Isolation of selenate from selenite, carbonate, phosphate, and arsenate solutions for δ¹⁸O-selenate determination

ABSTRACT

Selenium and oxygen isotope systematics can be useful tools for tracing sources and fate of Se oxyanions in water. In order to measure δ¹⁸O values of selenate, SeO₄^2– must first be sequestered from water by precipitation as BaSeO_4(s). However, other dissolved oxyanions insoluble with Ba²⁺ require removal. Dissolved selenate was separated from dissolved selenite, carbonate, phosphate, and arsenate by addition of Ce³⁺ cations that quantitatively removed these oxyanions by precipitation as insoluble Ce₂(SeO₃)_3(s), Ce₂(CO₃)_3(s), CePO_4(s), and CeAsO_4(s), respectively. δ¹⁸O-selenate (−8.19 ± 0.17 ‰) did not change after four replicates of selenite removal by Ce₂(SeO₃)_3(s) precipitation and Ce³⁺ removal by cation exchange (−8.20 ± 0.14, −8.32 ± 0.09, −8.17 ± 0.13, and −8.29 ± 0.13 ‰). δ¹⁸O-selenate values (−10.86 ± 0.45 ‰) were preserved also when selenate was pre-concentrated on anion exchange resin, quantitatively retrieved by elution, and processed with Ce³⁺ to remove interfering oxyanions (−10.77 ± 0.07 ‰). The extraction and purification steps developed here successfully isolated dissolved selenate from interfering oxyanions while preserving δ¹⁸O-selenate values. This method should be useful for characterizing δ¹⁸O-selenate when present with the co-occurring oxyanions above in laboratory experiments and field sites with high Se concentrations, although further research is required for methods to eliminate any co-occurring sulphate.

KEYWORDS:

• Analytical methods
• cerium
• ion exchange
• isotope geochemistry
• isotope measurements
• oxygen-18
• selenium

Acknowledgements

The authors thank Anastasia Maydenov and Emma Soucy for technical assistance in ion exchange methodology. Special thanks go to the Northeastern University Mass Spectrometry Core Facility for infrastructure support. The authors thank the anonymous reviewers whose suggestions greatly improved the report.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by Northeastern University and the National Science Foundation [grant number CBET-1236182].