## 1 Introduction
Thus, to assure a good dispersion of the CTNs, two approaches may be used: mechanical and chemical methods. The mechanical methodology consists of applying ultrasonic energy, which overcomes the Van der Walls forces and promotes the dispersion of CNTs [33]. The non-covalent chemical method consists of the addition of surfactants which are long molecules that can promote the dispersion of the CNTs due to physical and chemical interactions [34]. Both methods can be combined in order to promote a more efficient dispersion of the CNTs [35]. The use of clay materials to immobilize heavy metals in contaminated soils has been documented by several authors [36; 37; 38]. Clay materials have the potential to immobilize heavy metals in soils through adsorption mechanisms, retaining them on their surface, since they have a considerable high surface area. Their use has obvious advantages when compared to other options, because they are an environmentally friendly material, widely available, and non-expensive [36]. The most important properties to the potential adsorption capacity of a clay mineral are the specific area and cation exchange capacity (CEC). In general, higher values of CEC imply higher capacity to adsorb heavy metals. Clay minerals with higher surface area also have more adsorption potential, since they present more adsorption sites [39]. [PERSON] et al. [40] showed that the adsorption potential is more closely tied to the specific surface of the clay than to CEC. Clay minerals are usually used in their natural state, but nowadays, some authors are focusing on the use of modified clay minerals [41; 42; 43; 44]. The modification of clay minerals consists of adding chemicals or organic compounds to the clay minerals surface, attempting to improve their adsorption capacity. Binders have been used in solidification/stabilization to immobilize heavy metal-contaminated soils and are an effective technique according to several authors [2; 5; 10; 17; 18; 19; 21; 45; 46; 47; 48; 49]. In fact, they present the additional advantage of improving the mechanical properties of the soil while also immobilizing the contaminants [2; 19; 46; 50]. In this case, there are complementary chemical and physical interactions that cause a decrease of the overall mobility of the contaminants and an effective fixation on site [9; 10; 51]. The principal mechanism causing the immobilization of the heavy metals is chemical fixation of the contaminants onto the binder components, as, for instance, onto calcium silicate hydrates (CSH), i.e., the HMs are immobilized by the soil/binder matrix [9; 52]. The addition of binders to a soil has another beneficial effect on heavy metals immobilization: their addition can reduce the permeability of the soil which makes it harder for the heavy metals to migrate through the soil and contaminate adjacent areas [50]. They are an alkali material and mixing them with soil usually results in an increase of the pH [19; 53; 54], reducing the acidity of the soil and consequently reducing the mobility of the contaminants, since heavy metals show more mobility in acidic environments [6]. The combination of binders with different additives is a possibility recommended by some authors to more effectively immobilize contaminants [55]. However, the S/S technologies involving the use of binders, namely, Portland cement, limit the use of the soil for other purposes rather than construction. In this work, the final objective is to evaluate the capacity of the additives CNTs and clay mineral as an alternative or complement to the traditional additive Portland cement to reduce the mobility of heavy metals (lead, cooper, nickel, and zinc) in contaminated soil. Both CNTs and the clay mineral were previously treated applying ultrasonic energy and adding a surfactant to guarantee the good dispersion of the additives and an increase of the number of available adsorption sites.