Redox Processes of Chromium in Sea Water

Chromium exists in natural waters in two oxidation states, Cr(III) and Cr(VI), that are characterized by different chemical behaviour, bioavailability and toxicity. Dissolved Cr(III) has a strong tendency to adsorb to surfaces (Cranston and Murray 1978) and is characterized by a low solubility. Candidates for the solubility control of chromium(III) in natural waters are not fully understood. They include Cr(OH)3(s) (Baes and Mesmer 1976), mixed hydroxides of the type (Cr X Fe1−X )(OH)3(s) (Sass and Rai 1987; Rai et al.1989) and chromite (FeCr2O4) (Murray et al. 1983) and give variable concentrations from 10−7 to 10−10 M. The substitution of waters of hydration in aquated Cr(III) ions is extremely slow and the stability of the hydration sphere imposes limitations on the mechanism of electron transfer reactions. Cr(III) has a low toxicity toward organisms since its bioavailability is controlled by solubility limitations and the formation of strong complexes. The very low assimilation of Cr(III) has led to its use as an inert tracer of ingested particulate matter to measure carbon assimilation in marine invertebrates (Wang et al. 1997). At low levels Cr(III) is an essential element for mammals. Cr(VI) forms anionic oxy-compounds that also have an adsorption affinity for certain proton-specific mineral surfaces (Zachara et al. 1987). However, their adsorption is limited in sea water where both Cr(VI) species and particle surfaces have negative charges, and high concentrations of sulfate (0.028 M) compete with chromate for adsorption sites. Cr(VI) is much more bioavailable to aquatic organisms than Cr(III) and it is thought that this oxidized species is transported into the cells as a sulfate analogue (Riedel 1984). The competition between Cr(VI) and sulfate is responsible for significant changes in the bioavailability and toxicity of Cr(VI) in estuarine waters where sulfate concentrations increase markedly with salinity (Riedel 1984; Riedel 1985). Cr(VI) compounds are toxic due to their oxidation of intracellular compounds and are also known as a human carcinogen and mutagen.