Composite-polymer modified bentonite enhances anti-seepage and barrier performance under high-concentration heavy-metal solution

Modified bentonite (MB) was prepared with acrylic acid and sodium carboxymethyl cellulose as monomers, which was characterized with carboxyl and hydroxyl groups on a coverage film. The adsorption capacities for Cd2+ and Pb2+ were increased to 222.41 mg/g and 397.17 mg/g, respectively, indicating that Cd2+ was less competitive than Pb2+. Multiple adsorption kinetics and isotherms indicated that the Cd2+ adsorption by MB was dominated by ion exchange, while Pb2+ adsorption was a combination of physical adsorption and ion exchange. The positron annihilation lifetime (PAL) further indicated that the adsorption of both Cd2+ and Pb2+occurred in the interlayer micropores of MB. Regarding to the chemical compatibility of MB, its free swelling index (FSI) and the hydraulic conductivity reached 85 mL/2 g and 5.87 × 10−11 m/s in deionized water, respectively. In high-concentration Cd2+ and Pb2+ solutions, MB maintained a lower permeability in the Cd2+ solution. The mechanism can be speculated that appropriate adsorption capacity of Cd2+ by MB results in higher content of trapped polymers as polymer blockage in MB, which contributed to maintain impermeability under high-concentration heavy-metal solution.