## 2 Materials and Experimental Procedures  
### Materials  
The soil selected for this study was collected in the region of Coimbra, in the center of Portugal. It is an organic fine-grained soil composed mainly of silt and clay particles, as stated in Table 1, exhibiting poor mechanical behavior [53; 56]. The soft soil tested possesses a high organic matter content which combined with the particles composition induces potentially high adsorption capacity. However, since the soil is slightly acid, this may hinder the adsorption capacity of the soil, since all heavy metals present higher mobility in an acidic environment [15]. All tests were performed without modifying the pH of the natural soil or of any other material or suspension. In order to simulate real conditions, the soil was contaminated in the laboratory for the maximum HMs concentrations measured in contaminated soils found in Portugal (considered as the most challenging conditions) [58; 59], and was stored in a temperature-controlled room (20 \(\pm\) 2 \({}^{\circ}\)C) for 7 days, at least, to allow the HM to distribute evenly in the soil. Table 2 presents the maximum concentrations found in Portugal for the heavy metals under study (Pb, Cu, Ni, and Zn), and it is possible to verify that all the values are above the reference values recommended in the Ontario Soils Quality Guidelines [60], showing the presence in Portugal of highly contaminated soils. The values shown are referred to the dry weight of soil. The soil was contaminated in the laboratory by the following salts: lead nitrate (Pb(NO\({}_{3}\))\({}_{2}\)--supplied by Riedel-de-Haen), hydrated copper chloride (CuCl\({}_{2}\)2H\({}_{2}\)O--supplied by Merck), hydrated nickel sulphate (NiSO\({}_{4}\)6H\({}_{2}\)O--supplied by Fluka), and hydrated zinc sulphate (ZnSO\({}_{4}\)7H\({}_{2}\)O--supplied by Sigma). The carbon nanotubes used in this work were multi-walled carbon nanotubes (MWCNTs), with an indicative cost of 100 \(\upxi\)/kg, supplied by Nanocyl, (MWCNTs CN7000), considering that they are up to ten times cheaper than the single-walled carbon nanotubes (SWCNTs). According to the supplier and [PERSON] et al. [35], the MWCNTs are characterized by a diameter of 9.5 nm (average), a length/diameter ratio of 158, a specific surface area of around 275,000 m\({}^{2}\)/kg, a specific density of 1.7 g/cm\({}^{3}\) (on average), and zeta potential of around \(-\)25.2 mV. Portland cement type I, mechanical strength class 42.5R, was the binder chosen for the present work. This binder, supplied by Cimpor Portugal, and with an indicative cost of 0.08 \(\upxi\)/kg, is characterized by the chemical composition presented in Table 3. \begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{2}{c}{**Soil Characteristics**} \\ \hline Grain size distribution & Clay (\%) (_w/w_) & 21 \\ \cline{2-3}  & Silt (\%) (_w/w_) & 59 \\ \cline{2-3}  & Sand (\%) (_w/w_) & 20 \\ \hline Specific gravity & G (-) & 2.6 \\ \hline Natural water content & w (\%) (_w/w_) & 80.9 \\ \hline Liquid limit & w\({}_{\text{L}}\) (\%) & 54.1 (a) \\ \hline Plastic limit & w\({}_{\text{P}}\) (\%) & 39.8 \\ \hline Organic matter content & OM (\%) (_w/w_) & 7.4 \\ \hline pH & pH (-) & 5.4 \\ \hline Soil classification USCS (b) & (-) & OH \\ \hline \end{tabular}
\end{table} 
Table 1: Characteristics of the soil used in this work. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Heavy Metal**} & \multicolumn{2}{c}{**The Soil Geochemical Atlas of Portugal [59]**} & **Ontario Soil Quality Guidelines [60]** \\ \cline{2-3}  & **Maximum Value Registered (mg/kg)** & **Reference Values (mg/kg)** \\ \hline Lead & 585 & 120 \\ Copper & 245 & 230 \\ Nickel & 880 & 270 \\ Zinc & 589 & 340 \\ \hline \end{tabular}
\end{table} 
Table 2: Maximum values registered in Portuguese soils [59] and international guideline values [60] for the heavy metals studied in this work.