Environmental Engineering Reference
In-Depth Information
bacteria under aerobic and acidic conditions (pH 4) at temperature between 15 and
55 C, depending on strain promote the microbiological leaching of metals such as
copper, silver, uranium, and zinc by the oxidation followed by electron transfer to
oxygen inducing the metal solubilization (Gadd
2004
; Kavamura and Esposito
2010
). Leaching can be performed directly by oxidation of metal sulfides to
produce sulfuric acid, which then can desorb the metals on the soil by substitution
of protons. Indirect leaching involves conversion of Fe
2+
to Fe
3+
, which in turn
oxidizes sulfur minerals to Fe
2+
producing acidity. Several options are available
for bioleaching including heap leaching, bioslurry reactors, and in situ processes.
Anoxic sediments are more suitable for treatment since the bacteria can solubilize
the metal compounds without substantially decreasing the pH. Copper, zinc,
uranium, and gold have been removed by Thiobacillus species in biohydromet-
allurgical processes (Karavaiko et al.
1988
). Another fungus Aspergillus niger,
which can produce citric and gluconic acids has a potential for remediation of
metal contaminated soil. They can act as acids (pH 3.5) and chelating agents for
the removal of metals such as copper from oxide mining residues (Mulligan et al.
1999b
). Mercury and cadmium can be oxidized, while arsenic and iron can be
reduced by microorganism. Cr (VI) can be oxidized to Cr (III) that is less mobile
and toxic. Bacteria such as Bacillus subtilis and sulfate reducing bacteria in the
presence of sulfur can perform this reaction.
17.4.3 Biosorption
Biosorption is the process of binding of metals to cell surfaces (Vidali
2001
). It is a
biological treatment method, which involves the adsorption of metals into biomass
of algal or bacterial cells that can be dead or alive. With the use of Trichoderma
reesei adsorption and desorption of cadmium and copper were analyzed by Kim
et al. (
2003
). Pb(II) and Cd (II) were recovered employing biomass of Amanita
rubescens from aqueous solution using the ability of the macrofungus by Sari and
Tuzen (
2009
). If large scale, inexpensive production techniques for the biomass
are developed, this heavy metal treatment is promising (Hazardous waste con-
sultant
1996
).This method is only applicable for low concentrations of metals in
water. Therefore, the cells could potentially be placed in permeable barriers for
adsorption of metals in ground water.
17.4.4 Biodegradable Biosurfactants
Surfactants are a class of natural chemicals, which are amphiphilic in nature, that
promote the solubilization and emulsification of various types of organic and
inorganic contaminants. Bio-surfactants are produced by bacteria and yeast. Bio-
degradable biosurfactants (surfactin, rhamnolipid and sophorolipid) are used to
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