Abstract
Adsorption processes at mineral–water interfaces control the fate and transport of arsenic in soils and aquatic systems. Mechanistic and thermodynamic models to describe this phenomenon only consider inner-sphere complexes but recent observation of the simultaneous adsorption of inner- and outer-sphere arsenate on single crystal surfaces complicates this picture. In this study, we investigate the ionic strength-dependence of the macroscopic adsorption behavior and molecular-scale surface speciation of arsenate bound to gibbsite and bayerite. Arsenate adsorption decreases with increasing ionic strength on both minerals, with a larger effect at pH 4 than pH 7. The observed pH-dependence corresponds with a substantial decrease in surface charge at pH 7, as indicated by ζ-potential measurements. Extended X-ray absorption fine structure (EXAFS) spectroscopy finds that the number of second shell Al neighbors around arsenate is lower than that required for arsenate to occur solely as an inner-sphere surface complex. Together, these observations demonstrate that arsenate displays macroscopic and molecular-scale behavior consistent with the co-occurrence of inner- and outer-sphere surface complexes. This demonstrated that outer-sphere species can be responsible for strong adsorption of ions and suggests that environments experiencing an increase in salt content may induce arsenic release to water, especially under weakly acidic conditions.
Highlights
Adsorption is a critical control on the occurrence and fate of arsenic in soils and aquatic systems [1,2,3,4,5] and is utilized to remove this toxic element during water treatment [6,7]
The macroscopic uptake of this oxoanion onto oxide minerals has not been observed to display a dependence on ionic strength in prior studies [8,9,10], indicating a lack of outer-sphere complexes [11]
The gibbsite and bayerite particles have similar surface areas (21.8 m2 /g and 23.4 m2 /g, respectively) determined by BET analysis [22]. Both minerals used in this study were previously characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM), confirming they are consistent with the expected products of the published mineral synthesis methods, i.e., single phase, relatively monodispersed particles of the target mineral
Summary
Adsorption is a critical control on the occurrence and fate of arsenic in soils and aquatic systems [1,2,3,4,5] and is utilized to remove this toxic element during water treatment [6,7]. The macroscopic uptake of this oxoanion onto oxide minerals has not been observed to display a dependence on ionic strength in prior studies [8,9,10], indicating a lack of outer-sphere complexes [11]. Previous spectroscopic studies have widely observed that arsenate forms inner-sphere surface complexes on mineral surfaces [4,8,9,12,13,14,15,16,17]. Surface complexation models for arsenate adsorption predict the partitioning of this oxoanion between water and mineral surfaces using only such complexes [9,18,19,20]. A description of arsenate adsorption behavior based solely on an inner-sphere (ligand exchange) binding mechanism was established over the last few decades that is widely accepted today
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