The Application of Biochar as Heavy Metals Adsorbent: The Preparation, Mechanism, and Perspectives

With the increased activity from humans in agriculture and industry, a growing amount of waste containing heavy metals is discharged into the environment, which brings great risk to human health. Biochar, as a great absorbent for heavy metals remediation, has been extensively studied. The adsorption capability of biochar is affected by many factors, such as the species and properties of raw materials, the preparation methods (temperature, heating rate, and residence time), and functional sites introduced by the modification agent. However, how these factors determine the adsorption of heavy metals on biochar is not clear. The present work thoroughly reviewed the traditionally used methods for biochar preparation such as pyrolysis, hydrothermal carbonization and gasification, meanwhile, the emerging biochar preparation techniques (retort carbonization and torrefaction) are also explored. Accordingly, the commonly used modification methods (alkali modification, acid modification, ferromagnetic modification, microbial modification, etc.) are comprehensively investigated. The adsorption kinetics and isotherms are also discussed to demonstrate the adsorption mechanism from a theoretical basis. Notably, to facilitate the large-scale biochar application in practice, a discussion focusing on the factors associated with practical utilization is provided. Consequently, the review of environmental risk and the challenge regarding biochar disposal safety, a thorough economic analysis, detailed exploration of industrial-scale implementation challenges, enhanced life cycle assessment and sustainability analysis are included, aiming to contribute a better understanding of the practical implications of engineering biochar for application in heavy metals remediation.