Molecular-level insights into pH regulation of cation adsorption and exchange at clay particle edges

Abstract

Clay particle edges are often adsorption and catalytic centers while performances are strongly pH-dependent. Microscopic understanding of adsorption of single (Na+, K+, Cs+, Pb2+) and binary (Na+/Cs+, Na+/K+, K+/Pb2+, Na+/Pb2+) metal ions by montmorillonite particle edges under various pH conditions is addressed by molecular dynamics simulations, and mechanisms regarding how pH regulates adsorption of metal ions, ion-specific effects, impacts of co-ions, thermodynamics and kinetics of ion exchange, and immobilization of heavy metals are unraveled. Under all pH conditions, Na+ and Pb2+ with smaller ionic radii have much higher inner-sphere adsorption densities than K+ and Cs+, and pH elevation generates new adsorption configurations that greatly promote adsorption. The promoting extent is apparently larger for Na+ and Pb2+, while ion-specific sequence (Na+ > K+ > Cs+) remains, which is applicable to single and binary metal ions. Impacts of co-ions onto adsorption rely on their identities and become magnified at higher pH although with consistent trends at all pH conditions; e.g., Na+ always inhibits Pb2+ adsorption while the degree of inhibition is larger due to pH elevation. Whether to promote or suppress adsorption by co-ions can be predicted from relative adsorption preference (Na+ > Pb2+ > K+ > Cs+). Stronger competitive adsorption occurs for binary Na+/Pb2+ than other Aa+/Bb+ ions and for the particle edges than basal and interlayer surfaces. As indicated by selectivity coefficients, montmorillonite particle edges are always Na+-selective, while pH elevation significantly enhances Pb2+ selectivity. Specific adsorption at clay particle edges emerges at lower pH for Na+ than Pb2+ (5.3 vs. 6.9) and increases at higher pH, while the increasing extent is larger for Pb2+, indicating pH elevation favors the immobilization of heavy metals. Results significantly enrich the knowledge about the adsorption/exchange of metal ions and the immobilization of heavy metals by clay minerals.