*Bioremediation of Hazardous Wastes DOI: http://dx.doi.org/10.5772/intechopen.102458*

years [7, 11, 13, 14, 17, 18]. This involves the removal of metals, pesticides, solvents, explosives, and raw petroleum, as well as a variety of other pollutants from soils, water (surface and subsurface), and vaporous contaminants [7, 11, 14]. When the plants have accumulated enough toxins, they are harvested and disposed of. **Figure 1** shows a graphical presentation of different types of phytoremediation as each mechanism is explained as follows:



#### *1.1.7 Combinative bioremediation*

This is when two or more bioremediation methods work together to remove contaminants from the environment. This kind of bioremediation technique can be effectively applied in a multi-contaminated environment. The combinative strategy most likely to be suitable and effective in boosting bioremediation of bauxite residue is a combination of bioaugmentation (incorporation of inocula) [8, 11] and biostimulation (introduction of nutrients to enhance the activity of microorganisms) of the indigenous community in bauxite residue [11, 13].

In this scenario, for instance, biostimulation using organic or inorganic compounds can be applied as the first or basic treatment while bioventing or bioaugmentation using engineered microbes can be applied subsequently as a secondary or tertiary treatment to facilitate the removal or degradation of recalcitrant compounds. Combinations of bioaugmentation and biostimulation have also proven effective, albeit they do not always show significant improvements over bioaugmentation alone. Given the nearly consistent advancement seen with bioaugmentation technology, it is anticipated that bioaugmentation will improve on the outcomes obtained so far with biostimulation for bauxite waste cleanup (provided an appropriate choice of the microbes and adequate trials are prioritized). Based on the simplicity of obtaining and introducing the inoculum, the most suited approach for future research and field trials is combinative bioremediation using biostimulation and bioaugmentation technology.

#### **1.2 Bioremediation mechanisms for contaminant removal**

Several bioremediation mechanisms for reducing or oxidizing contaminants have been discovered over time, such as adsorption, physio-biochemical (biosorption and bioaccumulation) bioleaching, biotransformation, biomineralization, and molecular mechanisms [7, 11].

#### *1.2.1 Adsorption bioremediation mechanism*

Environmental pollutants (both organic and inorganic) can be absorbed by microorganisms at specific sites in their cell structure that do not require the dissipation of energy. There are many various kinds of chemicals connected with bacterial cell walls,

#### *Bioremediation of Hazardous Wastes DOI: http://dx.doi.org/10.5772/intechopen.102458*

but their extracellular polymeric substances (EPS) are of particular importance since they have been shown to have significant effects on corrosive base characteristics and metal adsorption [10, 26]. Several studies on the metal binding behavior of EPS have revealed that it has a remarkable capacity to absorb complex metals by a variety of processes that combine ion exchange and micro-precipitation of metals [10, 13]. Bioremediation research and application are still limited in the present scenario due to a lack of understanding of the genetic traits and genome-level properties of the organisms used in metal adsorption, the metabolic route, and the kinetics of metal adsorption [7].

#### *1.2.2 Physio-biochemical mechanism*

In microscopic organisms, inhibition is advanced through two mechanisms: detoxifying (changing the detrimental metal's state and rendering it inaccessible) and dynamic efflux (siphoning poisonous heavy metals from cells) [7, 9]. In wastewater or soil, the fundamental redox (oxidation and reduction) reaction occurs between hazardous metals and microorganisms. Additionally, microbes oxidize heavy metals, causing them to lose electrons, which are recognized by active electron acceptors (nitrate, sulphate and ferric oxides) [26]. Additionally, the biosorption process, which consists of a biosorbent's increased affinity for sorbate (metal ions), is repeated till a balance between the two components is established [18, 26]. For instance, *Saccharomyces cerevisiaeacts* as a biosorbent for Zn (II) and Cd (II) removal via the ion exchange process [10, 26]. *Cunninghamella elegans* is also reported as a potential sorbent against substantial metals delivered by textile wastewater [12, 17].

Bioaccumulation is a term referring to the combination of active and passive techniques of hazardous metal bioremediation. Additionally, bioremediation may entail aerobic or anaerobic microbial activity [10, 12, 13]. Aerobic degradation frequently involves the addition of oxygen atoms to the reactions via monooxygenases, dioxygenases, hydroxylases, oxidative dehalogenases, or chemically active oxygen molecules produced via catalysts including ligninases or peroxidases [10–13]. Anaerobic contaminant corruptions comprise initial enactment reactions followed by oxidative degradation with the assistance of anaerobic electron acceptors. The act of Immobilization refers to the process of reducing the activation of significant metals in a polluted environment by modifying their physical or synthetic state [7, 12]. Microbes muster metals from polluted sites through leaching, filtering, chelation, methylation and redox transformation of harmful metals [12, 17]. Since significant metals cannot be entirely eliminated, the cycle modifies their oxidation state or organic complex to make them more soluble, less poisonous and precipitated [9, 14].

#### *1.2.3 Bioleaching*

In bioleaching, naturally occurring microorganisms such as bacteria and fungi solubilize metal sulphides and oxides from ores and secondary wastes. Adsorption, ion exchange, membrane separation, and selective precipitation are some of the processes used to purify solubilized metals. It is a cost-effective and environmentally beneficial technique because it consumes less energy and produces no hazardous gases. It has been applied to leach metals from low-grade ores, and it now provides a substantial global business in the extraction of metals like copper, cobalt, gold, nickel, uranium, zinc, and other elements [27].

#### *1.2.4 Biotransformation*

This is the procedure for altering the structure of a chemical substance to produce a molecule with higher polarity. Moreover, this metal-microbe interaction process converts hazardous metal and organic chemicals into a less poisonous form. This mechanism has emerged in microorganisms to assist them in adjusting to variations in their surroundings. Bacterial cells have a significant surface-volume ratio, a rapid pace of proliferation, a rapid rate of metabolic activities, and are easy to keep sterile [27]. As a result, they are perfect for biotransformation. Various methods, such as condensing and hydrolyses, forming new carbon bonds, isomerization, inserting functional groups, and oxidation, reduction, and methylation, can be used to attain this objective. Metals may be volatized, reducing their lethal nature, as a result of these interactions.

#### *1.2.5 Biomineralization*

Biomineralization refers to the mechanisms by which microbes produce minerals, and it can lead to metal extraction from solution, which can be used for decontamination and biorecovery. Dead biota and related products may also serve as a model for mineral deposition, with physicochemical parameters determining whether the process is reversible or not. There are several prevalent microbe-precipitated biominerals with unique chemical features such as high metal sorption capacities and redox catalysis. However, some biominerals can be deposited at nanoscale dimensions, resulting in additional physical, chemical, and biological features that can be used in practical applications [28].

#### *1.2.6 Molecular mechanisms involved in bioremediation process*

Different components of genetically altered bacteria, such as Deinococcus geothemalis, are active in the removal of heavy metals [9, 14, 18]. Hg2+ reduction has been recorded at high temperatures as a result of the expression of meroperon from *Escherichia coli* coded for Hg2+ reduction [18]. Two distinct components for Hg reduction by microscopic organisms ((*Klebsiella pneumonia* M426) are mercury volatilization by the decrease of Hg (II) to Hg (0) and mercury precipitation as insoluble Hg attributed to unstable thiol (H2S) [7, 18]. Genetic of *Deinococcus radiodurans* (radiation tolerant bacterium) which usually decreases Cr (IV) to Cr (III) has been done for toluene (fuel hydrocarbon) reduction by cloned genes of *tod* and *xyl* operons of *Pseudomonas putida* [9, 11, 18]. Microbial metabolites including metal-bound coenzymes and siderophores are usually part of the degradation pathway.
