Preparation and application of CPC/Keggin-Al30 modified montmorillonite composite for Cr (VI) removal

Abstract

To enhance the adsorption performance of natural calcium-based montmorillonite (Ca-MMT), the polymeric hydroxyaluminum cation - Keggin-Al30 ([Al30O8(OH)56(H2O)24]18+) and cetylpyridinium chloride (CPC) were used to modify Ca-MMT for preparation of CPC-Al30 pillared montmorillonite composite (CPC-Al30PMT). The CPC-Al30PMT was characterized via analysis of its chemical and physical structure, morphology, surface properties, and zeta potential. The results showed that the Al30 polymer cation and CPC successfully intercalated into the interlayer of Ca-MMT, which increased the d001 value from 1.47 nm to 1.93 nm, and increased the pore volume correspondingly. The zeta potential was reversed from negative to positive after modification, presenting a possibility to adsorb anionic contaminants from water. The investigation of hexavalent chromium (Cr (VI)) adsorption on CPC-Al30PMT showed that the removal of Cr (VI) was positively correlated with the dosage of adsorbent, and negatively correlated with the initial concentration of Cr (VI), the ionic strength and the initial pH of the solution. CPC-Al30PMT showed good Cr (VI) removal performance, with the maximum adsorption (qe) of 42.44 mg/g. The adsorption process conformed to Freundlich model and pseudo-second-order kinetic model. Thermodynamics studies showed that adsorption was a spontaneous exothermic process. Electrostatic interaction, ion exchange and reduction were the main mechanisms of the removal.