Removal of thiocyanate from aqueous solutions by ion exchange

Abstract

The adsorption kinetics and equilibrium of thiocyanate in aqueous solutions onto an anion-exchange resin (Purolite A-250) were investigated in a batch-mode operation to assess the possible use of this adsorbent. The effect of various parameters such as initial thiocyanate concentration, contact time, pH, particle size, resin dosage and temperature were studied. A comparison of four kinetic models, the pseudo-first-order, second-order, Elovich and diffusion controlled kinetic models, on the thiocyanate-resin system was used to determine the rate constants and the adsorption mechanism. The kinetic results correlated well with pseudo-second-order model. The experimental parameters had also an effect on the pore and surface diffusivities. The optimum conditions for removal of thiocyanate were found to be pH 8, 2 g/l of adsorbent dosage, 355–500 μm of particle size and equilibrium time of 30 min, respectively. The column capacity and performance by the bed depth service time model using bed depth and flow rate as variables were evaluated. The adsorption isotherm data were fitted well to Langmuir and Freundlich isotherms. The adsorption capacity was calculated as 191.20 mg/g at 323 K. Thermodynamics parameters such as free Δ G 0 , Δ H 0 and Δ S 0 for the adsorption were evaluated. The positive value of Δ H 0 indicated that the process was endothermic in nature.