Competitive Adsorption of Metal Ions at Smectite/Water Interfaces: Mechanistic Aspects, and Impacts of Co-Ions, Charge Densities, and Charge Locations

Abstract

In natural environments, multiple metal ions are concomitant, and their adsorption at clay mineral/water interfaces is competitive and selective. In this study, the competitive mechanisms and impacts of co-ions for binary electrolyte solutions contacting smectite surfaces are probed by molecular dynamics simulations. Different binary metal ions (Na+/Cs+, K+/Cs+ of 1:1 type, Na+/Pb2+ of 1:2 type, and Pb2+/Ca2+ of 2:2 type) are considered, and impacts of charge densities and locations that are critical to ion adsorption are also subject to investigations. Impacts of charge densities and locations onto adsorption of metal ions are similar in single and binary systems, and increase of charge densities and shift of charge locations from octahedral to tetrahedral sheets may change the major surface complexes from the outer sphere to the inner sphere and greatly enhance the amounts and stabilities, with the enhancement degrees being Pb2+, Ca2+ > Na+ > K+ > Cs+. The major surface complexes of metal ions can be changed by charge densities and locations and are more affected by co-ions than the minor surface complexes. Stronger competition and higher adsorption selectivity occur (1) at higher charge densities; (2) for beidellite than montmorillonite; and (3) for the inner sphere than the outer sphere. As discussed, all binary systems show peculiarities regarding competition and adsorption selectivity, and the competition capabilities and adsorption selectivities may vary with charge densities and locations. These findings are consistent with experimental observations available and provide insights to unravel the complicated adsorption mechanisms at clay mineral/water interfaces and to manage contaminated soil sites.