When evaluating the selenium (Se) speciation at a site, sometimes things don’t add up – that is, the sum of detected Se species might be significantly lower than the associated dissolved Se result.  Incomplete mass balances are often concerning, since they raise questions about what “missing” forms of Se might be present in a sample. Given that certain Se species are known to be more bioaccumulative than others, increasing their potential toxicity, investigating the source of this discrepancy is often warranted. While a low mass balance could be caused by various factors, it often signifies that unanticipated forms of Se are present. The anionic species selenite (Se⁴⁺) and selenate (Se⁶⁺) predominate in most natural waters, but selenium exists in numerous other molecular forms, including volatile selenides and colloidal elemental Se (Se⁰). BAL has offered analyses for the volatile dimethylselenide (DMSe) and dimethyldiselenide (DMDSe) for over a decade, but a method for Se⁰ quantitation has been on our list of research priorities for years.  We’re happy to announce that BAL is now actively working to bring online a new method to support Se⁰ analysis.

In collaboration with select clients, BAL has been developing a special chromatographic separation method for screening samples for the presence of particulate and Se⁰.  Figure 1 contains a chromatogram demonstrating the separation of Se⁰ from the ionic Se species selenite (Se⁴⁺), selenate (Se⁶⁺), and selenomethionine (SeMet).

Figure 1. Chromatogram of elemental selenium spiked into a standard containing other selenium species

When the selenium speciation analysis of treated industrial wastewater shows a poor mass balance between dissolved selenium and sum of selenium species, additional testing for Se⁰ can reveal the culprit, as demonstrated in Figure 2. This method can serve as another tool for evaluating the performance of biological treatment systems for selenium. While such systems are generally designed to remove produced Se⁰ prior to discharge, those processes are not always completely effective. With parts-per-trillion (ppt) level detection limits, monitoring for Se⁰ can also help identify if natural attenuation is occurring in contaminated environments.

Figure 2. Chromatogram demonstrating the separation of elemental selenium from selenate in an industrial effluent sample.

Additional research currently underway at BAL is focusing on methods for characterizing the particle size distribution of the Se⁰ in samples using single particle-ICP-MS analysis.  An example size distribution is presented in Figure 3.  This type of information is usually accompanied with particle concentration (particles/L) and is expected to aid in studying the fate, transport, settleability, and transformations of nanoparticulate Se⁰ in the environment.

Figure 3. Particle size distribution of elemental selenium nanoparticles (NP-Se0) in a standard. The mean particle size is 48 nm.

These new methods are further evidence of BAL’s commitment to offering the most comprehensive speciation analysis services to address our client’s analytical needs. If you or your colleagues are interested in receiving updates as we continue to refine these methods, please use the form below to contact us!