# A Systematic Review on Earthworms in Soil Bioremediation
## 3 Results  
### Alone Inorganic Vermiremediation  
Vermiremediation is an expanding technology in recent years due to the pivotal role of earthworms in the soil. Of the 53 papers under study, 14 deal with vermiremediation of inorganic contaminants; considering the amount of data collected, it was necessary to carry out a graphic reworking of some (10) of these works to clarify the effects of each species of earthworm on the different PTEs reported in Figure 2.  
Among the research articles, the most used earthworm species involved in the PTEs vermiremediation is _Eisenia fetida_. _E. fetida_ is an epipeic earthworm employed in terrestrial ecotoxicology as the standard test organism for the following reasons: it is easy to breed in laboratory, has short generation times and is susceptible to chemicals [79; 80]. Seven research articles have been identified [81; 82; 83; 84; 85; 86; 87] that employ _E. fetida_ for bioremediation of PTEs, using different types of organic waste as the primary substrate with diverse contamination levels. The final aim of these experimental tests was to find a sustainable recycling system for these types of organic waste. Starting from Cd, one of the elements most analysed by the authors, the mean percentage reduction was 55.38%, with maximum values of 100% [86], 92.86% [85] and 91.27% [83]. The experiment performed by [PERSON] et al. [86] achieved the best reduction (100%), working on pharmaceutical industry sludge, adding from 50% to 75% of cow dung (CD). Also,  
Figure 2: Alone inorganic vermiremediation. Reduction percentages were averages of the same contaminant and earthworm species collected from different articles (Number of articles: 10). The reduction values reported for the NA series refer to the test in the absence of earthworms. [PERSON] et al. [85] obtained results close to total removal (92.86%) by mixing silk industrial sludge waste (SIS1) with CD in a 3:1 ratio. However, these matrices were generally poorly contaminated by Cd compared to the starting substrates of [PERSON] et al. [83], which also obtained satisfactory results by adding 50% CD to silk industrial sludge (SIS2), reaching a Cd restoration of 87.30%. For Cr, the SIS1 and SIS2 matrices gave the greatest reduction percentages, equal to 96.39% without the addition of CD; however, the result reached with the SIS2 matrix [83] is noteworthy as the starting concentration of this metal was considerably higher (16.62 mg kg\({}^{-1}\)) against 1.66 mg kg\({}^{-1}\) by SIS1 [85]. By adding 25% or 50% CD to SIS1 and SIS2, respectively, the Cr was reduced by about 93%; also, in this case, [PERSON] et al. [83] started from higher initial concentrations (7.76 mg kg\({}^{-1}\)). Regarding Pb, the combination of SIS with CD in a 1:1 ratio for both studies [83; 85] led to considerable reductions in the contaminant (80.10%, 86.82%). The most recent work [83] also evaluated vermibeds based on textile sludge (from the cotton industry, CTS1) as a substrate, in which earthworms reduced Cd concentrations by 91.27%. _E. fetida_ significantly detoxified Pb (74%); notably, this matrix was characterised by the highest initial Pb concentration (31.21 mg kg\({}^{-1}\)) among all the articles analysed. A good Pb reduction equal to 75.91% was obtained in another study [82] on a matrix composed of municipal solid waste. In general, the average reduction calculated for Pb and Cr were 33.14% and 48.35%, respectively, as shown in Figure 2.  
Observing the graph (Figure 2), the percentage of reduction of Zn is relatively low (19.3%); this is because in [PERSON] et al. [86] and [PERSON] et al. [84], the final values of this metal were recorded as higher than the starting concentrations. Despite the ability of _E. fetida_ to accumulate metals from the environment, the progressive mineralisation and decomposition of organic matter during vermicomposting can concentrate and increase the PTEs contents [88; 89]. As reported by [PERSON] et al. [86], also, Cu has negative percentages of reduction, which affected the final average (Figure 2).