## 5 Bioavailability of PAHs
### Factors Affecting PAH Bioavailability  
PAHs are characterized by their high hydrophobicity, consequently increasing their affinity for being adsorbed into soil organic matter and ultimately being less available for biological uptake. Different studies [70; 71; 72; 129; 130] have highlighted that the following factors have an essential role in determining the bioavailability of PAHs. First is contamination time (ageing): the irreversible sorption of PAH is exponentially proportional with contact time, thus decreasing the bioavailability of pollutants to microorganisms and therefore the rate and extent of biodegradation. For instance, the removal efficiency of anthracene from freshly- and age-spiked agricultural soil was investigated. The results revealed that 72% of anthracene was removed in freshly-spiked soil, while only 34% was degraded in aged soil [131]. Nevertheless, it is worth mentioning that recently contaminated soil would exhibit toxicity to or even inhibit indigenous microorganisms until they adapted to the new environment [132; 133]. The second factor for determining the bioavailability of PAHs is their physicochemical properties: PAHs' water solubility is considered a crucial factor regarding their bioavailability. It is inversely proportional with PAHs' molecular weight, which in turn reduces their accessibility to microorganisms (Section 2.1). The last factor is the physicochemical properties of the soil: organic matter, particle size, and shape have a major influence on PAHs' bioavailability. Mineral surfaces (i.e., clays) and organic matter of the soil matrix are characterized by their high affinity to adsorb PAHs.  
The addition of organic co-substrates to the composting mixture is believed to enhance the bioavailability of PAHs, which consequently increases the biodegradation rate [25; 50; 71]. [PERSON] et al. [106] demonstrated that water-extractable organic matter (WEOM) from cow manure compost was observed to increase the apparent solubility of phenanthrene, pyrene, and benzo[a]pyrene to 8.4, 34, and 89 times higher than their measured water values, respectively, thus promoting their solubility and biodegradation. Additionally, in a diesel-spiked soil, [PERSON] et al. [66] showed that compost addition initially decreased PAH removal by up to 89% because of the decreased bioavailability resulting from strong sorption. However, as time increased, compost amendment enhanced PAH removal by more than two-fold compared with unamended soil, to which 30% was contributed by desorption and 70% by degradation. In coal tar- and coal ash-contaminated soils, compost addition was beneficial overall for enhancing PAH removal up to 94%, and 40% of the total loss was due to the enhanced desorption [66].  
The stability of the used co-substrates plays a major role in stimulating bioavailability and biodegradation of PAHs (as discussed in Section 4.3). This type of substrates was found to have more humic matter [71]. In this context, humic matter was able to increase the microbial activities much more than those developed in humin (aged organic matter), demonstrating that humin is able to sequester organic contaminants in a stronger way [70]. An important observation is that the bioavailability of the more readily degradable or LMW PAHs was decreased due to competitive inhibition of the enzymes, which is associated with biodegradation when the enzymes present in a multiple-PAH mixture. However, the bioavailability of those usually more recalcitrant PAHs (HMW PAHs) was increased by producing inducible enzymes for catabolism [134].