Selective acid recovery and metal separation from acid mine drainage using a highly stable vermiculite membrane

Selective ion separation from acid mine drainage (AMD) under extreme acidic conditions presents a critical environmental challenge. While membrane-based technologies show promise for advancing water treatment processes, implementation in AMD treatment requires membranes that combine exceptional acid stability with precise ion sieving capabilities. Inspired by the ion retention characteristics of natural clay minerals, we have developed cation-selective membranes using vermiculite nanosheets as building blocks. The vermiculite membranes (VM) featured high acid stability and well-ordered two-dimensional nanochannels (2 Å), achieving a high H⁺ permeation rate of 3.24 mol m^-2 h^-1. The VM demonstrated exceptional selectivity between monovalent and metals ions, with a H⁺/Fe³⁺ selectivity factor of 1284 in single-cation transport process. In complex multi-ion environments, the VM maintained stable separation performance, achieving a H⁺/Fe³⁺ selectivity of 1000 in mixed solutions containing H⁺, K⁺, Na⁺, Ca²⁺, Mg²⁺ and Fe³⁺. Additionally, VM effectively blocked other common AMD metals, including Cu²⁺, Co²⁺, Ni²⁺ and Mn²⁺, while maintaining stable separation performance even at extreme low pH values (pH = 1). Through integrated theoretical calculations and experiments, we revealed that the synergistic effects of ultra-confined nanochannels (4∼5 Å) and surface charge created enhanced energy barriers for trivalent ion transport, resulting in high ion selectivity. Beyond providing an effective solution for AMD remediation, this work establishes vermiculite-based membranes as promising candidates for ion separation applications.