Biochar, a versatile environmental material, has gained significant attention for its exceptional physical and chemical properties. This comprehensive review explores the innovative preparation methods of element-doped biochar, highlighting their enhanced functionalities and groundbreaking applications across diverse fields. Drawing from conventional approaches, this study systematically investigates in-situ and exogenous doping techniques, examining their distinct advantages, limitations, and profound impacts on the morphological structure and surface chemistry of biochar. By integrating multiple elements, the research reveals how doping significantly improves the adsorption capacity, catalytic efficiency, and electrochemical performance of biochar, offering opportunities for its potential use in environmental remediation, soil enhancement, energy conversion, and even cosmetic applications. Moreover, this study introduces an original framework of “preparation–structure–performance–application”, emphasizing the importance of optimizing doping strategies and element selection to maximize versatility of biochar across multiple domains. Beyond basic insights into existing knowledge, this review provides novel perspectives for future research, particularly in areas such as carbon sequestration, pollutant adsorption, and advanced catalysis. This comprehensive synthesis not only synthesizes existing knowledge but also delivers fresh, innovative insights into the untapped potential of element-doped biochar, propelling transformative progress in sustainable materials science and beyond. Graphical Abstract

Coal gangue is a solid waste with low carbon content discharged during the course of the coal mining process. The resource utilization of coal gangue could solve environmental problems caused by its excessive production, such as soil contamination and land occupation. This study proposed to produce high-strength thermal insulation bricks using coal gangue as the primary material and three other mineral powders as auxiliary materials, including K-feldspar, CaCO₃ and fly ash. A systematic analysis was conducted to explore the optimum raw material addition ratio and optimum sintering temperature; then, the intrinsic structure of thermal insulation bricks and their sintering formation mechanisms were revealed. The results showed that the optimal ratios of coal gangue, K-feldspar, CaCO₃ and fly ash were 65 wt%, 15 wt%, 10 wt% and 10 wt%, respectively; the compressive strength of the thermal insulation brick produced under this ratio was 22.5 MPa; thermal conductivity was 0.39 W m⁻¹ k⁻¹. During sintering processes, mineral powders sufficiently fused to form a skeleton, and the CO₂ derived from CaCO₃ formed pores. The optimum sintering temperature was 1150 °C, because at this temperature, K-feldspar had the best effect in promoting the conversion of CaCO₃ to Ca-feldspar. The high level of the relative crystallinity of Ca-feldspar (about 76.0%) helped raise the Si–O network’s polymerization degree (NBO/T = 1.24), finally raising the compressive strength of thermal insulation bricks. The innovative method of using coal gangue to make thermal insulation bricks not only solved the environmental pollution caused by coal gangue but also provided excellent construction materials with high practical application value.

Converting biowaste into biochar and incorporating it into soil is recognized as an effective method for carbon (C) sequestration. However, biochar inevitably undergoes an aging process in soil, which influences its stability, and ultimately threatens its carbon sequestration ability. This study selected CaCl₂ as an exogenous additive of sewage sludge and bone dreg for producing Ca-rich biochars, and both surface C and bulk C stability in them were investigated under three aging processes (dry–wet aging, freeze–thaw aging, and natural aging in farmland soil). The results showed that the above three aging processes resulted in oxidized surface C in Ca-rich biochar decreasing by 10~23%, 28~41%, and 0~74%, respectively, compared with that in pristine biochar, while oxidized bulk C decreased by 6~10%, 0~1%, and 0~35%, respectively. This was attributed to the “protective effect” of Ca-containing crystals on the biochar surface, including CaO, Ca₅(PO₄)₃Cl, Ca₅(PO₄)₃(OH), Ca₈H₂(PO₄)₆·H₂O, and Ca₁₀(PO₄)₆(OH)₂, which intervened in the oxidation of C–C/C=C in biochar but also reduced the generation of a fragmented C structure. By comprehensively assessing surface C and bulk C stability under three aging processes, final C sequestration in Ca-rich biochar increased to 27~80%, compared to that in pristine biochar (23~74%). Therefore, Ca-rich biochar is more dominant than pristine biochar, considering C sequestration potential during long-term aging in soil.