A simple cation exchange model to assess the competitive adsorption between the herbicide paraquat and the biocide benzalkonium chloride on montmorillonite

Abstract

Since adsorption on clay minerals is one of the most popular water-pollutant removal method, it is important to consider the coexistence of several compounds that can affect pollutant retention. In this work, the competitive behaviour between cationic herbicide paraquat (PQ) and a benzalkonium chloride compound (BAC) on montmorillonite (MMT) was performed by adsorption experiments and analysed by a theoretical cation exchange model. Experimental studies included adsorption isotherms, XRD, FTIR and zeta potential (ζ). Binary systems (BAC + PQ) were performed at different initial concentrations ratio BAC/PQ = 0.1; 0.5; 1.0; 2.0 and 3.0. Results showed that at higher BAC initial concentration, the maximum adsorbed PQ amount decreases. X-Ray diffractograms showed that as the BAC/PQ ratio increases, the MMT basal spacing peak shifts to lower angles suggesting that the basal spacing is mainly controlled by the presence of BAC. Besides, the total adsorbed amount (adsorbed PQ plus adsorbed BAC) is closer to clay CEC value (0.91 mEq g−1), at this value ζ ≅ 0 mV suggesting that the adsorption occurs through a cation exchange mechanism. The theoretical model applied was originally developed to describe the exchange of simple cations on clay minerals, however it was effective in predicting the behaviour of larger cations such as PQ and BAC. In this scenario, thermodynamic parameters calculated through the model indicate that cation exchange process is spontaneous towards adsorbed BAC, suggesting that an adsorbed PQ cation is replaced by an equivalent amount of BAC cation (expressed in terms of electrical charge).