Thermodynamic, kinetic and equilibrium isotherm studies of As(V) adsorption by Fe(III)-impregnated bentonite

Arsenic removal has received much attention all over the world because of its toxicity and carcinogenicity. In the current research, the adsorption behavior of As(V) from aqueous solution onto Fe(III)-impregnated bentonite (Fe-Bent) was examined. Adsorption experiment results showed that the adsorption capacity of initial bentonite clay (Bent) was increased by Fe(III)-impregnation method. In Fe-Bent, the mononuclear Fe(III) cations, small oligomeric cluster and bulky FexOy particles were made such active sites identified by UV–Vis spectroscopy. X-ray diffraction results showed that Fe(III) species presented mainly in both crystalline and amorphous forms on the external surface of Fe-Bent. In an experimental study, the effect of different parameters such as contact times, arsenic concentrations, adsorbent dosages, pH and temperature conditions was investigated. Fe-Bent sample represented the maximum adsorption capacity of 10.06 mg L−1 to remove 5–20 mg L−1 of As(V) at pH = 6 and ambient temperature. In this experiment, the residual As(V) concentration decreased to less than 0.01 mg/L with the adsorption efficiency of 99% (i.e., below the limit of the WHO). The two- and three-parameter isotherm models revealed the best fit to the Freundlich, Sips, Toth, Temkin and Langmuir models, respectively. The isotherm models indicated the strong adsorption between arsenic and Fe-Bent. The adsorption kinetics was also considered by first-order, second-order and Elovich rate equations. It was noticed that the Fe-Bent adsorption followed the external surface reaction such as ligand exchange and surface complexation. The thermodynamics studies exposed that the process was spontaneous and endothermic for Fe-Bent.