Effect of Carbonate Alkalinity on the Solubility of Modern Marine Phosphorite

Abstract

The interaction of modern (Holocene) phosphorite nodules with solutions imitating pore waters of sediments of highly productive ocean regions with a high carbonate alkalinity was studied experimentally. The previously suggested increase in the dissolved phosphorus concentration with increasing carbonate alkalinity at a constant pH value was supported. At the carbonate alkalinity corresponding to that in seawater, fluorine is removed from the solution most likely due to predominance of apatite carbonatization according to a scheme like ${\text{C}}{{{\text{a}}}_{{{\text{10}}}}}{{{\text{(P}}{{{\text{O}}}_{{\text{4}}}}{\text{)}}}_{{\text{6}}}}{{{\text{F}}}_{{\text{2}}}} + x{\text{HCO}}_{{\text{3}}}^{ - } + x{{{\text{F}}}^{ - }} = {\text{C}}{{{\text{a}}}_{{{\text{10}}}}}{{{\text{(P}}{{{\text{O}}}_{{\text{4}}}}{\text{)}}}_{{{\text{6}} - x}}}{{{\text{(C}}{{{\text{O}}}_{{\text{3}}}}{\text{)}}}_{x}}{{{\text{F}}}_{{{\text{2}} + x}}} + x{\text{PO}}_{{\text{4}}}^{{{\text{3}} - }} + x{{{\text{H}}}^{ + }}.$ An increase in carbonate alkalinity results in decreased fluorine removal, which is probably associated with intensified apatite carbonatization according to a different scheme: ${\text{C}}{{{\text{a}}}_{{{\text{10}}}}}{{{\text{(P}}{{{\text{O}}}_{{\text{4}}}}{\text{)}}}_{{\text{6}}}}{{{\text{F}}}_{{\text{2}}}} + {\text{HCO}}_{{\text{3}}}^{ - } = {\text{C}}{{{\text{a}}}_{{{\text{10}}}}}{{{\text{(P}}{{{\text{O}}}_{{\text{4}}}}{\text{)}}}_{{\text{6}}}}{\text{C}}{{{\text{O}}}_{{\text{2}}}} + {\text{2}}{{{\text{F}}}^{ - }} + {{{\text{H}}}^{ + }}.$