The topic of selenium has garnered attention in various industries for decades now, but the challenge associated with removing selenium from wastewaters has not diminished. In order to understand the most effective form of treatment for this unusual element that is both an essential micronutrient but also toxic at relatively low concentrations, the molecular form, or speciation, of selenium must be understood.
The two most common inorganic forms present in industrial wastewaters and agricultural runoff are selenite and selenate. Numerous treatment options are available for selenite, but conventional options like iron co-precipitation are ineffective for selenate. Several reviews have identified biological treatment as one of the best options for selenate removal, in part because it is cost-effective and has been proven in full-scale installations. Therefore, it is prudent for industries needing to implement selenium treatment and collaborative service providers to characterize waste streams via speciation analysis before and during bench- or pilot-scale studies.
Systems for biological treatment of selenium-laden waters take many forms, but anaerobic bioreactors are one of the most common iterations. In most of these systems, reduction of selenite and selenate to elemental selenium (Se0) is the desired outcome. This elemental selenium is mostly insoluble and can be removed from the effluent via filtration or temporarily retained in the reactor until backflushing of the system can be performed. Well-maintained and optimized bioreactors have been shown to treat industrial wastewaters containing mg/L concentrations of selenium down to single-digit µg/L levels.
Depending on the bacterial strains involved, the chemical environment, proper operation and maintenance, and the availability of nutrients, other less-desirable selenium species may be formed by bioreactor systems. Figure 1 shows a chromatogram of an effluent from a bioreactor used to treat a wastewater initially containing only selenite and selenate. Of particular note are the appreciable concentrations of the reduced selenium species, selenosulfate and selenocyanate (SeCN), as well as the organic selenium species that are present. While the toxicity of all these species have not been fully characterized and their bioaccumulation can be influenced by other constituents in the water, it has been shown that the bioavailability of certain species like selenomethionine can be several orders of magnitude higher than those of inorganic selenium.
When routine analytical testing indicates that a sample contains elevated levels of arsenic, more detailed characterization is often warranted. Performing arsenic speciation analysis – where specific molecular forms of arsenic are individually quantified – is often critical; however, if you don’t select the most appropriate analytical method, you can end up without the data you need or paying too much for data that is not helpful. But with dozens of potential arsenic species and numerous analytical options available, deciding what analyses to request can sometimes seem overwhelming. Brooks Applied Labs has decades of experience offering advanced arsenic speciation services and can help you choose an appropriate approach to meet the objectives and budget of your project.
Differentiating between the two most common forms of inorganic arsenic, arsenite and arsenate, is generally required for determining an appropriate treatment method for water samples known to contain elevated arsenic concentrations. This is because arsenate is typically directly amenable to conventional treatments like adsorption or co-precipitation, but arsenite is not. While arsenite and arsenate may be the most common arsenic species in simple waters, those from reducing environments or with significant biological activity can contain more unique species such as thioarsenicals or organic arsenicals like monomethylarsonic acid, dimethylarsinic acid, and trimethylarsine oxide. Moreover, certain sample types like landfill gas condensates can even contain volatile arsenic species like trimethylarsine. Brooks Applied Labs has developed methods tailored for the collection, preservation, and analysis of not just simple waters, but also these more complex ones to ensure that our clients can trust the integrity of their data.
While individually quantifying arsenite and arsenate provides crucial information for water samples, it is often unnecessary for biota and food. Most promulgated or proposed regulations for these latter sample types only set limits on the inorganic arsenic content, which they define as the sum of arsenite and arsenate. Examples of such regulations are California’s Proposition 65, the US FDA’s proposed limits for juices and infant rice cereal, and the WHO-FAO and EC limits for inorganic arsenic in rice products. Therefore, requesting inorganic arsenic quantitation in food and biota, rather than differentiation between arsenite and arsenate, typically suffices and can save clients money. But quantifying other species beyond inorganic arsenic is vital as an additional quality control check: comparing the sum of detected arsenic species against the separately determined total arsenic concentration (i.e., determining the mass balance) helps evaluate the completeness and accuracy of the data. Measuring other less common arsenic species can also be important for projects investigating the bioavailability, toxicity, or metabolism of arsenic.
Brooks Applied Labs understands that each project is unique, and a one-size-fits-all approach is never in the client’s interest. We offer numerous preparatory and analytical methods, customized for the sample matrix and target analytes, and have experienced staff available to help you decide what fits your individual needs. Should your next project require more detailed characterization for arsenic in waters, biota, food, soils, sediments, or another matrix, please contact us to see how we can help!
Brooks Applied Labs will be exhibiting at the Business & the Environment Conference & Expo in Portland December 11-12. This conference, organized by the Northwest Environmental Business Council (NEBC), is the Northwest’s largest environmental conference, with educational sessions covering topics such as environmental protection, sustainable business practices, and trending policy issues. Elizabeth Madonick will be there to answer any questions you may have about how our specialty elemental analyses and speciation services can support your projects. If you are going to be at the conference, be sure to come by and say hello!
The SETAC North America 39th Annual Meeting will be held in Sacramento November 4-8 this year. Come learn more about the unexpected ways alligators can potentially cause you harm you when Elizabeth Madonick, BAL Technical Services Specialist, presents a poster on the concentrations of total mercury and methylmercury in alligator meat on Tuesday, November 6th! Contact us any time after November 8th if you would like to receive a PDF copy of this interesting research.
The same week, Russ Gerads and Jamie Fox will be attending the International Water Conference in Scottsdale, Arizona. Jamie will be presenting on “Compliance with Selenium Aquatic Life Criterion and the Importance of Speciation for Treatment Selection and Monitoring” during the session on Trace Contaminants: Detection, Removal, and Recovery on November 5th. If you are interested in receiving a copy of this presentation, please feel free to contact us after November 12th.