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Chromite ore processing residue (COPR) is generated by the roasting of chromite ores for the extraction of chromium. Leaching of carcinogenic hexavalent chromium (Cr(VI)) from COPR dumpsites and contamination of groundwater is a key environmental risk. The objective of the study was to evaluate Cr(VI) contamination in groundwater in the vicinity of three COPR disposal sites in Uttar Pradesh, India, in the pre-monsoon and monsoon seasons. Groundwater samples (n = 57 pre-monsoon, n = 70 monsoon) were taken in 2014 and analyzed for Cr(VI) and relevant hydrochemical parameters. The site-specific ranges of Cr(VI) concentrations in groundwater were <0.005 to 34.8 mg L (Rania), <0.005 to 115 mg L (Chhiwali), and <0.005 to 2.0 mg L (Godhrauli). Maximum levels of Cr(VI) were found close to the COPR dumpsites and significantly exceeded safe drinking water limits (0.05 mg L). No significant dependence of Cr(VI) concentration on monsoons was observed.

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Many soils in the region of Kanpur, North India, are heavily affected by the leather industry and its upstream supplier sector, as indicated by elevated chromium (Cr) contents. Under reducing conditions—for instance, at water saturation after monsoon rain or flood irrigation—the dynamic and species distribution of Cr may be affected due to changes in redox potential (EH). In this study, the influence of EH on the speciation and release of Cr from a contaminated agricultural soil was investigated. A soil sample that was affected by hyperalkaline leachate from chromite ore processing residue, was taken and packed in soil columns, and subjected to a saturation–drainage–saturation cycle. After initial water saturation, the EH dropped slowly to minimum values of around ‒100 mV (calculated to pH 7), while EH was controlled by CrO42−/Cr2O3(s), or CrO42−/(Fe,Cr)OOH redox couples. Soil drainage resulted in a quick return to oxidizing conditions; i.e., EH > 300 mV. The Cr species distribution and release showed a clear trend with EH. At the beginning of the experiment, under oxidizing and weakly reducing conditions (EH range from >100 to 300 mV), Cr(VI) was released in particular. However, under moderately reducing conditions (EH range from 100 to –100 mV), Cr was gradually immobilized and irreversible sequestered via reductive precipitation. The results presented in this study provide an improved understanding of the mobility of Cr(VI) in contaminated soils at varying water contents, which is essential for the evaluation of environmental risks in this region. Core Ideas Cr(VI) was highly mobile in the investigated soil. High Cr(VI) concentration prevented the reduction of Mn and Fe. Cr(VI) was mobilized under oxidizing soil conditions. Cr was immobilized under moderately reducing soil conditions.

Environmental context Chromate (CrO42–)-containing waste is illegally dumped in some places in the state of Uttar Pradesh, north India, although CrO42– is known to be toxic and carcinogenic. Because CrO42– is leached from the landfills, this study investigated the adsorption of CrO42– by soils. The results indicated that CrO42– is highly leachable and adsorption is inhibited, which leads to contamination of the groundwater and drinking water in this area. Abstract Chromite ore processing residue (COPR) is a harmful waste of the chromate (CrO42–) extraction roasting process. Nevertheless, deposition of COPR in uncontrolled surface landfills is still common practice in some countries. Leaching of carcinogenic CrO42– and contamination of groundwater is a key environmental risk arising from COPR sites. The objective of this study was to evaluate the adsorption behaviour of CrO42– from COPR eluates by soils. Prior to the adsorption experiments, batch studies at varying solid-to-liquid ratios were performed to evaluate the solubility of CrO42– from COPR. Chromate adsorption experiments were carried out in a batch system with eluates obtained from two different Indian COPRs to assess potential groundwater contamination by CrO42–. Three soils that originate from the surroundings of COPR dumping sites were chosen in order to provide realistic adsorption conditions. The data were evaluated with the Freundlich and Langmuir equation. Chromate adsorption was inhibited because of the high pH of both of the soils (pH 6.7 to 7.2) and the eluates (pH 12.3) as well as the high carbonate concentration of the eluates. The extent and behaviour of CrO42– adsorption from both eluates was similar. The main difference between the eluates was the solubility of CrO42– from COPR and thus the initial CrO42– concentration. The results presented in this study provide an improved understanding of the mobility of CrO42– in the affected area, which is important because the local population uses the groundwater not only for the needs of livestock but also as drinking water.