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Knowledge of calcium phosphate (Ca-P) solubility is crucial forunderstanding temporal and spatial variations of phosphorus (P)concentrations in water bodies and sedimentary reservoirs. In situ relationshipsbetween liquid- and solid-phase levels cannot be fully explained by dissolvedanalytes alone and need to be verified by determining particular sediment Pspecies. Lack of quantification methods for these species limits theknowledge of the P cycle. To address this issue, we (i) optimized aspecifically developed conversion–extraction (CONVEX) method for P speciesquantification using standard additions, and (ii) simultaneously determinedsolubilities of Ca-P standards by measuring their pH-dependent contents inthe sediment matrix. Ca-P minerals including various carbonate fluorapatite(CFAP) specimens from different localities, fluorapatite (FAP), fish boneapatite, synthetic hydroxylapatite (HAP) and octacalcium phosphate (OCP)were characterized by XRD, Raman, FTIR and elemental analysis. Sedimentsamples were incubated with and without these reference minerals and thensequentially extracted to quantify Ca-P species by their differentialdissolution at pH values between 3 and 8. The quantification of solid-phasephosphates at varying pH revealed solubilities in the following order: OCP > HAP > CFAP (4.5% CO3) > CFAP(3.4% CO3) > CFAP (2.2% CO3) > FAP.Thus, CFAP was less soluble in sediment than HAP, and CFAP solubilityincreased with carbonate content. Unspiked sediment analyses together withstandard addition analyses indicated consistent differential dissolution ofnatural sediment species vs. added reference species and therefore verifiedthe applicability of the CONVEX method in separately determining the mostprevalent Ca-P minerals. We found surprisingly high OCP contents in thecoastal sediments analyzed, which supports the hypothesis of apatiteformation by an OCP precursor mechanism

ocean science

Quantification of octacalcium phosphate, authigenic apatite and detrital apatite in coastal sediments using differential dissolution and standard addition