The geochemistry of chromium in various marine environments

• Main Author: Mugo, Robert K.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/6626

The geochemistry of chromium in different marine environments was investigated using a combination of laboratory and field studies. Chromium speciation and concentrations were determined using an electron capture detection gas chromatographic technique (ECD-GC) on samples from a broad range of environments. The distribution of this element in aquatic environments appears to be driven mainly by redox, sorption/co-precipitation and complexation processes. Dissolved chromium profiles in the Pacific and Atlantic oceans and in two B.C. inlets show a slight surface depletion (10-25 %) and an increase near the sediment-water interface. The surface depletion might suggest Cr involvement in nutrient cycling in the oceans, but the lack of clear Cr-nutrients correlation in both oceanic basins and also the lack of an inter-ocean nutrienttype gradient suggests that other factors may be important. Photoreduction of Cr(VI) coupled with removal via sorption may be important controls on Cr speciation in surface waters. Nearshore areas showed enrichment in total dissolved Cr values relative to the open ocean likely through elevated inputs via riverine run-off. In addition, higher dissolved organic matter (DOM) concentrations in coastal areas can lead to higher Cr(III) values by reducing Cr(VI) (directly or through the Fe photochemical cycle) and via complexation of Cr(III), allowing this species to remain in solution in excess of levels predicted by thermodynamic equilibrium. Chromium cycling in areas with redox gradients is controlled by speciation changes, sorption/co-precipitation and complexation processes. Under suboxic and anoxic conditions, Cr(VI) is reduced to Cr(III), much of which is either removed or is bound as non-labile colloidal Cr forms in solution. In Saanich Inlet, a seasonally anoxic fjord, more than 90 % of chromate is reduced to Cr(III) during stratification, roughly half of which is detected in either labile or nonlabile forms; the rest is removed presumably via sorption and/or co-precipitation processes. In the Black Sea, a permanently anoxic basin, Cr(VI) is reduced to Cr(III) at the redox interface, this species subsequently undergoes removal onto particles and is released in deep waters where it is stabilized by formation of organic and/or colloidal complexes. Cr residence time in the Black Sea interface zone is estimated to be ~ 40 yrs, much shorter than the ~ 1000 years calculated for the anoxic region where reduced Cr(III) species are quite stable. Total chromium values in hydrothermal vents areas over the Juan de Fuca Ridge (JDFR) showed approximately a two-fold enrichment over values in the open Pacific Ocean. These preliminary results suggest that hydrothermal activity may be a source of Cr for waters at the JDFR. Total Cr in vent fluids is estimated to be at least 80 ± 10 nM based on Si data. Laboratory studies of Cr transformations support field observations that processes such as Cr(VI) reduction, Cr(III) binding by DOM and sorption are important controls in regulating the distribution of Cr species in aquatic environments. Humic acids were shown to be capable of reducing Cr(VT) under a wide range of acidic pH conditions. They are also capable of binding Cr(III) in non-labile forms that require oxidative digestion before they can be detected by the ECD-GC technique. Preliminary results also showed that Fe(II) may be an even faster reductant for Cr(VI) in seawater, especially at higher pH typical of marine environments.