*1.1.1 Bioattenuation or natural attenuation*

There are a lot of different physical, chemical, and biological processes commonly termed bioattenuation, which make pollutants smaller in terms of their size and toxicity as well as how much of them there are. Some examples of these processes are sorption, volatilization, chemical or biological stabilization, and the transformation of contaminants. This entails removing pollutant concentrations from the surrounding through biological methods or perhaps incorporating (oxic and anoxic biodegradation, plant and animal sorption), physical occurrences (changes in weather conditions, dispersion, dilution, diffusion, volatilization, sorption/desorption), and chemical reactions (ion exchange, complexation, abiotic change) [6–8]. For instance, natural biodegradation and biotransformation are incorporated within the broader notion of common restriction [9, 10]. At the point when the site is contaminated with chemicals, the environment acts in 4 different approaches to facilitate remediation [11]:


• Certain chemicals, such as oil and solvents, can disappear, hence, they can transform from liquids to gases within the soil or groundwater. As a result, if these gases reach the earth surface via the air, they may be pulverized by sunlight.

Additionally, if natural attenuation is insufficiently rapid or complete, bioremediation will be accelerated or augmented via biostimulation, bioaugmentation, bioventing, or biopile [11, 12].

#### *1.1.2 Biostimulation*

This bioremediation approach invigorates the activity of native microbes by adjusting the environmental parameters or the introduction of nutrients [11, 13]. This is carried out with the incentive of natural or normally prevailing parasites or microbial communities [7, 11, 13]. Successive steps involve providing manures, development enhancements and minor elements. Also, by giving other natural prerequisites including pH, temperature and oxygen to enhance their digestion rate and degradation pathway [10, 12]. Similarly, the presence of pollutants even in small quantities can act as a stimulant by spinning for bioremediation proteins. Typically, this type of deterioration is followed by the provision of organic or inorganic nutrients and oxygen to promote the metabolism of native microbes for effective remediation [6]. These nutrients are the fundamental building blocks of life, enabling microorganisms to synthesize vital components such as enzymes, energy, and cell biomass required to degrade the toxin [6, 14]. However, nitrogen, phosphorous and carbon are significantly required to enhance metabolism.

#### *1.1.3 Bioaugmentation*

This procedure entails sequentially adding contaminant-degrading microbes (inherent/non-native/genetically modified) to improve the biodegradative efficiency of the native microbial community in the polluted site [8, 11]. Thus, to rapidly grow the natural microbial population and accelerate breakdown at the pollutant's location. Microorganisms that predominate in polluted sites on a global scale, may surely change significant amounts of harmful substances into non-poisonous structures. This process converts pollutants to by-products like carbon (IV) oxide and water, as well as metabolic intermediates that serve as critical nutrients for cell development [15, 16]. Microorganisms can also be isolated from the remediation environment, cultured autonomously, genetically engineered, and then reintroduced to the site [8, 11]. For persuade, all basic microbes are prevalent in locales where soil and groundwater are polluted with chlorinated ethenes, for example, tetrachloroethylene and trichloroethylene [7, 8, 11]. These are employed to facilitate the effective removal and conversion of these pollutants to non-poisonous ethylene and chloride by in situ microbes [10].

Additionally, genetically modified microbes have been shown to degrade a broad range of environmental contaminants effectively. Since the metabolic pathway can be altered to produce less puzzling and harmless end products [8, 17]. Genetically engineered microorganisms (GEM) have shown viability in bioremediation of soil, groundwater and activated sludge, proving effective degradation abilities of extensive integration of chemical and physical contaminations. GEMs have better enzyme abilities, which makes them better at breaking down a wide spectrum of aromatic hydrocarbons and making the soil more fertile [14, 18]. There are several types of hydrocarbon-degrading microorganisms that include the genera *Alanivorax* and

*Bacillus*, *Pseudomonas* and *Bravibacillus*, *Acinetobacter* and *Methylobacterium*, and *Candidauts* as well. Biodegradation of the benzene, toluene, ethylbenzene, and xylene (BTEX) isomers may be discovered in situ using the polymerase chain reaction, and nucleotide sequence analysis of BTEX degraders in the environment [7–9, 14, 18].

#### *1.1.4 Bioventing*

It is the practice of venting oxygen through the soil to encourage the development of natural or injected microbes and fungus in the soil by supplying oxygen to the soil microbes, which has been termed as bioventing [8, 11, 14]. The use of low air flowrates to supply sufficient oxygen to sustain microbial movement has long been a typical practice in aerobic degradation of substances, and it has been for many years. For example, several scientists have demonstrated that bioremediation of oil-contaminated soil utilizing bioventing may be achieved with reasonable success [19]. Consequently, petroleum residuals and their by-products are biodegraded, and volatile organic compounds, when destroyed, release vapors that slowly permeate through the biologically dynamic soil environment.

#### *1.1.5 Biopiles*

Biopile, also known as biocells, bioheaps, biomounds and composts piles are employed to minimize the toxicity of total petroleum hydrocarbon constituents via microbial respiration. Biopiles are an ex-situ bioremediation technology that consists of piling polluted soil onto a compost pile (biopiles) or cells (biocells) or mounds (biomounds) or heaps (bioheap) and stimulating oxic metabolism in the soil via aeration or introduction of minerals or nutrients, bulking agents, and subsequently confining it in a treatment bed with polyethylene material to avoid evaporation, surface runoff, and volatile emissions. Biopiles treatments can transform pollutants into low-toxic by-products through biological processes by utilizing already existing microorganisms to breakdown fuels and oils into carbon dioxide and water.

The biopile technology is made up of commercial roll-off dumpsters or containers that have been turned into fully contained bioremediation units. The biopile units have an impermeable liner to decrease the possibility of leachate movement to the subsurface ecosystem. Excavated soils are combined with soil additives and placed on a treatment area with leachate collecting devices and some type of aeration to maximize and regulate the rate of biodegradation. Air is introduced to the biopile mechanism of piping and pumps, which either power air into the heap under a specific tension or draw air through the heap under a negative tension [8, 20]. Microbial movement, for instance, can boost the adsorption and degradability of petroleum pollutants during funneling and siphoning operations. Biopiles, such as biocells, bioheaps, biomounds, and compost, might alleviate public concern about excavated soil contaminated by vigorously remediable hydrocarbons [8, 13, 19].

#### *1.1.6 Phytoremediation*

Utilizing plants for bioremediation is highly dependent on their ability to break down certain pollutants [21–24]. Phytoremediation is the process of utilizing plants to degrade, eliminate, or convert contaminants to less hazardous chemicals [25]. Even though plants have been used for soil purification for centuries, scientists have contributed to its advancement and expanded its scope of application throughout the
