Surface Enhanced Raman Spectroscopy for Rapid Selenium Monitoring and Speciation in Complex Water, the Example of Flue Gas Desulfurization Wastewater

Metals and Metals Speciation Analysis in Environmental Samples
Oral Presentation

Prepared by , M. Peterman

Contact Information: [email protected]; 510-229-9197


Coal fired power plants produce large levels of toxic Selenium in the wastewater streams discharged from wet scrubber systems, such as Flue Gas Desulfurization (FGD) units. Federal regulation requires wastewater streams to contain less than 12-parts-per-billion (ppb) in total selenium before they are released in the environment. To comply, plants install Biological Selenium remediation units, which commonly digest only specific selenium species. Selenium occurs as selenide, selenite, selenate and selenocyanate all of which are environmental hazard and life threatening.

Operators of FGD units are forced to send samples for expensive, time-consuming laboratory testing for selenium speciation and analysis. The cost, the distance and the delay limit the data available for accurate process control. To guarantee treatment performance the development of a method capable of selenium speciation and quantification in complex wastewater matrices is necessary, in particular the distinction between selenite and selenate. The standard laboratory technique is ion chromatography coupled to mass-spectrometry. It is an expensive and labor-intensive operation that is not adequate for a timely-response.

The quantitative detection of selenate down to ppb-levels is shown for lab and FGD field samples. The apparatus, the procedure, its optimization, and its challenges are described. A pretreatment process necessary to remove the major interfering ions and the ultra-stable calcium carbonate nanoparticles present in FGD is discussed. Specificity of the analytical procedure to the +6 oxidation state is demonstrated with complete independence toward other selenium species. The technique employed is based on Surface Enhanced Raman Spectroscopy (SERS), it used active gold- nanoparticles coated with a cationic monolayer to enhance specificity to selenate and used a selenate isotopologue as internal standard to ensure high accuracy and repeatability. Quantitative analysis is demonstrated down to the 5-ppb regulatory level in complex-matrices in lab and in various wastewater samples. A side-by-side comparison with standard analysis equipment proves the high accuracy of this technique.

The successful results obtained for both lab and field samples demonstrates the capabilities of this cost-effective analysis technology to generate quickly meaningful data that can be used to optimize the FGD treatment process.