Rhodium element Critical Material
Rhodium (Rh). The element is distinct from the Rh(III) chloride hydrate used as experimental feed, monometallic biomass-supported particles and mixed PdRh nanoparticles. The US 2025 list names the rhodium commodity individually; that designation is mapped to the element without extending it to every Rh-bearing compound, mineral or secondary resource. Curated evidence concerns laboratory transformation in synthetic solutions, not extraction from ore, purified-metal recovery or industrial deployment.
Identifiers & Classification
| System | Identifier | Label |
|---|---|---|
| CHEBI |
CHEBI:33359
|
rhodium atom |
Criticality Designations
| Jurisdiction | List Name | Edition / Document | Classification | Source |
|---|---|---|---|---|
| United States of America | U.S. Department of the Interior Final List of Critical Minerals | 2025 (published November 7, 2025; 90 FR 50494-50497) | critical |
usgs_2025
|
Microbial & Biochemical Mechanisms (2)
Resting-cell transformation of soluble Rh(III) to Rh(0)-bearing particles
redox transformation
Microbial
Associated Organisms:
Geobacter sulfurreducens (NCBITaxon:35554)
Washed resting Geobacter sulfurreducens cells supplied with acetate removed Rh from a synthetic single-metal solution and formed cell-associated particles identified as metallic Rh by XRD and selected area electron diffraction. Cell-free controls did not show comparable Rh removal. The observation supports cell-associated reduction under these conditions, without establishing complete conversion to Rh(0), a specific cytochrome requirement or an isolated-metal recovery yield.
Experimental Context & Analytical Limits:
Laboratory 10 mL batches in serum bottles contained 200 micromolar Rh supplied from a 50 mM RhCl3.xH2O stock in deionized water, 10 mM sodium acetate and 30 mM sodium bicarbonate. Bottles were flushed for at least 30 minutes with 80:20 nitrogen/carbon dioxide, pH 6.8-7.0, then received resting cells at OD600 0.4 and were incubated at 30 degrees C for up to 24 h. Biomass had been grown anaerobically at 30 degrees C with 20 mM acetate and 40 mM fumarate to late log phase, harvested at 4920 g and 4 degrees C for 20 minutes, and washed twice in 30 mM bicarbonate pH 7. The paper does not provide a culture-collection or strain accession; the species identifier does not resolve that omission. Rh remaining in solution was quantified by ICP-MS after 14800 g centrifugation for 20 minutes and nitric-acid dilution of the supernatant. Results 3.1.1 reports greater than 95% aqueous Rh removal within 1 h, whereas no-cell Rh controls retained the dissolved pool. This is centrifuged-supernatant depletion, not a quantitative Rh(0) fraction. Washed 24 h solids were assessed by XRD and TEM/SAED; the inferred crystallite size and image particle size have different measurement bases.
Laboratory 10 mL batches in serum bottles contained 200 micromolar Rh supplied from a 50 mM RhCl3.xH2O stock in deionized water, 10 mM sodium acetate and 30 mM sodium bicarbonate. Bottles were flushed for at least 30 minutes with 80:20 nitrogen/carbon dioxide, pH 6.8-7.0, then received resting cells at OD600 0.4 and were incubated at 30 degrees C for up to 24 h. Biomass had been grown anaerobically at 30 degrees C with 20 mM acetate and 40 mM fumarate to late log phase, harvested at 4920 g and 4 degrees C for 20 minutes, and washed twice in 30 mM bicarbonate pH 7. The paper does not provide a culture-collection or strain accession; the species identifier does not resolve that omission. Rh remaining in solution was quantified by ICP-MS after 14800 g centrifugation for 20 minutes and nitric-acid dilution of the supernatant. Results 3.1.1 reports greater than 95% aqueous Rh removal within 1 h, whereas no-cell Rh controls retained the dissolved pool. This is centrifuged-supernatant depletion, not a quantitative Rh(0) fraction. Washed 24 h solids were assessed by XRD and TEM/SAED; the inferred crystallite size and image particle size have different measurement bases.
Curated Evidence Points:
Support
Source:
xie_2026
Methods 2.1-2.3, Results 3.1.1 and Figure 1a distinguish the cell-containing and no-cell monometallic Rh assays. Results 3.2, Figure 2, Table 2 and Figure 4c identify Rh(0)-consistent diffraction in harvested material and particle agglomerates associated with cells. The phase claim relies on diffraction, not the black color or aqueous removal alone.
Partial
Source:
xie_2026
Results 3.3, Figure 3 and Table 1 report no discernible monometallic Rh XPS signal, so the monometallic surface Rh(0)/Rh(III) proportions are unresolved. XRD is dominated by one broad reflection; SAED supplies additional indexed planes, but neither quantifies all amorphous or dissolved Rh. The record does not transfer the bimetallic Rh XPS percentages to this preparation.
Partial
Source:
xie_2026
Methods 2.2 says controls without electron donor were prepared, but Rh-specific donor-free outcomes are not shown in Figure 1 or tabulated in the inspected article or supplementary Table S1. Replicate counts and definitions of Figure 1 error bars are not specified. A matched killed-cell comparison and direct genetic or protein-level demonstration of Rh-reducing cytochromes are not reported; acetate dependence, viability and a unique enzyme pathway therefore remain unestablished.
Rh transformation during simultaneous Pd/Rh exposure of resting cells
redox transformation
Microbial
Associated Organisms:
Geobacter sulfurreducens (NCBITaxon:35554)
A separate simultaneous exposure of resting Geobacter sulfurreducens to Pd(II) and Rh(III) generated cell-supported mixed nanoparticles. The authors' combined XRD, atomic-resolution imaging and elemental mapping support a PdRh alloy interpretation, with mixed surface oxidation states. This preparation is distinct from monometallic Rh particles and from sequential Pd-seeding protocols.
Experimental Context & Analytical Limits:
Methods 2.2 repeats the 10 mL, OD600 0.4, 10 mM acetate, 30 mM bicarbonate, pH 6.8-7.0, 30 degrees C anaerobic assay with 100 micromolar Pd plus 100 micromolar Rh added together, supplied from Na2PdCl4 and RhCl3.xH2O stocks. Results 3.1.2 reports 97% aqueous Rh removal within 1 h with cells; the cell-free mixture also lost approximately 20% Rh by the later endpoint, unlike the single-metal no-cell Rh control. The authors propose co-removal during Pd hydrolysis as an explanation. Characterized solids were collected after 24 h. No purified alloy product yield, full Rh mass balance, real waste-feed trial or deployment is established by this synthetic-feed experiment.
Methods 2.2 repeats the 10 mL, OD600 0.4, 10 mM acetate, 30 mM bicarbonate, pH 6.8-7.0, 30 degrees C anaerobic assay with 100 micromolar Pd plus 100 micromolar Rh added together, supplied from Na2PdCl4 and RhCl3.xH2O stocks. Results 3.1.2 reports 97% aqueous Rh removal within 1 h with cells; the cell-free mixture also lost approximately 20% Rh by the later endpoint, unlike the single-metal no-cell Rh control. The authors propose co-removal during Pd hydrolysis as an explanation. Characterized solids were collected after 24 h. No purified alloy product yield, full Rh mass balance, real waste-feed trial or deployment is established by this synthetic-feed experiment.
Curated Evidence Points:
Support
Source:
xie_2026
Figure 1d and Results 3.1.2 support faster and greater Rh depletion in the cell-containing mixture than in its no-cell control. Figures 2 and 5b and Results 3.5.2 combine an intermediate lattice spacing and Pd/Rh co-localization to support the authors' alloy assignment. The XRD/Vegard-law estimate of Pd71/Rh29 and EDS estimates near Pd60/Rh40 are method-dependent estimates, not one exact uniform particle composition; Figure 5 labels field averages.
Partial
Source:
xie_2026
Figure 3 and Table 1 fit the bimetallic surface Rh signal as 81% Rh(0) and 19% Rh(III). These are fitted surface proportions, not whole-feed reduction yields. The partial abiotic Rh removal and unknown donor-free outcomes preclude assigning all Rh depletion to metabolism. Proposed changes in complex speciation, reduction potential and cytochrome mediation were not directly resolved in this assay; no specific microbial enzyme is established.
Context_only
Source:
xie_2026
Methods 2.4, Results 3.6, Figure 6 and supplementary Table S1 concern subsequent NaBH4-driven 4-nitrophenol catalysis at room temperature, normalized to 4 mol% total metal relative to the initial organic substrate. Bio-PdRh performed better than monometallic bio-Rh in this benchmark, while bio-Pd had the fastest initial rate. These catalyst results are not Rh recovery percentages or demonstrations of microbial metabolism during the catalytic test.
Sources & References
-
chebi_rhodium: ChEBI:33359 - rhodium atom; Rh, charge 0, atomic number 45
https://www.ebi.ac.uk/chebi/CHEBI:33359 (accessed 2026-10-11) -
usgs_2025: U.S. Geological Survey (2025). Final 2025 List of Critical Minerals. Federal Register 90 FR 50494-50497, November 7, 2025; final commodity table, Rhodium row on page 50497.
https://www.govinfo.gov/content/pkg/FR-2025-11-07/html/2025-19813.htm (accessed 2026-10-11) -
xie_2026: Xie et al. Microbial synthesis of bimetallic Pd-Rh and Pd-Pt nanoparticle catalysts. Nanoscale Advances 8, 961-972 (2026); first published online December 16, 2025. DOI 10.1039/D5NA00861A. PMC12713794.2; Methods 2.1-2.4, Results 3.1-3.6, Figures 1-6, Tables 1-2 and supplementary Table S1.
https://doi.org/10.1039/D5NA00861A (accessed 2026-10-11) -
ncbi_geobacter: NCBI Taxonomy 35554 - Geobacter sulfurreducens; species, not a strain
https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=35554&mode=Info (accessed 2026-10-11)
Curation History
| Timestamp | Curator | Action | Summary |
|---|---|---|---|
2026-10-11T06:51:17Z |
Codex (GPT-6; Codex) | create | Created the rhodium element record from verified identity, dated US criticality and primary laboratory evidence. Preserved assay boundaries, controls, phase uncertainties and deployment limits; attached canonical CREATE history. Independent scientific review is separate. |
Record source: data/records/rhodium.yaml