Constructing ZIF-8-decorated montmorillonite composite with charge neutralization effect and pore structure optimization for enhanced Pb2+ capture from water

A zeolitic imidazole framework-8/montmorillonite (ZIF-8/MMT) composite was synthesized by a simple method through in-situ growth of ZIF-8 on MMT for efficient Pb2+ capture from water. The adsorption process of Pb2+ by ZIF-8/MMT composite followed the Langmuir model and the maximum adsorption could reach up to 336.89 mg/g, which was much better than that of the monocomponent of ZIF-8 (117.78 mg/g) or MMT (85.20 mg/g). Kinetics result suggested that Pb2+ adsorption by ZIF-8/MMT was a pseudo-second-order dynamic process, indicating a chemisorption process dominates. Ion exchange, electrostatic attraction and hydroxyl complexation were main pathways for Pb2+ removal by ZIF-8/MMT composite. The enhanced performance of ZIF-8/MMT composite should be attributed to following points. Firstly, the lamellar structure of MMT was in favor of ZIF-8 nanoparticles grow and disperse uniformly on the surface, thus avoiding the aggregation. Secondly, the positive charge on ZIF-8 was neutralized by the negative charge on the surface of MMT, and thus the adsorption capacity of ZIF-8 for Pb2+ is effectively released. Thirdly, the combination of MMT and ZIF-8 optimized the pore structure, which was favorable for Pb2+ adsorption and diffusion. Therefore, the synthesized ZIF-8/MMT composites is a prospective candidate for Pb2+ removal from wastewater.