Understanding and eliminating the reductant interference on Chromium VI measurement with USEPA method 3060A

Excessive reductants are used in engineering to ensure a reliable remediation effect of chromite ore processing residue (COPR), however, re-yellowing phenomenon of remediated COPR occurs after some time though the Cr(VI) content meets regulatory requirements after curing period. This problem is due to a negative bias on Cr(VI) determination using USEPA method 3060A. To address this issue, this study tried to reveal the interference mechanisms and proposed two methods to amend the bias. Results of ion concentrations, UV-Vis spectrum, XRD, and XPS together showed that Cr(VI) was reduced by ions (Fe²⁺, S₅^2-) in the digestion stage of USEPA method 3060A, and as a result, method 7196A would not reflect the true Cr(VI) concentration. The interference on Cr(VI) determination generated by excess reductants mainly occurs during the curing period of remediated COPR, but it decreases over time as reductants being oxidized gradually by the air. Compared with the thermal oxidation, the chemical oxidation with K₂S₂O₈ prior to alkaline digestion performs better to eliminate the masking effect brought by excess reductants. This study provides an approach on how to accurately determine the Cr(VI) concentration in the remediated COPR. It might be helpful to reduce the occurrence possibility of re-yellowing phenomenon.