Origin for superior adsorption of metal ions and efficient control of heavy metals by montmorillonite: A molecular dynamics exploration

Ion adsorption/exchange at charged solid/water interfaces plays a central role during a broad spectrum of chemical, environmental and engineered processes. Almost all clay minerals are featured by strong charge heterogeneity (ChHg), and in this study, critical importance and regulatory mechanisms of ChHg during adsorption/exchange of metal ions and control of heavy metals at montmorillonite interfaces have been unraveled. Although increase of charge densities promotes adsorption of metal ions, inner-sphere (InS) Pb2+ appears only at very high charge densities (1.50 e/uc); In contrast, ChHg causes considerable InS(Pb2+) at 0.75 e/uc. Compared to InS(Ca2+), InS(Pb2+) emerges at much lower ChHg: 0.03 vs. 0.11 C/m2, and is apparently superior regarding both amount and stability; Moreover, stronger ChHg further promotes InS adsorption; e.g., at 0.11 C/m2, InS(Pb2+) and InS(Ca2+) have adsorption densities of 0.34 and 0.06 μmol/m2 and potential energies of -121.3 and -81.5 kJ/mol, respectively. Pb2+ dominates InS adsorption of binary Pb2+/Ca2+ systems, and impacts of co-ions are consistent for all ChHg and magnified at stronger ChHg: Pb2+ always suppresses Ca2+ adsorption whereas Ca2+ always promotes Pb2+ adsorption. Selectivity coefficients amount to 0.87, 1.06 and 1.16 when ChHg equals 0.00, 0.05 and 0.11 C/m2, respectively. Increase of ChHg causes the switch from Ca2+-selectivity to Pb2+-selectivity, and adds kinetic difficulty of Pb2+ exchange by Ca2+. Accordingly, ChHg within montmorillonite becomes source for efficient adsorption of divalent metal ions and control of heavy metals. Results rationalize experimental observations at molecular level, and provide new insights about adsorption/exchange of metal ions and control of heavy metals at clay/water interfaces that are significantly beneficial to screening and synthesis of efficient clay-based adsorbents.