The fungal cell wall is composed of chitin (a long straight linear homopolymer of
beta-1,4-linked N-acetylglucosamine), glucan, mannan, proteins, and other different
polymers that have carboxyl, phosphoryl, hydroxyl, amino, amine, and imidazole
functional groups at the surface of fungus (Ramrakhiani et al. 2011). The fungal cell
wall has a multilaminate, microfibrillar structure. Ultrastructural studies have
revealed two phases: (1) an outer layer consisting of glucans, mannans, or galactans
and (2) an inner microfibrillar layer, the crystalline properties of which are conferred
by the parallel arrangement of chitin chains, sometimes of cellulose (a polymer of
D-glucopyranose) chains or in certain yeasts (Sağ 2001).
The main functional groups in fungal biomass that can participate in sorption are
electronegative groups such as hydroxyl or sulfhydryl groups, anionic groups such
as carboxyl or phosphate groups, and nitrogen-containing groups such as amino
groups. Carboxyl and phosphate groups are considered to be significant binding sites
for many toxic heavy metals (Tobin et al. 1994; Viraraghavan and Srinivasan 2011).
The metal absorbed using the cell wall has been comprehensively based on two
mechanisms: absorbed directly using functional groups like phosphate, carboxyl,
amine, and phosphate diester compounds. The second uptake mechanism results
from physicochemical inorganic interactions directed by adsorption phenomena
(Kapoor and Viraraghavan 1995; Pakshirajan et al. 2013).
Another group of heavy metal-binding compounds produced by fungi is melanins, which are fungal pigments that improve survival under different environmental
stresses. Fungal melanins are found inside and/or outside the cell walls where
electron-dense deposits and granules are observed. Granules might be released into
the external medium and be named “extracellular melanin” even though this is
generally of identical composition to cell wall-associated melanin. Fungal phenolic
polymers and melanins contain phenolic units, peptides, sugars, aliphatic hydrocarbons, and unsaturated fatty acids and along these lines possess many potential metalbinding sites (White et al. 1995). Fungal melanins can effectively adsorb overwhelming heavy metals, including zinc and lead, in the bioremediation of soil.
Comparative with sand materials, dehydrated biomass of melanized fungi brought
about 50–200 times more metal absorbs in terms of surface area (Cordero et al.
2017). Chitin and chitosan present in fungal cell walls can also sequester metal ions.
Zinc was adsorbed more actively on the outer layer of Rhizopus arrhizus, especially
on its chitin/chitosan components (Viraraghavan and Srinivasan 2011).
4.1.1 Ion Exchange
Ion exchange is the substitution of an ion in a solid phase in contact with a solution
by another ion (Bahafid et al. 2017). The functional groups present on the cell
surface cooperate through the metal ion known as an ion-exchange mechanism,
for the uptake of metal ions from the environment and decontaminated sites (Meena
and Busi 2018).
17 Mycoremediation: Fungal-Based Technology for Biosorption of Heavy Metals. . .
363
beta-1,4-linked N-acetylglucosamine), glucan, mannan, proteins, and other different
polymers that have carboxyl, phosphoryl, hydroxyl, amino, amine, and imidazole
functional groups at the surface of fungus (Ramrakhiani et al. 2011). The fungal cell
wall has a multilaminate, microfibrillar structure. Ultrastructural studies have
revealed two phases: (1) an outer layer consisting of glucans, mannans, or galactans
and (2) an inner microfibrillar layer, the crystalline properties of which are conferred
by the parallel arrangement of chitin chains, sometimes of cellulose (a polymer of
D-glucopyranose) chains or in certain yeasts (Sağ 2001).
The main functional groups in fungal biomass that can participate in sorption are
electronegative groups such as hydroxyl or sulfhydryl groups, anionic groups such
as carboxyl or phosphate groups, and nitrogen-containing groups such as amino
groups. Carboxyl and phosphate groups are considered to be significant binding sites
for many toxic heavy metals (Tobin et al. 1994; Viraraghavan and Srinivasan 2011).
The metal absorbed using the cell wall has been comprehensively based on two
mechanisms: absorbed directly using functional groups like phosphate, carboxyl,
amine, and phosphate diester compounds. The second uptake mechanism results
from physicochemical inorganic interactions directed by adsorption phenomena
(Kapoor and Viraraghavan 1995; Pakshirajan et al. 2013).
Another group of heavy metal-binding compounds produced by fungi is melanins, which are fungal pigments that improve survival under different environmental
stresses. Fungal melanins are found inside and/or outside the cell walls where
electron-dense deposits and granules are observed. Granules might be released into
the external medium and be named “extracellular melanin” even though this is
generally of identical composition to cell wall-associated melanin. Fungal phenolic
polymers and melanins contain phenolic units, peptides, sugars, aliphatic hydrocarbons, and unsaturated fatty acids and along these lines possess many potential metalbinding sites (White et al. 1995). Fungal melanins can effectively adsorb overwhelming heavy metals, including zinc and lead, in the bioremediation of soil.
Comparative with sand materials, dehydrated biomass of melanized fungi brought
about 50–200 times more metal absorbs in terms of surface area (Cordero et al.
2017). Chitin and chitosan present in fungal cell walls can also sequester metal ions.
Zinc was adsorbed more actively on the outer layer of Rhizopus arrhizus, especially
on its chitin/chitosan components (Viraraghavan and Srinivasan 2011).
4.1.1 Ion Exchange
Ion exchange is the substitution of an ion in a solid phase in contact with a solution
by another ion (Bahafid et al. 2017). The functional groups present on the cell
surface cooperate through the metal ion known as an ion-exchange mechanism,
for the uptake of metal ions from the environment and decontaminated sites (Meena
and Busi 2018).
17 Mycoremediation: Fungal-Based Technology for Biosorption of Heavy Metals. . .
363