XANES Determination of Adsorbed Phosphate Distribution between Ferrihydrite and Boehmite in Mixtures

Iron‐ and Al‐(hydr)oxide minerals are important sorbents for retaining PO 4 in soils. Our objective was to determine the distribution of adsorbed PO 4 between ferrihydrite and boehmite in aqueous mixtures of these minerals. Phosphate was adsorbed in aqueous suspensions up to maximum concentrations of 1860, 850, and 1420 mmol kg −1 for ferrihydrite, boehmite, and 1:1 (by mass) mixtures of these minerals at pH 6. The solids were analyzed as moist pastes using P K‐XANES (X‐ray absorption near edge structure) spectroscopy. The adsorption isotherm for the mixed‐mineral suspensions could essentially be described as a linear combination of Freundlich isotherm models for each single‐mineral system, indicating negligible mineral interactive effects on PO 4 adsorption in the mixtures. X‐ray absorption near edge structure spectra for PO 4 adsorbed on ferrihydrite or in ferrihydrite/boehmite mixtures showed a pre‐edge feature at approximately 2146 eV that was absent in boehmite systems. Linear combination fitting of the pre‐edge region of XANES spectra for mixtures with average spectra for PO 4 adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO 4 was adsorbed on ferrihydrite in the mixtures. With increasing concentration of adsorbed PO 4 in the mineral mixtures, the concentration adsorbed on the ferrihydrite component increased linearly. Phosphate distribution trends in the mixtures suggested an affinity preference for ferrihydrite at the lowest adsorbed PO 4 concentration (100 mmol kg −1 minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO 4 kg −1 ), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO 4 kg −1 ).