Characterization of chromium-containing wastes and soils affected by the production of chromium tanning agents

Abstract

Wastes of unknown composition derived from the production of trivalent chromium (Cr(III)) salts used as tanning agents are deposited in the area of Kanpur, India. The questions of whether these samples are chromite ore processing residue (COPR) and whether Cr occurs in its toxic hexavalent form (Cr(VI)) arise. Twenty-one samples from two disposal sites and surrounding soils were analyzed, specifically examining their elemental and mineralogical composition. Additionally, aqueous eluates with different liquid-to-solid ratios were performed and analyzed for Cr(VI). The samples were classified in accordance to the sum of silicon and aluminum and the sum of calcium and Cr contents: uncontaminated, moderately contaminated, and highly contaminated material. Highly contaminated material exhibited extremely alkaline pH values up to 12.5 and total Cr contents ranging from 65.7 to 110 g/kg, whereas uncontaminated material had comparatively moderate pH values and Cr contents <1 g/kg. In total, seven crystalline phases commonly found in COPR were identified in the contaminated samples, of which five phases (brownmillerite, hydrocalumite, hydrogarnet, magnesiochromite, and periclase) are known to be able to accommodate Cr whereas hydrogarnet and hydrocalumite are the main host phases for Cr(VI). Batch tests showed that dissolution controlled the Cr(VI) concentrations in the eluates. Six samples were clearly identified as highly Cr-contaminated COPR. Leaching of Cr(VI) and resulting contamination of soils and water bodies is a key environmental risk arising from these COPR sites especially during the monsoon season. This situation is of particular concern as the local population use the highly Cr(VI)-contaminated water not only for the needs of livestock and irrigation but also as drinking water due to the absence of alternative water resources.