# Preparation and Modification of Biochar Materials and their Application in Soil Remediation
## 2 Preparation and Modification of Biochar
### Preparation of Biochar
#### 2.1.2 Factors Affecting the Pyrolysis Process  
The raw materials for the preparation of biochar are abundant. Basically, any form of organic materials can be pyrolyzed [18]. Due to the large output of biomass solid waste resources, biomass is a common raw material for biochar, mainly including wheat straw, corn straw, wood chips, melon seed shell, peanut shell, rice husk, livestock and poultry manure, kitchen waste, sludge, fruit skin, etc. [19], Biochar prepared from different materials contains different proportions of cellulose, hemicellulose, and lignin, so its yield, element composition, and ash content are different [20; 21]. [PERSON] et al. [22] found that the ash content of straw biochar is higher than that of other biochar, which is mainly caused by the high Si content of straw. [PERSON] et al. [23] compared the physicochemical properties of biochar prepared from different feedstocks (the straws of canola, corn, soybean, and peanut). The ash content of biochar from corn straw prepared at 700 \({}^{\circ}\)C was the highest (73.30%), compared to canola, soybean, and peanut straw biochar (28.55%, 23.70%, 38.50%, respectively). The reaction temperature ranges of high temperature anoxic, hydrothermal synthesis, and flash carbonization for the preparation of biochar are 400-900 \({}^{\circ}\)C, 180-250 \({}^{\circ}\)C, and 300-600 \({}^{\circ}\)C, respectively [11]. In general, with the increase of pyrolysis temperature, the yield of biochar and the number of acidic functional groups (-COOH, -OH) decreased, while the alkaline functional groups, ash content, and pH increased. In addition, the effects of pyrolysis temperature on the surface area and pore volume are especially significant. [PERSON] et al. [24] showed that the specific surface area and total pore volume of sesame straw biochar increased from 46.9 to 289.2 m\({}^{2}\cdot\)g\({}^{-1}\), 0.0716 to 0.1433 cm\({}^{3}\cdot\)g\({}^{-1}\), respectively, with the pyrolysis temperature increased from 500 to 600 \({}^{\circ}\)C.  
According to the different heating rate, it could be divided into slow pyrolysis (SP) and fast pyrolysis (FP) [25]. SP is characterized by slow heating (minutes to hours) of the organic material in the oxygen-depleted atmosphere and relatively long solids and gas residence times [26; 27]. During the SP process, liquid and solid products such as char, bio-oil, and syngas (CO, CO\({}_{2}\), H\({}_{2}\)) are produced. The FP involves blowing small particles of organic material into a thermal reactor and exposing it to heat transfer in milliseconds to seconds [18]. Modern FP often takes place in fluidized bed systems, systems using ablative reactors, and systems using pyrolysis centrifuge reactors (PCR) [28]. Slow and fast pyrolysis results in biochars with different physicochemical properties, thus providing different effects on the soil environment upon application. Compared with the FP-biochar contained labile un-pyrolyzed biomass fraction, the SP-biochar can be pyrolyzed completely [26]. At the same pyrolysis temperature, the yield of biochar decreases with the increase of residence time. [PERSON] et al. [29] prepared orange peel biochar with the pyrolysis temperature of 700 \({}^{\circ}\)C and residence time of 6 h, and the biochar yield was only 5.93%. The specific surface area and pores of biochar increased with the extension of residence time. But the residence time is not as long as possible. [PERSON] et al. [30] found that the specific surface area and pores decreased from 2 to 3 h. The reason is that the increase of residence time is conducive to the development of biochar pores, but excessive residence time may cause damage to the pore structure [31]. The reaction atmosphere studied by scholars is dominated by inert gas, such as N\({}_{2}\), Ar, which mainly act to isolate oxygen. Besides, the atmosphere of CO\({}_{2}\), H\({}_{2}\)O, NH\({}_{3}\), O\({}_{3}\)[32] is also used to prepare biochar, which is known as physical activation, also called gas activation.