7. Bioprecipitation of Metals and Metalloids

Heavy metals are toxic, carcinogenic and unlike organic contaminants are not biodegradable, and thus accumulate in organisms. Approximately 60% of the polluted areas in the world, suffer from the harmful effects of metals including Cd, Ni, Cu, Pb, Zn, Hg and Co. Mining, fertilizer, tanneries, paper, batteries and electroplating industries are the main sources of heavy metal containing waters. For example, in China, the annual amount of heavy metal containing electroplating industry wastewater has exceeded 4 billion tons. Up to 1000 mg/kg heavy metal concentration in sediments has been reported due to repeated discharges. We reviewed the sources of heavy metal containing water and metal precipitation techniques including metal sulfide, hydroxide, ferrihydrite, geothite, jarosite as well as schwertmannite precipitation. Metal sulfide precipitation relies on the biological generation of H₂S and near complete metal removal is possible with both organic (i.e. ethanol) and inorganic (i.e. hydrogen) electron donors. The utilization of soluble electron donors provides high rate and dense metal precipitates with metal recovery of over 80% (usually 100%). Additionally, metals can be recovered separately as various metal sulfides by adjusting pH. Biological oxidation/reduction processes facilitate the formation of insoluble metal precipitates for uranium (U⁶⁺ to U⁴⁺); chromium (Cr⁶⁺ to Cr³⁺) or iron (Fe²⁺ to Fe³⁺). The major points extracted from the study are: (1) metal sulfide precipitation is fast, results in low residual metal concentrations and allows for selective recovery of various metals with a wide variety of different reactor configurations, (2) high rate biological metal recovery is possible with cultures which use metals as electron acceptors which eliminates the drawbacks such as chemical costs and huge sludge volume production in chemical reduction, (3) animal manure, leaf mulch, sawdust, wood chips, sewage sludge, cellulose could be used in passive treatment systems and therefore operational costs could be optimized, (4) some heavy metals can be precipitated through biological oxidation (i.e. Fe²⁺ to Fe³⁺) and (5) possible iron precipitates include hematite (Fe₂O₃); geothite (FeOOH); ferric hydroxide Fe(OH)₃; jarosite Fe₃(SO₄)₂(OH)₆; schwertmannite Fe₁₆O₁₆(SO₄)₂(OH)12.n(H₂O) and scorodite (FeAsO₄.2H₂O).