Ball-milling synthesis of biochar and biochar-based nanocomposites and prospects for removal of emerging contaminants: A review

The ball-milling of carbon-based materials with nanoparticles is currently emerging as a promising cost-effective alternative method of producing nanocomposites with great surface characteristics. Biochar-based nanocomposites are effective in the adsorption of heavy metals, dyes and emerging organic contaminants in water and wastewater because of the combined benefits of biochar and nanoparticles. Contaminants of emerging concern are increasingly being detected in the environment and there is, therefore, an urgent need to develop nanotechnology-based multifunctional and highly efficient processes that can remove a wide range of these chemicalsmost of which are persistent and may bioaccumulate. To date, not many reviews are available on the use of the ball-milling method for engineering biochar-based nanocomposites. Ball milling (solid-state synthesis) is a process extensively used in industry to synthesize nanomaterials. Ball milling process produces small particle size as small as 10 microns, coupled with the advantages of continuous operation. Thus, this review article aims to provide an overview of the efficiencies of the ball-milled biochar-nanoparticle composite. Ball milling approach improves the physicochemical properties of biochar and biochar-nanoparticle composite such as total and micropore surface of biochar which in turns enhance their sorption abilities. Different adsorption mechanisms of biochar and modified biochar have been reported, which include physisorption, chemisorption, ion-exchange, pore-filling, hydrophobic effects and pi-pi electron donor-acceptor (pi-pi EDA) interactions.