In general, pyrolysis refers to the thermal decomposition of organic matter in the absence of oxygen at temperatures over 400°C (Maschio et al., 1992; Thines et al., 2017). Three substances are produced, which consist of a residual solid biochar, a liquid product commonly known as bio-oil, pyrolysis oil, or biocrude, and a non-condensable gas known as syngas, which consists of carbon monoxide (CO), carbon dioxide (CO₂), hydrogen (H₂), and methane (CH₄). The pyrolysis process is generally divided into two stages. In the first stage, heat is transferred to the surface of ions by radiation, from the surface to the interior. During the main pyrolysis and preheating solution, the volatiles and syngas flow through the particles in the void due to the loss of biomass water due to the increase of temperature. The process of heat transfer is a function of time. The second stage of the pyrolysis process begins with the expansion of the solid voids and the conversion of biomass into gas (Blasi, 2008). The expansion of pores provides a site for volatile gases produced during the pyrolysis process. There are three main mechanisms for heat transfer, conduction inside the biomass particle, convection inside the pores of the biomass particle, and the convection and radiation from the surface of the final product.

Accordingly the different operations can be divided into three types: conventional pyrolysis, fast pyrolysis, and flash pyrolysis, as shown in Table 1. The heating rate of traditional pyrolysis is relatively slow which allows for the production of solid, liquid, and gaseous products in compelling portions (Goyal et al., 2008). Traditional pyrolysis can be divided into intermittent pyrolysis and continuous pyrolysis. In the latter stage, few internal repeats occur, such as the breaking of bonds, the appearance of free radicals, the elimination of water, and the formation of hydrogen peroxide (Shafizadeh et al., 1982). The main pyrolysis mainly takes place in the second stage of the high-rate decomposition of biomass. The third stage is the slow decomposition of the carbon to form a carbon-rich residue known as biochar. The overall yield of conventional pyrolysis would be approximately 35% biochar, 30% bio-oil, and 35% syngas by mass (Laird et al., 2009). Bio-oil mainly exists in the state of steam and aerosol, and cannot be separated from syngas. If this gas is discharged into the environment, it will cause serious environmental pollution. In contrast, when production is mainly concentrated on liquid or gas products, rapid pyrolysis is chosen (Liu et al., 2021d; Liu W. et al., 2021). In the process of rapid pyrolysis, high operating temperature, short contact time, fine particles, and other operating conditions are required if rapid heating is desired (Babu, 2008). The overall yield of fast pyrolysis would be approximately 50–70% bio-oil, 10–30% biochar, and 15–20% syngas by mass (Babu, 2008). Raw materials need to be dried and ground to a size less than 2 mm before entering the main system of the pyrolysis reactor. However, the temperature of rapid pyrolysis increases rapidly and the particles are sparse. So rapid pyrolysis to produce gaseous products is a good choice (Demirbas et al., 2002). The overall yield of flash pyrolysis would be approximately 60% biochar and 40% volatiles by mass. This process consists of a gasifier which allows a small, limited amount of oxygen to enter the reaction chamber which causes partial combustion of biomass, producing 5–15% char and traces of bio-oil which are referred to as “tar” (Laird et al., 2009). As for laboratory scale, the pyrolysis process is generally done either through conventional heating or microwave heating. The product decomposition diagram is shown in Figure 2.

Co-precipitation is the process by which a solute is separated from a solution by means of a carrier, by which the solute is bonded to the solute rather than dissolved in the solution. The solute is called an impurity. This kind of co-precipitation is used in purifying solution and industrial wastewater (Xu R. et al., 2021). A solute can be co-precipitated out from a solution through three common methods such as inclusion, occlusion, and surface adsorption (Kolthoff, 1932). Inclusion or also known as the formation of mixed crystals is a process which consists of the incorporation of solute in the crystal lattice of the carrier through a hole in which the regular structure of the carrier remains unchanged. This combination forms a mixed crystal in which the amount depends on the adsorption phenomena during the precipitation process.

Apart from that, during the occlusion process, the solute remains incorporated into the crystal lattice of the carrier, instead of the carrier being completely surrounded by the solute during the formation of the crystal matrix, entrapping the solute from returning to the solution. The surface adsorption process on the other describes how the solute is attached to the surface of the carrier and moves out from the solution only when the crystal matrix forms a large surface area and behaves like a flocculated colloid (Wang P. et al., 2020). In general, the precipitation process is a phenomenon that occurs when a substance reaches a certain saturation, followed by the slow growth of the nucleus as the solute diffuses to the surface of the crystal (Sugimoto, 2003). In addition, some organic ions or polymer surface complexing agents can be added to control the size of the magnetic material (Cai and Wan, 2007). Due to the simplicity of the co-precipitation process and the wide range of operating parameters, the required particle size and characteristics can be obtained for the production of magnetic biochar. Massart et al. (Massart, 1981) were first to perform a controlled preparation of superparamagnetic iron oxide particles utilizing the alkaline precipitation of FeCl₃ and FeCl₂ which managed to produce spherical magnetite particles with a diameter in the nanometer scale. In addition, another study by Babes et al. (Babes et al., 1999) showed that if the ratio of Fe²⁺ to Fe³⁺ increased, the particle size of the magnetic biochar increased, but the yield of magnetic biochar decreased (Wang et al., 2021). The extensive growth of this simple yet significant method which controls the particle size of magnetic particles being produced provided space for researchers to perform studies on the production of magnetic biochar based on various biomasses. In this regard, Saravanan et al. (Saravanan et al., 2012) discovered diamond glue, took out a little biological sample, and co-precipitated the ammonia solution (2:1) with Fe²⁺ and Fe³⁺ to produce magnetic iron oxide particles. This magnetic material had good thermal stability compared with the star polymer containing iron oxide particles. Similarly, Mohan et al. (Mohan et al., 2011) prepared magnetic biochar from almond shells by chemical precipitation (FeCl₃ and FeSO₄). The magnetic biochar had a spongy porous structure and the non-magnetic biochar had a porous shape. These remarkable characteristics provide a good source of magnetic biochar with good adsorption properties.

The reduction co-precipitation process is similar to the co-precipitation process, but the difference is that the transition metal is reduced by a reducing agent such as sodium borohydride or potassium borohydride in the process of binding to the biochar (Wang P. et al., 2020). When the reaction is terminated, the supernatant is removed and the residue is cleaned and dried in a true empty tank to achieve the magnetic biochar. Interestingly, this material is composed of nanoparticles and most of the zero-valent metals, making the magnetic biochar produced highly reductive, and also highly effective at contaminant removal. For example, Zhu et al. (Zhu S. et al., 2018) found that the Cr adsorption capacity of reduced co-precipitated magnetic biochar was 58.82 mg/g, and that most of the Cr (VI) was reduced to Cr (III). Therefore, this method can synthesize magnetic biochar with a better effect and stronger ability to remove pollutants. However, the used reducing agent is harmful and will produce hydrogen during the reduction process, which will pose a certain risk to safety when used on a large scale.

Hydrothermal carbonization refers to the heterogeneous reaction of biomass and metal ions, because the reaction temperature (100–300°C) is relatively low, the reaction pressure is generated by the reaction itself (An et al., 2019). These conditions are milder than the previous conditions, there is no need to add bases or strong reductants, making the reaction easier. For example, Zhang et al. (Ngarmkam et al., 2011; Bastami and Entezari, 2012) successfully synthesized magnetic biochar by this hydrothermal method from iron-containing sludge and sludge at 473°C, and used the resulting product as a Fenton-like catalyst to completely degrade methylene blue. Similarly, Nethaji et al. (Nethaji et al., 2013) found that magnetic biochar synthesized using this method had a maximum adsorption capacity for Cr (VI) of up to 142.86 mg/g, which was greater than that of most magnetic biochar prepared by co-precipitation (Baig et al., 2014), reductive co-deposition (Devi and Saroha, 2014), or impregnation pyrolysis (Quan et al., 2014).

Recently, other methods for preparing magnetic biochar have also been developed, such as ball milling, in which biochar is mechanically mixed with iron oxides in a solvo-free manner (Shang et al., 2016), direct pyrolysis of biomass/metal salts, and cross-linking of biochar and iron oxides (Mojiri et al., 2019). For example, Shang et al. (Shang et al., 2019) found that the adsorption capacities of magnetic biochar synthesized by ball-milling of biochar and iron oxide for the pharma-ceuticals carbamazepine (CBZ) and tetracycline were 62.7 mg/g and 94.2 mg/g, respectively. In addition, Dai et al. (Dai S.-j. et al., 2019) demonstrated that the herbicides dichlorophenol and atrazine were very efficiently removed by magnetic biochar synthesized by the molten salt method.

To improve magnetic biochar’s ability to heal in the environment more quickly, the researchers treated it with metal or acid or alkali solutions, and also reacted it with different chemical functional groups, in order to improve adsorption selectivity and capacity. A graphical summary of these various methods for the synthesis of magnetic biochar is shown in Figure 3.

Hydrothermal carbonization, reductive co-deposition, co-precipitation, pyrolysis, and other synthetic methods are also used to obtain magnetic biochar. Therefore, the nature of the raw material, the physicochemical properties of the pollutants, and the method operability should be carefully considered when selecting a synthetic approach for magnetic biochar.

Magnetic biochar has also been used as a catalyst for hydrogen peroxide, producing a highly reactive oxidizing substance that is used to degrade organic matter. For example, Chen et al. (Chen et al., 2018) found that the antibiotic ofloxacin could be efficiently degraded by magnetic biochar and persulfate. Meanwhile, Zhang et al. (Zhang et al., 2018) demonstrated that methylene blue can be completely removed by magnetic biochar coupled with hydrogen peroxide. Because different organisms have different physical and chemical properties, some organisms cannot remove organic pollutants at all by using female organisms. So, we can take advantage of the function of the female organism, the catalytic property, which is to produce hydroxyl radicals inside of the organism in time, to produce strong oxidizing radicals that attack organic pollutants freely, in a way that maximizes the function of the magnetic biochar. If it is difficult to remove organic pollutants by using magnetic biological carbon, its catalytic properties can be considered to achieve efficient degradation of organic pollutants.

In addition to the above functions, magnetic biochar can also be used as a photocatalytic carrier, for example, Li M. et al. (Li M. et al., 2019) prepared the Fe₃O₄/BiOBr/BC photocatalyst using magnetic biochar as a carrier and found that its removal efficiency of propranolol was nearly 95%. Similarly, magnetic biochar can effectively recover precious metals by adsorption and enrichment. For example, Zhang et al. (Zhang et al., 2019) found that the adsorption capacity of magnetic biochar for silver ions was 818.4 mg/g.

It is very interesting that Qin et al. (Qin et al., 2017) used magnetic organisms in the process of anaerobic digestion, but as an additive, they found that a small amount of magnetic biochar promoted the production of methane on anaerobic digestion, and finally found methanogenic bacteria that produced methane, which facilitated recycling. And finally, female organisms could not just be used for environmental remediation, but also for energy storage (Hu et al., 2019; Lu et al., 2019). For example, Thines et al. (Thines et al., 2017) found that the density capacitance and energy could be increased when magnetic biological carbon was used as an electric conductor. Meanwhile, Jiang et al.‘s (Jiang et al., 2019) research results showed that hydrogen could be effectively recovered in the process of removing nitrophenol. Although magnetic biochar has a good adsorption effect, due to the release of a lot of heat in the preparation process, the yield is low, and the adsorption may not be complete, difficult to recover, and may cause secondary pollution to the environment. The large-scale application of magnetic biochar is limited.