Other Redox Reactions

The redox state of sediments and associated porewaters influences the retention and release of trace elements and other compounds, as illustrated by chromium. It commonly exists in the environment in two oxidation states: trivalent chromium, Cr(III), which is an essential trace element, and hexavalent chromium, Cr(vi), which is carcinogenic. The latter species is also much more labile in most aquatic systems, and may be found at potentially toxic levels (>100 mgl- ) in some aquifers due to either natural processes or industrial pollution. While Cr(iii) is more thermodynamically stable under reducing conditions, it is generally only found at low levels because it exists in solution primarily as cations (Cr3+, Cr(OH)^, Cr(OH)2+) that have high affinities for sediments with a net negative surface charge (e.g., iron oxides and clay minerals) and Cr(iii) readily precipitates as Cr(OH)3 and Cr2O3 in the pH ranges commonly encountered in the environment. in contrast, Cr(vi) forms relatively soluble oxyanions (e.g., CrO4- and HCrO- ) that have less affinity for sediment surfaces and, therefore, may occur at higher concentrations in many aquifers. Although Cr(vi) may be naturally converted to Cr(iii) in reducing ground-water systems, the microbial oxidation of Cr(iii) to Cr(vi) at the aerobic/anaerobic interface in sediments has been associated with anomalously high levels of the potentially toxic species in some aquifers - as has the discharge of industrial Cr(vi) in other aquifers.

Sediment redox processes also strongly affect metalsulfide minerals, which can be solubilized under oxidizing conditions. The oxidation of metal-sulfides such as cinnabar (HgS), cuprite (CuS), galena (PbS), sphalerite (ZnS), chalcopyrite (CuFeS2), and chalcocite (Cu2S) by oxygen is similar to that of pyrite (FeS2):

The oxidation of the reduced Fe(ii) and sulfide in pyrite results in the formation of Fe(iii) in the form of iron oxide (Fe(OH)3) and sulfate in the form of sulfuric acid (H2SO4), respectively.

The H2SO4 produced by that oxidation is responsible for much of the acid mine drainage (pH < 5) generated from mines and mine tailings. While the abiotic oxidation of sulfidic minerals is thermodynamically favorable in aerated waters, the kinetics are generally very slow at ambient environmental conditions because of the large activation energy required. However, the oxidation of sulfidic minerals is markedly accelerated by bacteria (e.g., Thiobacillus ferrooxidans) which obtain energy from the oxidation of pyrite.

Acid mine drainage of sulfidic deposits is accompanied by the leaching and mobilization of other heavy metals. These may include arsenic, cadmium, copper, lead, manganese, mercury, selenium, and zinc, which are all relatively toxic to most aquatic organisms. Consequently, surface waters downstream from acid mine drainage may be nearly devoid of all but microbial life until pH levels increase and iron and manganese precipitate out as oxy-hydroxides that scavenge those toxic elements.

However, sediment burial following the deposition of iron and manganese oxyhydroxides leads to reducing conditions in subsurface sediments, which thermodyna-mically favor the reductive dissolution of those compounds. This process releases not only iron and manganese back into solution, but also metals and metalloids that had been adsorbed onto the oxyhydroxides. As a result, sediments with relatively high levels of contaminant metals that have been transported downstream from acid mine drainage constitute a potential source of contamination to porewaters and overlying waters. Similarly, other sediments that have relatively large amounts of metals (e.g., Cd, Cu, Pb) and metalloids (e.g., As, Se) -from natural or industrial sources - scavenged onto iron and manganese oxyhydroxides represent a potential source of pollution under reducing conditions. Again, those reducing conditions may be catalyzed by the deposition of organic matter, either following an algal bloom or the discharge of organic industrial, agricultural, and municipal wastes.

Heavy metals are not the only class of compounds which are readily adsorbed by iron and manganese oxy-hydroxides. Nutrients such as phosphate and nitrate also exhibit this same behavior under oxidizing conditions, and are thus released when the sediments become reducing and the Fe(iii) and Mn(iv) precipitates are solubilized to dissolved forms of Fe(ii) and Mn(ii). Thus, a change in the redox state ofsediments to more reducing conditions can result in the release of phosphate and nitrate to overlying waters where they can promote eutrophication. As a result, lakes which receive nutrient-rich runoff from urban or agricultural areas are often mechanically aerated in order to maintain oxygenated bottom waters and surface sediments to prevent the reductive dissolution ofiron and manganese oxyhydroxides and the subsequent release of nutrients. This method has also been employed successfully to promote the reformation of iron and manganese oxyhydroxides and recovery of lakes following eutrophication.

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