Cleaning up selenium
An EU-funded project has found that selenium-contaminated water can be effectively cleaned by refining a technology that is already in use. A study published in the Journal of Environmental Quality by researchers from Switzerland and the Netherlands showed that technology normally used in the treatment of industrial wastewater can be adapted to remove elemental selenium from contaminated water. This technology could be transferred to aid the treatment of contaminated domestic water. Selenium is an essential trace element; too little in the diet can lead to health problems. However, it is toxic when taken in excess, leading to gastrointestinal disorders, sloughing of nails, fatigue, neurological damage and, in extreme cases, cirrhosis of the liver and death. Selenium occurs naturally in soils and phosphate fertilisers, most often in a soluble form called selenate. Soluble forms of selenium pose a problem because they can accumulate in the body tissues of small organisms. They leach into rivers in agricultural run-off and pass up through the food chain. As concentrations increase, selenium poisoning occurs and causes reproductive failure and birth defects in birds and animals. Selenium contamination can also result from human activities such as coal burning and sulphide mining. Very high levels of selenium have been found in groundwater in Ireland, France and parts of eastern Europe. The current study evaluated the efficacy of using an Upflow Anaerobic Sludge Blanket (UASB) reactor to remove selenium from contaminated water. The UASB reactor is a 'digester', or a machine that is filled with microorganisms that break down industrial or agricultural wastes. In the reactor, a blanket of granular sludge is suspended in a tank. Wastewater flows upwards through the blanket to be degraded by anaerobic microorganisms. The reactors produce methane, which can be captured and used as an energy source, usually generating the electricity needed to heat the digestion tanks. They are efficient and relatively inexpensive, but are not currently adapted for cleaning drinking water or treating domestic sewage. Selenium-respiring microorganisms are not currently used in most UASB reactors because the selenium cycle is not widely understood. Without using these microorganisms, elemental selenium might be removed from the water but an acutely toxic form of selenium could also be created at the same time. This alkylated selenium is soluble, and could easily wash out into the environment. The challenge is to create solid, elemental selenium from the soluble forms that occur in soil and water and remove it completely. If the soluble selenium is not completely converted, it will remain in the water and be washed back into the environment. In the current study, the reactor was found to be effective in creating solid selenium from soluble selenium, but only when selenium-respiring microorganisms were used, the temperature and pH levels were kept constant, and the speed of the water rising up through the blanket was carefully controlled. When the temperature or pH inside the UASB reactor fluctuates during digestion, or when the flow of water up through the reactor is too strong, the elemental selenium that is achieved by the selenium-respiring microorganisms occurs in such small particles that these, too, can wash out of the system. This puts selenium back into the environment but also represents a lost opportunity. Selenium is valuable in its elemental form. It is commonly used in glass manufacturing, chemical production and pigments and is an essential material in the drums of laser printers and copiers. It is also used to form a black, vitreous material which is usually sold industrially as beads. Recapturing elemental selenium from contaminated water would be beneficial economically; however, the size of the particles produced in this process must be large enough so that the solids can actually be removed from the reactor. If UASBs are used to clean selenium-contaminated water, they will be effective only if the right bacteria are used. If elemental selenium is to be recovered from the process in a usable form, the environment within the reactor must be very carefully controlled. The researchers found that the selenium nanoparticles resulting from less-controlled conditions in the reactor might be harvested using a separate process; however, this would be more expensive than simply carefully controlling the UASB's operation. The study has wider implications for the treatment of irrigation, mining or even drinking water. The work was supported by the EU's Marie Curie Excellence TEAM grant, 'Novel biogeological engineering processes for heavy metal removal and recovery'.
Countries
Switzerland, Netherlands