12.2.1 Traditional Methods for Treating Environments
Contaminated by Heavy Metals
Environments contaminated by heavy metals are removed through means of conventional technologies based on physico-chemical principles and are ineffective and
costly. In one of the processes used to eliminate metals from aqueous solutions to
increase pH, reagents are added, which change metals from a soluble to insoluble
form (hydroxides) resulting in their precipitation. This method generates lots of mud
in the final wastewater with high concentrations of metals which is difficult to
remove. While other complicated methods have single or several stages like (1) precipitation with hydroxides, carbonates or sulphides; (2) redox chemistry; (3) sorption
(adsorption with activated carbon/ion exchange); (4) utilization of membranes
(ultrafiltration, electrodialysis and RO, i.e. reverse osmosis); (5) recovery of
electrolytes; (6) evaporation; (7) liquid-liquid extraction and (8) electrodeposition.
But, these days, a new technology bioremediation has emerged as a substitute
because of its simplicity, effectiveness and low cost (Goyal et al. 2003; Tabak
et al. 2005; Hameed 2006; Machado et al. 2008; Wang and Chen 2009; Elen Aquino
Perpetuo et al. 2011).
Bioremediation utilizes plants or microorganisms, viable or non-viable, natural or
genetically engineered to remediate environments contaminated with organic
molecules which are difficult to degrade (xenobiotics) and to lessen the damage of
harmful heavy metals, by changing them into non-toxic or less toxic elements,
thereby forming harmless products (Cunningham and Berti 1993; Davis et al.
2003; Prasad and Aranda 2018). Various strategies can be applied to improve the
bioremediation which can be dependent on the nature of the contaminated environment. One such method, biostimulation, encourages the growth of indigenous
microorganisms through the addition of nutrients in the contaminated site. As a
result, the rate of biodegradation/bioremediation can be increased. In another
method, i.e. bioaugmentation or bio addition, microbial populations are added to
local, outside or genetically modified organisms (GMO), at places with shortage of
native microbes or where they are not able for competition.
Microbes have developed various methods to overcome the damages of metals
and metalloids, through storage and resistance or by limiting their bioavailability or
toxicity through biomethylation as well as transformation (Thakare et al. 2021). The
high level of waterborne heavy metals is a big environmental problem in aquatic
ecosystems globally. Many of these heavy metals move to groundwater, and others
are stored in seafood or in plants and are the main toxic source for humans. The
rhizosphere has increased microbial activity which increases storage, transformation,
degradation and biomethylation of Se with other trace elements. Microbes residing
in the rhizosphere easily remove harmful heavy metals or metalloids coming through
wastewaters by biosorption, sulphide precipitation, and biotransformation (reduction, volatilization).
A lot of microorganisms like bacteria, fungi, yeasts and algae have been studied
for utilizing in bioremediation processes, and some of them are used as heavy metal
biosorbents (Deeb and Altalhi 2009; Dobson and Burgess 2007; Dowdy and Volk
318
N. Srivastava
Contaminated by Heavy Metals
Environments contaminated by heavy metals are removed through means of conventional technologies based on physico-chemical principles and are ineffective and
costly. In one of the processes used to eliminate metals from aqueous solutions to
increase pH, reagents are added, which change metals from a soluble to insoluble
form (hydroxides) resulting in their precipitation. This method generates lots of mud
in the final wastewater with high concentrations of metals which is difficult to
remove. While other complicated methods have single or several stages like (1) precipitation with hydroxides, carbonates or sulphides; (2) redox chemistry; (3) sorption
(adsorption with activated carbon/ion exchange); (4) utilization of membranes
(ultrafiltration, electrodialysis and RO, i.e. reverse osmosis); (5) recovery of
electrolytes; (6) evaporation; (7) liquid-liquid extraction and (8) electrodeposition.
But, these days, a new technology bioremediation has emerged as a substitute
because of its simplicity, effectiveness and low cost (Goyal et al. 2003; Tabak
et al. 2005; Hameed 2006; Machado et al. 2008; Wang and Chen 2009; Elen Aquino
Perpetuo et al. 2011).
Bioremediation utilizes plants or microorganisms, viable or non-viable, natural or
genetically engineered to remediate environments contaminated with organic
molecules which are difficult to degrade (xenobiotics) and to lessen the damage of
harmful heavy metals, by changing them into non-toxic or less toxic elements,
thereby forming harmless products (Cunningham and Berti 1993; Davis et al.
2003; Prasad and Aranda 2018). Various strategies can be applied to improve the
bioremediation which can be dependent on the nature of the contaminated environment. One such method, biostimulation, encourages the growth of indigenous
microorganisms through the addition of nutrients in the contaminated site. As a
result, the rate of biodegradation/bioremediation can be increased. In another
method, i.e. bioaugmentation or bio addition, microbial populations are added to
local, outside or genetically modified organisms (GMO), at places with shortage of
native microbes or where they are not able for competition.
Microbes have developed various methods to overcome the damages of metals
and metalloids, through storage and resistance or by limiting their bioavailability or
toxicity through biomethylation as well as transformation (Thakare et al. 2021). The
high level of waterborne heavy metals is a big environmental problem in aquatic
ecosystems globally. Many of these heavy metals move to groundwater, and others
are stored in seafood or in plants and are the main toxic source for humans. The
rhizosphere has increased microbial activity which increases storage, transformation,
degradation and biomethylation of Se with other trace elements. Microbes residing
in the rhizosphere easily remove harmful heavy metals or metalloids coming through
wastewaters by biosorption, sulphide precipitation, and biotransformation (reduction, volatilization).
A lot of microorganisms like bacteria, fungi, yeasts and algae have been studied
for utilizing in bioremediation processes, and some of them are used as heavy metal
biosorbents (Deeb and Altalhi 2009; Dobson and Burgess 2007; Dowdy and Volk
318
N. Srivastava