The mobility of arsenate and phosphate in soils and aquatic systems is strongly governed by adsorption to mineral surfaces. As common naturally occurring adsorbents, aluminum hydroxides play a critical role in regulating the environmental mobility of arsenate and phosphate because of their high adsorption affinities for these oxoanions. These minerals display surface functional groups in a series of coordination states, each of which differ in their adsorption affinities for these oxoanions. The distribution of functional group types varies among distinct surfaces of aluminum hydroxides and we thus hypothesize that oxoanion adsorption behavior and mechanisms will show a dependence on particle morphology. To test this hypothesis, we investigated individual and competitive adsorption of arsenate and phosphate on two aluminum hydroxide polymorphs, gibbsite and bayerite, with distinct particle morphologies at pH 4 and 7. Synthetic gibbsite platelets expose large (001) basal surfaces terminated predominantly by doubly coordinated functional groups (&gt;Al<sub>2</sub>OH). In contrast, synthetic bayerite microrods display mainly edge surfaces dominated by singly coordinated functional groups (&gt;AlOH). Macroscopic adsorption studies show that both oxoanions behave similarly in individual adsorption experiments with gibbsite adsorbing less arsenate or phosphate per unit surface area than bayerite. The addition of phosphate reduces arsenate adsorption on both minerals via competitive adsorption. Phosphate has a greater effects on arsenate adsorption to gibbsite than that to bayerite. These observations suggested that &gt;Al<sub>2</sub>OH groups have affinities for arsenate and phosphate that differ from the affinities displayed by &gt;AlOH groups. EXAFS spectroscopy reveals that arsenate adsorbed to gibbsite has a lower number of Al neighbors on average than on bayerite, suggesting either a greater proportion of outer-sphere arsenate complex or a distinct coordination environment. These differences are largely preserved during competitive adsorption with phosphate. These results demonstrate that macroscopic and molecule-scale arsenate adsorption behavior is affected by surface functional group coordination state.

Impacts of surface site coordination on arsenate adsorption: Macroscopic uptake, competitive adsorption, and binding mechanisms on aluminum hydroxide surfaces

251st National Meeting (2016)

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