An overview of sulfur-functional groups in biochar from pyrolysis of biomass

Biochar is a solid material obtained from the pyrolytic carbonization of biomass in an oxygen-free/limited environment. Sulfur-containing biochar has a wide range of applications, such as adsorptive removal of pollutants (e.g., Hg, Cd, and Ni) and acting as a solid acid catalyst or as an electrode of Li-S battery. To date, many methods have been developed to strengthen the function of biochar by introducing sulfur-containing groups to promote the application and commercialization of biochar. This review aims to present the formation, analysis, engineering, and application of sulfur-functional groups in biochar. The sulfur-functional groups such as organic sulfur (e.g., C-S, -C-S-C-, C=S, thiophene, and sulfone) and inorganic sulfur (e.g., sulfate, sulfide, sulfite, and elemental S) can be determined through Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near edge structure (XANES). The sulfur-functional groups can be obtained through selecting biomass composition, pyrolysis process, S-doping, and post-treatment of biochar, but the engineering is challenging. The positive effect of sulfur-functional groups in the application is also analyzed in this paper, such as the complexation and electron transfer between sulfur-functional groups and heavy metal (e.g., Hg, Cd, and Ni) on improving biochar adsorption capacity. However, there are still challenges in directional synthesis, precise determination, and regulation of application performance. Based on the research gaps identified, future prospective investigation directions on analysis, engineering, and application of biochar S-functional groups were presented in this review.

Automated summary

Biochar is a solid material obtained from the pyrolytic carbonization of biomass in an oxygen-free/limited environment. Sulfur-containing biochar has a wide range of applications, such as adsorptive removal of pollutants (e.g., Hg, Cd, and Ni) and acting as a solid acid catalyst or as an electrode of Li-S battery. To date, many methods have been developed to strengthen the function of biochar by introducing sulfur-containing groups to promote the application and commercialization of biochar.