Pretreatment method for the analysis of phosphate oxygen isotope (δO) of different phosphorus fractions in freshwater sediments.

Accurate measurement of the oxygen isotopic composition of dissolved phosphate (δO) of different phosphorus (P) fractions in lacustrine sediments is very difficult because of the influence of large amounts of impurities. In this study, we developed a five-step method for obtaining high purity AgPO sample for the analysis of δO of different P fractions in freshwater sediments. Sedimentary P was divided into NaHCO-P, NaOH-P and HCl-P by chemical sequential extraction. Pretreatment procedures for different sedimentary P fractions were improved in the following respects: 1) abandonment of the magnesium-induced coprecipitation method to avoid the introduction of impurity ions, such as Mg and Cl, and reduce the loss of P; 2) use of a small amount of non-phosphate activated carbon powder to efficiently remove organic matter in extracts of NaHCO-P and NaOH-P, and reduce the loss of P; 3) adjustment of the HCl-P extract pH to 4 in order to form Fe(OH)-PO coprecipitate, thereby removing most of metals and Cl. This method reduces the pretreatment steps, simplifies the operation and increases the recovery of phosphate (90.98%-96.69%). The high purity AgPO sample can be obtained and the repeatability and accuracy of measured δO is better than 0.3‰, demonstrating high reliability and accuracy. This new method was used to analyze the δO of different P fractions in sediments of a eutrophic lake in southwestern China. The preliminary results indicated that the δO in the sediments can be used to identify different P sources, and provide new insights into sedimentary P cycling. The method established in this study provides a powerful tool for investigating the sources and biogeochemical cycle of P in freshwater sediments.