Insight into modified biochars and their immobilizing effects on heavy metal(loid)s in contaminated soils: Mechanisms and influencing factors

Soil contamination by heavy metal(loid)s is a considerable environmental concern, and immobilization is a promising way to reduce toxicity. In recent years, modified/engineered biochars have gained enormous attention for their use in soil remediation, and various studies have reported notable results from their application and their ability to immobilize heavy metal(loid)s. In this review, a summary of publications on the utilization of modified biochars is presented to address the heavy metal(loid) threat in soils. Various modified/engineered biochars were described from the review of relevant publications. Modification causes great changes in biochar surface chemistry, such as increases in pore volume, surface functional groups, and metal binding sites, which can be observed through various analytical techniques, including Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and magnetism. Such analytical approaches elucidate immobilization mechanisms of adsorption, precipitation, surface complexation, and cation exchange between biochar and metal(loid) ions. In addition, the performance of biochar in remediating heavy metal(loid)s also leads to considerable improvements in soil conditions. Additionally, many factors that influence metal(loid) immobilization by biochar in soil, such as pH, redox potential, microorganisms, and climate regime, are highlighted. Finally, this paper emphasizes that using modified biochars as an immobilizing agent for remediation of heavy metal(loid)-polluted soils is promising and would be practicable if a comprehensive mechanism of their long-term stability in soil is well elucidated.