Iron Speciation and Solubility in Sea Water

Abstract

An investigation of ferric ion complexing has been conducted in synthetic media and seawater at 25°C. In a potentiometric study, formation constants were determined for the species FeCl2+, FeCl2+, Fe(OH)2+ and Fe(OH)2+ at an ionic strength of 0.68 m. Formation constants for the ferric chloride complexes were determined as ClB1 = 2.76 and ClB2 = 0.44. In a study of the reaction Fe3+ + n H2O ⇌ Fe(OH)n(3-n)+ + n H+ in NaClO4, NaNO3 and NaCl the formation constants *B1 and *B2 were shown to be relatively independent of medium when the effects of nitrate and chloride complexing were accounted for. The average values obtained for these constants are *B1 = 1.93 x 10-3 and *B2 = 8.6 x 10-8. Reasonable agreement with these values was obtained when these constants were determined directly in seawater by accounting for the effects of chloride, fluoride and sulfate complexing. Iron solubility equilibria were investigated in seawater at 36.22 0/00 and 25°C using various filtration and dialysis techniques. In simple filtration experiments using 0.05 micron 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+] greater than 6.0, and the solubility product, [Fe3+][H+]-3 = *KSO, of freshly precipited hydrous ferric oxide was determined as *KSO = 4.7 x 105. Solubility studies conducted by determination of the rates of dialysis of various seawater solutions and by filtration of acidified seawater solutions were indicative of the existence of the species Fe(OH)3°. The formation constant for this species can be calculated as *B3 = 2.4 x 10-14. Although the species Fe(OH)4- appeared to be negligible relative to Fe(OH)2+, some species other than Fe(OH)2+ appears to be significant at p[H+] greater than 8.5.