Long-Term Leaching Behavior of Chromite Ore Processing Residue as Backfill Material and the Propagation of Chromium in the Surrounding Soil

Abstract

Leaching characteristics of chromium ore processing residue (COPR) for the long term (simulating a 20-year leaching scenario) were investigated using laboratory flow-through column testing with synthetic rainwater and groundwater for 250 pore volumes of flow (PVFs). Four saturated column experiments simulated the long-term leaching of chromium (Cr) from two types of COPR [unweathered, gray-black (GB) and weathered, hard-brown (HB)] via advective groundwater and rainwater flow. The release of total [Cr(T)] and hexavalent Cr [Cr(VI)] from GB and HB COPR showed peak concentration before 5.5 PVFs and decreased significantly thereafter. The concentrations of Cr(T) and Cr(VI) from GB and HB COPR reached a steady state (5–20 mg/L) after 150 PVFs under groundwater and rainwater permeation. A substantial residual (>65%) of Cr(T) in the COPR was found after the column tests and continued releasing for an even more extended period (>250 PVFs). Higher concentrations of Cr(T) and Cr(VI) were released from COPR when permeated with groundwater relative to those with rainwater due to the highly alkaline (pH = 12.0) environment. Amorphous compounds may be a vital sink related to the release of the Cr from COPR, which can explain the higher release of Cr from HB COPR (33.2% and 22.5% cumulative leaching in groundwater and rainwater, respectively) than that from GB COPR (21.3% and 19.8% cumulative leaching in groundwater and rainwater, respectively). The temporal leaching behavior of Cr(T) obtained from the experiment was used as the input of a numerical model to investigate the long-term transport of Cr(T) through different soil substrates (sand or clay) as well as a bentonite cutoff wall. In an area with a sandy substrate of high permeability, a sand–bentonite cutoff wall is shown to control the transportation of Cr in the long term effectively.