that soils contaminated with both PAHs and heavy metals had a stronger inhibitory effect on microorganisms and plant growth than soils amended with only heavy metals or only PAHs, while the inhibitory effect was more significant during the early stage of microorganisms and plant growth. Shen et al. [235] found that the interaction effect between heavy metals (Zn and Cd) and PAHs were strongly dependent on the time of pollution. Palanisami et al. [236] observed that the co-existence of PAHs and heavy metals significantly inhibited soil enzyme activities, and the co-contamination not only reduced the microbial diversity but also exerted selective pressure on microorganisms, with a few distinctive species being observed with a higher microbial activity. The combined effects of heavy metals and other PHCs in oily sludge on soil microbial activity have rarely been reported. However, the studies of such impacts can have important implications to oily sludge treatment since a considerably large amount of oily sludge were disposed in landfills or landfarms [237].Overall, few attention has been paid to handle various heavy metals in oily sludge and/or its disposal treatment by products although many methods are available. For oily sludge incineration byproducts such as sludge ash and fly ash, the sta-bilization/solidification technology could be used to prevent heavy metals from leaching [238]. For oily sludge ash that contains relatively high levels of valuable heavy metals such as vanadium (V) and nickel (Ni), metal recovery technologies (e.g., solvent extraction) could be attractive [230,239]. For example, Nabavinia et al. [230] applied sulfuric
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