## 1 Introduction  
Although soil is an essential asset to humans, its preservation is quite often not respected. Still, soil health is becoming an increasing concern, namely, at the European level. In this context,the existence of heavy metals (HM) in contaminated soils can be a serious problem to soil health and thus to modern societies, affecting food and water sources and biodiversity. Heavy metal pollution can occur by natural or anthropogenic sources. Among the various anthropogenic activities, some must be highlighted due to their frequency and expressive way they contribute to the soil's contamination: the use of pesticides and fertilizers, smelting, mining, and emissions from industries [1; 2]. Natural cases of soil contamination are unusual but can have severe effects, like, for example, forest fires that originate dusts rich in heavy metals, which deposit on the soil [3]. Heavy metals cannot be degraded and tend to contaminate the water and food chain, since, besides compromising the soil quality, they can leachate and threaten the quality of vegetables and cattle, and ultimately human health [4; 5]. The mobility of the heavy metals in soils is very dependent on the soil characteristics, namely, the organic matter content, grain size distribution, and pH of the soil. In general, all heavy metals are more soluble and have more mobility in acidic environments, thus acidic soils tend to favor the spread of the contaminants, while more alkali soils tend to accumulate heavy metals [6]. Fine grained soils, with large quantities of silt and clay, show better capacity to retain heavy metals unlike coarse soils that allow the infiltration of water, being less capable to immobilize the heavy metals [7]. In general, the organic matter present in soil contributes to the retention of HM due to complex interactions established between the constituents of the organic matter and the HM [5; 8]. Since heavy metals are not biodegradable and cannot be destroyed through high temperatures or the use of chemicals, ordinary soil remediation techniques are ineffective. The traditional method used to remediate HM-contaminated soils consists in excavation and relocation to a landfill. The high costs, limited landfills availability [9], and possibility of hazardous exposure of this traditional remediation method, led to the development of new methods that are more cost-effective and less harmful to the environment, like the solidification/stabilization (S/S) of soil [10]. S/S consists of mixing additives with the soil with the objective to reduce the potential migration of HM and thus the risk of contaminating adjacent areas. This technique has been supported by various associations due to its potentialities [11; 12; 13; 14]. Some of the mixing additives used in the S/S technique are binders, clay minerals, alkaline materials, organic compounds, and more recently, nanomaterials [10; 15; 16; 17; 18; 19; 20; 21]. Carbon nanotubes (CNTs) are materials with reduced size (nanoscale) and composed by cylindrical graphite sheets rolled up forming a tube. They can be single-Walled (SWCNTs) or multi-walled carbon nanotubes (MWCNTs) depending on the number of layers of graphene sheets [22], being applied in a wide range of scientific areas [23; 24; 25]. Several studies have proven that CNTs possess high adsorption capacity [26; 27; 28; 29; 30], which means, in the present context, that this nanomaterial has the capacity to retain large quantities of heavy metals on its surface. Although studies, so far, were done on wastewaters [26; 27; 28; 29; 30], they raised the possibility of CNTs being added to a soil, potentially increasing its adsorption capacity and efficiently immobilizing contaminants, which was already validated in a previous study from the authors [31] and will be further explored in the present paper for different conditions. Due to the high surface area, Van der Walls forces become important, which cause CNTs to form aggregates and lose part of their advantages [32].