Formatted Title
CrVI Remediation at Chrome Plating Facilities and Chromite Ore Processing Residue (COPR) Facilities
Background/Objectives
Chromite ore processing residue (COPR) is a byproduct of the high lime-based roasting chromate production process and is a hazardous industrial waste impacting soil and groundwater quality in many countries. One of its greatest threats to the environment is the release of hexavalent chromium (CrVI), which is highly mobile and classified as a human carcinogen. CrVI is also the primary contaminant at chrome plating facilities which litter the country in almost every metropolitan city. CrVI is very toxic and must be remediated. However, traditional chemistries are ineffective when high levels of CrVI are present. We present the results for CrVI reduction at multiple sites and a COPR site.
Approach/Activities
Bench-scale treatability studies are recommended to evaluate the treatment effectiveness of commercial remediation agents and other industrial chemistries. In some cases, sulfide chemistry works fine either biotically or abiotically. In others, ZVI or iron reduction works well to meet the goals. However, when CrVI concentrations are very high (up to 48,500 mg/kg in soil), traditional chemistry dosage demands (15wt%, wt20% and higher) render those options cost prohibitive and even when financially viable, fail the longevity test and result in reoxidation of CrIII to CrVI within years.
Based on a rigourous study of all known CrVI remediation chemistries, the best performing reagent chemistry and most sustainable results are derived by sulfer based chemistries plus other geochemical conditioning reagents to promote stable conversion of the highly toxic hexavalent chromium Cr(VI) to trivalent chromium Cr(III) in a precipitate, which is less toxic, less soluble, and essentially immobile. This reduction is long term resilient solution, because Cr(III) is not easily reoxidized to Cr(VI) under current and future conditions in groundwater or in the vadose zone (generally aerobic). This mineral phase lowers chromium solubility to <0.10 mg/L at pHs as low as ~4.5 range. This approach is engineered to meet site specific challenges and requirements.
Results/Lessons Learned
Project performance results demonstrate reduction of up to 100% of the hexavalent chromium concentrations using a reagent dosage of 6-12% (weight to weight).