Lei Zhao,Zhong-Fang Sun,Xiao-Wen Pan,Jing-Yan Tan,Shan-Shan Yang,Jie-Ting Wu,Chuan Chen,Yuan Yuan,Nan-Qi Ren
Sewage sludge derived biochar for environmental improvement: Advances, challenges, and solutions
Sludge biochar,Environmental improvement,Application barriers,Strategies and prospects
Highlights • The current application status of sludge biochar was comprehensively reviewed. • The mechanism and capacity of sludge biochar were discussed. • The key challenges in the sludge biochar application were summarized. • The strategies for overcoming sludge biochar application barriers were put forward.
Sewage sludge derived biochar for environmental improvement: Advances, challenges, and solutions

• The current application status of sludge biochar was comprehensively reviewed. • The mechanism and capacity of sludge biochar were discussed. • The key challenges in the sludge biochar application were summarized. • The strategies for overcoming sludge biochar application barriers were put forward.

With the advancement in the world's economy and urbanization, sewage sludge as the main solid waste of urban sewage treatment plants has been increasing on a yearly basis. For example, in 2021 alone, the total amount of sludge in China reached 80.6 million tons (China, 2021). It is estimated that the annual amount of sludge may be higher than 90 million tons by 2025 (Zhang et al., 2020d). According to the definition of the U.S. Environmental Protection Agency (EPA) (USEPA, 2007), sewage sludge is reusable as it is rich in organic matter, nutrients, and various waste biomass materials containing trace elements. Yet, on the other hand, sewage sludge contains pathogens and heavy metal pollutants, as well as a large multitude of toxic organic chemicals that are harmful to humans and the ecological system. If treated improperly, it can cause serious secondary pollution to the environment. Presently, the most widely used sludge treatment methods include biological technologies (anaerobic digestion, aerobic composting), dewatering, and drying. The final destinations of disposed sludge were agricultural use, landfill, and construction materials. Table 1 shows how sewage sludge is disposed of in Europe, America, and Japan. As can be seen, turning sludge into resources is a future trend in these areas and all over the world. Biochar is an easy-to-prepare porous material with an aromatic compound structure. Compared to traditional incineration/combustion, which requires higher temperature (300°C–1000°C) and significant energy input, hydrothermally treated sludge is preferable due to the lower energy consumption, higher yield, and oxygen-containing functional group content (Chen et al., 2021). Previous studies have also found that sludge pyrolysis has great advantages over incineration in reducing heavy metal pollution at a pyrolysis temperature lower than 600°C (Chen et al., 2020). The properties of sludge-derived biochar are usually affected by factors such as moisture content, pyrolysis temperature, residence time, heating rate, etc. (Han et al., 2022; Hu et al., 2022; Sun et al., 2022b). Gaur et al. (2020) observed the carbon content decreased significantly with the increase of retention time at each tested temperature. Zheng et al. (2019a) showed that with the increase in reaction temperature, volatile matter decreased obviously. For example, when the pyrolysis temperature increased from 300°C to 700°C, the yield of sludge biochar decreased from 91.1% to 81.2% (Zhang et al., 2022b). In addition, recent researches reveal that improving pyrolysis temperature usually results in a larger biochar surface area. When the pyrolysis temperature rises from 500°C to 900°C, the porosity of the biochar increases from 0.056 cm3/g to 0.099 cm3/g, while the specific surface area increases from 25.4 cm2/g to 67.6 cm2/g (Patel et al., 2020). This phenomenon should be attributed to that the tar produced at low temperatures adheres to the surface of biochar and blocks the pores, while the tar gradually decomposes when the temperature rises, thus accelerating the formation speed of pores and increasing the specific surface area (Zhang et al., 2022a). While in some cases, the specific surface area and porosity decrease when the pyrolysis temperature rises. It may be due to the melting and collapse of the pore wall at excessively high temperatures. Similar to specific surface area and porosity, the pH of biochar also changes with variations in pyrolysis temperature and materials. The pH value of biochar has a significant effect on its function as an adsorbent to remove pollutants. When biochar is used to remove pollutants from water, the pH value of the solution significantly affects the surface charges of biochar. At higher pyrolysis temperatures, the alkali in sewage sludges is released from the structure, and the oxygen-containing functional groups decompose, resulting in a decrease in the number of acidic functional groups (Wang et al., 2019). It is also reported that the pH value of biochar is more significantly affected by its aromatization degree (Khan et al., 2020). With a rise in pyrolysis temperature, the alkalinity of organic anions in biochar also increases. The above changes in the properties of biochar also affect the high adsorption of biochar. For example, He et al. (2022) found that fine biochar (with a large specific surface area) has significantly higher NH4+-N adsorption capacity than medium and coarse biochar. However, Lee et al. (2022) found that the adsorption capacity of anions decreases with the increase of pH, while the adsorption capacity of cations increases with the increase of pH. With the advantages of high absorbability, stable chemical properties, large specific surface area, rich porous structure, and numerous surface functional groups, sludge derived biochar has a wide prospect of application. Despite numerous studies published on the production and properties of biochar, to the best of our knowledge, no extensive review is available on the application of sludge derived biochar on environmental improvement. Therefore, the objective of this paper is aimed to, in brief, provide a state-of-the-art review of current and emerging sludge biochar application fields that were effective in water pollution control, soil remediation, carbon sink and carbon emission reduction as shown in Fig. 1. Emphasis is put on the key challenges and new strategies for overcoming sludge biochar application barriers to realizing highly efficient pollutants removal, soil conditioning, and carbon dioxide mitigation.

The multifunctional characteristics of biochar show a very effective potential in the field of water environment remediation (Ahmad et al., 2014). Recent studies showed that biochar can be used as an adsorbent to remove a number of organic pollutants in water (Table 2). Generally, the absorption process and types of pollutants that can be absorbed by biochar depend on the physical and chemical structures of the biochar surface. The absorption mechanisms mainly include hydrophobic interaction, pore filling, electrostatic adsorption, and hydrogen bond (Qiu et al., 2022), as provided in Fig. 2.

Relevant studies show that if a pollutant has hydrophobic functional groups such as the methyl group, then it can be removed by biochar through hydrophobic interaction (Dai et al., 2019). Theoretically, the higher the pyrolysis temperature (> 500°C), the higher the hydrophobicity and aromaticity of biochar, the larger the specific surface area, and thus the easier it is to absorb hydrophobic and nonpolar organic pollutants. For example, when the pyrolysis temperature is increased from 350°C to 750°C, the surface functional groups, hydrophobicity, and crystallinity of biochar are increased, making it more effective to remove tetracycline (Choi et al., 2020). Compared with low-temperature pyrolyzed biochar, high-temperature biochar (700°C) with high aromaticity enhanced its antibiotic adsorption capacity (Premarathna et al., 2019). In particular, a high dose of biochar will produce more active sites, which will further enhance the adsorption of tetracycline (Kang et al., 2023), but excessive active substances may also cause self-quenching, leading to the removal rate of tetracycline decreases (Zhu et al., 2019).

Pore filling is considered another main mechanism by which biochar absorbs organic matter. Based on pore sizes, pores can be structurally divided into micropores (< 2 nm), medium pores (2–50 nm), and large pores (> 50 nm). Studies found that pore structures have a significant effect on pollutant absorption in water. Generally, biochar with medium pores is more effective in absorbing organic matter, while larger pores help increase water permeability. The pore size increases with the increase of pyrolysis temperature, and the pores are easily filled with organic pollutants to achieve a high removal rate. Xu et al. (2022) found that the larger the void volume of biochar, the larger the specific surface area, the smaller the average pore diameter, the stronger the pore-filling effect, and thus the better the removal of organic pollutants. During the pollutant removal in the liquid phase, the absorption of organic pollutants is related to the micro and medium pores on biochar. When removing microplastics, larger microplastics are stuck in the gap of biochar, and others are trapped in the pore of biochar. The colloidal property of biochar winds the microplastics to increase their size, so as to fix the microplastics for removal (Wang et al., 2020d). Moreover, the interaction between medium pores and hydrophobic pore walls also plays a significant role in removing pollutants.

Electrostatic adsorption is also one of the mechanisms for pollutant removal, which mainly rely on charges of the biochar surface and the electrical property of pollutants. There are clay minerals with high ion exchange capacity on the surface of biochar, which stimulate the adsorption of organic dyes, antibiotics, and other pollutants through the ion exchange (Premarathna et al., 2019). Recent studies showed that the functional groups on the surface of sludge biochar could affect the removal rate of anionic and cationic dyes. Wang et al. (2020a) demonstrated that electrostatic adsorption was the main driving force for the adsorption of acridine orange (AO) by biochar. Wang et al. (2021a) also concluded that the electrostatic interaction between biochar and microplastics dominates the removal of microplastics. To improve the removal of organic pollutants, the surface of biochar needs to be modified by functional groups with different electrical properties. In addition, given the charges on biochar surface are significantly affected by pH, the absorption of organic pollutants is also related to variations in pH. When pH < 7.95, the surface of sludge biochar activated by peroxydisulfate (PDS) is negatively charged, resulting in electrostatic repulsion between PDS and tetracycline, which reduces the removal rate of tetracycline. The degradation rate of tetracycline is the highest when pH is 5 (Kang et al., 2023). It is generally believed that low pH (i.e.< 3) is essential for the dissolution of heavy metals (Yu et al., 2022). Moreover, it should be noted that the electrostatic adsorption effect is mainly related to charges on the biochar surface and the electrical properties of heavy metal ions. As the biochar surface is largely negatively charged, positively charged ions such as Pb2+, Cd2+, and Cu2+ can be easily absorbed onto biochar due to electrostatic interaction. Lima et al. (2022) found that the maximum absorption of Cd2+ and Zn2+ by sludge biochar was 56.9 mg/g and 26.1 mg/g, respectively.

Hydrogen bonding represents another mechanism by which biochar absorbs organic matter. The ability of biochar to form hydrogen bonds with hydrogen atoms on pollutants (except heavy metals) is related to the existence of functional groups in biochar (Rangabhashiyam et al., 2022). Zhang et al. (2022b) found that when the pyrolysis temperature of biochar is lower than 350°C, more hydrogen bonds on the material surface can effectively interact with polar organic pollutants. Similarly, biochar prepared at a temperature lower than 500°C has more -C=O groups, leading to a higher degradation of sulfamethoxazole (Wang and Wang, 2021). In addition, the removal performance of biochar against organic pollutants can also be improved by modification. Xu et al. (2022) prepared biochar from sludge and modified sludge biochar with montmorillonite and nano zero-valent iron (nZVI), which produced more effective and stable functional groups on the surface of biochar, promoted the aggregation of free radical cations and improved the adsorption capacity of nitrogen and phosphorus to 34.8 mg/g and 294.1 mg/g, respectively. The adsorption mechanism is related to ligand exchange, electrostatic attraction, and ionic bonds. In addition, phosphorus (P) can be removed by chemical precipitation and co-precipitation of iron corrosion products by nZVI modification. Liu et al. (2021) activated sludge with sodium hydroxide and prepared sludge biochar by dry mixing method. The results showed that the biochar has good tetracycline adsorption performance and the maximum adsorption capacity can reach 379.8 mg/g. Further, the biochar prepared from Fe-Mn activated sludge has good absorptivity and biodegradability for Orange G (75.2%) (Hao et al., 2020). For example, Zhang et al. (2021b) formed Fe0/Fe3O4/biochar composite by pyrolysis of sludge and walnut shell. The composite has a porous structure and a large specific surface area (109.9 m2/g), and more -OH is produced on the surface, which can promote the degradation of methylene blue. Hung et al. (2022) also found that the oxidation of electron transfer, hydroxyl addition, free radicals (HO·), and non-free radicals in the system was enhanced after the use of biochar, which improved the degradation rate of phthalate esters (90%), indicating that the redox property of biochar and chemical structures of pollutants have a certain effect on the redox process of organic pollutants. Mian et al. (2020) found that N-doped sludge biochar can be used as a non-metal activator to effectively activate peroxymonosulfate (PMS) and thus oxidize and remove organic matter. Further, given the complicated composition of biochar, biochar materials may contain redox-active metals (e.g., Cu, Fe, and Mn), which can experience certain morphological changes in the biomass pyrolysis process and participate in the reaction in the activation of oxidant, thereby promoting degradation of organic matters. Therefore, biochar is a superior absorbent for organic pollutants and can effectively activate a number of oxidants to oxidize and decompose organic pollutants. In view of the influence of the synthesis method and substrates of biochar on its application, we summarized the elemental composition and characteristics of biochar in order to analyze its application value. It can be seen from Table 3 that sludge biochar produces more ash, which can be used as a biostimulator to remediate the environment (Zhang et al., 2020a). Therefore, sludge biochar has advantages in promoting pollutant remediation.

From the analysis of heavy metal content and leachable amount in biochar, it is believed that sewage sludge biochar might be also suitable for soil improvement (Phoungthong et al., 2016).Biochar can be stored in the soil for a long time due to its chemical and thermal stability (Shen et al., 2021). The inherent characteristics of biochar have a positive impact on the physical, chemical, and biological characteristics of the soil amendments, including soil aggregation, pH regulation, nutrient retention, etc., and lead to changes in microbial community structure (He et al., 2021). Its adsorption mechanism diagram is shown in Fig. 3. Meanwhile, by virtue of more adsorption active sites and functional groups of biochar itself, it can be used to remove and immobilize pollutants in the soil to achieve the purpose of soil remediation.

The improvement of soil by adding biochar is mainly reflected in the following aspects. Generally, the porous structure and large specific surface area of biochar reduce the soil bulk density, increase the porosity and aggregate structure, and improve the water-retention and supply capacities and hydrophobicity (Adhikari et al., 2022; Zheng et al., 2022). Tokova et al. (2020) added 20 t/ha biochar to a silver loam haplic luvisol, which significantly reduced the bulk density to 12% and increased the soil porosity to 12%. Zhang et al. (2020c) showed that biochar improved the soil aggregates, the micro aggregates and large aggregates in the soil increased by 65–86% and 67–89%, respectively, compared with the control. This led to an increase in soil physical structure, accelerated the growth of crop roots, and increased the total crop yield (10–16%). Recent studies have shown that the effect of biochar on improving water retention capacity is related to the biochar particle size and concentration. Mao et al. (2019) showed that adding 5% biochar can significantly improve the water-holding capacity and hydrophobicity of red and yellow soils. Vilas-Boas et al. (2021) added 10% biochar to sandy soil to achieve 345 g/kg water retention capacity. As a carbon-rich material, biochar can improve the organic carbon content in the soil. Additionally, the mineral elements rich in biochar can improve the soil nutrient content and effectively regulate the circulation of nutrients in the soil. Zhang et al. (2020c) added 20–40 t/ha biochar in the rice-wheat rotation field, the organic carbon content in the soil increased by 26–53%, and the content of other nutrients in the soil such as total nitrogen and phosphorus increased by 14–16% and 6–19%, respectively. Fang et al. (2020) used Mg/Ca modified biochar to absorb phosphorus in the acid extract of incineration sewage sludge ash for fertilization. Further research suggested that the modified biochar enhanced soil nutrient circulation and organic matter decomposition, improved the germination index of plant seeds, and promoted plant growth and development (Ahmad et al., 2022). It was found that the presence of mineral elements and functional groups in biochar promotes the alkali exchange capacity, thereby increasing the pH of the soil (Wu et al., 2020). In addition, the presence of a large number of hydroxyl and carboxyl groups and aromatic ring structures provides exchange sites for ions on biochar, thus improving the cation exchange capacity (CEC) of soil and affecting the absorption of nutrients by crops. Alves et al. (2021) added sludge biochar to the sandy loam and increased the CEC value from 3.04 to 6.34. Finally, a beet yield of 7.17 times higher than that without biochar was obtained. Biochar can be used as a slow nutrient release fertilizer for soil remediation and crop growth, because biochar can reduce the migration of water-soluble ions, nutrient loss through leaching, and use of soluble mineral sources (Luo et al., 2021). Therefore, the nutrient can be slowly released into the soil for a long time, thus contributing to improving nutrient absorption and crop productivity. Fachini et al. (2022) studied the release of potassium (K) from the sewage sludge biochar-based fertilizer rich in potassium chloride to the natural silica sand. The biochar reduced the potassium release rate by 77%, improved the potassium utilization efficiency, reduced the leaching amount of potassium, and reduced the risk of this nutrient polluting the groundwater.

In recent years, the application of biochar to remove various pollutants from soil has received extensive attention and has been considered an environmentally friendly and cost-effective curing agent, which can directly remove pollutants from the soil by adsorption of available active sites on the surface of materials (El-Naggar et al., 2021; El-Naggar et al., 2020). Wang et al. (2021b) studied the use of Pb adsorption sites composed of phosphorus compounds on the surface of sewage sludge/cotton straw biochar to fix the Pb in the soil. In addition, biochar increased the content of total organic carbon in the soil and formed more Pb complexes, thus further improving the stabilization efficiency of Pb. The application of biochar can also reduce the content of exchangeable heavy metals and their biological effects, improve nutrient utilization, and promote crop growth. Hale et al. (2020) reduced the content of Al3+ significantly after adding biochar to aluminum-rich soil. Furthermore, it is worth noting that the metal is concentrated during the preparation of biochar. The number of metals in biochar introduced into the soil is therefore presumed to be much more than that in the original sludge. However, in the study of Zhang et al. (2021a), the use of biochar can significantly reduce the accumulation of metals in the edible part of crops (corn and radish), making it meet the edible content standard (hazard index < 1). The formation of oxygen-containing functional groups (C-O/C-OH and C=O) and the change of pH in the soil after adding biochar can realize the adsorption and removal of heavy metals. Aihemaiti et al. (2022) used ferrous sulfate modified sludge biochar to migrate vanadium (V) in the contaminated soil of the mining area. The iron modified biochar provides an amorphous iron oxide with high adsorption capacity, thus further fixing V in the soil. The results showed that the water extractability and acid solubility of vanadium in the contaminated soil was reduced by 99% and 95%, respectively, and the concentration of vanadium was reduced from 340 mg/kg to 14.2 mg/kg. For the degradation of PPCPs in soil, the effect of biochar has also been studied. Chang et al. (2021) used iron manganese oxide modified biochar to increase the residual amount of phthalate in the soil, which is related to the improvement of bacterial community structure by biochar. It has also been confirmed that the removal of polyfluoroalkyl substances (PFAS) from the soil by sewage sludge biochar is also an economic and environmentally sustainable waste management option (Krahn et al., 2022).

Biochar can provide shelter, nutrition, and a comfortable living environment for soil microorganisms (Zheng et al., 2022). The porous structure and high specific surface area of biochar can protect the attachment, and growth of soil microorganisms from the impact of external adverse conditions such as soil pollutants. In addition, nutrient elements (C, N, P, K), ash, and minerals are additional nutrient sources to promote microbial growth. The addition of sludge biochar can improve the state of microorganisms to achieve the degradation and fixation of pollutants in soil. For example, Aihemaiti et al. (2022) have proved that adding biochar can promote the abundance of Preoteobacteria and significantly reduce the migration of V (V(V) and V(IV)) in soil. Diao et al. (2022) added sludge biochar to paddy soil, which affected the diversity and species of soil microorganisms (Actinobacteriota, Pontibacter, and Alkaliphilus), and reduced the bioavailability of Cd and Pb. Ali et al. (2019)found that the addition of sludge biochar to the soil had a strong impact on the accessibility of organochlorine pesticides (OCPs) (8–69%), and enriched the soil microbial community (Proteobateria, Firmitutes, Gemmatimonades, and Actinobacia). The improvement of enzyme activity also proves the efficiency of biochar. Paz-Ferreiro et al. (2012) monitored the activity of several enzymes that can reflect the soil quality index, such as dehydrogenase, beta-glucosidase, phytoestrase, and arylsulphatase, and found that the enzyme activity was promoted after the addition of sludge biochar. Du et al. (2019) also confirmed that the addition of sludge biochar can improve the activity of aryl sulfase, beta-glucosidase, and dehydrogenase enzymes, thereby improving the removal efficiency of total carbon, N, and S. In addition to improving microbial activity and enzyme activity, biochar, with its redox-active functional groups, graphitized structure, and redox-active metals, can also be used as an electronic medium to promote the electron transfer process, so as to achieve microbial degradation of pollutants in soil. An et al. (2022) found that the addition of biochar improved the electron transfer ability of microorganisms, and concluded that the electrochemical properties of biochar were important factors affecting the microbial community structure. Tan et al. (2019) used iron-containing biochar to significantly accelerate the electron transfer from microbial cells to Orange G, thereby increasing the reduction removal rate (72–97%). It is proposed that the conductive domain and the charge discharge of surface functional groups in the biochar are the key points for microorganisms to reduce Orange G.

With increasing greenhouse gas emissions and climate change, the question of how to control the emissions of greenhouse gases has been a worldwide crucial problem. Research shows that the continuously increasing carbon content in the atmosphere is mainly caused by the loss of soil carbon sink during large scale land use (Wani et al., 2022). As a stable carbon-rich substance, biochar can fix carbon elements in both preparation and storage processes to prevent carbon from entering the atmosphere, thereby effectively improving soil carbon sink. Adding biochar to the soil will reduce methane and carbon dioxide emissions and fix carbon in the soil, but this is limited to soil with only low organic carbon (Schimmelpfennig et al., 2014). In the study of Hu et al. (2023), the fixed amount of CO2 in soil increased by 50% after adding biochar, because the carbon in biochar is difficult to be degraded, the mechanism behind the carbon change remains to be solved. Meanwhile, adding biochar to soil has a significant impact on the decomposition of organic matter in the soil, and further improves the storage of soil organic carbon. This can be confirmed by several aspects: (1) Compared with organic carbon in natural soil, unstable carbon remaining in biochar obtained under incomplete pyrolysis is preferentially used; (2) Biochar induced the change of microbial community related to soil organic carbon degradation (Yang et al., 2022); (3) Biochar promotes soil aggregation and reduces enzyme activities related to carbon degradation (Gross et al., 2022).

CO2 capture is considered a potential strategy to reduce the amount of CO2 released into the atmosphere (Guo et al., 2022). Sludge is converted into biochar through pyrolysis, rather than being directly burned or mineralized, which also substantially reduces carbon dioxide emissions. Additionally, biochar is a kind of renewable resource with favorable adsorption performance. As an efficient and environment-friendly CO2 adsorbent, it has many advantages such as low energy demand, low cost, multi-porous structure, and high wide availability (Qiao and Wu, 2022). Compared with other adsorbents such as amine-supported silica, carbonaceous materials, zeolites, and metal-organic frameworks, biochar is more stable, highly aromatized, and carbon-rich (Lee et al., 2020), and is expected to become an economically and environmentally friendly substitute (Guo et al., 2022). The adsorption of carbon dioxide (CO2) by biochar is classified as physical adsorption and chemical adsorption. The adsorption mechanism diagram of biochar in CO2 mitigation is shown in Fig. 4. (1) CO2 physisorption The excellent pore structure of biochar play an important role in the physical adsorption of CO2. The pore structure, especially the micropores with a diameter of less than 1 nm, is closer to the dynamic diameter of CO2 molecules. Moreover, because of the overlapping adsorption force and potential field of adjacent pore walls, it has a stronger attraction to CO2 (Guo et al., 2016). Therefore, the physical adsorption can be enhanced by adjusting the pore distribution of biochar (Guo et al., 2022). Serafin et al. (2017) found that by adjusting the pyrolysis temperature and pressure, the biochar showed different micropore sizes, resulting in different sensitivity of carbon dioxide adsorption. At 0°C and 1 atm, only the micropores within the range of 0.30–0.86 nm are most effective for carbon dioxide capture. When the temperature rises to 25°C, the optimum pore size is 0.30–0.33 nm. In the study of Igalavithana et al. (2020b), the sludge posed a more microporous structure of 0.5 nm after pyrolysis at 550°C and steam activation for 45 min, achieving a CO2 adsorption capacity of 2.5 mol/kg. According to the adsorption kinetics test, the CO2 adsorption is finally in equilibrium, indicating that the physical adsorption of biochar is stable. Liu et al. (2022) prepared sludge biochar at the pyrolysis temperature of 600°C. The micropore volume was 0.019 cm3/g, respectively, and the CO2 adsorption capacity reached 28.4 mg/g. Proper adjustment of sewage sludge can promote biochar utilization as an adsorbent. Liu et al. (2022) selected two commonly used dehydration regulators (cationic polyacrylamide and polymer aluminum chloride) to treat raw sludge. The results showed that the micropore volume and micropore surface area of biochar were significantly increased, reaching 0.025 cm3/g and 41.2 m2/g, 0.022 cm3/g and 40.1 m2/g, respectively. Meanwhile, the CO2 adsorption capacity of 48.5 mg/g and 31.9 mg/g were achieved. The CO2 absorbed into the pores of biochar can be released through desorption for further utilization, such as direct utilization (fire extinguishing, cooling), indirect utilization (carbonation, curing of cement), and conversion into value-added compounds. In addition, the recyclability of carbon dioxide can be improved through multiple adsorption-desorption cycles (> 99%) (Igalavithana et al., 2020a). (2) CO2 chemisorption The CO2 chemisorption depends on the surface chemical properties of biochar, such as alkaline earth metals, surface functional groups, alkalinity, hydrophobicity, non-polarity, and aromaticity (Igalavithana et al. 2020b; Shafawi et al., 2021), and CO2 is adsorbed by the heterogeneous interaction between CO2 and the biochar surface. However, it is worth noting that the interaction between CO2 and adsorbent decreases with the increase of CO2 load. Therefore, the adsorption capacity of biochar for CO2 will get a discount when over CO2 volume is loaded (Serafin et al., 2017). Increasing the surface alkalinity will change the surface functional groups to improve the interaction with CO2. The CO2 can be converted into carbonaceous compounds, while reducing the oxygen and hydrogen content, the hydrophobicity and low polarity of biochar were greatly improved, which in turn improved the CO2 adsorption capacity of biochar. In addition, the larger specific surface area provides more active sites for CO2 adsorption, which is conducive to CO2 reduction. The Lewis acid-base interaction and condensation between nitrogen-containing and oxygen-containing functional groups on biochar and CO2 is the most important chemisorption interaction (Shafawi et al., 2021). Alkali elements and alkaline earth metals (Ca, Mg, K, Na, etc.) can improve the alkalinity of biochar, thereby improving the affinity for acidic CO2 and enhancing the adsorption capacity for CO2. Xu et al. (2016) pyrolyzed sludge to produce alkaline biochar, in which Fe, K, Na and other mineral components can induce chemical adsorption of CO2 through mineral reaction. For example, FeOOH in biochar adsorbs CO2 and converts it into Fe(OH)2CO3, and its adsorption capacity can reach 18.2–34.4 mg/g. Different materials and pyrolysis conditions determine the existence or types of functional groups in biochar. Basic functional groups and oxygen-rich functional groups play a role in promoting the CO2 adsorption capacity of biochar. In addition, common nitrogen-containing functional groups (amide, imine, pyridine, pyrrole, and lactam groups) in biochar can also be used to improve its CO2 adsorption capacity. Li et al. (2022a) reported that more nitrogen content was detected on the surface of biochar produced by sludge pyrolyzed at 700°C, forming more nitrogen-containing functional groups and strengthening its adsorption of CO2. After KOH modification, the adsorption rate of biochar on CO2 is faster, and the adsorption capacity almost increases vertically with time, finally reaching a plateau. Xu et al. (2020) found the maximum CO2 adsorption capacity of N-doped biochar was 49.2 mg/g, which was 55.2% higher than that of original biochar. The presence of heteroatoms (O, K, N, and S) in biochar also plays a positive role in CO2 adsorption (Li et al., 2022b), in which the presence of N and S on the surface of the biochar plays a greater role (Igalavithana et al., 2020a). This is mainly because heteroatoms can further modify the surface area and pore structure of biochar, enhance the hydrophobicity of biochar and improve the electron transfer rate (Ochedi et al., 2020; Petrovic et al., 2022).

The pollutants in soil and water, and the CO2 emissions are typically in the range of 791.71–2071.31 kg/ha, 7.27–42.76 t/day, 13.3–16.2 kt (Song and Pang, 2021; Tian et al., 2022; Yirong, 2022), respectively, while the adsorption capacity of sludge-derived biochar, is generally less than 379.8 mg/g (Table 2). A higher adsorption rate is required to make the industrial application of sludge-derived biochar more economically viable and technically feasible. Therefore, one of the major challenges for sludge-derived biochar on environmental improvement is the poor adsorption performance from the target compounds to the biochar, caused by the limited adsorption capacity. This issue is particularly serious for a large number of pollutants due to the limited biochar preparation. Even massive sludge-derived biochar is introduced into systems, biochar is only available to superficial margins of the target compounds due to limited binding sites in the biochar matrix The typical equation for describing the rate of biochar adsorption is shown in Eq. (1):where Qe represents the equilibrium adsorption capacity, mg/g, C0 and Ce represent the initial and equilibrium pollutant concentrations respectively, mg/L, V represents the solution volume, mL; m represents the mass of the added biochar, g (Chen et al., 2022). Moreover, proper biochar recycling equipment is needed, since unused biochar is easily left in the system. The resultant low biochar adsorption capacity will significantly increase the cost of biochar supply, therefore decreasing the economic benefits.

The sludge-derived biochar on environmental improvement has the advantage of simplicity. However, restrictions associated with possible toxicity, energy consumption, and cost input are big issues in sludge-derived biochar applications. The possible toxicity of biochar on the environment and human health is necessary to determine whether biochar can be applied on a large scale. As given in Table 4, the presence of pollutants and toxic substances in biochar, such as volatile organic compounds (VOCs), heavy metals, potentially toxic elements (PTEs), environmentally persistent free radicals (EPFRs), polycyclic aromatic hydrocarbons (PAHs), and dioxins depend on the source of raw sludge and pyrolysis conditions (Zheng et al., 2019b). In particular, the biochar pyrolyzed from sludge also contains heavy metals, toxic substances, and a large amount of nitrogen and phosphorus, which is easy to cause water eutrophication and soil contamination, resulting in threats to the safety of animals, plants, or human beings (He et al., 2019; Hu et al., 2022; Qian et al., 2022). Therefore, when using sludge biochar for pollutant removal, soil remediation, or carbon emission reduction, the composition and quantity of pollutants should be thoroughly determined. The cost and energy consumption of sludge biochar pyrolysis is a significant part of the application. The preparation of biochar from sludge pyrolysis are multiple processes, such as feedstock supply, biochar pyrolysis, transportation, maintenance, storage, and even regeneration. For instance, it was found that the lower raw material input is beneficial for transportation and storage, and is more conducive to achieving higher pyrolysis energy efficiency (Li et al., 2023). Moreover, the pyrolysis temperature largely determines the output of biochar (Zhang et al., 2020e). This phenomenon means how to balance energy consumption, biochar output, and biochar application efficiency is also an issue that needs to be considered in the biochar application.

(1) Biochar modification  Original biochar can be directly used to adsorb pollutants, heavy metals, and CO2, but it usually shows poor adsorption performance, which limits the large-scale application of biochar for adsorption. Therefore, in practical application, it is necessary to modify biochar to improve its physical and chemical properties, such as improving the specific surface area, increasing pore structure, and embellishing surface functional groups (Serafin et al., 2021; Shafawi et al., 2021). At present, biochar modification includes physical modification, acid modification, alkali modification, mineral modification, nitrogen doping modification, etc. (Hu et al., 2022). In the study of Beckinghausen et al. (2020), the sludge biochar was steam activated, and the results showed that the specific surface area increased by nearly 2.5 times. Ma et al. (2020) modified sludge biochar with phosphoric acid, its specific surface area increased from 14.0 m2/g to 41.6 m2/g, and its adsorption capacity for ciprofloxacin increased from 11.2 mg/g to 15.0 mg/g. KOH modification could increase the porous characteristics of biochar, including both surface area and porosity. It was found by (Li et al., 2022b) that the CO2 adsorption capacity of the original biochar is 35.5–42.9 mg/g, while the value increased to 136.7–182.0 mg/g after KOH activation. The specific surface area of the biochar is 3.9–14.5 times increased, and the micropores formed also assist in enhancing the adsorption capacity of CO2. Another study used 4 mol/L NaOH solution to modify sludge biochar, the specific surface area was increased from 10.9 m2/g to 135.6 m2/g, and its maximum Cd2+ adsorption capacity reached to 5.8 mg/g. Wang et al. (2020b) demonstrated that sulfur doping can promote the formation of metastable biochar persulfate complex and enhance the degradation of bisphenol A. (2) Co-pyrolysis The effect of biochar is dependent on the type of contaminants. Therefore, it is also worth mixing the sludge with other catalysts or co-pyrolysis with multiple substrates to produce biochar. For example, zero-valent iron (ZVI) can be used as a filler to eliminate heavy metals due to its excellent reduction and adsorption properties (Song et al., 2021). Studies have shown that mixing biochar with ZVI can remediate polluted paddy soil mildly and moderately (Qiao et al., 2019). The combination of composite materials can change biochar into different shapes such as porous biochar, biochar-based beads, and biochar-based hydrogels, so as to adapt to special environments that may lead to the loss of biochar (Sun et al., 2022b). The separate pyrolysis of sewage sludge has the disadvantages of low energy efficiency and high nitrogen content. While the co-pyrolysis of sewage sludge and biomass can eliminate these problems by virtue of the synergy effect (Qiu et al., 2020). In the study of Naqvi et al. (2019), the co-pyrolysis of sewage sludge and rice husk showed lower activation energy (45.1 kJ/mol–65.8 kJ/mol), that is, the biochar can be synthesized faster with lower energy consumption. Additionally, synergistic pyrolysis could also recover organic components more effectively. Zhang et al. (2020b) found that the co-pyrolysis of sludge and rice husk is more suitable for immobilization with metals, and the co-pyrolysis of sludge and bamboo sawdust is conducive to the increase of the aromatization of biochar. Furthermore, the co-pyrolysis of sewage sludge and other biomass can achieve more carbon fixation, improve carbon storage, and reduce carbon emissions into the environment (Sun et al., 2022a). Igalavithana et al. (2020b) co-pyrolyzed the sludge and pine sawdust at 550°C gave the pore diameter of 0.4–0.5 nm and surface area of 581.7 m2/g, which resulted in the CO2 adsorption capacity increased to 0.7 mmol/g. Konczak et al. (2020) produced biochar by co-pyrolysis of sludge and CO2, which can effectively reduce the toxicity of biochar and alleviate the greenhouse effect caused by CO2.Although modification methods can greatly improve the feasibility and effectiveness of biochar application, these strategies mean additional process and resource consumption. Therefore, developing more economical and feasible sludge biochar modification methods is an important development direction in the future.

The major challenge for the application of sludge-derived biochar is the generation of toxic compounds. The toxic compounds are generally produced through the following two pathways. Surface reactions that occur between biochar and the target compounds may lead to the release of harmful substances (Xiang et al., 2021), which further cause potential risks to the ecological environment. The free radicals of biochar itself may also induce reactive O2 species that are toxic to aquatic plants released into the water. To solve the above issues, several strategies can be addressed: (I) Pretreatment of sludge prior to pyrolysis (such as acid-elutriation (Yu et al., 2022), advanced oxidation (Wen et al., 2022), hydroxylamine enhanced Fenton oxidation (Yu et al., 2020) (Sun et al., 2022b). (II) Using aging and thermal post-treated biochar to remove polycyclic aromatic hydrocarbons and volatile organic compounds (Han et al., 2022). (III) Choosing precursors with fewer pollutants for pyrolysis. In general, proper pyrolysis unit design can also reduce the content of PAHs, VOCs, dioxins, and persistent free radicals (PFRs), so as to produce biochar safely (Guo et al. 2022; Han et al. 2022). The key factors to evaluate the feasibility of biochar application are the environmental impact, energy consumption, and economic analysis involved in the sludge derived biochar pyrolysis. With the characteristics of no gas turbine and no reagent cleaning, sludge biochar greatly reduces the emissions of greenhouse gases, ozone, and high concentrations of Zn and Cl, thus showing lower environmental impact than most traditional sewage sludge treatments (Huang et al., 2022). It should be noteworthy that the market price of biochar is still not comparable to the production price (0.78 CAD/kg biochar) (Huang et al., 2022). Therefore, the economic benefits and energy recovery benefits of biochar production from sewage sludge can be improved by introducing bio-oil, natural gas and other sewage sludge by-products into the pyrolysis process (You et al., 2020).

Pyrolyzing sewage sludge to biochar improves the physicochemical properties of sewage sludge and broadens the range of applications. Compared to the challenges of traditional methods that need expensive catalysts or intensive energy, sewage sludge derived biochar is promising particularly from a long-term perspective, considering that the input-to-output ratio is continuing to decline. Technical bottlenecks for implementing large-scale sewage-derived biochar for environmental improvement include adsorption capacity and application limitations. In order to address these issues, improved biochar modification and appropriate co-pyrolysis are necessary to enable biochar adsorption capacity improvement and pollutants removal selectivity. Moreover, integrating pretreatments and post-treatments with biochar preparation is a promising strategy to avoid toxic compound contamination during environmental improvement. Finally, the whole life cycle assessment of the aforementioned biochar preparation and application process is required to further evaluate its economics and environmental feasibility. This review summarizes recent trends in the research on sewage sludge biochar. It is concluded that sludge biochar has high efficiency, strong applicability, and environment-friendly significance in the field of environmental improvements such as water pollution control, soil remediation, and carbon emission reduction. In view of the challenges in applying biochar, new strategies such as feedstock selection and pretreatment, co-pyrolysis, and biochar modification are proposed. It is expected that the insights given in this review will promote more efforts contributed to sludge biochar preparation and modification, thus boosting its application in environmental improvement.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.