One-nanometre-resolution evidence of As(III) anoxic and oxic transformations on the surfaces of expandable clay minerals

Arsenic (As) is a priority pollutant found in water bodies, sediments and soils; therefore, mineral surfaces are critical for the environmental distribution of this element. The objective of this work was to analyse expandable clay minerals [hectorite (Fe-poor) and nontronite (Fe-rich)] exposed to As [as As(III), the most toxic form of As; 10 −4 M sodium arsenite in water, pH 0 = 7.2, 25 ± 1 °C] with 1 nm precision using hydride generation–cryotrapping–atomic absorption spectrometry, high-resolution scanning electron microscopy and energy-dispersive spectrometry. In supernatant solutions, As(III) underwent no transformation. The accumulation of As(III) was registered in hectorite (4%) and nontronite (2.7%). The outcome of this work expanded on current knowledge on the interaction between As(III) and expandable clay minerals. The higher content of structural Fe in nontronite vs. hectorite favoured the retention of As(III). Arguably, adsorbed As(III) on nontronite surfaces (1) transformed to arsenical ferrihydrite or (2) bonded with naturally occurring S; both processes facilitated its nonreversible surface bonding. In addition, adsorbed As(III) on nontronite and hectorite surfaces reversibly bonded to surface O.