An Isotope Labeling Technique to Investigate Atom Exchange during Phosphate Sorption and Desorption

Iron and manganese oxides are common minerals in a wide range of soils, and they largely control the fate of P. It is often implicitly assumed that phosphate does not exchange O atoms with minerals and water during adsorption, desorption, and precipitation reactions, but this has not yet been fully verified. In this study, we applied an ¹⁸O stable isotope labeling technique to individually label O atoms in Fe and Mn oxides, water, and phosphate, and tested the exchange of O among them during phosphate sorption and desorption reactions. Physico-chemical properties of ¹⁸O-labeled goethite, hematite, hausmannite, and birnessite minerals were characterized using X–ray diffraction, Brunauer–Emmett–Teller (BET) surface area and pore size distribution, and scanning electron microscopy. All sorption and desorption results showed fast initial kinetics, with hausmannite having the highest sorption capacity, followed by a relatively slower sorption–desorption reaction before reaching equilibrium. Oxygen isotope ratios in Fe and Mn oxides, phosphate, and water showed no exchange of O between phosphate and mineral or water during either sorption or desorption reactions. These results attest to the integrity of the P–O bond during these reactions and thus enable the phosphate O isotope composition to be used as a reliable tool for tracing phosphate sources in soils and other environments.