CMMMech
CMMMech > Secondary resource > Chromite ore processing residue

Chromite ore processing residue secondary resource

Chromium-bearing waste from chemical processing of chromite ore, distinct from the chromite mineral species and physical-beneficiation tailings. The curated case is a cementitious, high-lime-process residue from a Glasgow waste-fill site that received material from the Rutherglen Chromite Processing Works. High-lime processing oxidized ore Cr(III) to soluble Cr(VI), followed by aqueous extraction. The 2015 study sampled field-moist material from an exploratory borehole during the M74 motorway project and initially identified it tentatively as COPR from its pale granular appearance and high-pH yellow leachate. Subsequent XRF and XRD characterized a mixed residue rather than proving a pure feed. XRD identified cementitious and hydrated phases, including calcite, portlandite, brownmillerite, pyroaurite, hydrogarnet and brucite; remnant chromite was not detected by that assay. Table 4 reports 3.8 wt% Cr in the XRF composition, without loss-on-ignition correction; this is neither a mineral fraction nor a field-wet feed concentration. This record does not confer a constituent element's criticality designation on the residue or assert commercial microbial processing.

Microbial & Biochemical Mechanisms (1)

Chromium reduction and stabilization by separately produced biogenic magnetite

redox transformation Microbial

Associated Organisms: Geobacter sulfurreducens (NCBITaxon:35554)

Preformed nanoscale magnetite produced through G. sulfurreducens reduction of ferrihydrite lowered aqueous Cr(VI) release and reduced part of the bulk Cr(VI) in the residue. The microbial contribution is production of the Fe(II)-bearing reagent in a separate incubation. The particles were washed to remove bacterial cells before treatment; this is not evidence of live Geobacter growth or direct enzymatic Cr(VI) reduction in the hyperalkaline waste. Complete removal of cells was not independently demonstrated. Fe(II) oxidation coupled to Cr(VI) reduction is supported by chemical and spectroscopic changes, with residual inaccessible Cr(VI) and subsequent remobilization. Chromium was stabilized in the solid system, not recovered as an isolated saleable product.

Experimental Context & Analytical Limits:
Laboratory duplicate microcosms contained 25 g field-moist COPR and 50 mL ultrapure water in 120 mL serum bottles, degassed with filtered N2 and held in the dark at 20 degrees C for 100 days. Magnetite doses were 0, 0.5, 1, 2, 5 and 10% by mass of field-wet COPR, not dry-feed percentages. The unamended residue was the no-addition control; parallel commercial NANOFER 25S zero-valent-iron treatments were a nonmicrobial comparator. pH stayed between 12.5 and 12.9. Samples for XANES and XRD were taken after 50 days. Aqueous Cr(VI) was measured with diphenylcarbazide, with a reported 1 micromolar detection limit; readily extractable Fe(II) used 0.5 M HCl and ferrozine. Error bars represent standard deviations of duplicate microcosms. The reagent production used source-labelled G. sulfurreducens grown anaerobically at 30 degrees C on acetate/fumarate, then washed cells at OD600 0.4 in 20 mM acetate, 50 mM ferrihydrite, 30 mM bicarbonate and 10 micromolar AQDS, incubated at 30 degrees C in the dark for one week. Resulting magnetic particles were washed three times and stored anoxically. No strain accession is specified in this paper. Waste and groundwater in the separate aqueous experiments came from different Glasgow sites; their results are not pooled.
Curated Evidence Points:
Sections 2.1, 2.3-2.5, 2.8, 3.5-3.6 and Figures 6-8 support the residue identity, separately produced biogenic reagent and reductive stabilization. With 5 or 10% magnetite, aqueous Cr(VI) fell below 1 micromolar within one day; no-addition controls remained about 1.3 mM during the 100-day experiment. Lower magnetite additions left measurable aqueous Cr(VI). Linear combination fits to day-50 Cr K-edge XANES gave 26% of total Cr as Cr(VI) in the control and 7% after either 5 or 10% magnetite. These are fitted oxidation-state proportions, not isolated-product yields or mass fractions of the entire residue. Spectral shapes were consistent with Cr(III) hydroxide-like products, while XRD did not resolve a new crystalline Cr(III) phase; no unique product mineral or complete conversion is assigned.
Sections 3.5-3.6 preserve both analytical and treatment limits. Alkaline extraction indicated only 7% of initial total Cr as Cr(VI), versus the 26% XANES fit. The authors attribute this to incomplete extraction of resistant Cr(VI); the methods are not treated as interchangeable. Remaining Cr(VI) after excess reagent is interpreted as poorly soluble or inaccessible, not absent. The synthetic zero-valent-iron comparator required a smaller dose for aqueous removal and reached fitted bulk Cr(VI) proportions of 4-7%; that range is not assigned to the biogenic magnetite arm. No viable-cell versus killed-cell comparison establishes a living-cell contribution during COPR treatment.
Sections 2.9 and 3.7 and Figure 9 describe a separate air challenge after the initial 100 days: 25 mL portions were transferred to 250 mL flasks, shaken at 100 rpm in the dark at 20 degrees C, with headspace re-equilibrated every 2-4 days, and followed for another 100 days. All magnetite-dose arms showed some aqueous Cr(VI) remobilization; the authors describe the higher dose increases as less than 10% of the original Cr(VI) values. They explicitly leave oxidation of Cr(III) versus altered solubility of residual Cr(VI) unresolved. The broader discussion's stability language does not establish no rebound, complete detoxification or indefinite field stability.
Figure SI 1 supports magnetite as the detected crystalline phase in the biogenic reagent and mixed metallic iron/magnetite in the synthetic comparator. Figure SI 2 fits early aqueous removal in model chromate solutions and separately sampled groundwater; these fitted rates are not COPR-treatment rates.
Sections 3.3-3.4 and 4, Figures 1-5 and Tables 2-3 show incomplete electron utilization, stronger passivation in complex groundwater and inhibition by oxygen. Proposed in situ injection and reactive barriers were not demonstrated; the authors identify reagent-dose, clogging and passivation obstacles to deployment.

Sources & References

  1. watts2015: Watts, Coker, Parry, Pattrick, Thomas, Kalin and Lloyd (2015). Biogenic nano-magnetite and nano-zero valent iron treatment of alkaline Cr(VI) leachate and chromite ore processing residue. Applied Geochemistry 54:27-42; available online 11 December 2014. DOI 10.1016/j.apgeochem.2014.12.001; PMID 26109747; PMCID PMC4461148. Complete 16-page published article, Sections 2-5, Tables 1-4, Figures 1-9; source stabilization principally Sections 2.8-2.9 and 3.5-3.7.
    https://doi.org/10.1016/j.apgeochem.2014.12.001 (accessed 2026-10-14)
  2. watts2015_supplement: Supporting Information to Watts et al. (2015). Original publisher DOCX, Figures SI 1-2: nanoparticle XRD and fitted aqueous Cr(VI) kinetics. These are associated artifacts of the same experiment, not independent replication or direct COPR recovery measurements.
    https://ars.els-cdn.com/content/image/1-s2.0-S088329271400300X-mmc1.docx (accessed 2026-10-14)
  3. ncbi_sulfurreducens: NCBI Taxonomy 35554, Geobacter sulfurreducens, species-level entry. Resolves the source's species label without authenticating an experimental strain; the 2015 paper does not specify a strain accession.
    https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=35554&mode=Info (accessed 2026-10-14)

Curation History

Timestamp Curator Action Summary
2026-10-14T04:37:43.260843+00:00 codex (GPT-6; codex) CREATE Created a distinct chemical-processing residue with laboratory chromium stabilization by separately produced biogenic magnetite, preserving wet-feed dosing, analytical disagreement, incomplete reduction and remobilization.

Record source: data/records/chromite-ore-processing-residue.yaml