Costly metal extracted from power plant waste

A PhD student of Wageningen University has developed a technology that recovers the costly mental selenium from the waste streams of power plants. ‘In this way, you kill two flies with one blow’, says drs. Markus Lenz of the Sub-department of Environmental Technology. ‘You recover a resource for which there is a growing demand, and you prevent a toxic substance from entering the environment.’

Selenium is toxic, but is also essential to health. Too much selenium results in hair loss, fingernail loss and typical garlic breath, but at the same time the body requires selenium for producing detoxifying enzymes ‘There is no single element for which the margin between deficiency and toxicity is so small’, says Lenz. ‘Most Europeans consume too little selenium, but there are also areas where the population consumes excessive amounts of selenium. Many countries add selenium to animal feed or artificial fertiliser, but selenium is too hazardous to simply discharge into the environment.’

The price of the metal is increasing, especially due to demand from Asia. Most of the selenium on the market is obtained as a byproduct of copper refining, but there is also a need for new sources. ‘Selenium is also present in coal and oil and is released during combustion’, says Lenz. ‘Coal and oil fired power plants that purify their flue gases to prevent acid rain also capture selenium in the process. I have tried to extract the selenium by means of anaerobic purification installations.’

The waste streams with which the environmental technologist worked contained the metal in the form of selenate and selenite, which are toxic. He succeeded in having bacteria in reactors convert these compounds of selenium into the less toxic elementary metal. Lenz: ‘A problem we are still wrestling with is that a portion of the selenium continues to float in suspension in the reactor in very tiny particles. These particles are so small that water animals can consume them, or they can simply revert to their more toxic form once they are returned to the environment. A PhD student in our group attempted to change the process so that the selenium precipitated out of solution in larger pieces.’

Markus Lenz defended his doctoral thesis on 27 May; his doctoral supervisors were Prof. Cees Buisman, Professor of Biological recycling technology, and Prof. Piet Lens, Professor of Environmental Biotechnology. / Willem Koert