# A Systematic Review on Earthworms in Soil Bioremediation
## 3 Results
### Combined Vermierendiation for Organic Pollution
The lowest rates of bioremediation were recorded in treatments with _L. terrestris_ alone, either at high or low densities; however, both organisms employed gave promising results in the degradation of chlorpyrifos. [PERSON] et al. [153] inoculated an immature compost with hydrocarbon-degrading bacteria (S + M) and _E. fetida_ (S + E), individually and in combination (S + E + M). The matrix was artificially contaminated with different amounts of petroleum oily sludge to obtain three levels of TPHs pollution (5, 10 and 20 g kg\({}^{-1}\)). As can be seen from Figure 5, treatments with the microbial consortium were effective for all concentrations tested, demonstrating high tolerance and resistance. Earthworms, instead, were more sensitive to this contaminant and showed a dose-dependent mortality; the reduced removal rate is statistically visible in S + E compared to S + M at 20 g kg\({}^{-1}\). The constantly higher reduction percentages (91.2%, 90.9%, 85.35%) in combined treatments proved the synergistic effect of _E. fetida_ and microorganisms. The nanomaterials are defined as particles of 100 nm or less in at least one dimension and represent an innovative strategy suitable for remediating either organic or inorganic contaminated sites [154; 155]. A 28-day microcosm experiment [156] was conducted using the nano zerovalent iron (nZVI) and earthworms to remediate an agricultural soil contaminated by a mixture of three representative polychlorinated biphenyls (PCBs): 2,4,4\({}^{\prime}\)-Trichlorobiphenyl (PCB28), 2,2\({}^{\prime}\),5,5\({}^{\prime}\)-tetrachloropiphenyl (PCB101). The bioaccumulation of PCBs in _E. fetida_ reached equilibrium after 14 days of exposure; the accumulation was rapid, and no mortality was recorded, indicating a high endurance of earthworms to these compounds. The addition of nanomaterials to the substrate sig nificantly increased the concentration of PCBs in _E.__fetida_, reaching over 60 mg kg\({}^{-1}\) in tissues, compared with a maximum accumulation of 55.3 mg kg\({}^{-1}\) in the absence of nZVI. In conclusion, nanomaterials could be a good solution for enhancing earthworms' vermiremediation potential. The efficiency of vermiremediation is limited by the low bioavailability of PAHs and their biotoxicity [157]. Therefore, surfactants could be a promising method to improve the earthworms' accessibility to these low-soluble and recalcitrant compounds [158]. Surfactants are amphipathic substances capable of absorbing and altering the conditions of the interfaces [159] and are widely used in remediation technologies. However, the role of these helpers in vermiremediation has rarely been studied. [PERSON] et al. [160] used Tween-80 (20, 100 mg kg\({}^{-1}\)) and rhamn (25, 100 mg kg\({}^{-1}\)) surfactants in contaminated fluoranthene soil (25, 50 mg kg\({}^{-1}\)). Both Tween-80 and rhamn significantly increased the bioconcentration of fluoranthene in _E.__fetida_ by 35-64.1% and 34.5-44.2%, respectively. These results proved that surfactants promote the earthworms' pollutant uptake, intensifying fluoranthene bioavailability. Surfactants statistically reduced residual PAH in the soil in all treatments; however, it emerged that this attitude is closely related to the surfactant level, which should be added at an optimal concentration. For example, at 50 mg kg\({}^{-1}\) of fluoranthene, the lowest dose of rhamnipid gave better removal results than the highest dose. [PERSON] et al. [161] added biochar as a helper to vermicompost, contaminated with di-(2-ethylhexyl) phthalate (DEHP), to verify its impact on the earthworm restoration process. In the present study, biochar and _E.__fetida_, either alone or combined, significantly enhanced the DEHP degradation compared to treatment with only soil. The best results were obtained by treatment with earthworm alone, followed by combined and biochar ones.