Unlike organic compounds, metals cannot be broken down into non-toxic components. However, biological orgainisms can naturally reduce their toxicity through processes such as chelation and precipitation.
Chelates like mugineic acid readily bind to metals as shown below. Chelates are compounds that contain many ligand sites (the colored atoms) that attach to the metal. The tight bond of the chelate then renders the metal non-toxic.
+ Cu =
Chelates are used by many organisms including plants and bacteria to aid the absorption and transportation of essential metal nutrients. However, their binding properties can often be used to stabilize toxic metals as well. Thus chelates are critical compounds for bioremediation, especially in phytoremediation.
Other chelates include metallothionens and phytochelatins shown below which facilitate metal transportation in organisms. While metallothionens exist in many kinds of organisms, phytochelatins are only produced by certain species of plants.
Metallothionens:
Phytochelatin binded to Cd:
Chemical structure of a phytochelatin:
The incredible binding power of chelates allow even the most toxic metals to be absorped by plants and microbes. Much metal bioremediation research focuses on enhancing the production of these chelates or applying them artificially to increase absorbtion.
Metals can be removed from acid mine effluents as solid precipitates by anaerobic bacteria. Through redox reactions the bacteria reduce the oxidation state of the metal, which usually causes it to form a harmless solid precipitate. For instance, iron reducing bacteria like Geobacter who normally gain energy by reducing Fe (III) to Fe (II) can reduce U (VI) to U (IV) instead. The reduced form of the metal then forms a non-toxic solid precipitate.
Sulfur reducing bacteria such as Desulfovibrio (pictured below) can also be used for bioremediation, though the chemistry requires an extra step.
First the bacteria reduces the sulfate producing hydrogen sulfide:
SO4-2 + 2CH2O H+ = H2S + H2O +CO2
Hydrogen sulfide (H2S) then reacts with metals to form a sulfide that preciptates out of the effluent. Additionally, some bicarbonate (HCO3-) produced along with the CO2 in the sulfur redox reaction that acts to neutralize the acid in the effluent. With the acidity reduced and the metals now existing as a harmless precipitate the effluent is effectively remediated.
Though precipiation effectively remediates these toxic metals, the environmental conditions are not always suitable for this process. Sometimes and electron donor is missing to reduce the metal. In this case bioremediation strategies often add nutrients to provide an electron source. In some situations there is a more suitable electron acceptor such as oxygen. If this is the case bioremediation strategies might focus on creating an anaerobic environment such as a constructed wetland.
Sources:
Anderson (2003); Ibeanusi (2001); Salt (1998)