## 3 Results and Discussion
### Adsorption Tests
#### 3.1.1 Addition of MWCNTs  
Figure 3 and Table S1 in Supplementary Materials present the evolution of the heavy metals adsorption with time by the soil alone (reference), suspension of MWCNTs, and for the suspension of the soil enriched with MWCNTs. It is apparent from the results obtained that adsorption of the heavy metals is initially very rapid, tending quickly to equilibrium which is reached at the end of 5 min. Thus, after this initial period, no significant changes on the amount of adsorbed material were detected for all the situations and HMs tested. Figure 3: Results from the adsorption tests. Individual adsorption of lead **(a)**, copper **(b)**, nickel **(c)** and zinc **(d)** by the soil, MWCNTs and suspension of soil with the addition of MWCNTs. Looking at the results of the tests where only MWCNTs were used (aqueous suspension of MWCNTs), it can be concluded that the heavy metals lead (Pb\({}^{2+}\)) and nickel (Ni\({}^{2+}\)) were adsorbed in larger amounts than the heavy metals copper (Cu\({}^{2+}\)) and zinc (Zn\({}^{2+}\)). It is known that certain metals show higher affinity to CNTs than others as reported by [63]. In the present study, the amount of each individual heavy metal adsorbed is clearly related to its electronegativity and atomic radius. According to the Pauling scale, the electronegativity of Pb\({}^{2+}\) and Ni\({}^{2+}\) are respectively 2.33 and 1.91, higher than those of Cu\({}^{2+}\) (1.90) and Zn\({}^{2+}\) (1.65), while the atomic radius follows the sequence Pb \(>\) Ni \(>\) Cu \(>\) Zn [64], explaining, in this way, the higher adsorption of lead and nickel by the MWCNTs, both having higher electronegativity and atomic radius (the higher the atomic radius the lower the mobility of the heavy metal). For these tests, adsorption percentages of 82.3 and 76.4 were obtained for Ni\({}^{2+}\) and Pb\({}^{2+}\), respectively, while for Cu\({}^{2+}\) and Zn\({}^{2+}\), the maximum values were 20.4 and 21.8, respectively. These results are in agreement with the findings of [PERSON] et al. [31], but in the present study, the MWCNTs showed a higher adsorption capacity which can be explained by the combined effect of the surfactant selected (SDBS) that promotes the dispersion of the MWCNTs and further trapping of the heavy metals, considering the opposite charge of both materials. Examining the results of adsorption by soil particles alone, it may be concluded that the soil alone can capture a significant quantity of heavy metals, possibly as a result of the fine grain size and high content of organic matter. The heavy metal exhibiting a higher degree of adsorption was lead, followed by copper, nickel, and zinc. When comparing with the adsorption by MWCNTs alone, all metals, except nickel, presented a higher tendency to be adsorbed by the natural soil. Copper was the HM where this tendency was clearer, with 82.5% of adsorption in soil compared to the 18.4% obtained when only a dispersion of MWCNTs was used. This can be attributed to the high content of organic matter in the soil tested, which had a particularly strong effect on the adsorption of copper. The organic matter present in soils can help retaining heavy metals through both physical and chemical interactions. It has been proved previously [65] that this interaction (organic matter/heavy metals) is stronger towards copper than to other heavy metals. Considering that the soil tested has a high organic matter content, copper adsorption by the soil alone was higher than in the case of Ni\({}^{2+}\) and Zn\({}^{2+}\) adsorption. Pb\({}^{2+}\) maintained a high adsorbing tendency, as already observed when using the MWCNTs alone, due to its high electronegativity, which decreases its mobility. When comparing adsorption by soil alone with adsorption by the soil additivated with MWCNTs (0.01% _w/w_), an increase of the adsorption capacity of the additivated soil is always observed, except in the case of Zn\({}^{2+}\).