Effects of Ionic Strength on Arsenate Adsorption at Aluminum Hydroxide–Water Interfaces

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.

Effects of Phosphate Competition on Arsenate Binding to Aluminum Hydroxide Surfaces

“…Besides the above differences, the EXAFS-derived CN Al and As–Al interatomic distances R Al showed no change with varying phosphate concentrations for all samples explored on each mineral under various experimental conditions (Figure , Table S3). All of the EXAFS results indicate that arsenate adsorption mechanisms on gibbsite and bayerite are similar to results in our prior studies, , and these observed differences are largely preserved during competitive adsorption with phosphate (Figure S6). Therefore, the EXAFS results suggest that the suppression of arsenate adsorption due to phosphate addition occurs because of uniform competition at all binding sites and that no additional arsenate species form.…”

“…Different experimental conditions, such as pH variation, arsenate coverage, the addition of different amounts of phosphate, and mineral adsorbents, did not produce any distinctive patterns in EXAFS spectra (Figure S6), indicating the similar arsenate adsorption mechanisms on all samples explored. Structural model fits of the EXAFS spectra for all samples result an interatomic distance of around 1.7 and 3.2 Å for the first As–O and second As–Al shells, respectively, consistent with the bidentate binuclear inner-sphere arsenate surface complexes, identified in most previous studies. ,,,,− Ideally, if arsenate only forms bidentate binuclear inner-sphere surface complexes, the coordination number for the Al neighbors associated with this species (CN Al ) should be exactly 2. However, our EXAFS fitting results showed the CN Al is much smaller than 2 for both minerals regardless of pH and arsenate surface coverage (Table S3), indicating the presence of an additional type of arsenate surface species such as monodentate inner-sphere or outer-sphere complexes.…”

“…Our recent work exploring arsenate adsorption to aluminum hydroxide polymorphs, gibbsite and bayerite, suggested that bidentate inner-sphere arsenate surface complexes co-occur with outer-sphere species . The presence of outer-sphere species was further supported by the observation of classical ionic strength dependence of macroscopic arsenate adsorption onto both minerals at pH 4 . In addition, we found no readily apparent relationship between particle morphology and binding affinity or capacity for different mineral surfaces, suggesting uncharacterized variations of surface complexation modes of arsenate on aluminum hydroxides.…”

“…However, our EXAFS fitting results showed the CN Al is much smaller than 2 for both minerals regardless of pH and arsenate surface coverage (Table S3), indicating the presence of an additional type of arsenate surface species such as monodentate inner-sphere or outer-sphere complexes. Our EXAFS model refinements did not reveal monodentate inner-sphere binding configurations in the samples, suggesting the possible co-occurrence of arsenate outer-sphere surface complexes. ,, The EXAFS-derived CN Al on average is lower for gibbsite than that for bayerite, and statistical test ( t -tests) showed that this difference is significant. Thus, the lower CN Al suggests a greater proportion of outer-sphere arsenate complexes in the gibbsite system.…”

Spectroscopic Quantification of Inner- and Outer-Sphere Oxyanion Complexation Kinetics: Ionic Strength and Background Cation Effect on Sulfate Adsorption to Hematite

“…Mixed inner- and outer-sphere species have been observed on other Fe-oxide (e.g., ferrihydrite and goethite) and Al-oxide surfaces, suggesting that a similar kinetic behavior of sulfate surface species may also be relevant to these minerals. ,− Given the large contribution of Fe- and Al-oxide minerals to sulfate adsorption by soils, this work may better inform not only sulfate adsorption on these minerals but soils on the whole . Furthermore, the experimental framework established here may be relevant to adsorption kinetics of other oxyanions which may form coexisting surface species on soil mineral surfaces, such as selenate, chromate, and arsenate. ,,, This interpretation is particularly relevant to further development of oxyanion adsorption kinetic models which rely on molecular-level assumptions regarding reaction mechanisms that we show may be experimentally verified, despite parameterization with an experimental kinetic model.…”