18O Isotope Labeling Technique to Investigate Phosphate Sorption and Desorption Mechanisms on Iron and Manganese Oxides.

Iron and manganese oxides are commonly present in a wide range of soils and sediments and often control the fate of phosphate in these environments. Question regarding whether oxygen atom from mineral or phosphate exchanges during sorption, desorption, and mineral transformation has not been fully understood. In this study, we applied ¹⁸O stable isotope labeling technique to identify oxygen atom exchange between mineral, water, and phosphate and thereby explore the mechanisms of phosphate sorption and desorption on and from Fe/Mn oxides. We synthesized ¹⁸O labeled goethite, hematite, manganite, and birnessite minerals and characterized their physico-chemical properties by using X-ray diffraction, BET surface area and pore size distribution, and FIB scanning electron microscope. We monitored the oxygen isotope exchange among mineral, phosphate, and water by measuring corresponding changes in their d¹⁸O values during sorption, desorption, and mineral transformation. SEM images verify the purity of synthesized minerals with characteristic mineral morphologies such as needle shaped goethite, ball shaped hematite and manganite, and flowery layer-structured birnessite. Surface area and pore size distribution of these minerals vary from 38 to 60 m²/g, and 50 to 250 nm, respectively. Results from sorption experiments show that these minerals have strong sorption affinity for phosphate, with goethite having the highest sorption capacity. All sorption isotherms show fast initial sorption kinetics with half-life of sorption varying from 40 to 240 min, followed by relatively slow sorption kinetics before reaching to equilibrium. Further measurement of oxygen isotopes of Fe/Mn oxides, phosphate, and water at the different stages of the experiments will enable us to understand oxygen atom exchange during sorption, desorption, and mineral transformation.