Removal of Mixed Pesticides from Aqueous Solutions using Organoclays: Evaluation of Equilibrium and Kinetic Model

Removal of mixed pesticides, namely alachlor, metolachlor, chlorpyriphos, fipronil, alpha-endosulfan, beta-endosulfan, p,p'-DDT and two metabolites p,p'-DDE and endosulfan sulphate from aqueous solution by batch adsorption onto three commercial organo-modified montmorillonite clays [modified with octadecylamine (ODA-M), modified with dimethyl- dialkylamine (DMDA-M) and modified with octadecylamine and aminopropyltriethoxysilane (ODAAPS-M)] were investigated. Effect of process variables, mainly contact time and initial concentration of mixed pesticides, on adsorption phenomenon were evaluated. To understand the adsorption kinetic pseudo-first-order and pseudo-second-order models were tested. The pseudo-second-order model provided the best fit for explaining adsorption kinetics, on the basis of high correlation coefficient (r) and normalized percent deviation values. The adsorption equilibrium was explained by the Freundlich isotherm (r = 0.951-0.992). High values (0.17-0.52 mg g(-1)) of Freundlich constant (K-f) indicated higher affinity of pesticides towards all three organoclays, as a result of hydrophobic interaction between the adsorbent/adsorbate systems. Pesticides with high octanol-water partition coefficient (K-ow) and low water solubility showed faster adsorption with higher K-f values as compared to the pesticides with low K-ow and high water solubility. The order of organoclays for removal efficiency of mixed pesticide was ODAAPS-M > DMDA-M > ODA-M. These findings may find application to decontaminate or treat mixed pesticide contaminated industrial/agricultural waste waters.