Conversion of biological solid waste to graphene-containing biochar for water remediation: A critical review

The introduction of graphene into biochar can effectively and economically improve the capabilities of biochar for environmental applications. Graphene-containing biochar, derived from biological solid waste such as sugar cane trash, coconut shell, old corrugated cardboard packages and chicken feather, possess a lot of improvement in physicochemical properties (e.g., surface area, pore volume, surface functional groups, thermal stability, electron transfer capability, and catalytic activity) as compared to raw biochar. Consequently, graphene-containing biochar demonstrates a greatly enhanced environmental remediation potential. However, the production of graphene-containing biochar from biological solid wastes and its characterization and potential use for the remediation of organic and inorganic pollutants in the environments have not been critically reviewed up to date. Therefore, we aim to summarize the state-of-art knowledge about graphene-containing biochar preparation and its environmental applications. Furthermore, five types of preparation techniques for graphene-containing biochar and their applications for water remediation have been reviewed and discussed: (1) iron-based catalytic methods for graphene-like biochar; (2) carbonization with natural multilayered materials for graphene-like biochar; (3) hummers method (oxidation and exfoliation by strong acid and KMnO4) for graphene-like biochar; (4) thermal treatment of graphene oxide-pretreated biomass for graphene-biochar composite, and (5) biochar coated with exfoliated graphene. Moreover, we critically reviewed the removal efficiency and mechanisms of organic and inorganic pollutants using graphene-containing biochar. This review demonstrates the overarching scientific opportunities for a comprehensive understanding of using graphene-containing biochars as emerging and promising biosorbent for the remediation of contaminated water. We conclude that the conversion of biological solid waste to value-added graphene-containing biochar for environmental remediation could provide an alternative advanced way for the management of waste biomass.