Controls on iron(III) hydroxide solubility in seawater: The influence of pH and natural organic chelators

Abstract

Iron solubilities of Fe(III) hydroxide in coastal and oceanic waters and in the ultraviolet (UV)-irradiated seawaters over a pH range of 5.7-8.2 at 20°C were determined by a simple filtration (0.025 μm) involving γ- activity measurement of 59Fe. At pH ranges of 5.7-7.2 (coastal water) and of 5.7-7.6 (oceanic water), only the Fe(OH)2+ species is significant. The calculated solubility products, log*K(so), for coastal and oceanic waters were 4.8-5.0 and 4.4-4.6, respectively. The solubilities within the pH range of 7.8-8.2 are relatively independent of pH and aging time. Solubility in the oceanic water was about one order of magnitude lower than that in the coastal water, and UV irradiation reduced solubility to ≤0.1 nM. The vertical profiles of ambient Fe(III) solubility (pH 8.0-8.2) in oceanic waters have the following features in common: solubility in the surface mixed layer is high and variable (0.3-0.6 nM), generally corresponding with the depth of high chlorophyll a concentrations; solubility minima (0.15-0.2 nM) occur at a depth of 50-200 m. These results suggest that natural organic Fe(III) chelators exist in significant concentrations and control the dissolved iron concentration in seawaters.