performance of MC using batch adsorption and fixed-bed column experiments to understand both equilibrium behavior and continuous-flow use. Batch adsorption studies showed that the best results occurred at pH 3.5 and 25 °C, with initial concentrations of 20 mg/L for PHY and 40 mg/L for TC. Under these conditions, removal efficiencies reached 84% for PHY and 93% for TC. This was achieved with an adsorbent dosage of 4.0 g/L and a contact time of 45 minutes. The adsorption efficiency increased with a higher adsorbent dosage and lower temperature, indicating that the process is exothermic. Additionally, using smaller particle sizes (less than 75 µm) and heating the clay to 550 °C improved adsorption due to increased surface area and active sites. The equilibrium data fit best with the Langmuir isotherm model, showing R² values of 0.9827 for PHY and 0.9713 for TC, which suggests monolayer adsorption. Kinetic analysis indicated that the pseudo-second-order model best represented the data, implying that strong physical adsorption was the main mechanism. Fixed-bed column experiments confirmed that MC works well under dynamic conditions. The results showed that both pH and flow rate significantly affected adsorption performance. Maximum removal efficiencies were found at an acidic pH of around 3.5, supporting the batch results due to improved electrostatic interactions. At a flow rate of 2.5 mL/min, the column had much higher removal efficiency and delayed breakthrough compared to faster flow rates, which improved contact time and mass transfer. Increasing the flow rate led to earlier breakthrough and lower overall removal efficiency for both TC and PHY. The column system maintained stable adsorption performance across multiple runs, showing reliable contaminant removal during continuous operation. Furthermore, the modified clay effectively captured pollutants at initial concentrations of 80 mg/L for PHY and 100 mg/L for TC, proving its strength under higher loading conditions. A key aspect of this work is combining adsorption with thermal regeneration (thermolysis). The used MC was regenerated at 550 °C for 120 minutes, completely breaking down adsorbed contaminants into harmless substances like CO₂ and H₂O. The regenerated adsorbent retained high efficiency across five consecutive adsorption-thermolysis cycles in both batch and column systems, with little drop in performance, emphasizing its excellent reusability and structural stability. Extensive characterization through XRD, SEM, EDS, FT-IR, and TGA confirmed the successful modifications, improved surface properties, and high thermal resistance of MC. Overall, this combined adsorption-thermolysis method provides a strong, eco-friendly, and scalable solution for effectively removing persistent pharmaceutical contaminants from water, showing great potential for continuous treatment in real-world applications.