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

Oxmann, Julian F. and Schwendenmann, L. (2014) Quantification of octacalcium phosphate, authigenic apatite and detrital apatite in coastal sediments using differential dissolution and standard addition Ocean Science, 10 (3). pp. 571-585. DOI 10.5194/os-10-571-2014.

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Abstract

Knowledge of calcium phosphate (Ca-P) solubility is crucial for understanding temporal and spatial variations of phosphorus (P) concentrations in water bodies and sedimentary reservoirs. In situ relationships between liquid-and solid-phase levels cannot be fully explained by dissolved analytes alone and need to be verified by determining particular sediment P species. Lack of quantification methods for these species limits the knowledge of the P cycle. To address this issue, we (i) optimized a specifically developed conversion-extraction (CONVEX) method for P species quantification using standard additions, and (ii) simultaneously determined solubilities of Ca-P standards by measuring their pH-dependent contents in the sediment matrix. Ca-P minerals including various carbonate fluorapatite (CFAP) specimens from different localities, fluorapatite (FAP), fish bone apatite, synthetic hydroxylapatite (HAP) and octacalcium phosphate (OCP) were characterized by XRD, Raman, FTIR and elemental analysis. Sediment samples were incubated with and without these reference minerals and then sequentially extracted to quantify Ca-P species by their differential dissolution at pH values between 3 and 8. The quantification of solid-phase phosphates 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 solubility increased with carbonate content. Unspiked sediment analyses together with standard addition analyses indicated consistent differential dissolution of natural sediment species vs. added reference species and therefore verified the applicability of the CONVEX method in separately determining the most prevalent Ca-P minerals. We found surprisingly high OCP contents in the coastal sediments analyzed, which supports the hypothesis of apatite formation by an OCP precursor mechanism.

Document Type: Article
Keywords: CONVEX; SEDEX; Sequential extraction method; Fractionation; Marine phosphorus cycle; Phosphorus speciation; Successive or direct crystallization; Ostwald step rule; Octacalcium phosphate precursor mechanism; Early diagenesis; Calcium phosphate solubility; Saturation state and formation kinetics; Solubility criteria; Dissociation constant; Apparent solubility product; Phosphorite deposit; Sediment phosphorus species; Francolite; Staffelite; Holmbushite; Collophane; Fish bone apatite; Carbonate inclusion; X-ray absorption near-edge structure spectroscopy; XANES; Iron oxyhydroxides; Featherstone
Research affiliation: OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
Refereed: Yes
DOI etc.: 10.5194/os-10-571-2014
ISSN: 1812-0784
Date Deposited: 09 Jan 2015 08:30
Last Modified: 19 Feb 2015 08:13
URI: http://eprints.uni-kiel.de/id/eprint/26860

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