Quantification of individual phosphorus species in sediment: a sequential conversion and extraction method

Common sequential phosphorus (P) extraction methods are not specific to particular chemical species and have several limitations. This work presents the first chemical method for quantification of individual mineral and sorbed P species. It was developed by combining a conversion technique with a sequential extraction procedure. Mangrove sediments with different characteristics were incubated in pH-adjusted 0.01 M CaCl 2 with and without reference material additions of octacalcium phosphate (Ca 8 H 2 (PO 4 ) 6 .5H 2 O; OCP), hydroxyapatite (Ca 5 (PO 4 ) 3 OH), strengite (FePO 4 .2H 2 O) or variscite (AIPO 4 .2H 2 O). The changes in soluble phosphate concentration were measured in the supernatant solution, while pH-induced variations in P composition were determined by subsequent sequential extraction of the sediments. Dissolved phosphate concentration was controlled by adsorption below pH 7.8. Above this pH, soluble phosphate concentration was governed by OCP, which was qualitatively determined by plotting the experimental values of pH + pH 2 PO 4 and pH - 0.5 pCa on a solubility diagram including the isotherms of known crystalline phosphate compounds. In contrast to the often-predicted slow dissolution rate of crystalline phosphates in soils or sediments, drastic changes in P composition by dissolution, precipitation and adsorption processes were detected after 7 days. These were mainly not observed indirectly by changes in dissolved phosphate through adsorption effects, but were determined quantitatively by subsequent sequential extraction, thus enabling the quantification of individual species. Evaluation of the method was performed by standard addition experiments. Besides P species quantification, the method provides the means for other applications, such as the determination of P mineral dissolution kinetics in soils and sediments, the prediction of P composition in changing environmental settings and the refinement of theoretical models of phosphate solubility in soil and sedimentary environments.