Abstract: Phosphorus is the limiting element of the trophic states for most freshwater systems. Therefore, the identification of the
main phosphorus sources is essential to the efficient control of phosphorus pollution as well as the prevention of eutrophication. Phosphate oxygen isotope (δ¹⁸OP) is proved to be effective in source identification. However, freshwater habitats are always of low phosphorus concentrations and complex compositions. For example, water bodies in silicate rocks exposed areas can be characterized by having little phosphorus while abundant silica. The later can easily precipitate and adsorb PO₄^3-, making it difficult to obtain valid δ¹⁸OP through existing methods. In this study, the widely used Mclaughlin pretreatment method was modified to deal with the phosphorus-poor, silicon-rich water, the phosphorus-poor sandy sediments and soils. The efficient enrichment and purification of PO₄^3- in freshwater are achieved with the following improvements: (i) reduction in the proportion of reagents added at the PO₄^3- enrichment step; (ii) graded pH adjustment prior to CePO₄ precipitation; (iii) multiple steps of fine pore membrane filtration to reduce the silica content; (iv) dynamic ion-exchange to remove cations. For phosphorus-poor sandy sediments and soils, an increase of the sample amount and the concentration of the extraction solutions, combined with the purification steps of water samples have been used to precipitate sufficient pure Ag₃PO₄ for δ¹⁸OP determination. The phosphate recovery of significant procedures relating to precipitation or cation are above 80%, and the precision is ±0.6‰ . The analytical accuracy was evaluated by comparing the δ¹⁸O_P of Ag₃PO₄ generated with the modified methods and the pure phosphate solution directly. The modified methods have been applied to the δ¹⁸O_P determination of samples from Danjiangkou Reservoir basin, providing technical support for tracing phosphorus in this basin.

Key words: phosphate oxygen isotope (δ18OP), pretreatment methods, phosphorus-poor and silicon-rich freshwater system;
phosphorus sources identification