Biochar has received considerable attention as an effective and sustainable amendment to immobilize heavy metals (HMs) in contaminated soils. Its ability to stabilize toxic metals is, however, strongly influenced by its physicochemical properties and dynamic soil environmental factors. Despite extensive studies on biochar applications, a systematic understanding of how pyrolysis conditions regulate biochar characteristics and how these characteristics control the mechanisms of metal immobilization is still incomplete. Addressing this knowledge gap is essential for optimizing the pyrolytic design and practical deployment of biochar in soil remediation.

This review comprehensively summarizes the evolution of biochar physicochemical properties under various preparation processes and elucidates the dominant mechanisms involved in heavy-metal stabilization. Biochar immobilizes HMs mainly through four pathways: pore structure adsorption, cation exchange, surface complexation via oxygen-containing functional groups, and mineral precipitation. The relative contribution of each mechanism varies with feedstock type, pyrolysis temperature, residence time, and atmospheric conditions during carbonization. For instance, higher pyrolysis temperatures often enhance aromaticity and surface area, promoting metal precipitation, whereas lower temperatures preserve oxygenated groups, favoring complexation and ion exchange. Understanding these relationships enables a more predictable control over biochar functionality for targeted remediation outcomes.

Aging processes further complicate biochar–metal interactions. Chemical oxidation, microbial degradation, and physical weathering can lead to structural collapse, decomposition of functional groups, and release of unstable carbon fractions. These changes alter surface charge, pore connectivity, and binding energy, thereby affecting metal retention efficiency. Importantly, the effect of aging is not uniformly detrimental: certain oxidative aging processes can introduce new oxygen-containing groups, enhancing the sorption affinity for specific metals. Hence, evaluating both beneficial and adverse aging effects provides a more realistic understanding of biochar performance under long-term field conditions.

By integrating insights into pyrolysis regulation, surface chemistry, and aging mechanisms, this work provides a conceptual framework for enhancing biochar stability and reactivity. Controlling pyrolysis parameters and developing tailored post-treatment or activation strategies could substantially improve biochar’s persistence and metal-binding capacity. Such optimization will help reduce environmental variability in remediation performance and facilitate the design of biochar with predictable and durable functions.

Future research should prioritize multi-scale and interdisciplinary approaches, combining advanced characterization techniques, in situ monitoring, and long-term field trials. Standardized evaluation criteria are also needed to link laboratory findings with large-scale applications. Overall, this study highlights the necessity of understanding biochar’s dynamic nature—from pyrolytic formation to environmental transformation—to realize its full potential as a robust, low-carbon solution for heavy-metal immobilization and sustainable soil restoration.