# A Systematic Review on Earthworms in Soil Bioremediation
## 3 Results
### Combined Vermierendiation for Inorganic Pollution
Interestingly, AMF alone caused an increase in plant tissues' Cd content only at low and moderate contamination levels, while earthworms increased it at moderate and high levels. Lastly, the results of this study demonstrated that the treatment with the most significant impact on the phytoremediation abilities of _S. nigrum_ is the combined one, followed by _R. irregularis_ and earthworm inoculation. Biochar is a carbon-rich material obtained from biomass pyrolysis at high temperatures (200 to 700 \({}^{\circ}\)C) under limited oxygen conditions [142]. Biochar can bind PTEs through precipitation, complexation and ion exchange mechanisms [143]. To obtain a high-quality organic fertiliser, some authors add different amounts of this amendment to improve the vermicomposting process, achieving a product with an acceptable PTEs concentration. A recent study by [PERSON] and [PERSON] [144] investigated the combined effects of biochar and _E. fetida_ on some PTEs (Cd, Cr, Cu and Pb) removal from sewage sludge. Different concentrations of biochar were evaluated (O1 2%, O2 4% and O3 6%); however, the 4% biochar treatment with earthworms (S + E + O2) appeared to be the most efficient in lowering the PTEs concentration, with a reduction of Cd 55%, Cr 28%, Cu 30% and Pb 21% (Figure 4). The substrate with the highest percentage of biochar (S + E + O3) was adverse to earthworm fitness and remediation efficiency despite this amendment being recognised as a stabiliser and sequester [145]; furthermore, treatments with only biochar (S + O1) did not improve the reduction processes. Consequently, this study demonstrated that earthworms are the real protagonists in the bioremediation of these inorganic contaminants. Also, [PERSON] et al. [146] evaluated the effect of different types of biochar applied at 10% in a vermicomposting process on a base matrix composed of sewage sludge and kitchen waste. The biochar from the poplar plant residues (PPB) was the most effective since the concentration of all the metals analysed (Cr, Cu, Pb, Zn) decreased significantly compared to the treatment without biochar, except for Cd. _E. fetida_ bioaccumulated these elements within its tissues, bringing to that decrease. [PERSON] et al. [139] instead used agricultural topsoil as substrate, artificially contaminated with Cd, to evaluate co-remediation effectiveness with 2% biochar and _E. fetida_ (S + E + O). In this case, the removal rate in the combined treatment (34.4%) did not statistically differ from the one with earthworm alone (30.5%), as reported in Figure 4. However, Cd accumulation in earthworm tissue increased by 13.1% in S + E + O, probably due to the ingestion of metal-bound biochar particles. In conclusion, from the papers examined, it emerges that biochar helps the earthworm in the bioremediation activity; however, attention must be paid to the application rate because a high dose of biochar could be toxic for these organisms. [PERSON] et al. [147] performed an experimental study using paper mill sludge (PMS) by adding cow dung (CD) and tea waste (TW) at different combinations with _E. fetida_. For the authors, the best mixture in PTEs removal was PMS:CD in equal ratio (S2 + E), followed by S1 + E (PMS:CD ratio 1:2) and S2 + E + O1 (PMS: TW:CD ratio 1:1:1). In fact, as observable from Figure 4, the S2 + E treatment showed the highest reduction percentage for Cd (58.21%), Cu (79.83%), Ni (81.9%) and Pb (83.48%).