Biomass-based carbon microspheres for removing heavy metals from the environment: a review

Heavy-metal residues in the environment that harm organisms can be removed using various methods. Carbon microsphere materials can be effectively used for heavy metal adsorption because of their large specific surface area, rich pore structure, and excellent physical and chemical stability. In this review, we focus on water purification by heavy metal removal using carbon microsphere materials. First, the methods for preparing carbon microspheres are discussed, focusing on the process and mechanism of preparing biomass derived carbon microspheres using the hydrothermal method. Then, we summarize the common functionalization methods of carbon microspheres. Compared to other functional groups and heteroatoms, carbon microspheres have the advantages of increased adsorption capacity, high removal rate, specific selective adsorption capacity, and good reusability for treating heavy metals in water. They can be used for removing specific heavy metals, notably uranium, which is a radioactive element. The adsorption mechanisms of carbon microspheres for removing heavy-metal pollutants include electrostatic attraction, complexation, and chelation owing to coordination reaction, ion exchange, and redox reaction. However, because of the different sources of raw materials, defects in preparation processes, varying functionalization methods, and material aging, carbon microspheres face a series of problems and challenges with respect to their practical application. These issues need to be addressed in future studies.