An improved method for phosphate oxygen isotope analysis in water using zirconium-loaded resin enrichment

The phosphate oxygen isotope (δ¹⁸O_P) tracing analysis method is crucial for identifying the sources of phosphorus pollution in water bodies. However, existing methods face challenges in areas such as presample collection, which demands substantial water volumes, as well as in processing, separation, and purification. This study developed a new δ¹⁸O_P testing method using zirconium (Zr)-loaded resin for in situ enrichment, elution, and purification (ZR-ISEEP). The Zr-IRC748 resin, made by loading Amberlite IRC-748 with Zr, shows phosphate adsorption behavior consistent with the Freundlich isotherm (R² = 0.98) and the pseudo-second-order kinetic model (R² = 0.99). In addition, its phosphate adsorption is not significantly affected by common water anions (Cl^-, F^-, NO₃^-, and SO₄^2-). The addition of same phosphorus source, X-ray diffraction analysis, and comparison with the conventional method confirm that the ZR-ISEEP method produces high-purity Ag₃PO₄ without significant isotope fractionation, verifying its feasibility. Furthermore, in a river with phosphate concentrations of 0.047-0.056 mg/L, using 100 mL of resin for 4 days' enrichment, yielded high-purity Ag₃PO₄. No significant differences were observed between instantaneous sampling and ZR-ISEEP method results (P > 0.05). These findings offer a new approach for δ¹⁸O_P monitoring in water bodies, providing a solid foundation for future studies.