# Remediation of Metal/Metalloid-Polluted Soils: A Short Review
## 3 Toxicology
### Presence and Distribution into the Environment  
Several studies report on the distribution of heavy metals and metalloids in soil [9; 10; 27; 28]. Some soil properties facilitate the accumulation of these pollutants and increase their concentrations. pH controls the adsorption and solubilization; if the soil pH is acidic, the solubility and the bioavailability of heavy metals/metalloids increase [29]. Organic matter content (OMC) is another factor that influences their presence since organic substances with a high molecular mass have a high affinity with heavy metals and form water-insoluble metal complexes [30]. In general, the distribution of heavy metals and metalloids is high in topsoil and decreases as the depth increases [31]. Some studies evaluated their concentrations in soils close to industrial activities, and some examples are reported in Table 3. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Country/ Organization** & **Type of Soil** & **Hg** & **Cd** & **Pb** & **Cr(VI)** & **Ni** & **Unit** & **Ref.** \\ \hline WHO & Agricultural soil & 0.08 & 0.003 & 0.1 & 0.1 & 0.05 & ppm & [20] \\ \hline China & Agricultural soil & 0.3–1.0 & 0.3–0.6 & 80 & 150–300 & 40–60 & ppm & [20] \\ \hline US & Agricultural soil & 1.0 & 0.43 & 200 & 11 & 72 & ppm & [21] \\ \hline Italy & Residential soil & 1 & 2 & 100 & 2 & 120 & mg\(\cdot\)kg\({}^{-1}\) & [22] \\ Industrial soil & 5 & 15 & 1000 & 15 & 500 & mg\(\cdot\)kg\({}^{-1}\) & [22] \\ \hline Finland & Threshold value & 0.5 & 1 & 60 & 100 & 50 & mg\(\cdot\)kg\({}^{-1}\) & \\ Lower guideline value & 2 & 10 & 200 & 200 & 100 & mg\(\cdot\)kg\({}^{-1}\) & [23] \\ Higher guideline value & 5 & 20 & 750 & 300 & 150 & mg\(\cdot\)kg\({}^{-1}\) & \\ \hline Canada & Agricultural soil & 0.8 & 3 & 200 & 250 & 100 & mg\(\cdot\)kg\({}^{-1}\) & [24] \\ \hline Germany & Agricultural soil & 5 & 5 & 1000 & 500 & 200 & mg\(\cdot\)kg\({}^{-1}\) & [2] \\ \hline Spain & Soil pH \textless{} 7 & 1 & 1 & 50 & 100 & 30 & mg\(\cdot\)kg\({}^{-1}\) & [25] \\ Soil pH \textgreater{} 7 & 1.5 & 3 & 300 & 150 & 112 & mg\(\cdot\)kg\({}^{-1}\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Limit concentrations of heavy metals and metalloids in the soil. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  
Very often, in industrial areas, the concentrations of heavy metals and metalloids are higher than the threshold limits allowed by national legislation. Some authors compared the concentration values of the same soil, before and after the presence of anthropological sources. They demonstrated that an increase of concentration was precisely due to these activities [8]. In agricultural soils, high concentrations are due to the use of fertilizers and pesticides [29]. Regarding the European Union, an interesting study was done by [PERSON] et al. [5]; where they monitored As, Cd, Cr, Cu, Hg, Pb, Zn, Sb, Co, and Ni concentrations. Results showed that, in most soil samples taken from various regions, concentrations were above the threshold limits, both on their entire land area and on agricultural lands. In Western Europe and Mediterranean regions, the concentrations were higher than those in North-Eastern Europe and Eastern-Central Europe. The measured values of arsenic exceeded threshold values by 10% to 90%, and the most polluted regions were France, Italy, and Spain. For cadmium, the values were 10% to 70% above threshold values, and the highest concentrations were found in Ireland and Greece. A few European regions were found with cobalt values higher than the threshold values, among them, France and Greece. Zinc, mercury, and copper values were not higher than 10% of guideline values and most regions had concentrations below 2% of the threshold limits. Nickel and antimony contaminations