Coal combustion for energy generation has traditionally been associated with environmental pollution since the onset of the Industrial Revolution. A waste stream (Flue Gas Desulfurization, FGD) derived from coal combustion in modern power plants is particularly enriched in chemical elements . Using a biologically and environmentally friendly approach, FGD can be cleaned up, generating resources and profit at the end of the pipe .
Although coal is considered a “dirty” fuel, its abundance and high energy density make it a resource widely used nowadays to generate energy. Once coal is burned, an important component of the power station is devoted to cleaning up the flue gas. Fly ashes are separated using filter bags or electrostatic precipitators (ESP), while the gaseous pollutants such as NOx and SO2 are removed using Selective Catalytic Reduction (SCR) and Flue Gas Desulfurization (FGD) systems, respectively (see Figure 1). NOx (nitrogen oxides) are involved in the formation of smog and acid rain; SO2 (sulfur dioxide) is a toxic gas and a net contributor to acid rain.
For removing nitrogen oxides, urea or ammonia are often used. On the other hand, for sulfur dioxide removal, a slurry of lime/limestone is sprayed in the flue gas. Calcium from the slurry reacts with SO2, producing gypsum (CaSO4), a marketable product, plus a water stream (FGD) enriched in chemical elements such as cadmium, mercury, nickel, selenium, zinc, and others. Of these, selenium (Se) is known as a powerful toxicant for the environment, especially for aquatic ecosystems (fish and birds). Notable examples of Se poisoning were documented throughout the USA and Canada which led to the virtual collapse of entire fish or aquatic bird populations.
However, we tend nowadays to no longer consider wastes as undesirable products of human activity but rather as potential resources. This approach falls under a novel strategy called the circular economy. Unlike the linear economy strategy which is based on the obsolete mode of production that disposes of the waste generated at the end of the production cycle, the circular economy endeavors to recover and reuse part of the raw materials and energy employed .
A very interesting way to clean up and recover resources from these FGD streams is by means of microbial metabolism. At the end of the 1980s, scientists unraveled a novel strategy used by bacteria to generate cellular energy in the form of ATP by respiring selenium. This process is analogous to the way we humans respire oxygen, except that selenium respiration appears to be an ancient strategy used by bacteria in an oxygen-poor atmosphere.
Luckily, this discovery was shown to have practical applications when using bioreactors inoculated with microorganisms able to metabolize selenium in this fashion. This approach is known as bioremediation, since a biological entity, in this case, bacteria, is used to clean up the waste. What makes the story even more interesting are the potential benefits of harnessing this metabolic capacity. The metabolism of selenium by bacteria leads to nanoparticles and, also, to biogas, when the wastewater is additionally charged in organic matter. Specialized methanogenic microorganisms present in the microbial community seeded in the bioreactor are responsible for transforming organic matter to methane (see Figure 2).
Selenium is widely used in the industry and has numerous uses in domestic applications. Solar cells need selenium to efficiently convert light into electrical currents, the glass industry employs it as a red pigment, and even certain types of steel have this element in their composition. At home, selenium is often present in antidandruff shampoos or in dietary supplements.
All these make selenium a valuable resource for our society, warranting its recovery and reuse from FGD effluents. The other resource generated from FGD, biogas, a mixture of methane and other gases, is burned to generate energy and is considered a regenerable fuel, unlike finite fossil fuels such as coal and oil. Overall, by applying the bioremediation approach, the burden of selenium pollution can be changed into profit, in addition to providing an ecological service by discharging cleaner effluents to the environment.
These findings are described in the article entitled Flue gas desulfurization effluents: An unexploited selenium resource, recently published in the journal Fuel. This work was conducted by Dr. Patricia Cordoba from Instituto de Diagnóstico Ambiental y Estudios del Agua, Consejo Superior de Investigaciones Científicas (IDAEA-CSIC), Barcelona, Spain and Dr. Lucian Staicu from Polytechnic University of Bucharest, Romania.
- Cordoba, Patricia. “Partitioning and speciation of selenium in wet limestone flue gas desulphurisation (FGD) systems: A review”. Fuel 202 (2017): 184-195.
- Cordoba, Patricia and Staicu, Lucian. “Flue Gas Desulfurization effluents: an unexploited selenium resource”. Fuel 223 (2018): 268-276.
- Ni, Thomas, Staicu, Lucian et al. “Progress toward clonable inorganic nanoparticles”. Nanoscale 7 (2015): 17320-17327.