**7. Bioavailability**

Bioavailability refers to the fraction of a chemical that can be taken up or transformed by living organisms from the surrounding bio-influenced zone where organism mediated biochemical changes occur [83, 84]. The success of any rhizoremediation process depends on the bioavail‐ ability of the specific pollutant and root microbial modifications of their solubility, physio‐ chemical properties of the pollutant, soil properties, environmental conditions, biological activity and chemical speciation in the rhizosphere [59, 67]. Bio surfactants increase the bioavailability of hydrocarbons resulting in enhanced growth and degradation of contami‐ nants by hydrocarbon degrading bacteria present in polluted soil [85].

*Pseudomonas putida* is a good candidate for metabolic engineering and genetic manipulation applications for expression of genes encoding several degradative enzymes [88]. Therefore, a *P. putida* strain was engineered to increase the efficiency of degradation of naphthalene and salicylate [88]. Similarly, another study demonstrated the efficiency of the naphthalene degradation process performed by different microbial strains of the genera *Pseudomonas* and *Burkholderia* in soil model systems [90]. Previous, studies have shown that bacteria can degrade BaP when grown on an alternative carbon source in liquid culture experiments [91]. Sor‐ khoh*et al.* [93] isolated spore forming PAHs degrading bacteria and reported their subsequent genetic studies of their degradation pathways which may lead to the discovery of novel genes involved. Various studies showed the significance of the rhizospheric effect on degradation of

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In recent past, Bisht *et al*. [44] reported four bacteria from non-contaminated rhizospere of *P. deltoides* which were able to degrade 80-90% degradation of anthracene and naphthalene within 6 days. The maximum degradation pathways were reported from *Mycobacterium vanbaalenii* because of its exceptional ability to degrade a great variety of low and high molecular weight PAHs oxidatively in soil versatility of this species makes it probable

*Calotropis* sp. as a dominant and common desert plants that grows widely in warm and urbanizing regions and has a high capacity for taking heavy metals into its tissues due to their abilities to absorb and tolerate heavy metals [96]. The use of the leaf biomass of *Calotropis procera* can be employed as good bio-sorbent for the removal of Cr (III) from aqueous solutions

The rate and degree of PCB degradation decreases with the increase of chlorination degree. For example, 62% 2-Cl-PCBs, 28% 3-Cl-PCBs, 24% 4-Cl-PCBs, and 18% 5-Cl-PCBs were degraded during the two-month active treatment phase [98]. The reversibly sorbed PCBs will be bio stabilized within 5, 6, 12 and 15 years, respectively, during the passive phase. The rate of biodegradation of PAHs is highly erratic and is dependent not only on PAH structure, but also on the physicochemical parameters of the site as well as the number and types of micro‐ organism present. The rate and degree of PAH degradation decreases with the increase of number of benzene ring. PAHs sorb to organic matter in solid and sediments, and the rate of their sorption strongly controls the rate of which microorganism can degrade the pollutant. Much of the current PAH research focuses on techniques to enhance the bioability and therefore, the degradation raters of PAHs at polluted site. The sequential active passive biotreatment approach is an effective scheme for degradation of both PAHs and PCBs in the land treatment systems. The quantitative model, together with laboratory and field testing, can be a useful tool for the plan, design and operation of similar land treatment systems.

inoculants in the remediation of PAHs contaminated sites [95].

and as an alternative method of their removal from industrial effluent [97].

organic pollutant molecules [94].

**9. Rate of PAH Biodegradation**

The important pollutant properties controlling their fate in the environment include the vapour pressure and the Henry's constant [67]. The vapour pressure indicates whether or not a pollutant is easily volatilised in dry soil conditions, the Henry's constant provides a better measure of the volatilisation potential in wet and flooded soil. As the residence time in soil of highly volatile compounds such as chloroethene will be short, they are not a primary target of rhizodegradation. The solubility of a pollutant is further modified by soil properties. Organic matter quality and content, clay content, mineral composition, type of mineral surface, pH and redox potential are known as important controls of organic pollutant solubility, with hydro‐ phobic, nonpolar organic matter being of particular importance for binding organic pollutants [86]. Binding of organic pollutants to the soil matrix is known to progress as the contact time increases, rendering pollutants less bioavailable [67]. This phenomenon is known as "ageing" and is attributed to sorption onto minerals and organic matter in soil, and subsequent interparticle diffusion in minerals and entrapment within humic complexes, nano- and microspores [83, 86].

Apart from the absorption capability of the organisms (biology), the bioavailability of a pollutant in soil not only depends on its solubility (chemistry), but also its diffusion and mass transport (physics) towards the sites and niches where degrader populations are abundant [83]. It is well established that bioavailability is one of the most limiting factors in bioreme‐ diation of persistent organic pollutants in soil [37, 86]. In bioreactor systems this problem is often addressed by agitation and mixing and addition of surfactants [34]. In recent past several microbes have been reported to be chemotactic towards different organic pollutants, for example toluene acting as chemoattractant to *Pseudomonas putida* [87]. Chemotactic bacteria might be more competent for bioremediation than their non-chemotactic counterparts [87].
