Impacts of Surface Site Coordination on Arsenate Adsorption: Macroscopic Uptake and Binding Mechanisms on Aluminum Hydroxide Surfaces

Aluminum hydroxides play important roles in regulating the fate and transport of contaminants and nutrients in soils and aquatic systems. Like many metal oxides, these minerals display surface functional groups in a series of coordination states, each of which may differ in its affinity for adsorbates. The distribution of functional group types varies among distinct surfaces of aluminum hydroxides, and we thus hypothesize that the adsorption behavior and mechanisms will show a dependence on particle morphology. To test this hypothesis, we investigate arsenate adsorption on two aluminum hydroxide polymorphs with distinct particle morphologies, gibbsite [γ-Al(OH)₃] and bayerite [α-Al(OH)₃], at pH 4 and 7. Synthetic gibbsite platelets expose large (001) basal surfaces predicted to be terminated by doubly coordinated functional groups (>Al₂OH). In contrast, synthetic bayerite microrods display mainly edge surfaces (parallel to the c axis) containing abundant singly coordinated functional groups (>AlOH₂). Macroscopic adsorption studies show that gibbsite adsorbs less arsenate per unit surface area than bayerite at both pH values and suggest that two surface complexes form on each material. Similar electrokinetic behavior is displayed at the same relative coverages of arsenate, suggesting that similar reactive surface groups (>AlOH₂) control the surface charging on both particles. EXAFS spectroscopy shows that there is no variation in arsenate surface speciation on a given mineral with surface coverage or pH. Whereas bidentate binuclear inner-sphere species are the dominant complexes present, the EXAFS result suggest that outer-sphere species also occur on both minerals, with a greater abundance on gibbsite. This binding mode likely involves adsorption to >Al₂OH sites, which have a slow ligand exchange rate that inhibits inner-sphere binding. These results demonstrate that adsorption mechanisms and capacity, even when normalized for specific surface area, vary with metal oxide particle morphology because of the distribution of distinct functional groups.