Mineralogical and Geochemical Controls on Polyphosphate Transformation and Mineralization in Marine Sedimentary Environments

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Wan, Biao
Tang, Yuanzhi
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Phosphorus (P) is an essential and limited micronutrient regulating marine primary productivity. Despite the critical roles of marine P cycle in global biogeochemical processes, the mechanisms leading to marine P removal as authigenic apatite are not fully understood. This dissertation investigates the mineralogical and geochemical controls on marine polyphosphate (polyP) transformation and mineralization under laboratory controlled experiments and sediment incubations. PolyP is a group of polymeric molecules with at least three phosphate groups joined by phosphoanhydride (O–P–O) bonds and is widely synthesized by most microorganisms in aquatic environments. PolyP release to marine environments can be a potential P source to induce the nucleation of calcium (Ca) phosphate minerals, such as amorphous Ca-phosphate (ACP) and crystalline apatite phases. The degradation of PolyP to orthophosphate is catalyzed by three groups of environmental metal oxide minerals, such as iron (Fe), aluminum (Al), and manganese (Mn) oxides, and this process is significant enhanced by the presence of Ca2+. A terminal-only pathway via one-by-one cleavage of terminal phosphate groups is the dominant mechanism for such mineral-catalyzed polyP hydrolysis. Mineral type and structure, particle size, metal cations, and solution pH are important factors affecting the hydrolysis rate and extent. At circumneutral pH conditions, ACP precipitates upon polyP hydrolysis in the presence of Ca2+ and transforms to crystalline hydroxyapatite upon long-term aging. By normalizing the hydrolysis rates and considering the abundance and activity of common metal oxide minerals (hematite, boehmite, and birnessite) and phosphate enzymes (acid and alkaline phosphatases) in soils and sediments, the hydrolysis rates of polyP by abiotic and biotic factors can be directly compared, and the rates by hematite and birnessite are comparable to those by acid and alkaline phosphatases. In mesocosm sediment incubation, polyP hydrolysis extents and rates roughly follow the order of alkaline phosphatase > acid phosphatase ≥ birnessite > hematite > boehmite ≈ raw sediment solids. Upon polyP hydrolysis, ACP forms first but cannot transform into crystalline Ca-phosphate minerals in artificial seawater due to the presence of highly concentrated Mg2+. When Mg2+ is removed, equilibrium P concentrations are significantly lower due to significant ACP formation, which may transform into hydroxyapatite after 150-day sediment incubation. This study greatly advances our current understanding of marine P burial under the effects of mineralogical and biological controls and provides new insights for understanding the occurrence of crystalline Ca-phosphate minerals in marine environments.
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