Biological Removal Biotransformation and Biosorption of Metal Ions from Contaminated Wastewater

Whereas solid organic and inorganic material in wastewater or sludge can be removed by sedimentation, soluble organic pollutants and xenobiotics should be eliminated from the aqueous environment by microbial mineralization or anaerobic degradation to gaseous products, with a varying portion (5%-50%) being used as substrates for bacterial growth. Most of the inorganic components present in wastewater are soluble and are ionized. Trace amounts of many cations (e.g., Na+, K+, Ca2+, Fe2+, Ni2+, Co2+, Mn2+, Zn2+, etc.) and anions (e.g., PO3-, Cl-, S2-) are essential micronutrients for bacterial growth. Other cations such as ammonia may also be required for bacterial growth, but the surplus amount must be oxidized to nitrate, and the nitrate denitrified to gaseous nitrogen, for N elimination from wastewater. Under anaerobic conditions sulfate is reduced to sulfide, low amounts of which are required for growth of bacteria. The sulfide not required for growth is toxic for bacteria if present as H2S in high concentrations. In anaerobic reactors at a slightly alkaline pH, most of the sulfide is precipitated as heavy metal sulfides; then it is harmless to microorganisms and their environment. To support detoxification, heavy metal ions may also be precipitated chemically.

Metal ion contaminants in wastewater can be removed by microorganisms by either a direct or indirect influence on the redox state of the metal ions or through biosorption of metal ions on the cell surface (Lovley and Coates, 1997). Some microorganisms have also developed resistance mechanisms against toxic metals by changing the oxidation state without supporting anaerobic growth. Certain bacteria, yeasts, fungi, and algae can actively accumulate intracellular metal ions against a gradient. The process of bioaccumulation of metal ions depends on living, metabo-lically active cells, whereas biosorption is a passive, energy-independent process that can be mediated also by inactive cell material. Biosorption of metal ions includes mechanisms such as ion exchange, chelation, matrix entrapment, and surface sorption (Unz and Shuttleworth, 1996). After biosorption or active removal of metal ions from wastewater or contaminated soil, the heavy metal-containing biomass must be separated and incinerated or regenerated by desorption or remobilization of the metals. As an example of successful biosorption, wastewater from the galvanizing industry that contained 29 mg L-1 Zn and 10.5 mg L-1 Fe was purified in a three-stage stirred-reactor cascade. With 15 g biomass, 7.5 L of wastewater was decontaminated (Brauckmann, 1997). The removal of metal ions by biologically catalyzed changes in the redox state is an alternative. The altered speciation (valence status) of metals can lead to precipitation, solubilization, or volatilization of the metal ions.

Many bacteria use metal ions as electron acceptors for anaerobic respiration. Examples are the reduction of Fe3+, Cr6+, Mn4+, Se6+, As5+, Hg2+, Pb2+, or U6+ (Table 1.6). The dissimilatory metal-reducing bacteria can use H2 or organic pollutants (e.g., xenobiotics) as electron donors and are capable of simultaneously removing organic and inorganic contaminants. Metal ions are reduced and precipitated by sulfide that is generated by sulfate-reducing bacteria.

Solubilization of most heavy metal precipitates is favored at acid pH, which is the favorable pH for soil or sludge decontamination, whereas an alkaline pH is favorable for precipitation of heavy metal ions to decontaminate wastewater.

An example of the formation of precipitates or soluble compounds at different re-dox states occur in ferrous and manganese compounds. Under aerobic conditions Fe(III) (ferric iron) and Mn(IV) ions form insoluble Fe(III) and Mn(IV) oxides or hydroxides, but under anaerobic conditions they form soluble Fe(II) compounds (ferrous iron) or Mn(II) compounds. Most organisms that grow with energy conserved during reduction of Fe(III) or Mn(IV) are members of the Geobacteriaceae. Transfer of electrons from the terminal reductase, localized in the outer membrane or at the

Table 1.6 Metals as electron acceptors for anaerobic respiration.

Reaction

Microorganism

Reference

2 Fe3+ + H2 o 2 Fe2+ + 2 H+

Geobacter metallireducens

Lovley and Lonergan (1990)

Pelobacter carbinolicus

Lovley et al. (1995)

Mn4+ + H2 o Mn2+ + 2 H+

Geobacter metallireducens

Lovley (1991)

mixed culture

Langenhoff et al. (1997)

2 Cr6+ + 3 H2 o 2 Cr3+ + 6 H+

Desulfovibrio vulgaris

Lovley and Phillips (1994)

Bacillus strain QC1-2

Campos et al. (1995)

Se6+ + H2 o Se4+ + 2 H+

Thauera selenatis

Macy et al. (1993)

strains SES-1; SES-3

Se6+ + 3 H2 o Se0 + 6 H+

Oremland et al. (1989)

Te4+ + 2 H2 o Te0 + 4 H+

Schizosaccharomyces pombe

Smith (1974)

Pb2+ + H2 o Pb0 + 2 H+

Pseudomonas maltophila

Lovley (1995)

As5+ + H2 o As3+ + 2 H+

Geospirillum arsenophilus

Ahmann et al. (1994)

Hg2+ + H2 o Hg0 + 2 H+

Escherichia coli

Robinson and Tuovinen (1984)

Thiobacillus ferrooxidans

U6+ + H2 o U4+ + 2 H+

Shewanella putrefaciens

Lovley et al. (1991)

cell surface, to the insoluble Fe(III) or Mn(IV) oxides outside the cells can proceed either in a direct way (contact between the oxides and the cells) or by 'soluble electron shuttles' (e.g., by humic substances) between the metal-reducing microorganism and the mineral (Lloyd, 2003). Since Fe is an essential element for microorganisms, aerobic bacteria must excrete siderophores, which bind Fe3+ to their phenolate or hydroxamate moiety and supply the cells with soluble Fe2+. To accelerate Fe3+ reduction in biotechnological processes, the chelator nitrilotriacetic acid can be added. In addition to their use in synthesis of cell components (e.g., cytochromes, ferredox-in, etc.), Fe2+ salts can be electron donors for nitrate reduction (Straub et al., 1996).

The reduction of Hg2+ to metallic Hg by Escherichia coli or Thiobacillus ferrooxi-dans facilitates Hg separation and prevents methylation reactions under aerobic or sulfate-reducing conditions. The mechanism includes nonenzymatic transfer of methyl groups from methylcobalamin to Hg2+ to form methyl mercury or dimethyl mercury, which are both neurotoxins and become enriched in the food chain. Selenium and arsenic can be transformed microbiologically by methylation to dimethyl selenide or to di- or trimethyl arsine in a volatile form. Methylated arsenic compounds are less toxic than nonmethylated arsenic compounds (White et al., 1997).

Changing the redox state of natural and anthropogenic radionuclides by metal-reducing microorganisms offers a possibility to control their solubility and mobility by converting, e.g., U6+ to U4+, Pu5+ to Pu4+, or Np5+ to Np4+. The tetravalent metals can be removed by chelators (e.g., EDTA) or by immobilization onto biomass from the contaminated environment (Lloyd, 2003).

In contrast to bacteria, fungi are capable of leaching soluble as well as insoluble metal salts, because they excrete organic acids such as citric acid, fumaric acid, lactic acid, gluconic acid, oxalic acid, or malic acid, which dissolve metal salts and form complexes with the metal ions. The leaching efficiency depends on the soil microflora. Some soil microorganisms seem to be able to degrade the carbon skeleton of the metal-organic complex and thus immobilize the metal ions again (Brynhildsen and Rosswall, 1997).

Mixed bacterial cultures or Wolinella succinogenes use perchlorate or chlorate as electron acceptors for respiration (Wallace et al., 1996; van Ginkel et al., 1995) and thus detoxify these chemicals.

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