Clay Mineral Structure Dictates Arsenic(III) Oxidative Fate: Mechanistic Insights from Contrasting 1:1 and 2:1 Phyllosilicates

The oxidative transformation of arsenite (As(III)) by iron-bearing clay minerals is a critical process mitigating arsenic mobility and toxicity in aquifers. However, the molecular-scale mechanisms underlying the distinct efficiencies among different clay types remain poorly constrained. Herein, employing synthetic Fe(II)-rich analogues (2:1-type montmorillonite (SIP-1) and 1:1-type kaolinite (SIP-2)), we demonstrate that the expandable 2:1 structure of SIP-1 facilitates a sustained and multi-pathway oxidation process, achieving superior As(III) oxidation (60%) and removal (>99%) under neutral-to-acidic conditions. A suite of evidence confirms that its nanoconfined interlayer microenvironment not only enables direct electron transfer from structural Fe(II) to As(III) but also stabilizes reactive intermediates (ROS and Fe(IV)), thereby inhibiting surface passivation. In contrast, the non-expandable 1:1 structure of SIP-2 constrains oxidation to external surfaces, leading to rapid deactivation via irreversible passivation, with its maximum efficiency shifting to alkaline conditions (76% removal, pH 8). Our findings establish that the crystal structure dictates the interfacial oxidation mechanism and long-term performance, overriding the influence of iron content alone. This mechanistic understanding provides a predictive framework for arsenic fate in heterogeneous subsurface environments and a fundamental principle for designing high-performance, clay-based remediation materials.