of the cell wall. Complexation is the sole process which is accountable for calcium,
magnesium, cadmium, zinc, copper and mercury storage through P. syringae. Production of various organic acids like citric acid, oxalic acid, gluonic acid, fumaric
acid, lactic acid and malic acid by microbes may chelate harmful metals and produce
metallo-organic molecules. These organic acids assist in solubilizing metal
compounds and discharging through their surfaces. Carboxyl groups present in
polysaccharides and other polymers of microbes may biosorb or complex metals.
12.5.4.4 Precipitation
Precipitation can be of two types, cellular metabolism-dependent and cellular
metabolism-independent. In the first case, metal are eliminated from solutions
usually through the active defence system of the microorganisms. They respond to
the presence of those compounds which form harmful metal and supports the
precipitation procedure. Chemical interaction of metal with the cell surface results
in cellular metabolism-independent precipitation. These different biosorption
procedures can happen at the same time.
12.5.4.5 Physical Adsorption
Metal ion of solutions attaches itself to microbial cell wall polyelectrolytes with the
help of various interactions like electrostatic, Van der Waals, covalent bonding,
redox and biomineralization to attain electroneutrality during physical adsorption.
This method is independent of metabolism as well as reversible and has great
potential due to many advantages, particularly treatment of huge level of wastewater
having low amount of contaminants (Ahluwalia and Goyal 2007; Kuroda and Ueda
2010; Nishitani et al. 2010). Cell wall potential negatively attracts the metal ions and
depends on pH in physical adsorption. pH affects metal storage in yeasts, algae and
bacteria similarly. As in yeast, at pH < 2, the metal storage is practically zero due to
the linkage of cell wall active sites to protons at low pH, which limits the metal
cation strategy causing repulsion. Due to this, pH rises, and a greater number of
groups such as acetamide chitin, structural polysaccharides of fungi, phosphate and
amino groups of nucleic acids, amino and carboxyl groups of proteins and hydroxyl
groups of polysaccharides are substituted by negative charges which further raises
the attraction between metallic cations and cell surface adsorption. Due to this, there
is a decline in metallic ions solubility which ultimately decreases bioavailability, and
precipitation takes place (Esposito et al. 2002; Chen and Wang 2008; Nishitani et al.
2010; Kuroda and Ueda 2011). Hence, pH of 4–8 is usually termed as “good” for
heavy metal biosorption for roughly every kind of biomass (Borro and Fein 2005;
Wang and Chen 2006; Machado et al. 2010). The function of extracellular polymeric
substances (EPS) excreted by bacteria for eliminating heavy metal ions through
adsorption has also been investigated (Gupta et al. 2000). According to Kuyucak and
Volesky (1988), electrostatic forces of metal ions of solutions and microbial cell
walls are responsible for biosorption of various metals like uranium, cadmium, zinc,
copper and cobalt by dead biomass of algae, fungi and yeasts. Electrostatic
attractions are also performing copper biosorption by Z. ramigera bacterium and
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
327
magnesium, cadmium, zinc, copper and mercury storage through P. syringae. Production of various organic acids like citric acid, oxalic acid, gluonic acid, fumaric
acid, lactic acid and malic acid by microbes may chelate harmful metals and produce
metallo-organic molecules. These organic acids assist in solubilizing metal
compounds and discharging through their surfaces. Carboxyl groups present in
polysaccharides and other polymers of microbes may biosorb or complex metals.
12.5.4.4 Precipitation
Precipitation can be of two types, cellular metabolism-dependent and cellular
metabolism-independent. In the first case, metal are eliminated from solutions
usually through the active defence system of the microorganisms. They respond to
the presence of those compounds which form harmful metal and supports the
precipitation procedure. Chemical interaction of metal with the cell surface results
in cellular metabolism-independent precipitation. These different biosorption
procedures can happen at the same time.
12.5.4.5 Physical Adsorption
Metal ion of solutions attaches itself to microbial cell wall polyelectrolytes with the
help of various interactions like electrostatic, Van der Waals, covalent bonding,
redox and biomineralization to attain electroneutrality during physical adsorption.
This method is independent of metabolism as well as reversible and has great
potential due to many advantages, particularly treatment of huge level of wastewater
having low amount of contaminants (Ahluwalia and Goyal 2007; Kuroda and Ueda
2010; Nishitani et al. 2010). Cell wall potential negatively attracts the metal ions and
depends on pH in physical adsorption. pH affects metal storage in yeasts, algae and
bacteria similarly. As in yeast, at pH < 2, the metal storage is practically zero due to
the linkage of cell wall active sites to protons at low pH, which limits the metal
cation strategy causing repulsion. Due to this, pH rises, and a greater number of
groups such as acetamide chitin, structural polysaccharides of fungi, phosphate and
amino groups of nucleic acids, amino and carboxyl groups of proteins and hydroxyl
groups of polysaccharides are substituted by negative charges which further raises
the attraction between metallic cations and cell surface adsorption. Due to this, there
is a decline in metallic ions solubility which ultimately decreases bioavailability, and
precipitation takes place (Esposito et al. 2002; Chen and Wang 2008; Nishitani et al.
2010; Kuroda and Ueda 2011). Hence, pH of 4–8 is usually termed as “good” for
heavy metal biosorption for roughly every kind of biomass (Borro and Fein 2005;
Wang and Chen 2006; Machado et al. 2010). The function of extracellular polymeric
substances (EPS) excreted by bacteria for eliminating heavy metal ions through
adsorption has also been investigated (Gupta et al. 2000). According to Kuyucak and
Volesky (1988), electrostatic forces of metal ions of solutions and microbial cell
walls are responsible for biosorption of various metals like uranium, cadmium, zinc,
copper and cobalt by dead biomass of algae, fungi and yeasts. Electrostatic
attractions are also performing copper biosorption by Z. ramigera bacterium and
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
327