Environmental Effects of Silicon within Biochar (Sichar) and Carbon-Silicon Coupling Mechanisms: A Critical Review

Biochar is increasingly gaining attention for its potential environmental benefits. In addition to carbon (C), silicon (Si) is a major elemental component in biochar with abundant precursor sources and remarkable properties. Due to the abundance and utilization of silicon-rich biochar (Sichar), as well as the significant function of Si in agricultural production and environmental remediation, it is indispensable to understand the environmental effects of Si within Sichar. Therefore, this review focused on carbon-silicon coupling in Sichar and summarized the advanced studies on Si within Sichar regarding characterization, soil improvement, pollution remediation, and C-Si coupling interactions. After an understanding of Si content, morphology, species and releasing behaviors, the environmental effects on soil Si balance, the plant uptake of Si, and remediation potentials of inorganic pollutants (Al, As, Cd, and Cr) were summarized. The C-Si coupling interactions were highlighted in the processes of Sichar preparation, pollution remediation, and soil C sequestration. The coupling relationship of C and Si from biomass under natural, pyrolysis and geological processes for the biogeochemical cycling of C and Si can obtain four "F" benefits of farm, food, fuel, and finance. To better understand the environmental effects and maximize the benefits of the designed utilization of Sichar, more investigations are required with an extension to microbes and more interactions with different ions via quantitative modeling.