Kinetics, equilibrium and thermodynamics studies on natural and heat treated clays for the removal of arsenate ions from aqueous solution

This study investigates the adsorption properties of arsenic (As(V)) from aqueous solutions using natural clay (NAC) and heat-treated natural clay (HTNAC), highlighting its novelty and significance as a sustainable approach to arsenic removal. The unique adsorption behaviors of the clays, enhanced by heat treatment, provide valuable insights into the influence of surface modification on adsorption efficiency. Characterization of the clays revealed significant changes in surface area, pore structure, and functional groups upon heat treatment, which improved their adsorption capacity. Functional groups such as Si-O-Fe, Si-Si-OH, Al-Al-OH, and Si-O-Al which were very useful for the arsenate ions uptake were identified from the Fourier-transform infrared (FT-IR) analysis, while X-ray diffraction (XRD) examination revealed peaks corresponding to illite, kaolin and quartz by the clay mineral. Utmost adsorption was attained at an interactive time of 150 and 180 min for HTNAC and NAC, pH of 5.0, temperature of 45 °C, and adsorbent dosage of 35 mg for NAC and 30 mg for HTNAC respectively. The data from equilibrium study of NAC matched the Langmuir model, whereas those from HTNAC matched the Freundlich model. The NAC kinetic data, correlate better to the first-order model, whereas the second-order model best described the kinetic data for HTNAC. The maximum sorption capacity found are 150.658 ad 197.662 mg/g for NAC and HTNAC respectively which revealed that the heat-treated clay exhibited superior performance compared to untreated clay, attributed to enhanced surface activity and reduced structural impurities. Thermodynamics values of ΔG at various temperatures were found to be in the range of - 26.58 to - 93.01 kJ/mol for NAC and - 32.17 to - 87.04 kJ/mol for HTNAC suggesting the sorption of As(V) to be feasible and a spontaneous process. This study underscores the importance of utilizing heat-treated natural clay as a low-cost, efficient, and environmentally friendly adsorbent for arsenic.