**4. Biochar systems for synergistic valorization of wood waste and contaminated soil**

Contamination of soils from human activities is a widespread environmental problem around the globe [47]. Only in EU-28, it has been estimated that 2.8 million potentially contaminated sites exist [48]. A widely applied technique for remediating contaminated sites worldwide is the "dig and dump" technique, where the contaminated soil is excavated and landfilled, and the excavated site is usually backfilled with virgin material [49]. However, this technique is not sustainable because of high-energy requirements, scarcity of landfill space, high costs, and decreasing availability of natural resources for backfilling [49]. Hence, various alternative techniques are being explored, including the application of biochar to contaminated soils.

Biochar exhibits good sorption properties for organic compounds, such as polycyclic aromatic hydrocarbons (PAHs), and inorganic substances, such as heavy metals, because of their large surface area, porous structure, and cation-exchange capacity [9, 46, 47]. Therefore, the mixing of biochar with soils contaminated with these substances is considered a potential option for stabilizing the contaminants. The efficacy of this technique depends on the properties of the utilized biochar and the type and concentration of contaminants in the soil [50]. For example, the efficacy of biochar for sorption of PAHs and heavy metals, such as Cd, Zn, Pb, and Cu, have been reported as good [51, 52], while for negatively charged metal(loid)s, such as As and Mo, the sorption capacity of biochar is low [47, 50]. Furthermore, the interplay between positive and negative effects has been reported for contaminated soils

with multiple contaminants [46]. This indicates that the utilization of biochar for remediation of contaminated soils may not be suitable for all types of contaminated soil and thus case–specific assessments are generally required.

To explore the potential of using biochar for remediating contaminated soils with PAHs, heavy metals and metal(loid)s in Sweden, the research project "Biochar-RE: Source" was carried out between 2018 and 2020 [53]. The purpose of the project was to test and assess a new technique for remediation of contaminated soils excavated in urban areas, which is based on biochar made from urban wood waste. As part of the research, different biochar systems that use pyrolysis were designed and their environmental performance was assessed and compared to that of the "dig and dump" technique, which is the prevailing method for handling contaminated sites in Sweden [54]. The assessment of these systems is described by Papageorgiou et al. [55]. The following sections of this chapter (4.1 and 4.2) describe these systems and provide an overview of the methodological approach followed for the assessment and a summary of the results of the assessment. For more details see Papageorgiou et al. [55].
