## 1 Introduction
In spite of the fact that various studies have demonstrated the efficiency of using different washing agents (e.g., HCl, H\({}_{2}\)SO\({}_{4}\), HNO\({}_{3}\), oxalic acid, citric acid, saponin, ethylenediaminetetracetic acid-EDTA, nitrilotriacetate, and ethylenediaminetiscinic acid), adverse effects may result from their use or toxicity, such as the release of additional contaminants into the environment and changes in soil pH, soil structure, soil productivity, soil microorganism activity, or soil microbial health [13; 24; 25; 26; 27; 28]. The key point in the development of remediation technology for heavy metal-contaminated soil is identifying a washing reagent with a high pollutant removal rate, low toxicity, low damage to soil properties [14], low cost, and high biodegradability [22]. Therefore, for the washing of soils contaminated with heavy metals, it might be attractive to use natural materials that do not pose a secondary risk to the environment. In this regard, chitosan, which is a common and biodegradable chelating polymer composed of randomly distributed \(\beta\)-(1-4)-linked D-glucosamine (deacetylated unit) and N-acetyl-D-glucosamine (acetylated unit) and which is made by treating shrimp and other crustacean shells from the food processing industry with the alkali sodium hydroxide [29], may have the potential of becoming an extracting agent in soil washing technology. Chitosan contains hydroxyl and amine functional groups which can react with various heavy metal ions [30; 31]. However, the efficiency of heavy metals' removal by soil washing using chitosan as a washing agent is influenced not only by the interaction between heavy metals and the functional groups of chitosan, but also by other factors affecting the decontamination process, such as the liquid/solid ratio, the concentration of washing solution in chitosan, the duration of soil washing, the pH of the washing solution, the type of pollutants present in the soil, etc. [21; 32; 33]. Chitosan is an effective adsorbent for the removal of heavy metals from various media [34]. The maximum adsorption capacity was reported at a pH of 5.0 and 6.0 [35] with a removal efficiency of 93.76% [36] and 100%, respectively [37]. For example, [PERSON] et al. [4] pointed out that Zn, Cu, and Pb were extracted from soil in a higher rate after 4 h of washing with HCl, while As and Fe were better extracted after 8 h. Besides this, all of the studied heavy metals were extracted better from soil at a HCl concentration of 6 molL\({}^{-1}\), while As and Cu were extracted better at a HCl concentration of 2 molL\({}^{-1}\) and 3 molL\({}^{-1}\), respectively [14]. Also, the removal of Cu and Ni from the soil increased with increasing concentrations of chitosan in the washing solution from 0.10 to 0.40 gL\({}^{-1}\)[38]. Moreover, with the increase of the liquid/solid ratio from 5 to 20, the removal efficiency of Cu increased from 26.34% to 47.22%, while the removal efficiency of Ni increased from 41.75% to 66.13% when the soil was washed with chitosan [38]. [PERSON] and [PERSON] [39] found that the extraction of Pb from millpond sludge did not seem to increase significantly beyond the 0.05 M EDTA concentration, being observed a decrease beyond the 0.10 M EDTA. Consequently, the elaboration of a mathematical model is essential in order to predict and estimate the effectiveness of the soil decontamination process by soil washing depending on the variables affecting this process. Regression analysis is a widely used statistical technique for modelling the relationships between variables [40]. Moreover, to our knowledge, so far, no prediction models can be found in the literature for the estimation of the effectiveness of the soil decontamination process using washing with chitosan.