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

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, ionspecific 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.