SOLUBILITY OF FE(III) IN SEAWATER

Abstract

Recently Kuma et al. [K. Kuma, J. Nishioka, K. Matsunaga, Controls on iron (III) hydroxide solubility in seawater: The influence of pH and natural organic chelators, Limnol. Oceanogr. 41 (1996) 396-407] made some careful measurements of the solubility of Fe(III) in UV and non-UV irradiated seawater as a function of pH (5-8). They showed that organic compounds can increase the solubility (32-65%) at pH=8.1, apparently due to the formation of Fe(III) organic complexes. In this paper I have examined how these results can be quantified using a speciation model for Fe(III). The results indicate that the effect of pH (2-9) on coastal and open ocean waters by Kuma et al. and the earlier filtration measurement of Byrne and Kester [R.H. Byrne, D.R. Kester, Solubility of hydrous ferric oxide and iron speciation in sea water, Mar. Chem. 4 (1976) 255-274] can be adequately represented by considering the formation of FeOH2+ and Fe(OH)2+ using the hydrolysis constants (K*1=10-2.62, K*2=10-6.0) determined by Millero et al. [F.J. Millero, W. Yao, J. Aicher, The speciation of Fe(II) and Fe(III) in natural waters, Mar. Chem. 50 (1995) 21-39]. The solubility measurements [Kuma et al., 1996] on unaltered coastal and open ocean waters appear to require the consideration of the formation of Fe(OH)30 (K*3=10-13.3-10-14.3). A more careful look at these measurements indicates that the curvature between pH 7 and 8 can be attributed to the formation of complexes of Fe3+ with organic ligands (FeL). Model speciation calculations (pH 6-8) yield total ligand concentrations of [L]T=1.2 nM and 0.17 nM for unaltered coastal and open ocean waters, respectively, assuming K'FeL=1021. These estimates are in good agreement with the values found for ocean waters by voltammetric methods. The model calculations for the solubility of Fe(III) (0.2 nM at pH=8.1 and 0.6 nM at pH=7.65) are in good agreement with measured open ocean surface (0.2 nM) and deep waters (0.6 nM) determined by various workers. Since these calculations are strongly dependent upon the total Fe and organic ligand concentrations in the waters, future studies should include solubility and voltammetric measurements on the same sample to clarify these relationships.