Equilibrium and thermodynamic studies for the removal of As(V) ions from aqueous solution using dried plants as adsorbents

This paper reports the feasibility of using dried plants to remove As(V) from aqueous solution under different experimental conditions. For this purpose, micro-particles of both Calami rhizoma and Withania frutescens plants, have been separately used without pre-treatment as natural adsorbents. Effect of various process parameters, namely adsorbent dose, contact time, initial As(V) concentration, temperature, and pH has been studied. The experimental data were analyzed using Freundlich, Langmuir, and Temkin isotherm models to determine the mechanistic parameters related to the adsorption process. It was found that the Langmuir and Freundlich models fitted well. Thermodynamic parameters, viz, free energy change (ΔG0), enthalpy change (ΔH0), and entropy change (ΔS0), were also determined. The negative values of free energy change indicated the spontaneous nature of the adsorption and the positive values of enthalpy change suggested the endothermic nature of the adsorption process. The presence of some competing ions like Cl−, NO3-, Mg2+, Cd2+, Cu2+, and Zn2+ did not affect the arsenate uptake or removal, whereas HPO42- strongly interfered negatively.