Marine Science Faculty Publications

Solubility of Hydrous Ferric Oxide and Iron Speciation in Seawater

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Iron solubility equilibria were investigated in seawater at 36.22‰ salinity and 25°C using several filtration and dialysis techniques. In simple filtration experiments with 0.05 μm filters and Millipore ultra-filters, ferric chlorides fluorides, sulfates, and FeOH2+ species were found to be insignificant relative to Fe(OH)2+ at p[H+] = −log [H+] greater than 6.0. Hydrous ferric oxide freshly precipitated from seawater yielded a solubility product of ∗Kso=[Fe3+][H+]−3= 4.7 · 105">∗Kso=[Fe3+][H+]−3= 4.7 · 105 . Solubility studies based on the rates of dialysis of various seawater solutions and on the filtration of acidified seawater solutions indicated the existence of the Fe(OH)30 species. The formation constant for this species can be calculated as ∗β3=[Fe(OH)30] [H+]3/[Fe3+] = 2.4 · 10−14">∗β3=[Fe(OH)30] [H+]3/[Fe3+] = 2.4 · 10−14 . The Fe(OH)4 species is present at concentrations which are negligible compared to Fe(OH)2+ and Fe(OH)30 in the normal pH range of seawater. However, there is at least one other significant ferric complex in seawater above p[H+] = 8.0 (possibly with bicarbonate, carbonate, or borate ions) in addition to the Fe(OH)2+ and Fe(OH)30 species.

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Marine Chemistry, v. 4, issue 3, p. 255-274