Chapter 7 - Chromium in the Ocean

Abstract

Several of the essential trace nutrient metals, including chromium, are considered important environmental contaminants because of their toxicity and potential to be mobilized by human activities. Because they are essential micronutrients, marine organisms have evolved mechanisms for accumulating them from water and food and for regulating their forms, distribution, and concentrations in tissues and body fluids. However, there is relatively little published information on the bioaccumulation of chromium by marine organisms. Chromium can occur in nature in nine oxidation states between -2 and +6. At the pH, Eh, and ionic strength of seawater, chromium exists in the oceans primarily in two valency states: particle-active trivalent chromium, Cr (III), and more soluble hexavalent chromium, Cr (VI). However, Cr (II) may also occur under some conditions. From a thermodynamic standpoint, only hexavalent chromium is present in oxygenated seawater. Most of the chromium in oceanic surface waters is, in fact, hexavalent. However, in near shore and estuarine waters, and hypoxic ocean waters (for example, the minimum-oxygen layer in the ocean), the dissolved Cr (VI)/Cr (III) ratio usually ranges from 1 to 20 and appears to be controlled by biological activity.