Twice-milled magnetic biochar: A recyclable material for efficient removal of methylene blue from wastewater

[Display omitted] •An optimized magnetic biochar was developed with both excellent sorption and re-use characteristics.•The highly monodispersed ultrafine magnetite avoided the over-wrapping of ball milled biochar.•With the help of the extra ball milling process, surface functional groups and specific surface areas were restored, thus leading to a 93.8% recovery of adsorption.•The ball milled magnetic biochar can be recycled for at least 4 times. Although magnetic modification has potential for preparing recyclable biochar, the traditional preparation methods of loading magnetic materials on biochar will probably lead to pore blockage and consequently remarkable adsorption recession. Herein, a preparation method was developed in which ball milled biochar was loaded with ultrafine magnetite and then milled for a second time, thus generating a magnetic, recyclable biochar with minimal pore blockage. The deposits of magnetite did not significantly wrap the biochar, although a decreased sorption performance was still detectable. Benefitting from the extra milling step, surface functional groups and specific surface areas of the adsorbents were largely restored, thus leading to a 93.8 % recovery adsorption of 84.6 ± 2.5 mg/L on methylene blue. Meanwhile, the recyclability of the material was not affected. The adsorption was driven by multiple interactions. These twice-milled magnetic biochar is quite outstanding for sustainable removal of aqueous contaminants with its recyclability and high sorption efficiency.