Chromium speciation and fractionation in ground and surface waters in the vicinity of chromite ore processing residue disposal sites

Chromium concentrations of up to 91 mg l⁻¹ were found by ICP-OES for ground water from nine boreholes at four landfill sites in an area of S.E. Glasgow/S. Lanarkshire where high-lime chromite ore processing residue (COPR) from a local chemical works had been deposited from 1830 to 1968. Surface water concentrations of up to 6.7 mg l⁻¹ in a local tributary stream fell to 0.11 mg l⁻¹ in the River Clyde. Two independent techniques of complexation/colorimetry and speciated isotope dilution mass spectrometry (SIDMS) showed that Cr was predominantly (>90%) in hexavalent form (Crⱽᴵ) as CrO₄²⁻, as anticipated at the high pH (7.5–12.5) of the sites. Some differences between the implied and directly determined concentrations of dissolved Crᴵᴵᴵ, however, appeared related to the total organic carbon (TOC) content. This was most significant for the ground water from one borehole that had the highest TOC concentration of 300 mg l⁻¹ and at which <3% of Cr was in the form of Crⱽᴵ. Subsequent ultrafiltration produced significant decreases in Cr concentration with decreasing size fractions, e.g. <0.45 µm, <100 kDa, <30 kDa and <1 kDa by the tangential-flow method. As this appeared related more to concentrations of humic substances than of TOC per se, horizontal bed gel electrophoresis of freeze-dried ultrafilter retentates was carried out to further characterise the Crᴵᴵᴵ-organic complex. This showed for the main Cr-containing fraction, 100 kDa–0.45 µm, that the Cr was associated with a dark brown band characteristic of organic (humic) matter. Comparison of gel electrophoresis and FTIR results for ultrafilter retentates of ground water from this borehole with those for a borehole at another site where Crⱽᴵ predominated suggested the influence of carboxylate groups, both in reducing Crⱽᴵ and in forming soluble Crᴵᴵᴵ-humic complexes. The implications of this for remediation strategies (especially those based on the addition of organic matter) designed to reduce highly mobile and carcinogenic CrⱽᴵO₄²⁻ to the much less harmful Crᴵᴵᴵ as insoluble Cr(OH)₃ are discussed.