Surface functional groups of carbon-based adsorbents and their roles in the removal of heavy metals from aqueous solutions: A critical review

Carbon-based adsorbents such as graphene and its derivatives, carbon nanotubes, activated carbon, and biochar are often used to remove heavy metals from aqueous solutions. One of the important aspects of effective carbon adsorbents for heavy metals is their tunable surface functional groups. To promote the applications of functionalized carbon adsorbents in heavy metal removal, a systematic documentation of their syntheses and interactions with metals in aqueous solution is crucial. This work provides a comprehensive review of recent research on various carbon adsorbents in terms of their surface functional groups and the associated removal behaviors and performances to heavy metals in aqueous solutions. The governing removal mechanisms of carbon adsorbents to aqueous heavy metals are first outlined with a special focus on the roles of surface functional groups. It then summarizes and categorizes various synthesis methods that are commonly used to introduce heteroatoms, primarily oxygen, nitrogen, and sulfur, onto carbon surfaces for enhanced surface functionalities and sorptive properties to heavy metals in aqueous solutions. After that, the effects of various functional groups on adsorption behaviors of heavy metals onto the functionalized carbon adsorbents are elucidated. A perspective of future work on functional carbon adsorbents for heavy metal removal as well as other potential applications is also presented at the end.