Water Treatment Class 3-A Practice Test

Under anaerobic conditions, the lack of oxygen drives reduction of the metal oxides. Microbes use organic matter as an electron donor and reduce iron and manganese oxides (Fe(III) and Mn(IV)) to soluble Fe2+ and Mn2+. Those reduced ions enter the pore water and can move into the overlying water, so the metals go into solution rather than remaining as solids in the sediments. In contrast, if oxygen were present, these metals would tend to oxidize and precipitate as oxides, staying in the sediments, and carbonate precipitation would require different chemical conditions.