Pretreatment method for the analysis of phosphate oxygen isotope (δ18Op) of different phosphorus fractions in freshwater sediments

Accurate measurement of the oxygen isotopic composition of dissolved phosphate (delta O-18(p)) 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 Ag3PO4 sample for the analysis of delta O-18(p) of different P fractions in freshwater sediments. Sedimentary P was divided into NaHCO3-P, NaOH-P and HCI-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 Mg2+ 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 NaHCO3-P and NaOH-P, and reduce the loss of P; 3) adjustment of the HCI-P extract pH to 4 in order to form Fe(OH)(3)-PO43- 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 Ag3PO4 sample can be obtained and the repeatability and accuracy of measured delta O-18(p) is better than 0.3%., demonstrating high reliability and accuracy. This new method was used to analyze the delta O-18(p) of different P fractions in sediments of a eutrophic lake in southwestern China. The preliminary results indicated that the delta O-18(p), 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. (C) 2019 Elsevier B.V. All rights reserved.