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