# Application of Biochar for the Restoration of Metal(loid)s Contaminated Soils
## 2 Materials and Methods  
### Soils and Biochar  
Two soils were used for this study: a soil with high concentration of metal(loid)s (RA) to be amended and a non-contaminated soil (NC) used as control.  
The RA soil was collected from the area known as Guadiamar Green Corridor (37\({}^{\circ}\)29\({}^{\prime}\)42\({}^{\prime\prime}\)\(\mathrm{N}\)\(6^{\circ}\)13\({}^{\prime}\)12\({}^{\prime\prime}\)\(\mathrm{W}\)), near to the pyrite mine of Aznacollar (Seville, Spain) (Figure 1A), where highly metal(loid)s contaminated soils exist due to the mining accident which occurred in 1998 [30]. Despite the efforts for restoring the area, metal(loid)s contamination persists more than 20 years after the accident. This soil is classified as eutric Regosol [31] and characterized by the absence of vegetation (Figure 1C) due to the presence of residual pollution [29; 32].  
The NC soil was collected from an area close to the mining spill but non-affected by metal(loid)s contamination (37\({}^{\circ}\)30\({}^{\prime}\)00\({}^{\prime\prime}\) N 6\({}^{\circ}\)13\({}^{\prime}\)11\({}^{\prime\prime}\) W) (Figure 1A). This soil is classified also as eutric Regosol and covered with vegetation as seen in Figure 1B.  
Soil samples were collected from the surface layer (0-10 cm) using the methodology described in [PERSON] et al. [33]. Briefly, five soil subsamples from a square area of 1 \(\times\) 1 m were obtained, one subsample from each vertex and one from the center of the square. The subsamples were homogenized by manual mixing and then, sieved through 2 mm sieve to obtain a representative sample of each soil. The main soil properties, pH, electrical conductivity (EC) and organic carbon (OC) content of the two soils and their total content of different metal(loid)s were analyzed (Table 1).  
The biochar (BS) was supplied by Swiss-biochar (Belmont-sur-Laussanne, Switzerland) and was elaborated from the pyrolysis of oak pruning at 650 \({}^{\circ}\)C. The specific surface area of the biochar was 250-350 m\({}^{2}\) g\({}^{-1}\). Other physical and chemical properties of the biochar, determined as described below, are included in Table 1. The biochar was ground and sieved by 500 \(\mu\)m before use.  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & **RA** & **NC** & **BS** & **RA4B** & **RAB** \\ \hline
**pH** & 3.67 \(\pm\) 0.02 & 7.86 \(\pm\) 0.02 & 9.83 \(\pm\) 0.02 & 4.75 \(\pm\) 0.06 & 5.69 \(\pm\) 0.12 \\ \hline
**EC** (dS/m) & 3.49 \(\pm\) 0.02 & 0.69 \(\pm\) 0.01 & 1.39 \(\pm\) 0.01 & 3.20 \(\pm\) 0.03 & 3.28 \(\pm\) 0.03 \\ \hline
**OC** (\%) & 0.51 \(\pm\) 0.03 & 0.51 \(\pm\) 0.04 & 82.50 \(\pm\) 0.01 & 3.81 \(\pm\) 0.04 & 7.11 \(\pm\) 0.05 \\ \hline
**Pb** (mg/kg) & 446.55 \(\pm\) 7.51 & 103.85 \(\pm\) 0.63 & 1.88 \(\pm\) 0.33 & 463.00 \(\pm\) 7.64 & 473.93 \(\pm\) 34.73 \\
**As** (mg/kg) & 244.56 \(\pm\) 1.49 & 28.50 \(\pm\) 0.77 & 0.05 \(\pm\) 0.02 & 246.34 \(\pm\) 2.14 & 232.96 \(\pm\) 9.92 \\
**Zn** (mg/kg) & 161.15 \(\pm\) 0.77 & 189.44 \(\pm\) 3.71 & 16.84 \(\pm\) 1.57 & 172.84 \(\pm\) 2.15 & 166.80 \(\pm\) 2.78 \\
**Cu** (mg/kg) & 112.17 \(\pm\) 4.92 & 83.65 \(\pm\) 1.99 & 10.28 \(\pm\) 0.32 & 133.18 \(\pm\) 2.77 & 110.54 \(\pm\) 1.31 \\ \hline \end{tabular}
\end{table}
Table 1: pH, electrical conductivity (EC), organic carbon (OC) content and total content of different metal(loid)s in non-contaminated (NC), metal contaminated (RA) soils, biochar (BS), and contaminated soil amended with 4% (RA4B) and 8% (RA8B) of biochar (mean \(\pm\) standard error).  
Figure 1: (**A**) Satellite image of the nearest area to Aznalcollar’s mine in the Guadiamar Green Corridor (Seville, Spain). The light spots in the left image (**A**) represent highly contaminated soils. White squares indicate the sampling sites of the two soils used in this study. (**B**) Non contaminated soil (NC). (**C**) Soil contaminated with metal(loid)s (RA).