CO2-Uptake Enhances Chromate Release from Fresh Chromium Ore Processing Residues (COPR)

Chromium ore processing residue (COPR) still contains considerable chromate which can lead to groundwater contamination at uncontrolled dumpsites. The alkaline pH and presence of portlandite introduced in the high-lime roasting render COPR a potential CO2-trap. To assess whether this feeds back on chromate mobility, fresh COPR samples from two Indian production sites for leather tanning salts were examined. Carbonation was performed at near atmospheric pressure and 100 vol.-% CO2 for 48 h. The pH, electric conductivity, chromate and bulk anions were determined in aqueous batch leachates of the carbonated and native samples. In addition, the latter were spiked with Na2CO3 and titrated with HNO3 to the pH observed in the carbonated material to distinguish mere pH effects on chromate release from carbonation. The samples sequestered up to 5.0 wt.-% CO2, decreasing the pH and increasing chromate release by up to 270 %. The mobility enhancement was stronger in the carbonated than the pH-adjusted samples. Weathering of COPR and the concomitant uptake of CO2 may thus increase the risk of groundwater contamination at COPR dumpsites.