Iridium element Critical Material
Iridium (Ir), distinct from experimental hexachloroiridate(IV), metallic alloys and iridium-bearing ores or waste streams. The US 2025 list names iridium individually; this designation is mapped to the element without extending it to every Ir-bearing material. Curated evidence concerns laboratory adsorption and desorption using lyophilized unicellular algal biomass in synthetic solutions, not reduction to Ir metal or deployment.
Identifiers & Classification
| System | Identifier | Label |
|---|---|---|
| CHEBI |
CHEBI:49666
|
iridium 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 (1)
Reversible iridium adsorption by lyophilized Galdieria 074W biomass
biosorption
Microbial
Associated Organisms:
Galdieria javensis (NCBITaxon:3569544)
Lyophilized biomass of strain 074W (NIES-3638) removed dissolved Ir from acidic synthetic solutions relative to no-adsorbent controls, and the loaded Ir could subsequently be eluted. The study calls the organism Galdieria sulphuraria; the collection now identifies the same strain as Galdieria javensis. The observation concerns inactive microbial biomass, not metabolic uptake. Adsorption and eluate production do not establish purified elemental Ir recovery.
Experimental Context & Analytical Limits:
Biomass was cultivated in 500 mL modified Allen's medium at pH 2.5 and 40 degrees C under continuous illumination, harvested at OD750 2 and lyophilized. The sorption assay used 20 mg/mL adsorbent in 1 mL HCl solutions at 25 degrees C for 30 min, with Ir supplied as H2IrCl6. Single-metal assays included 10 mg/L Ir and 0.1-1 M HCl; separate high-loading and mixed-metal assays must not be pooled with them. ICP-MS measured supernatant Ir after dilution; the authors' recovery efficiency is the difference from the no-adsorbent supernatant divided by that control concentration. Figure 1 reports reduced sorption as acidity increases. Activated carbon and an anion-exchange resin were comparator adsorbents, not microbial controls. For Table 1 desorption, biomass loaded for 30 min in 0.2 M HCl with 10 mg/L Ir was collected by centrifugation, washed once with water and incubated in eluent for 30 min at 25 degrees C. Desorption efficiency uses the adsorbed pool as denominator, not the starting feed. Main figure captions specify means and SDs from three replicate assays; Table 1 does not separately define its uncertainty or replicate count.
Biomass was cultivated in 500 mL modified Allen's medium at pH 2.5 and 40 degrees C under continuous illumination, harvested at OD750 2 and lyophilized. The sorption assay used 20 mg/mL adsorbent in 1 mL HCl solutions at 25 degrees C for 30 min, with Ir supplied as H2IrCl6. Single-metal assays included 10 mg/L Ir and 0.1-1 M HCl; separate high-loading and mixed-metal assays must not be pooled with them. ICP-MS measured supernatant Ir after dilution; the authors' recovery efficiency is the difference from the no-adsorbent supernatant divided by that control concentration. Figure 1 reports reduced sorption as acidity increases. Activated carbon and an anion-exchange resin were comparator adsorbents, not microbial controls. For Table 1 desorption, biomass loaded for 30 min in 0.2 M HCl with 10 mg/L Ir was collected by centrifugation, washed once with water and incubated in eluent for 30 min at 25 degrees C. Desorption efficiency uses the adsorbed pool as denominator, not the starting feed. Main figure captions specify means and SDs from three replicate assays; Table 1 does not separately define its uncertainty or replicate count.
Curated Evidence Points:
Support
Source:
minoda_2023
Methods 2.1-2.3, Figure 1 and Table 1 support adsorption by lyophilized biomass and subsequent chemical desorption. Table 1 reports 98 +/- 7% desorption with 6 M HCl, but 5.3 +/- 0.8% with 1 M thiourea in 0.1 M HCl; the latter is low, not literally zero despite the narrative wording. Figure 4 shows declining sorption across reuse cycles. These percentages describe distinct assay stages, not a complete-process product yield.
Partial
Source:
minoda_2023
Results 3.1.5 and the Figure 4 caption specify 4 M HCl for reuse elution, whereas the abstract says 6 M; the exact repeated-cycle acid strength remains internally inconsistent. Figure 3 includes Cd although the adjacent mixture prose omits it; its Fe and Cd bar heights also differ visibly from quoted text percentages. Both presentations show substantial competing-metal adsorption, so universal Ir selectivity is unsupported. Supplementary Table S1 reports an Ir/Fe separation coefficient of 1.3 for this biomass; this is an enrichment ratio under the stated mixed-feed assay, not product purity. A reconciled mixed-feed recovery percentage is not adopted from the conflicting presentations.
Partial
Source:
minoda_2023
Methods 2.4-2.5 and Figures 5-7 associate contrast changes and Ir-Cl-like spectra with loaded biomass. Imaging used 1000 mg/L Ir, whereas main-text XAFS used 10 mg/L and Figure S3 specifies 200 mg/L for its supplemental fit; these conditions must not be pooled. The localization images do not directly establish the low-loading distribution. The authors' outer-sphere and amine-group interpretation remains a proposed binding model; no specific cellular ligand or active transporter is demonstrated. Supplementary Table S2 gives a model-fitted capacity rather than an observed saturation yield; its rounded Langmuir constant and the printed equation do not permit a reliable independent reconstruction, so that capacity is not adopted. No Ir(0) product, ore/waste-feed experiment or industrial operation is demonstrated.
Context_only
Source:
nies_3638
The strain collection explicitly connects NIES-3638, the 074W alias, current G. javensis identification and former G. sulphuraria name. This preserves the experimental strain while avoiding assignment to every member of the historically broader species. The collection's routine cultivation conditions are not substituted for the experiment's reported cultivation or sorption conditions.
Sources & References
-
chebi_iridium: ChEBI:49666 - iridium atom; Ir, charge 0; CHEBI:33360 is an alternative ID
https://www.ebi.ac.uk/chebi/CHEBI:49666 (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, Iridium row on page 50496. Its Yes column concerns byproduct production, not a separate criticality classification.
https://www.govinfo.gov/content/pkg/FR-2025-11-07/html/2025-19813.htm (accessed 2026-10-11) -
minoda_2023: Minoda et al. Reversible adsorption of iridium in lyophilized cells of the unicellular red alga Galdieria sulphuraria. RSC Advances 13, 14217-14223 (2023). DOI 10.1039/D3RA01249B; Methods 2.1-2.5, Results 3.1-3.2, Table 1 and Figures 1-7; supplementary methods, Figures S1-S4 and Tables S1-S2.
https://doi.org/10.1039/D3RA01249B (accessed 2026-10-11) -
nies_3638: NIES Microbial Culture Collection, NIES-3638; current name Galdieria javensis, former name Galdieria sulphuraria, other strain number 074W.
https://mcc.nbrp.jp/strainList.do?strainId=3636&strainNumberEn=NIES-3638 (accessed 2026-10-11) -
ncbi_galdieria_javensis: NCBI Taxonomy 3569544 - Galdieria javensis; species identifier
https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id=3569544&mode=Info (accessed 2026-10-11)
Curation History
| Timestamp | Curator | Action | Summary |
|---|---|---|---|
2026-10-11T07:57:17Z |
Codex (GPT-6; Codex) | create | Verified element identity, dated individual US criticality, current strain taxonomy and primary biomass-sorption evidence including supplements; preserved conflicting conditions and limits. Attached canonical CREATE history; independent scientific review is separate. |
Record source: data/records/iridium.yaml