Method for phosphate oxygen isotopes analysis in water based on in situ enrichment, elution, and purification

The phosphate oxygen isotope (δ¹⁸O_P) ratio has been proven to be an effective tool to trace the sources and biogeochemical cycles of phosphorus (P) in aquatic ecosystems. However, the enrichment of phosphate (PO₄) and the removal of impurities are quite complex and easy to cause PO₄ loss in current δ¹⁸O_P analytical methods. Moreover, the δ¹⁸O_P value obtained by the commonly-used instantaneous sampling method is more of the instantaneous information of P, which is accidental or uncertain for accurate identification of the P source. In this study, a new method of in situ enrichment, elution, and purification of PO₄ (ISEEP) was developed for δ¹⁸O_P analysis in waters. This method utilized a PO₄ binding phase (Zr-Oxide gel) to selectively in situ adsorb PO₄ in water and exhibited an adsorption capacity per unit area of up to 789.3 μg P/cm². The PO₄ on the gel was eluted easily with a 1 M NaOH solution. More than 99.7% of the common anions, cations, and dissolved organic matter (DOM), as well as more than 90% of the trace elements were removed synchronously after adsorption and elution of PO₄. The recovery rate of PO₄ in the whole procedure was as high as 92.8%. The XRD and SEM examinations showed that the ISEEP can obtain high-purity Ag₃PO₄ solid for the δ¹⁸O_P measurement. The reliability of the ISEEP method is confirmed by the measured δ¹⁸O_P value and standard deviation of parallel samples from different types of natural waters obtained by both the ISEEP and the current popular McLaughlin (2004) method. It provides a good prospect of this new method for tracing the P sources and their biogeochemical cycling in aquatic ecosystems.