## 2 Biochar--Preparation and Unique Physiological Characteristics  
Biomass is converted into biochar through the carbonization processes consisting of pyrolysis, gasification, and hydrothermal carbonization [31]. Among them, the so-called \"pyrolysis\", a thermochemical process, is a common method to prepare biochar with temperatures ranging from 300 to 900 \({}^{\circ}\)C in an oxygen-deficient environment [36]. The pyrolysis process transforms biomass into a stable form of carbon that is unlikely to escape into the atmosphere [37]. Biochar is largely composed of fixed carbon (85%, organic C of biochar), but it may also contain oxygen, hydrogen, and ash (inorganic component of biochar), depending on the amount of ash present in the original biomass [38]. Pyrolysis is generally divided into three primary categories, namely fast pyrolysis, intermediate pyrolysis, and slow pyrolysis [39]. Compared to other methods, slow pyrolysis has been observed to generate the highest biochar yield [36; 39]. Biochar is primarily produced from various organic wastes, including wood chips, wood pellets, tree bark, crop residues, and municipal solid waste [31; 40]. The benefits that biochar offers are closely linked to its unique physiochemical characteristics including high porosity, large specific surface area, functional groups, and cation exchange capacity [31]. Among them, the surface area and porosity play an important role in improving water-holding capacity and adsorption capacity [41]. The hydration process during pyrolysis, which involves water loss and removal of volatile compounds from biomass, creates the porous structure of biochar [42]. The surface area of biochar has a strong relationship with its porosity, with micropores having the most significant impact. Feedstocks and pyrolysis conditions determine the physiochemical characteristics of biochar. For example, increasing the pyrolysis temperature produces biochar with a higher micropore volume, larger surface area, and greater carbon content for better adsorption of heavy metals and organic pollutants [36; 43]. However, the biochar yield also decreases when increasing temperatures [39; 44]. Furthermore, a number of harmful pollutants named \"polycyclic aromatic hydrocarbons\" (PAHs) are generated during the pyrolysis process. The leaching of these pollutants from biochar-GR systems into the discharge raises concerns regarding human health and the environment [45]. Biochar manufactured at high pyrolysis temperatures of 600-800 \({}^{\circ}\)C often has higher PAHs than the acceptable limit. Hence, pyrolysis at 500 \({}^{\circ}\)C is preferable with less negative impacts [46; 47]. Regarding feedstock, [PERSON] and [PERSON] [31] recommended using feedstocks with low volatile contents to improve the biochar yield. [PERSON], et al. [48] carried out a comprehensive analysis of biochar feedstock and the pyrolysis temperature to achieve a high biochar porosity and large surface area. They concluded that wood and woody feedstock and an intermediate pyrolysis temperature (400-700 \({}^{\circ}\)C) provide optimal conditions for the development of a porous biochar structure [39]. In addition to the primary factors of feedstock and pyrolysis temperature, a thorough understanding of other variables such as reactor type, heating rate, residence time, and biochar modification treatments is necessary to achieve the desired biochar characteristics. The ability of biochar to retain water is strongly linked to its highly porous structure [49]. The internal porosity of biochar functions as millions of tiny sponges, holding a substantial amount of water. Additionally, the high porosity and aeration resulting from biochar's large surface area (ranging from 800 to 5000 mm\({}^{2}\)/g) indirectly enhance microbial diversity and survivability in soil [50]. Biochar can effectively reduce the leaching of organic molecules and nutrients because the functional groups on its surface enhance its cation and anion exchange capacities [51].