Thermodynamic and kinetic parameters of arsenic and mercury adsorption on chromic acid-activated-bentonite in wastewater treatment

This study investigated one of those methods by using the chromic acid-activated bentonite clay for the removal of arsenic and mercury from wastewater. The activated bentonite was characterized using XRF, BET, and SEM analyses, revealing an increased surface area (339 m2/g) and pore volume (1.0 cm3/g) compared to unactivated clay. Batch experiments were conducted at a contact time of 40 min, an adsorbent dose of 0.05 g/0.5 l, and an equilibrium pH of 6.2. At 298 K, maximum adsorption capacities (qmax) were 270 mg/g and 243 mg/g for arsenic and mercury, respectively. The adsorption behavior was best described by the Freundlich isotherm model, with correlation coefficients of 0.996 and 0.994 for arsenic and mercury, respectively, indicating multilayer chemisorption. The thermodynamic parameters – enthalpy change (ΔH), entropy change (ΔS), and Gibbs free energy change (ΔG) – were estimated to evaluate the nature of the adsorption processes. Arsenic adsorption exhibited a negative ΔG, indicating spontaneity, while mercury adsorption showed a positive ΔG, suggesting a non-spontaneous process. The positive ΔH values confirmed the endothermic nature of adsorption for both metals, and the positive ΔS values reflected increased randomness at the solid–liquid interface. We estimated the standard enthalpy change (ΔH), standard entropy change (ΔS∘), and Gibbs free energy change. Thermodynamic analysis revealed that arsenic adsorption was spontaneous (negative ΔG), while mercury adsorption was not (positive ΔG). Positive ΔH values confirm endothermic adsorption, while positive ΔS values indicate greater randomness at the solid–liquid interface. The adsorption capacity increased with temperature. It reached to 544 mg/g for arsenic and 477 mg/g for mercury at 328 K. These findings demonstrated the potential of acid-activated bentonite clay as an efficient and sustainable adsorbent for removing heavy metals like arsenic and mercury from wastewater, offering demonstrating potential applications for environmental remediation efforts.