Chemical Processes Involved in the Bioremediation of Heavy Metal Contaminated Soils

Bioremediation involves the use of microorganisms to aid with the destruction of contaminants, a process called microbial metabolism. This process involves biochemical reactions or pathways that result in organism activity and growth, as well as the reproduction of the organism. The chemical processes involved in microbial metabolism make use of reactants, contaminants, oxygen, or other electron acceptors to convert metabolites to well-defined products. The microbial metabolism system enables organisms to retrieve carbon, electrons, and other vital components to survive. In some cases, the contaminant may be transformed while the microorganisms seek other sources of energy or carbon. This reaction is described as "cometabolism," indicating that the transformation of the contaminant yields little or no benefit to the cell. Secondary utilization is another way to describe a nonben-eficial biotransformation. This transformation of the contaminant is an incidental reaction that is catalyzed by enzymes present in the cell's metabolic system (National Research Council 2003).

Aerobic respiration is a process that involves the use of oxygen by microorganisms to oxidize sources of carbon that may also exist in the contaminant. Thus, many microorganisms use aerobic respiration as a way to destroy organic contaminants. During the process of aerobic respiration, oxygen is reduced, resulting in the formation of water. Thus, a drop in oxygen concentration occurs when aerobic microbes are active, and this is instrumental in the reproduction of some living organisms. Microorganisms that can live without oxygen use anaerobic respiration as a metabolic process. Unlike aerobic respiration, where oxygen serves as the main electron acceptor, anaerobic respiration uses inorganic compounds such as nitrate, sulfate, and iron as electron acceptors.

Inorganic molecules such as ammonium, nitrite, as well as reduced iron, can serve as electron donors. When these molecules are used as electron donors, they are oxidized and their electrons are transferred to electron acceptors (usually oxygen) to produce energy for cell synthesis. Microorganisms that use inorganic molecules as their primary electron donors must obtain their carbon from carbon dioxide - a process called carbon dioxide fixation. Nitrogen gas, hydrogen sulfide, and reduced forms of metals are other by-products of anaerobic respiration. In circumstances where metals are used as electron acceptors by anaerobic organisms, the metal precipitates, which decreases its concentration and mobility in groundwater (National Research Council 2003). In general, under anaerobic conditions, concentrations of electron acceptors (such as nitrate and sulfate) will decrease. Today, research is focusing on metal contaminants that convert into a precipitated form when exposed to microorganisms. Metals that undergo this procedure offer promising solutions concerning the removal of persistent metal contaminants in the environment.

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