# Application of Biochar for the Restoration of Metal(loid)s Contaminated Soils
## 1 Introduction  
Soil pollution has become a great concern all over the world, and is considered the third threaten for the soil function by the Intergovernmental Technical Panel on Soils [1]. The main sources of organic and inorganic pollutants are related to industrial, urban, and agricultural activities. In the last few decades, mining has become the primary entrance of metal(loid)s into the environment with important harmful implications for human health [2] due to the immobile nature of these compounds and the lack of biodegradation processes, which makes them highly persistent pollutants [3]. In this context, there is an increasing interest in new and sustainable solutions for the prevention, mitigation, or remediation of soil contamination caused by metal(loids) [4]. Assisted natural remediation (ANR) is a novel concept that consists in enhancing natural processes by, for example, adding to the soil amendments that favor the immobilization of contaminants and the improvement of soil properties [5], and therefore promoting the natural restoration of vegetation. Calcium carbonate, iron, and organic matter rich amendments are the most used to reduce metal(loid)s mobility and toxicity by precipitation, complexation, and adsorption, with proved and high efficiency at field scale [6; 7]. Biochar is a carbon rich material obtained from the pyrolysis of organic wastes at low temperatures (from 400 to 700 \({}^{\circ}\)C) under O reducing conditions [8]. This process stabilizes the organic carbon and makes it more resistant to chemical and biological degradation [9]. Biochar is characterized by its high content of aromatic compound, high porosity, and surface area [9]. It can be produced from different biomasses such as agro-industrial,forestry and urban wastes, manure, and sewage sludge [10]. The pyrolysis conditions and the nature of the biomass determine the physical and chemical properties of the biochar [11; 12; 13]. The main and most spread application of biochar is as soil amendment [9], since it improves the physical and chemical soil properties and consequently the crop yield [14; 15]. Biochar has been shown to increase the carbon holding capacity of soils, reduce the emission of greenhouse gases [9; 16; 17], promote the soil permeability and the water holding capacity [15], favor the lining of acid soils [14], and supply essential nutrients such as phosphorous [18; 19]. In addition, several studies indicated the ability of biochar to immobilize organic and inorganic contaminants, decreasing their bioavailability and toxicity [20; 21]. The adsorption capacity of biochar is a consequence of its porous structure and the presence of different surface functional groups [22; 23]. Besides adsorption capacity, biochar can increase pH values in acid soils and has high cationic exchange and buffer capacity [11; 24]. These features make biochar a feasible amendment for contaminated soils, especially for those with metal(loid)s [6; 25]. Different studies analysed the different mechanisms that govern the sorption of metal(loid)s into biochar depending on the type of elements and the biochar and soil properties [26; 27; 28]. However, more information is still needed to determine efficiency of biochar application in soil remediation practices. In this study we explore the effect of biochar as an organic amendment to immobilize metal(loid)s and promote the vegetation restoration of a highly contaminated soil from the Guadiamar Green Corridor (Seville, Spain), an area affected by the Aznalcollar's pyrite mining spill where residual pollution is still detected more than twenty years after the accident [29] due to the oxidation of the remaining tailing in the soil. For this purpose, a pot experiment under greenhouse conditions was carried out. The total concentration of metal(loid)s and their soluble and bioavailable fractions were analyzed in amended and non-amended soil to assess the effect of biochar on metal(loid)s mobility and bioavailability. Phytotoxicity to _Trifolium pratense_ was also evaluated.