Application of biochar for the adsorption of organic pollutants from wastewater: Modification strategies, mechanisms and challenges

As an efficient and low-cost adsorbent, biochar has been widely used in the adsorption and removal of dyes, antibiotics and phenols from wastewater. Biochar is produced with gas and bio-oil during biomass pyrolysis and has many advantages as an adsorbent compared to activated carbon. In addition to sustainable soil remediation, it can also remove organic and inorganic contaminants from water/wastewater after physical/chemical modification. The adsorption capacity of biochar is often increased by acid/base and metal impregnation. This article first reviews the main physical and chemical properties of biochar, such as pH, specific surface area and porosity, surface functional groups and stability. Subsequently, three methods for preparing biochar by pyrolysis and the effects of raw materials, pyrolysis temperature, heating rate, and residence time on the properties of biochar are summarized. Due to the performance deficiencies of biochar, this article discusses the preparation of biochar composites by magnetic modification, acid modification, alkali modification, water vapor modification, nano metal oxide/hydroxide modification and heteroatom doping. The removal mechanisms of organic wastewater pollutants are summarized and analyzed, including pore filling, electrostatic interactions, hydrogen bonding, hydrophobic interactions and π-π bonding. The factors affecting the removal effect mainly include the kinetic parameters, pH value of the solution, adsorption temperature, coexisting ions, biochar particle size, initial concentration and dose and reaction time. Finally, the removal effects and adsorption mechanisms of biochar for dyes, antibiotics and phenols from typical organic wastewater are discussed.