In this review, the update progress of biochar, regarding the structural characterization, reactivity investigation, and functionalization and device of biochar, as well as their applications in environmental fields were summarized. For the first time, structural properties of biochar were discussed at macroscopic and microscopic levels. From the macro-scopic view, biochar is made of particles with different sizes including bulk biochar, dissolved biochar and nano biochar. From the microscopic view, the multilevel struc-tures of elements, phases, surface chemistry, and molecular were proposed. The structure and properties of biochar play an important role in determining its reactivity, which con-sisted of chemical activity, such as sorption, catalyst, and redox reaction, and biological activity in terms of interac-tion with microbes. There is a close relainterac-tionship between biochar structure and its reactivity. Particularly, how the qua-ternary structure of biochar influences the chemical activity is discussed. To overcome the shortages (weak mechanical strength, difficult to separate, easy transportation, and poten-tial toxicity) of direct environmental application of pristine biochar particles, functionalization and device of biochar are highlighted. The synthesis methods and environmental application of magnetic biochar, biochar-based 2D mem-brane and 3D macrostructure, immobilized microorganisms on biochar, and biochar-based biofilters are illustrated. The developed biochar-based materials and devices afford new solutions to challenging environmental problems.
The full understanding of biochar’s structure and reac-tivity, and functionalization of biochar and its applications are still wanted. Until now, no undisputed molecular struc-tural model has been put forward for biochar despite some dedicated efforts (Xiao and Chen 2017). This hinders the in-depth understanding of the reactivity of biochar and the functionalization of biochar for environmental applica-tion. Many scientific and technological challenges remain and attract considerable research attention. Figure 10
Table 2 Comparison of biochar and other substrate or amendment in biofilters
Property and function comparison Soil Gravel Compost Activated carbon Biochar
Source and availability ★★★ ★★★ ★★ ★ ★★
Low price ★★★ ★★★ ★★ ★ ★★
Energy requirement ★★★ ★★★ ★★★ ★ ★★
Greenhouse gas emission ★★★ ★★★ ★★ ★ ★★
Specific surface area and porosity ★ ★ ★ ★★★ ★★★
Hydraulic conduction ★★ ★★★ ★ ★★ ★★
Controllable synthesis ★ ★ ★ ★★ ★★★
Adsorption of organic pollutants ★ ★ ★★ ★★★ ★★★
Adsorption of inorganic pollutants ★ ★ ★★★ ★★ ★★★
Seed germination and plant growth ★ ★ ★★★ ★ ★★★
Microbial growth ★★ ★ ★★★ ★★ ★★★
Fig. 9 a Schematic diagram of the enhanced trace BPA and other contaminants removal in stormwater by biochar-amended biofilters (Lu and Chen 2018).
b Comprehensive mechanisms of biochar to enhance contami-nants removal in biofilters is highly related to their physi-cal, chemiphysi-cal, and biological structure and properties, which could be tuned in the synthesis process
24 Biochar (2020) 2:1–31
summarizes some of the critical research areas involved to make further progress in the comprehensive knowledge of the relationship of the structure–reactivity–functionali-zation–application and the development of biochar-based materials and devices for environmental application.
1. More efforts should be made to elucidate the multiple and multilevel structure of biochar; there are two direc-tions to forward. One is the mechanisms of biochar for-mation that underlie the pyrolysis of complex biomass, and the other one is applying and combining advanced techniques to characterize the structure of biochar. Thus, advanced instruments and proper combined techniques should be used to understand the physical and chemi-cal composition of biochar from both macroscopic and microscopic directions.
2. The sorption and catalytic performance, and their mech-anism of biochar towards a wider range of organic com-pounds and metal ions should be well studied. Mean-while, the redox property and its effect on direct reaction with contaminants and indirect reaction with contami-nants through biochar–microbe interactions, for exam-ple, electron transfer between biochar (shuttle), contami-nants, and microbes should be systematically elucidated.
Since reactivity is closely correlated with the structural properties of biochar, the reactivity of biochar affected by aging process should also be taken into consideration.
3. The concept of biochar by design was proposed for years (Abiven et al. 2014) and its large-scale produc-tion should be improved with the goal of environmental
protection. Eco-friendly and cost-effective approaches to modify and functionalize biochar for a specific purpose are important. Furthermore, integrating functionalized biochar within water treatment plants and soil pollution remediation should be given priority in future research.
4. It should be noted that most of the studies regarding bio-char-based materials or systems are still in the lab scale;
thus, more works in field should be paid to. Researches about biochar should change from material or environ-mental science to environenviron-mental engineering. Further scrutiny considering underlying mechanisms and long-term application performance and economic analyses of mature cases are still necessary.
In summary, more attention should be paid to the struc-ture–reactivity–functionalization–application relationship of biochar. In-depth structure elucidation, a better understand-ing of the reactivity of biochar, precise functionunderstand-ing and long-term field application evaluation are needed. We anticipate the biochar-based materials with intelligent functionalization and device achieve the greatest agricultural and environmen-tal benefits possible.
Acknowledgements This project was supported by the National Natural Science Foundations of China (21621005, and 21537005, 21425730), and the National Key Technology Research and Develop-ment Program of China (2018YFC1800705).
Compliance with ethical standards
Conflict of interest The authors declare no conflict of interest.
Fig. 10 Future research and per-spectives for the structure–reac- tivity–functionalization–appli-cation relationship of biochar
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