# A Systematic Review on Earthworms in Soil Bioremediation
Phytoremediation techniques can be classified as follows: phytoextraction which uses hyperaccumulator plants to take up contaminants from the soil, concentrating them in aboveground biomass (shoots) [51; 52]; phytostabilisation to decrease the bioavailability of xenobiotics, avoiding their mobilisation [53]; phytotransformation which uses the ability of plant enzymes to convert organic pollutants into less toxic forms [54]; rhizdegradation, in which the root exudates improve and promote the activity of the microorganisms in the rhizosphere, transforming organic pollutants [55]; phytovoltatilisation which uses plants to volatilise organic contaminants and subsequently release them into the atmosphere by their transpiration process [56; 57]; phytofiltration where hydroponically grown and aquatic plants can be used to remove xenobiotics from the water [58]. Bioaugmentation is a green technology that removes organic contaminants by inoculating, in different matrices, bacteria and/or fungi strains or their consortia [59], which have specific degradation abilities to the target pollutants [60]. Microorganisms suitable for bioaugmentation must have specific characteristics, such as rapid growth, easy maintenance in laboratory conditions, adaptability and resistance to strongly contaminated substrates [61]. Microbial strains used in this technique can be indigenous or allochthonous. Autochthonous strains are isolated from the contaminated environment and, once cultivated, are re-inoculated in the original site [59; 62]. The other category of microorganisms is taken from areas with similar contaminations and then introduced into the site to be remediated [63]. Most studies have focused on the bioremediation techniques described above, while the use of soil fauna, such as earthworms, as first actors in decontamination is a poorly studied field that has to be explored [64]. Vermieremediation was first defined by [PERSON] et al. [65] and utilises earthworms' biotic and abiotic interaction, life cycle, burrowing and feeding activities to transform, degrade or remove contaminants from the soil environment [66]. The role of earthworms in the terrestrial ecosystem is pivotal because they positively influence soil's physical, chemical and biological properties [67]. These organisms actively contribute to pedogenesis, soil turnover and the increase in soil porosity; consequently, they are termed ecosystem engineers [68; 69]. Thanks to their excavation and soil ingestion activities, earthworms play a pivotal role in organic matter dynamics, redistributing the elements within the soil profile, and can modify the pore system architecture [70]. Furthermore, earthworms are widely used as sentinel organisms by the international scientific community [71]; thanks to their permeable cuiticles and large ingestion of surface soil, they are directly in contact with the soil particles and are sensitive to contamination [72; 73]. Earthworms are able to remove contaminants both directly, through absorption and digestion, and indirectly by stimulating the growth of beneficial microorganisms for soil remediation [74]. Consequently, there are several mechanisms involved in vermieremediation: vermierextinction is the process in which earthworms remove contaminants from the substrate by dermal and intestinal absorption and accumulate them in their bodies [64; 75]; vermitransformation changes organic pollutants, passing through the earthworm's gut, by decomposing into less toxic forms by the microorganisms and enzymes that colonise it [66; 75]; the drildegradation occurs in the 2 mm thick zone around earthworms' burrow walls, called drillosphere [76]. Due to the release of earthworm residues, mucous secretions and casts, this area becomes a microbial hotspot with a great potential for the degradation of organic compounds [77; 78]. This review deals with current literature on the vermieremediation of inorganic and organic xenobiotics in solid matrices (such as natural or artificial soils, sludges, manure, etc.).