Atp

Figure 13.27 Energetic proton gradient for acidophilic iron-oxidizing chemolithotrophs.

interestingly, these bacteria both have stalks, although they are not related phylogeneti-cally. Ferroglobus, an archaea, is also of interest in that it oxidizes Fe(II) to Fe(III) under anoxic conditions, using nitrate as the electron acceptor.

13.4.3 Iron in Environmental Engineering and Science

Iron cycling plays a role in a number of issues of relevance to environmental engineers and scientists. These include problems of acidification, particularly acid mine drainage, and drinking water.

1/2O2

Figure 13.28 Gallionella-type bacteria causing iron precipitation in wells.

Acidification, Including Acid Mine Drainage Oxidation of soluble ferrous iron appears to consume acidity:

However, the subsequent precipitation of ferric hydroxide has the effect of producing 3 mol of acid, so that the overall reaction can be written as

During mining of coal and other minerals, anaerobic deposits containing pyrites (FeS2) are commonly exposed to the air. This begins a complex process involving both abiotic and biochemical mechanisms that leads eventually to oxidation of both the iron and sulfur. Acidification initially results from the oxidation of reduced sulfur to sulfate (sulfuric acid), which then makes oxidation of the iron favorable. The first step, or initiator reaction, is typically abiotic at the starting neutral pH:

The production of acid leads to a drop in pH and the development of favorable conditions for bacteria such as Thiobacillus ferrooxidans. The overall reaction can be written as

FeS2 + 3.75O2 + 3.5H2O ! Fe(OH)3 + 2SO42~ + 4H + (13.23)

Drainage from such areas can thus be highly acidic, with a pH of 2 not uncommon. This may have a drastic effect on the ecosystem of any nearby streams. An iron sulfate mineral known as yellow boy may also precipitate and color a stream bed dramatically.

Pyrites are also found in some clay minerals. This has been found to be a problem with the clay caps that are sometimes put over landfills—acid formation inhibits establishment of cover vegetation. Leachate from landfills containing reduced iron and/or sulfides may also become acidified through similar processes.

Drinking Water With certain groundwater supplies, iron-oxidizing bacteria such as Gallionella in wells can cause clogging of the screens, resulting in a decrease in pumping capacity. Beyond the growth itself, the large amounts of precipitated iron they produce may be responsible for much of the problem. A similar effect can occur with the oxidation of reduced manganese (Section 13.5.3), but iron concentrations are typically much (1000 x) higher. Clogging of sand filters is also possible if the feed water contains sufficient reduced iron.

High iron concentrations can provoke consumer complaints about the aesthetic quality of drinking water. Dissolved iron can create taste problems, particularly with hot beverages such as coffee or tea. Also, the oxidation of reduced iron over time may produce a noticeable brownish discoloration (mainly from ferric hydroxide) on porcelain surfaces such as kitchen sinks, toilet bowls, and bathtubs, and even in clothes during washing. Additionally, there may be concern among customers over the safety of the "rusty" water itself. For these reasons, the secondary standard for iron in drinking water is 0.3 mg/L.

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