Speciation of dissolved chromium and the mechanisms controlling its concentration in natural water

The circulation of chromium at naturally occurring concentration levels (i.e., 1μgdm−3 or lower) was examined using solid-phase spectrophotometry and inductively coupled plasma-mass spectrophotometry (ICP-MS) to determine the Cr(VI) and Cr(Total) concentrations, respectively. Natural water and stream sediments were collected from areas with various types of geologic features, such as metamorphic rocks, volcanic rocks and limestone in Japan and Indonesia. Cr(VI) was predominant in weakly alkaline natural waters, and the Cr(III) concentration was less than 1μgdm−3, which was considerably lower than that expected based on the solubility of Cr(OH)3. The dissolution of chromium in natural water was described by the leaching of Cr(VI) from Cr(III)-containing minerals under oxic conditions. The Cr(VI) concentration in a solution that was in contact with chromite, FeCr2O4, linearly increased with the reaction time. The results indicated that under oxic conditions at PO2=0.21atm, the leaching rate of Cr(VI) was pseudo zero-order at a fixed pH and was higher in alkaline than in acidic solutions. The removal of Cr from natural waters was due to the reduction of Cr(VI) to Cr(III) by organic matter, such as humic substances, as well as to the adsorption of Cr(III) onto suspended matter and river sediments. The reduction of Cr(VI) to Cr(III) followed the equation −d[Cr(VI)]/dt=[H+]a[Cr(VI)][DOC] with a=0 at pH<4 and a=−1 at pH>4.5. The removal of Cr(III) by river sediments and/or suspended particulate matter was rapid and the adsorbability of Cr(III) was dependent on pH, which suggests that the active species were Cr(OH)2+ and Cr(OH)3. Schematic models of the circulation of dissolved chromium in natural water were proposed under acidic and alkaline conditions.