AQUEOUS GEOCHEMISTRY OF CHROMIUM - A REVIEW

The aqueous geochemistry of chromium is reviewed, especially with reference to the recent work of Rai et al. (Report EPRI EA-4544, 1986, Report EPRI EA-5741, 1988). The chromium content of natural waters is generally low whereas rocks exhibit a wide range of Cr concentrations. In natural environments, the most stable oxidation states of chromium are Cr(III) and Cr(VI). The following minerals can control the dissolved chromium content of natural waters: Cr(OH)3(s) and (Fe, Cr)(OH)3 (ss) for Cr(III). Major Cr(III) disolved species are CrOH2+ and Cr(OH)3(0), as well as several organic and a few inorganic complexes, whereas HCrO4- and CrO4(2-) are the main forms of Cr(VI) in natural waters. The chromium distribution is also controlled by redox processes. Oxidation of Cr(III) to Cr(VI), poor with dissolved oxygen, is very effective in the presence of solid MnO2. Fe(II)- and organic matter-rich environments favour the reduction of Cr(VI) to Cr(III). Interaction with solid phases can also regulate the chromium content of water. Cr(III) exhibits a typical cationic sorption behaviour. Its adsorption increases with pH, but decreases when competing cations are present. Chromium (VI), on the other hand, exhibits a typical anionic sorption behaviour. Its adsorption decreases with increasing pH and when competing dissolved anions are present. The migration of chromium is determined by the competition between complexation, dissolution/precipitation, redox processes and adsorption/desorption mechanisms. Cr(III) will migrate under acidic conditions and/or if present as dissolved organic matter complexes. Hexavalent chromium generally migrates rapidly but its mobility is inhibited when the Fe(II) and organic matter concentrations are high and when sorption processes are favoured (low pH).