## 3 Results and Discussion
### Percolation Tests
#### 3.2.2 Addition of Montmorillonite  
The addition of montmorillonite to the soil was studied in two distinct ways: montmorillonite on its natural state was added to the soil in a dosage of 25 kg of montmorillonite per cubic meter of soil (to compare directly with the additive Portland cement alone); modified montmorillonite (previously dispersed with ultrasounds in aqueous solution enriched with the surfactant SDBS) was applied in a concentration of 0.01% _w/w_ (corresponding to 0.2 kg of montmorillonite per cubic meter of soil) and 1% _w/w_ (corresponding to 22 kg of montmorillonite per cubic meter of soil). In Figure 6 and Table S4 in Supplementary Materials, the results of the addition of montmorillonite to the soil are presented.  
The results show that mixing the soil with montmorillonite on its natural state does have a positive effect on the retention of some of the heavy metals. The addition of montmorillonite has almost no effect on the retention of the heavy metals Pb\({}^{2+}\) and Cu\({}^{2+}\) when compared with the adsorption by soil alone (Table S3), mainly due to the high affinity of lead and copper to the soil used in this work. The results for Ni\({}^{2+}\) and Zn\({}^{2+}\) show that the addition of montmorillonite on its natural state led to an increase in the amount of these HMs adsorbed compared to adsorption on soil alone; however, the adsorption values are far from satisfactory (what justifies the study, for these two heavy metals, with montmorillonite previously dispersed by ultrasounds and surfactant addition).  
The addition of modified montmorillonite resulted, in general, in an increase of the adsorption capacity of the soil for the heavy metals nickel and zinc (compare Tables S3 and S4). However, the montmorillonite concentration of 0.01% (_w/w_), equal to the one used for the MWCNTs, is too low, and a concentration of 1% (_w/w_) has to be used to immobilize Ni\({}^{2+}\) and Zn\({}^{2+}\) almost completely (even in the case of Zn\({}^{2+}\) in spite of its high mobility), corresponding to an increase of the adsorption efficiency, when compared with the use of montmorillonite in its natural state, especially for  
Figure 6: Results from the percolation tests. Immobilization of heavy metals by the soil and the soil with the addition of different quantities of montmorillonite (natural state or modified).  
Ni\({}^{2+}\). Additionally, the amount of montmorillonite corresponding to this concentration is still slightly lower than the one used for montmorillonite in its natural state.  
The results here reported lead to the conclusion that clay particles chemically modified, in this case with the application of ultrasounds to an aqueous solution enriched with the surfactant SDBS, have a higher capacity to retain heavy metals, even when lower amounts of clay are used (1% _w/w_). This can be related to the fact that the ultrasounds and surfactant promote the dispersion of the montmorillonite particles, increasing the surface area and consequently its adsorption capacity. The adsorption efficiency obtained using dispersed montmorillonite (1% _w/w_) is similar to the one obtained using MWCNTs (0.05% _w/w_), for conditions similar to the ones found in a real ground.