## 4 Bioremediation of PAH-Contaminated Soil by Composting
### Effect of PAH Characteristics and Concentrations  
The physical and chemical properties of PAHs have a considerable effect on their biodegradation rate. Microbial assimilation and biodegradation of these compounds basically depends on their solubility. Nevertheless, most of compounds belonging to this group are characterized as poorly soluble in water, especially with their increasing molecular weight and angularity (Table 1, Figure 1), which thus increase their hydrophobicity [69, 70, 71, 72]. This was obvious in many studies dealing with the biodegradation of different PAHs. For instance, [PERSON] et al. [73] investigated the application of different agricultural waste on the biodegradation of aged PAHs in soil microcosms over 90 days. The initial concentration of total PAHs in the soil was 36.1 mg kg\({}^{-1}\) dry soil, where four-ring PAHs comprised 41.7% of the total PAHs. The results demonstrated higher degradation rates of 40.7-61.2% for PAHs with low molecular weight (LMW), compared to 18.7-33.1% for those with high molecular weight (HMW) in all soil microcosms. Similarly, [PERSON] et al. [74] showed that LMW-PAH removal was more favorable in the mesophilic phase, with 11% and 15% residues in the soil, than in the thermophilic phase, with 29% and 31% residues. Additionally, more resistance to degradation was observed for HMW PAHs, resulting in a decrease in the total removal, which was less than 50% for both benzo[a]pyrene and benzo[k]flouranthene, in all treatments [75, 76]. In this regard, even though both compounds have the same number of benzene rings (five) and molecular weights, the higher octanol-water partition coefficient (log \(K_{ow}\)) of benzo[k]flouranthene increased its hydrophobic properties and consequently its degradation rate under the same conditions. Indeed, higher log \(K_{ow}\) leads to a higher potential of bioaccumulation, which is the main factor responsible for the lower biodegradability of such compounds [77]. Obviously, and according to the obtained result in different studies, there is a consistent relationship between the persistence of PAHs in the environment and increasing their numbers of benzene rings, which ultimately affects their biodegradation rate.  
The concentrations of PAHs also have a substantial influence on the microbial activity in such treatments, since high concentrations would lead to toxic or inhibition conditions. Meanwhile, low concentrations could be below the rate needed to stimulate microbial cultures to degrade these contaminants [78, 79]. This was obvious in the study conducted by [PERSON] et al. [78], in which the PAH concentrations had a crucial effect. Low concentrations were found to be less than the rates that are assumed to initiate the degradation process, since microbial communities prefer the utilization of readily available nutrients, which are consumed quickly before initiating biodegradation of the target PAHs. The same results were obtained by [PERSON] et al. [80], where low concentrations of PAH did not degrade, even when the system was supplanted with additional carbon sources. [PERSON] et al. [66] showed that compost addition is an effective approach for enhancing PAH removal from soils, but increasing the ratio of added compost does not necessarily help to increase removal. Nevertheless, enhanced removal by compost addition seems more effective for higher initial PAH concentrations. Inthis regard, [PERSON] et al. [81] demonstrated that the degradation rate of a compound is proportional to its concentration, especially for highly soluble compounds, and argued that the degradation of hydrocarbons is governed by first-order kinetics. However, this argument may be validated to some extent, as high concentration may become detrimental to microbial activity and disturb nutrient balance, especially when LMW PAHs are present [78].