Insights into the Mechanism of Fe(II) Adsorption and Oxidation at Fe-Clay Mineral Surfaces from First-Principles Calculations

Interfacial reactivity of redox-active iron-bearing mineral surfaces plays a crucial role in many environmental processes including biogeochemical cycling of various elements and contaminants. Herein, we apply density functional theory (DFT) calculations to provide atomistic insights into the heterogeneous reaction between aqueous Fe(II) and the Fe-bearing clay mineral nontronite Fe2Si4O10(OH)(2) by studying its adsorption mechanism and interfacial Fe(II) Fe(III) electron transfer (ET) at edge and basal surfaces. We find that edge-bound Fe(II) adsorption complexes at different surface sites (ferrinol, silanol, and mixed) may coexist on both (010) and (110) edge facets, with complexes at ferrinol FeO(H) sites being the most energetically favorable and coupled to proton transfer. Calculation of the ET activation energy suggests that interfacial ET into dioctahedral Fe(III) sheets is probable at the clay edges and occurs predominantly but not exclusively through the complexes adsorbed at ferrinol sites and might also involve mixed sites. No clear evidence is found for complexes on basal surface that are compatible with ET through the basal sheet despite this experimentally hypothesized ET interface. This study suggests a strong pH-dependence of Fe(II) surface complexation at basal versus edge facets and highlights the importance of the protonation state of bridging ligands and proton coupled electron transfer to facilitate ET into Fe-rich clay minerals.