## 4 Conclusions  
This experimental work assessed the efficiency of different additives (MWCNTs, montmorillonite, and Portland cement) to immobilize the heavy metals lead, cooper, nickel, and zinc in soil in conditions similar to a real case scenario. The choice of the HMs tested took into consideration the existent contamination scenario in Portugal, aiming also at testing HMs with different degrees of mobility. The study revealed that even if the adsorption tests are a valid tool to evaluate the performance of the additives, only percolation tests can take into consideration the influence of the soil's properties, reproducing the conditions of a real field situation in a better way. The results obtained allow to conclude that the different HMs have different affinities to the soil studied, with a high organic matter content; the affinity order identified was Pb > Cu > Ni > Zn. The use of Portland cement in a low dosage (25 kg of cement per cubic meter of soil), without any other additive, proved to be very efficient to immobilize the HMs studied with the advantage of the mechanical enhancement of the soil. The combination of Portland cement with other additives is not advised, since the addition of MWCNTs results in an increase of the cost without significant technical benefits regarding HMs immobilization, and the combination of Portland cement with montmorillonite is counterproductive. If the aim is not the mechanical enhancement of the soil, then the  
Figure 8: SEM images of the powder obtained from a leachate sample collected at 3 days: (**a**) contaminated soil with heavy metals; (**b**) contaminated soil with heavy metals enriched with MWCNTs. use of montmorillonite as a single additive allowed an increase in the immobilization of the HMs studied in this work, when compared with the soil alone adsorption capacity. The use of chemically modified and well-dispersed montmorillonite was more efficient than the use of montmorillonite without any previous treatment, when nickel and zinc are considered, even when lower amounts of montmorillonite are used. The addition of MWCNTs in a concentration of 0.05% (_w/w_), as a single additive to the contaminated soil, revealed to be efficient for the immobilization of the HMs studied, presenting results similar to the ones obtained with montmorillonite for Ni\({}^{2+}\) and Zn\({}^{2+}\), but using a much lower concentration of MWCNTs. Moreover, it was verified that the nanoparticles were not released into the water when the soil with MWCNTs was put in contact with water up to 14 days. Since all the options studied in this work revealed to be technically efficient in the immobilization of heavy metals in soils, it may be concluded that MWCNTs and clay mineral are a valid alternative to the traditional addition of Portland cement even when tested in conditions similar to a real case scenario, unless stabilization of the soil is also required. The designers' and decision-makers' choice of the best additive will depend mainly on the environmental/economic compromise. The following are available online at [[http://www.mdpi.com/2076-3417/10/22/7950/s1](http://www.mdpi.com/2076-3417/10/22/7950/s1)]([http://www.mdpi.com/2076-3417/10/22/7950/s1](http://www.mdpi.com/2076-3417/10/22/7950/s1)), Table S1: Results of the individual adsorption tests--Adsorption of the heavy metals under study by the Soil, suspension of MWCNTs in water, and suspension of Soil with addition of MWCNTs (MWCNTs = 0.01%, _w/w_). Table S2: Results of the individual adsorption tests--Adsorption of the heavy metals under study by a suspension of Montmorillonite in water and suspension of Soil with addition of Montmorillonite (montmorillonite = 0.01%, _w/w_) (results for soil alone as reference are given in Table S1). Table S3: Results of the percolation tests--adoption of the heavy metals under study by soil with or without the addition of MWCNTs. Table S4: Results of the percolation tests--adsorption of the heavy metals under study by soil with or without the addition of Montmorillonite.