Structural Biochemistry/Bioremediation of Metals

Bioremediation is a technology that utilizes microorganisms to reduce, eliminate, contain, or transform pollutants present in sediment, water, and air. The microorganisms used to perform bioremediation are called bioremediators.

Metals are significant natural components of all soils, where their presence in the mineral fraction constitutes a pool of potentially mobile metal species, many essential nutrients for plants and microbes, and important solid components that can have a fundamental effect on soil biogeochemical processes; for example: clays, minerals, iron and manganese oxides. Metals are also present in organic fractions--frequently in bound forms--with some metal recycling occurring as a result of organic matter degradation. The aqueous phase provides a mobile medium for metal transfer and circulation through the soil and to organisms, and to the aquatic environment. Metals such as mercury, lead, and arsenic, potentially can be toxic to the kidneys, decrease mental capabilities, and cause weakness, headaches, abdominal cramps, diarrhea, and anemia.

Mobility of Metal Contaminants
The immobility of metals is primarily caused by reactions that cause metals to precipitate or chemical reactions that keep metals in a solid phase. Chemical and physical properties affect the mobility of metals in soils and groundwater. Under acidic conditions (pH ranging between 4.0-8.5), metal cations are mobile while anions tend to transform to oxide minerals. At high pH levels, cations adsorb into mineral surfaces and metal anions are mobilized. Hydrous metal oxides of iron, aluminum, and manganese can affect metal concentrations because these minerals can subtract cations and anions.

When a microorganism oxidizes or reduces species, this reaction causes metals to precipitate. Mercury is an example of a metal that can be precipitated. The process begins when mercury (Hg2+) is reduced to mercuric sulfide causing mercury to transform to a precipitated form. Chromium is another metal that can convert to a precipitated form with the use of microorganisms. The process involves the reduction of hexavalent chromium (Cr 6+) to trivalent chromium (Cr 3+), which then can precipitate to chromium oxides, sulfides, or phosphates. Research today is focusing on other metal and radioactive contaminants that can undergo precipitation processes.