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 + /Pb 2+ of 1:2 type, and Pb 2+ /Ca 2+ 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 Pb 2+ , Ca 2+ > 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.

Electrical double layers (EDL) exist for all interfaces and determination of EDL thicknesses remains a difficult task. In this study, molecular dynamics (MD) simulations are employed aiming to develop a simple method for effective evaluation of EDL thicknesses. The EDL thicknesses at clay minerals/water interfaces are calculated by MD simulations and empirical Debye lengths, considering different electrolyte solutions (NaCl, KCl, CsCl, PbCl2 and CaCl2), electrolyte concentrations (0∼1.28 mol/L) and charge locations (tetrahedral and octahedral substitutions). Both methods predict the comparable EDL thicknesses at very low concentrations and more EDL compressions at higher concentrations, while MD simulations show the significant improvements in three aspects, as more rational declining trends with increase of concentrations (Critical for estimating EDL thicknesses especially in highly concentrated solutions), inclusion of ion‐specific effects (More EDL compression for divalent than univalent metal ions, and distinct EDL thicknesses for different alkali ions) and reflection of strong dependence on charge locations (EDL thicknesses differ for montmorillonite and beidellite and have a milder declining rate for montmorillonite). Insights are also offered regarding the microstructure of EDLs, and results promote the understanding of EDLs that determine all interfacial properties and processes.

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