## 2 Materials and Experimental Procedures
### Experimental Procedures
#### 2.2.2 Percolation Tests  
To evaluate the effectiveness of the different additives to immobilize heavy metals in conditions closer to a real on-site situation, percolation tests were conducted in four different situations: using only the soil (reference tests) and also using the soil additivated with MWCNTs or montmorillonite or mixing the soil with Portland cement. The tests consisted of using small samples of contaminated soil (37.6 cm\({}^{3}\)), additivated or not, and to percolate water through them, using an hydraulic charge of 5 m (\(\approx\)50 kPa). The heavy metal concentration in the leachate liquid was measured using atomic absorption spectrometry (Perkin Elmer atomic absorption spectrometer 3300). The procedure adopted for the percolation tests started with the laboratory contamination of the soil (stage 1), followed by the addition of the additive materials (stage 2) and ended with the percolation tests followed by the measurement of the heavy metals that percolated through the contaminated sample which allowed calculation of the amount of heavy metals immobilized (stage 3). In stage 1, four equal soil portions (5 kg each) were taken from the natural soil, air dried, and carefully mixed with a solution of lead, copper, nickel, or zinc, so that after mixing with the soil, all four portions possessed the original water content (80.9%). Each contaminated soil portion was stored for 7 days to ensure a uniform distibution of the HMs. In stage 2, for each of the four soil portions, a representative small sample was collected and mixed  
Figure 1: Procedure adopted for the adsorption tests in suspension conditions for the different types of tests: only soil, only MWCNTs/Montmorillonite in water and soil with the addition of MWCNTs/Montmorillonite.  
with a suspension of MWCNTs (in the same way as described for the adsorption tests), or mixed with montmorillonite in its natural state or in suspension, or still mixed with a slurry of Portland cement (corresponding to a binder quantity of 25 kg per cubic meter of soil, i.e., a small amout of binder was chosen, smaller than the amount usually used for soil stabilization (100 to 250 kg/m\({}^{3}\)[54]), because the main goal is the soil's remediation only [2; 46] and not its mechanical improvement). The samples with Portland cement were stored submerged in water in a temperature-controlled room (20 \(\pm\) 2 \({}^{\circ}\)C) during a curing time of 7 days. For the samples additivated with montmorillonite, a quantity of 25 kg of montmorillonite was used, in its natural state, per cubic meter of soil (equal to the binder amount to allow direct comparison), or, alternatively, a suspension of montmorillonite was prepared, as described for the adsorption tests, and added to the soil. In the third stage, the samples contaminated with the different heavy metals, additivated or not, were introduced in PVC tubes with 35 mm of height and 37 mm of diameter. The samples were protected from solid particles loss by a paper filter placed on the top plus two perforated PVC discs (on the top and bottom of the samples) to allow water to flow through the cell. After sealing the tubes, the percolation tests started with a flow of water under an hydraulic charge of 5 m, and the leachate was collected in a beaker. When the leachate volume was at least equal to the sample's volume, the percolation test ended. After filtering the leachate, atomic absorption spectrometry tests were performed to evaluate the amount of heavy metals in the leachate, using the same methodology described for the adsorption tests. A schematic of the percolation tests arrangement is shown in Figure 2. The arrangment constructed allowed for different samples to be tested simultaneously, either of soil contaminated with the same HM or testing the soil's samples contaminated with different HMs.  
In all percolation tests, the samples were always saturated with water.