The exposure of sulfide-containing minerals to atmospheric conditions due to natural or anthropogenic activities enables abiotic and/or biotic oxidation of related sulfur species to sulfate via reactions that produce high amount of acidity. The formed sulfate-rich acidic water is known as acid mine drainage (AMD). This acidic water leaches metals from minerals efficiently and AMD is therefore toxic not only due to the high acidity, but also due to the high metal content. In addition, the release of sulfate increases the risk of eutrophication and uncontrolled formation of toxic hydrogen sulfide (H2S). AMD is thus a severe threat to the environment not only at the site of formation but also at distant locations downstream from the point of discharge. Moreover, AMD is often a problem at mining sites still after the mining activities stop, as the AMD formation can continue in the waste rock heaps and tailings for decades.
The high acidity, high sulfate content and high metal content make the treatment of AMD challenging. As mining waters are very low in organic compounds, conventional bioreactors treating AMD often require external substrate. By integrating biological and electrochemical processes, however, efficient AMD treatment can be achieved due to simultaneous neutralization, sulfate removal and metal removal. With bioelectrochemical systems, substrate for biological sulfate reduction can be produced from natural sources (H+, CO2) and the (bio)electrochemical generation of hydrogen or microbial electrosynthesis of acetate will assist in neutralization of the water. In the presence of sulfide generated by the sulfate reducing bacteria, metal ions will readily precipitate as metal sulfides. Thus bioelectrochemical systems enable comprehensive treatment of AMD, and as no external substrates, neutralizing agents or chemicals are needed, the external input required for the process remains low.