Phosphate Bonding on Noncrystalline Al/Fe-Hydroxide Coprecipitates

Poorly crystalline minerals have high sorption capacities for environmentally important chemical species, but molecular-level mechanisms of sorption on complex mineral assemblages remain largely unknown. We determined the distribution of orthophosphate (PO4) bonding between Al and Fe in relation to structural properties of Al/Fe-hydroxide coprecipitates. Phosphate was sorbed at concentrations between 0.042 and 0.162 mol P mol(-1) Al+Fe on coprecipitates containing 0, 20, 50, 75, or 100 mol % of metal as Al. Phosphorus XANES analyses showed preferential bonding of PO4 for Al on coprecipitates with 20 and 50 mol %Al, and no preference for either metal at 75 mol % Al, consistent with X-ray photoelectron spectroscopy (XPS) analyses of near-surface metal distributions. Structural ordering and the Fe-hydroxide domain size in coprecipitates decreased with increasing Al proportion, as shown by X-ray diffraction (XRD) and Fe EXAFS analyses. Structural interactions in coprecipitates imparted unique PO4 sorption properties compared with isolated Al- or Fe-hydroxide.