Characterization of Phosphate and Arsenate Adsorption onto Keggin-Type Al30 Cations by Experimental and Theoretical Methods

Keggin-type aluminum oxyhydroxide species such as the Al30 (Al30O8­(OH)56­(H2O)2618+) polycation can readily sequester inorganic and organic forms of P­(V) and As­(V), but there is a limited chemical understanding of the adsorption process. Herein, we present experimental and theoretical structural and chemical characterization of [(TBP)2Al2(μ4-O8)­(Al28(μ2-OH)56(H2O)22)]14+ (TBP = t-butylphosphonate), denoted as (TBP)2Al30-S. We go on to consider the structure as a model for studying the reactivity of oxyanions to aluminum hydroxide surfaces. Density functional theory (DFT) calculations comparing the experimental structure to model configurations with P­(V) adsorption at varying sites support preferential binding of phosphate in the Al30 beltway region. Furthermore, DFT calculations of R-substituted phosphates and their arsenate analogues consistently predict the beltway region of Al30 to be most reactive. The experimental structure and calculations suggest a shape–reactivity relationship in Al30, which counters predictions based on oxygen functional group identity.