The failure of using equilibrium adsorption of fosthiazate onto montmorillonite clay particles to predict their cotransport in porous media as revealed by batch and column studies

PurposeAdsorption of fosthiazate onto montmorillonite clay particles (MPs) and cotransport of fosthiazate and MPs in sand and soil porous media were investigated for the understanding of the influence of MPs on transport of fosthiazate in subsurface environments.Materials and methodsThe adsorption of fosthiazate onto MPs was first examined by batch experiments and described by isotherm equations. Then, column experiments were conducted to investigate cotransport of fosthiazate and MPs in sand or soil porous media. Finally, the distribution of fosthiazate between colloidal (i.e., MPs) and solution phases during their cotransport in the porous media was compared to that in the batch experiments.Results and discussionThe isotherms for the fosthiazate adsorption were well fitted by Freundlich equilibrium model. The adsorption was endothermic and spontaneous, which was attributed to electrostatic attraction of the electronegative groups (phosphate ester) on the fosthiazate at the positively charged sites on the MPs. Almost complete breakthroughs were observed for the fosthiazate in sand porous media due to lack of positive surface charges for the adsorption. The addition of MPs did not enhance the retention of fosthiazate, i.e, by attachment of the MPs associated with fosthiazate, because the attachment rate of MPs on the sand surfaces was much greater than the adsorption rate of fosthiazate. However, the MPs remained in the pore water causing a distribution of fosthiazate between the colloidal and solution phases. Such distribution was dependent on MP concentration and collector property (i.e., sand or soil) and was different from that in batch experiments.ConclusionsOur study was the first to unambiguously show that the equilibrium adsorption cannot be used in transport models to predict the mobility of MP-associated organophosphorus pesticide fosthiazate due to greater attachment rate of MPs on soil collectors than adsorption rates of the pesticide on the MPs.