5.5 Bioremediation – The Emerging Sustainable Strategy

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Extracellular Precipitation Microbes are able to produce a wide variety of extracellu-

lar, specific and non-specific substances, which can bind to the heavy metal in the

ambient environment by converting them to less toxic nature. Extracellular poly-

meric substances (EPSs) produced by the biofilm-producing bacteria play a crucial

role in heavy metal removal. EPS comprising a mixture of polysaccharide, proteins,

mucopolysaccharide, and nucleic acids are able to bind with the heavy metals and

can effectively remove metals from the environment.

Adsorption on the Cell Surface Microbes can also sequester toxic heavy metals by cell

surface adsorption mechanism. It is a rapid physicochemical process involving both

live and dead cell biomass and has been considered as an effective biotechnological

process for the removal and/or recovery of toxic heavy metals. In this process, metal

ions get quickly attached to the cell within a few minutes. Several mechanisms such

as ion exchange, chelation, and diffusion through cell walls and membranes con-

tribute to bioadsorption process [1, 5].

Volatilization Microbes can often convert metals to its less toxic and less soluble form

by volatilization process. Volatilization can be achieved by a variety of processes such

as reduction, oxidation, methylation, and demethylation of the compounds.

Metal Efflux A wide variety of metal efflux transport systems are present in

microbes that are involved in the excretion of metals out of the cell. Most of them

are non-specific being involved in efflux of a wide range of molecules. These

transporters are originally identified as multidrug transporters but are also involved

in transportation of heavy metals, organic acids, and many other non-specific

compounds. The ABC transporters are involved in the efflux of Mn in Streptococcus

gordonii [30]. The P type ATPase is another efflux protein associated with the efflux

of Cd in Staphylococcus aureus.

5.5.2

Plant-Assisted Bioremediation (Phytoremediation)

Since contamination of soils and waters by toxic heavy metals is a serious environ-

mental problem, therefore effective remediation methods are necessary. Phytoreme-

diation is the use of plants and associated soil microbes to reduce the concentrations

or toxic effects of ambient heavy metal contamination. It can be used for the removal

of heavy metals and as well as for organic pollutants. It is a novel, cost-effective,

efficient, environment and eco-friendly, in situ applicable and solar-driven remedia-

tion strategy. The method is reported to be economically sustainable than traditional

physical and chemical methods, which are categorized into phytoextraction, phy-

todegradation, phytostabilization, phytovolatilization, rhizodegradation, rhizofiltra-

tion, and other methods (Table 5.5). The effects of phytoremediation were previously

reported to be observed in short time.

The plants, categorized as metal hyperaccumulators and wild, are able to remove

heavy metals many times higher compared to the ones that are cultivated. Explo-

ration of hyperaccumulators as a potential agent of phytoremediation is the most