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

Abstract

Iron‐ and Al‐(hydr)oxide minerals are important sorbents for retaining PO4in soils. Our objective was to determine the distribution of adsorbed PO4between 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−1for 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 PO4adsorption in the mixtures. X‐ray absorption near edge structure spectra for PO4adsorbed 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 PO4adsorbed on boehmite or ferrihydrite alone, indicated that 59 to 97% of the PO4was adsorbed on ferrihydrite in the mixtures. With increasing concentration of adsorbed PO4in 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 PO4concentration (100 mmol kg−1minerals), no affinity preference for either mineral at intermediate concentrations (200 to 600 mmol PO4kg−1), and the possibility of a surface precipitate involving Al at the highest concentration (1300 mmol PO4kg−1).