Potential leaching of Cr(VI) from laterite mines and residues of metallurgical products (red mud and slag): An integrated approach

We here report the chemical composition of (a) Fe-Ni laterite ores from central and northern Greece and bauxite ores from Parnassos-Ghiona deposit, (b) the corresponding high volume residues (slag and red mud) produced during the metallurgical processing of ferronickel and alumina, (c) experimentally produced water (natural and/or seawater) leachates of these ores and metallurgical leachates and (d) chromium isotope composition (expressed as delta Cr-53 values) of the leachates, seawater and of Cr (VI) contaminated natural water, in an attempt to assess the role of industrial mining processes on the Cr(VI) contamination of groundwater in ultramafic rock dominated terrain. The five week lasting laboratory leaching experiments, performed under atmospheric conditions, showed that only small amounts of Cr(VI) (<-2 mu g.L-1) from Fe-Ni laterites of Lokris (Central Greece) were mobilized into the aqueous phase. In contrast, Cr(VI) concentrations in leachates from Fe-Ni laterites of Kastoria reached values as high as 1300 mu g.L-1. In addition, unexpectedly high Cr(VI) concentrations were measured in water leachates from red mud samples (2100 mu g.L-1). The leachates from red mud are also characterized by relatively high V (6200 mu g.L-1) and Ga (90 mu g.L-1) concentrations. In contrast to leachates from red mud, the Cr(VI) concentrations in leachates from slag remained below similar to 2 mu g.L-1. The delta Cr-53 values measured in natural water leachates of Fe-Ni-laterite from Kastoria range between + 0.03 0.06 parts per thousand and 0.21 +/- 0.08 parts per thousand and differ from the more positively fractionated value of +1.01 +/- 0.05 parts per thousand obtained for the laterite from Lokris. The delta Cr-53 values in natural water and seawater leachates from red mud returned delta Cr-53 values of + 0.83 +/- 0.05 parts per thousand and + 0.73 +/- 0.08 parts per thousand, respectively, and thus are statistically indistinguishable. Although there is no clear-cut relationship between Cr(VI) concentrations and delta Cr-53 values in the diverse leachates, there is an indication that the laterite leachates from Kastoria with high Cr(VI) concentrations yielded less positively fractionated delta Cr-53 values, compared to the laterite leachates from Lokris. The agreement between chromium isotope compositions of artificial leachates of red mud and that of contaminated seawater from the Assopos river extrusion (estuary) forming a heavily Cr(VI) contaminated point source, suggests that the release of Cr(VI) from the red mud rather than from the primary bauxite ore is a major source of toxic Cr(VI), and that insignificant back-fractionation (reduction) of this mobile and toxic chromium along its pathway to the sea takes place. (C) 2015 Elsevier B.V. All rights reserved.