Sequential modification of montmorillonite with dimethyl dioctadecyl ammonium chloride and benzyl octadecyl dimethyl ammonium chloride for removal of perchlorate

A novel method is reported for surfactant modification of montmorillonite (Mt), and then the performance of the surfactant-modified Mt was investigated for the removal of perchlorate (ClO₄⁻). Mt was first modified with dimethyl dioctadecyl ammonium chloride (DDAC) under microwave irradiation, which was followed by a second modification with benzyl octadecyl dimethyl ammonium chloride (BODAC). The influence of the added amount of DDAC in terms of ClO₄⁻ adsorption to the modified Mt was intensively investigated. In addition, sequential and simultaneous-mixing methods were compared for preparation of the modified Mt in terms of ClO₄⁻ removal performance. Using sequential modification, addition of DDAC corresponding to 0.05 times the cation exchange capacity of Mt significantly increased BODAC uptake, resulting in the highest adsorption capacity for ClO₄⁻ (1.08 mmol/g). This can be explained by the good balance between the decrease of steric hindrance and the formation of a hydrophobic interface because of adsorbed DDAC. In contrast, for a composite synthesized with the same ratio of DDAC to BODAC, the simultaneous-mixing method resulted in modified Mt with much poorer ClO₄⁻ removal performance than the sequential modification method because of the negligible contribution of DDAC to BODAC uptake by Mt. Moreover, a high binding affinity of ClO₄⁻ to the sequentially modified composite was obtained because of hydrophobicity of interlaced alkyl chains.