The authors measured tungsten in hot spring waters and sediments of the Rehai geothermal field in southwestern China and ran sorption experiments with goethite and pyrite. Neutral and alkaline springs carried more dissolved tungsten, but acidic springs had far more tungsten in their sediments. Formation of polytungstates at low pH and their strong sorption to goethite explained the enrichment.
Key numbers
- Dissolved tungsten in 16 water samples: 0.10 to 76.98 micrograms/L; acidic springs (pH 2.78 to 5.52) 0.10 to 12.3 micrograms/L; neutral and alkaline springs (pH 7.20 to 9.15) 30.88 to 76.98 micrograms/L (Table 1, p. 6).
- Sediment tungsten: 991 micrograms/g at the ZZQD1 vent (pH 3.46, water 12.0 micrograms/L), 160 micrograms/g at HMZP-L (pH 7.20, water 30.9 micrograms/L) and 11.0 micrograms/g at YJQ (pH 9.15, water 68.3 micrograms/L) (Section 3.1, p. 7).
- The authors conclude that aqueous tungsten not much below 10 micrograms/L, acidic polytungstate formation and goethite-rich sediment together drive extreme enrichment; the critical aqueous level is placed between about 0.5 and 10 micrograms/L (Section 4; Conclusions).
Methods (brief)
Water was filtered at 0.45 micrometers and the tungsten split was left unacidified to avoid tungsten oxide precipitation; tungsten was measured by ICP-MS (iCAP RQ). Sediments were digested with HF and HNO3 for ICP-MS. Polytungstate formation was followed by UV-Vis (peaks near 275 and 320 nm) and thermodynamic modeling, and sorption of tungstate and polytungstates to synthetic goethite and pyrite was tested in laboratory reactors with XRD, SEM-EDX and XPS characterization.
Evidence fitness
Supports statements about tungsten levels in geothermal waters and sediments at one Chinese field and about pH-dependent speciation and sorption. The authors note that rivers receiving hot spring water serve as local drinking water sources, but tungsten in drinking water itself was not measured, so the paper does not support drinking-water occurrence or human exposure estimates.
Limitations
One geothermal field. Six of the water values and part of the sediment data come from the authors’ earlier study. Sediment values cited here are from the text; the full sediment table is supplementary. Health statements in the introduction summarize other literature and were not tested.
Related evidence
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