Authigenic apatite and octacalcium phosphate formation due to adsorption–precipitation switching across estuarine salinity gradients

[摘要] Mechanisms governing phosphorus (P) speciation in coastal sediments remainlargely unknown due to the diversity of coastal environments and pooranalytical specificity for P phases. We investigated P speciation acrosssalinity gradients comprising diverse ecosystems in a P-enriched estuary. Todetermine P load effects on P speciation we compared the high P site with alow P site. Octacalcium phosphate (OCP), authigenic apatite (carbonatefluorapatite, CFAP) and detrital apatite (fluorapatite) were quantitated inaddition to Al/Fe-bound P (Al/Fe-P) and Ca-bound P (Ca-P).Gradients in sediment pH strongly affected P fractions across ecosystems andindependent of the site-specific total P status. We found a pronounced switchfrom adsorbed Al/Fe-P to mineral Ca-P with decreasing acidity from landto sea. This switch occurred at near-neutral sediment pH and has possiblybeen enhanced by redox-driven phosphate desorption from iron oxyhydroxides.The seaward decline in Al/Fe-P was counterbalanced by the precipitationof Ca-P. Correspondingly, two location-dependent accumulation mechanismsoccurred at the high P site due to the switch, leading to elevatedAl/Fe-P at pH < 6.6 (landward; adsorption) and elevated Ca-P atpH > 6.6 (seaward; precipitation). Enhanced Ca-P precipitation byincreased P loads was also evident from disproportional accumulation ofmetastable Ca-P (Ca-P_meta) at the high P site. Here, sedimentscontained on average 6-fold higher Ca-P_meta levels compared withthe low P site, although these sediments contained only 2-fold more totalCa-P than the low P sediments. Phosphorus species distributions indicatedthat these elevated Ca-P_meta levels resulted from transformationof fertilizer-derived Al/Fe-P to OCP and CFAP in nearshore areas.Formation of CFAP as well as its precursor, OCP, results in P retention in coastal zones and canthus lead to substantial inorganic P accumulation in response toanthropogenic P input.

[学科分类] 地球化学与岩石