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Comparison between Hydride Generation and IC-ICP-MS for Selenium Speciation (pdf)

Trace (ppt level) Total Arsenic and Selenium Analysis (pdf)

Treatment Plant Optimization for Trace Metal Removal (pdf)

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Selenium speciation analysis was performed on a sample generated from a flue gas desulfurization wastewater treatment plant by ion chromatography inductively coupled plasma mass spectrometry (IC-ICP-MS). Quantified species included selenite, selenate, selenocyanate, methylseleninic acid, and selenomethionine.

Sample pretreatment was limited to filtration (0.45 um) without preservation to minimize species conversion prior to analysis. Aliquots of each sample were injected onto an anion exchange column and are mobilized by a basic (pH > 7) gradient. The eluting selenium species were then introduced into the ICP-MS. Various selenium isotopes were monitored to confirm Se in each species. The Se species were identified by matching their retention times to known standards. Chromatograms of a standard mixture and an FGD wastewater sample are presented in Figures.

As part of the Quality Control protocol, four (4) preparation blanks, a certified reference material, matrix duplicate, one matrix spike and matrix spike duplicate set, one analytical duplicate, one analytical spike and analytical spike duplicate, and isotopic monitoring for species confirmation was used. Total selenium analysis via inductively coupled plasma dynamic reaction cell mass spectrometry was also performed for confirmation of the sum of species.

A cursory review of the chromatogram for the FGD discharge identifies that the quantified selenium species account for only a fraction of the total selenium present in the sample. Especially the concentration of selenite and selenate was less than 5% of the total selenium in the sample. At the same there were more than 13 different unidentified peaks which illustrate the extremely complex chemistry of selenium in wastewaters. Although the sum of the identified selenium species did not correlate well with the total selenium analysis by ICP-DRC-MS, integration of all the peaks (unknowns included) comes within experimental error of the total concentration (RPD less than 15%).

Samples from this plant was also analyzed for selenium speciation by a different commercial laboratory using HG-AAS. Unfortunately, the results showed significant biases for Se(IV) which also resulted in biased results for selanate since the Se(VI) concentration is calculated by difference.

This brief study confirms that the complexity of industrial waste, especially from flue gas desulfurization plants, requires a method such as IC-ICP-MS to obtain detailed but most importantly, accurate information and make better decisions.

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