Isolation of selenate from selenite, carbonate, phosphate, and arsenate solutions for δ 18 O-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 δ ¹⁸ O values of selenate, SeO ₄ ^2- must first be sequestered from water by precipitation as BaSeO _4(s) . However, other dissolved oxyanions insoluble with Ba ²⁺ require removal. Dissolved selenate was separated from dissolved selenite, carbonate, phosphate, and arsenate by addition of Ce ³⁺ cations that quantitatively removed these oxyanions by precipitation as insoluble Ce ₂ (SeO ₃ ) _3(s) , Ce ₂ (CO ₃ ) _3(s) , CePO _4(s) , and CeAsO _4(s) , respectively. δ ¹⁸ O-selenate (-8.19 ± 0.17 ‰) did not change after four replicates of selenite removal by Ce ₂ (SeO ₃ ) _3(s) precipitation and Ce ³⁺ removal by cation exchange (-8.20 ± 0.14, -8.32 ± 0.09, -8.17 ± 0.13, and -8.29 ± 0.13 ‰). δ ¹⁸ O-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 Ce ³⁺ to remove interfering oxyanions (-10.77 ± 0.07 ‰). The extraction and purification steps developed here successfully isolated dissolved selenate from interfering oxyanions while preserving δ ¹⁸ O-selenate values. This method should be useful for characterizing δ ¹⁸ O-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.