Cr(VI) removal from industrial effluents by adsorption on an indigenous low-cost material

Feasibility of an indigenous low-cost clay mineral, wollastonite, has been examined for the removal of Cr(VI) from aqueous solutions and wastewaters. The removal is found to be dependent on concentration of Cr(VI) and low concentrations favour the uptake. The uptake increased from 41.7 to 69.5% by decreasing the concentration of Cr(VI) in solution from 2.0×10−4 to 0.5×10−4 M at 0.01 M NaClO4 ionic strength, 2.5 pH and at 30 °C. Rate of uptake was found to be 3.0×10−2 min−1 under optimum conditions and the process is governed by first-order kinetic equation. The process involves both film and pore diffusion, and the value of rate constant for intraparticle diffusion was found to be 5.0×10−4 mg g−1 min−1/2 under favourable conditions, while mass transfer coefficient for film mass transfer was found to be 2.31×10−3 s−1 at 0.5×10−4 M Cr(VI) concentration at optimum conditions. The process is a typical example of endothermic adsorption and therefore higher temperatures favour the uptake. pH has pronounced effect on process of removal and removal is higher in lower pH range, maximum (69.5%) being at 2.5 pH and minimum (12.3) at pH 8.0. Various predictive empirical models have been developed.