Measurement of selenite in sediment extracts by using hydride generation atomic absorption spectrometry

Measurement of selenite (Se [IV]) is the most important step to determine selenium (Se) speciation in water, and in soil–sediment extracts by using hydride generation atomic absorption spectrometry (HGAAS). Determination of Se [IV] commonly involves sample acidification and application of XAD resins for removal of sample organic interference. The effects of sample acidification and application of XAD resins on determination of Se [IV] in sediment samples have not been evaluated in detail. In this study, we evaluated these effects by examining Se concentrations in six different extracts (0.01 M HCl, de-ionized water, 0.1 M NaOH, 0.1 M NaH2PO4, 0.1 M Na2HPO4, and 0.1 M Na2HPO4+0.1 M NaOH) from three seleniferous sediments. Results showed that the amount of extracted Se [IV] was pH-dependent, and was 3–15 times higher in a 0.1 M NaOH extract than in 0.01 M HCl and de-ionized water extracts. Phosphate can replace Se [IV] for adsorption sites, thus resulting in an increase in extracted Se [IV]. Acidification of a humic substance (HS)-containing sample caused a significant loss of free Se [IV], especially in 6 N HCl, due to a complexion of HS-Se [IV]. Hydrophobic and hydrophilic characteristics of HS-Se [IV] were found in these sediment extracts, resulting in the accumulation of HS-Se [IV] on XAD resins when the pH 2 extracts were allowed to pass through XAD-7 columns and a significant loss of spiked Se [IV] in the pH 2 extract elutes. These results suggest that determination of Se [IV] in a HS-containing sample by using HGAAS must be evaluated with caution during acidification and application of XAD resins for removal of sample organic interferences.