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

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.