MICROBIAL OXIDATION AND REDUCTION OF MANGANESE - CONSEQUENCES IN GROUNDWATER AND APPLICATIONS

In the natural environment, manganese is found as reduced soluble or a dsorbed Mn(II) and insoluble Mn(III) and Mn(IV) oxides. Mn oxidation h as been reported in various microorganisms. Several possible pathways, indirect or direct, have been proposed. A wider variety of Mn-reducin g microorganisms, from highly aerobic to strictly anaerobic, has been described. The mechanisms of Mn reduction can be either an indirect pr ocess resulting from interactions with organic or inorganic compounds, or a direct enzymatic (electron-transfer) reaction. The role of micro organisms in Mn cycle is now well demonstrated by various methods in s uperficial natural environments, and research has been initiated on su bsurface sediments. Observations in vivo (Rhone valley) and under in v itro suggested that bacterial activities are the main processes that p romote manganese evolution and migration in shallow aquifers. After th e building of hydroelectric dams, the stream of the Rhone was modified , giving rise to mud deposition on the bank. In the mud, bacteria are stimulated by the high organic content and consume oxygen. The redox p otential drops. The manganese oxides previously formed under aerobic c onditions are reduced and soluble manganese (Mn(II)) migrates into the aquifer. If the subsurface sediments are coarse-grained, the aquifer is well aerated, allowing the re-oxidation of Mn(II) by the oligotroph ic attached bacteria in aquifer sediments. If the aquifer is confined, aeration is not sufficient for Mn-reoxidation. Mn(II) remains in a re duced state and migrates to the wells. Furthermore, the presence of or ganic matter in subsurface sediments results in the reduction of previ ously formed Mn oxides. Pseudo-amorphous manganese oxides, which were probably recently formed by bacteria, are more readily reduced than ol d crystalline manganese oxides. Although the concentrations of soluble manganese found in groundwaters are not toxic, it still is a problem since its oxidation results in darkening of water and plugging of pipe s in drinking or industrial water systems. Soluble manganese can be re moved from water by biological processes involving manganese-oxidizing bacteria, either in situ, or in sand filters after pumping. Various p rocedures are mentioned.