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Strontium element Critical Material

Strontium (Sr), represented as an element distinct from dissolved Sr(II), strontianite or other mineral species, and strontium-bearing commodities or wastes. Laboratory cultures of Bacillus sp. TK2d lowered soluble Sr from a defined saline SrCl2 medium while Sr-bearing carbonate-rich material formed. The EU 2024 critical-raw-material designation names strontium; it is not inherited by every strontium compound or waste.

Identifiers & Classification

System Identifier Label
CHEBI CHEBI:33324 strontium atom

Criticality Designations

Jurisdiction List Name Edition / Document Classification Source
European Union Critical raw materials, Regulation (EU) 2024/1252, Annex II, Section 1 2024 original Official Journal text (May 3, 2024), point (ad) critical eu_2024

Microbial & Biochemical Mechanisms (1)

Culture-associated strontium removal into carbonate-rich precipitates

biomineralization Microbial

Associated Organisms: Bacillus sp. TK2d (NCBITaxon:1630002)

Bacillus sp. TK2d cultures removed Sr from the filtered liquid fraction and produced extracellular Sr- and Ca-bearing precipitates. Electron diffraction supported aragonite-like carbonate material, while Sr XANES fits supported a changing mixture of SrCl2-like and SrCO3-like local environments. Ureolysis and cell-surface association are consistent contributors, but an obligatory molecular pathway and a pure strontianite product are not established. The result concerns a defined laboratory salt medium, not treatment of radioactive wastewater or an isolated strontium product.

Experimental Context & Analytical Limits:
Horiike et al., Medium and cultivation conditions and Sr removal test: marine broth 2216 at 3.74 g/L (described as 10% broth), final NaCl 30 g/L, autoclaved then filtered through 0.45 micrometre pores, supplemented with urea at 20 g/L and 1.0 mM Sr as SrCl2. A 0.5 mL, OD600 0.1 preculture inoculated 50 mL in a 100 mL flask, incubated at 30 degrees Celsius and 120 rpm in triplicate. Parallel salinity tests used source-labelled 0.3%, 3%, 5% and 10% NaCl. The isolation plate was set to pH 7.6; the paper does not provide a time-resolved broth pH series or a pH-matched abiotic arm. Daily aliquots were centrifuged at 21,000 x g for 5 min; the ICP-AES procedure included 0.2 micrometre filtration and triplicate analytical readings per sample. The two centrifugation descriptions specify 4 and 20 degrees Celsius, respectively; their relationship is not resolved. Uninoculated Sr-containing medium was the negative control. Microscopy, diffraction and XANES examined collected solids at different times; these are not measurements of one identical final sample. Reagent SrCl2 was used with no radionuclide activity stated; the Discussion explicitly leaves actual contaminated seawater and 90Sr tests for future work.
Curated Evidence Points:
Results, Isolation of a urease-producing microorganism, and Figures 1-2: at 3% NaCl the text reports soluble Sr declining from 1.0 mM to 0.13 mM at day 2 and below 1.0e-7 mol/L after 4 days. The stated bound is retained rather than converted to zero or an exact 100% recovery. The Methods describe uninoculated controls; the screening Results report little decline in the negative control and four other isolates. Figures 1-2 show culture time courses rather than overlaid full negative-control curves. Ammonium accumulation accompanies Sr decline but is not a urease knockout or inhibition test.
Figures 3-5 and the solid-characterization Results report Sr/Ca in precipitates, P-containing dense regions associated with cells at day 2, and extracellular mineral material at later times. The reported electron-diffraction spacings are consistent with aragonite. Table 1 gives two-standard XANES fitted SrCO3-like fractions of 52% after 5 days and 87% after 10 days for the 1 mM starting-Sr cultures; the remaining fitted fractions are SrCl2-like. These are model-dependent spectral fractions, not crystalline-phase purities, fractions of total initial Sr recovered, or proof that all solid Sr is strontianite.
Figure 2 and its Results show much weaker Sr removal at 5-10% NaCl than at 0.3-3%, despite growth at some higher salinities. Urease-positive isolates did not all remove Sr effectively. The chemical comparison also formed Sr-bearing carbonate without bacteria, using a different Na2CO3/CaCl2/SrCl2 recipe; it is not a chemistry-matched control proving microbial necessity for precipitation. The data do not establish a specific Mg ion-channel mechanism or universal Sr selectivity.
Figure 1b shows a modest Mg concentration change although the Results describe no decrease; the defensible comparison is much stronger depletion of Sr and Ca, not exact Mg exclusion from all phases. EDS nondetection in selected carbonate material has an approximately 0.1% weight/weight detection limit and does not establish zero Mg throughout the precipitate. Long-term leaching stability, isolated-metal yield and deployment are untested in this study. The linked supplement was not acquired after bounded PMC, publisher and Europe PMC attempts; claims here rely on the inspected main text, Table 1 and main Figures 1-6, not an independent reassessment of supplementary micrographs or chemical-control curves.

Sources & References

  1. horiike_2017: Horiike, Dotsuta, Nakano, Ochiai, Utsunomiya, Ohnuki and Yamashita (2017). Removal of Soluble Strontium via Incorporation into Biogenic Carbonate Minerals by Halophilic Bacterium Bacillus sp. Strain TK2d in a Highly Saline Solution. Applied and Environmental Microbiology 83(20), e00855-17. DOI 10.1128/AEM.00855-17; PMID 28802269; PMC5627000.
    https://doi.org/10.1128/AEM.00855-17 (accessed 2026-10-11)
  2. eu_2024: European Parliament and Council. Regulation (EU) 2024/1252 of 11 April 2024; original Official Journal text published May 3, 2024. Annex II, Section 1, point (ad): strontium, a critical raw material.
    https://eur-lex.europa.eu/eli/reg/2024/1252/oj/eng (accessed 2026-10-11)
  3. chebi_strontium: ChEBI:33324 - strontium atom; Sr, net charge zero.
    https://www.ebi.ac.uk/chebi/CHEBI:33324 (accessed 2026-10-11)
  4. ncbi_tk2d: NCBI Taxonomy 1630002 - Bacillus sp. TK2d. An unresolved Bacillus isolate entry; the database species rank does not establish a formally named species.
    https://api.ncbi.nlm.nih.gov/datasets/v2/taxonomy/taxon/1630002/dataset_report (accessed 2026-10-11)
  5. sequence_tk2d: DDBJ/GenBank LC034238.2 - Bacillus sp. TK2d partial 16S ribosomal RNA gene, 1463 bp; strain TK2d, marine sediment, Japan. Links taxon 1630002 and Horiike et al. 2017.
    https://www.ncbi.nlm.nih.gov/nuccore/LC034238.2 (accessed 2026-10-11)

Curation History

Timestamp Curator Action Summary
2026-10-11T06:51:22.828619Z Codex (GPT-6; Codex) create_record Created elemental strontium with dated EU criticality and TK2d carbonate-associated removal; retained mixed XANES fits, salinity dependence, control and supplement limits.

Record source: data/records/strontium.yaml