NaOH–ball-milled co-modified magnetic biochar and its oil adsorption properties

A magnetic biochar adsorbent (BM–OH–BC) was prepared through NaOH–ball milling co-modification with walnut shells as the raw material, and its oil adsorption characteristics were investigated. The adsorption properties of oil pollutants were compared before and after the NaOH-ball-milling co-modification, and the mechanism of NaOH-ball-milling on oil pollutants was investigated by scanning electron microscope-energy dispersive X-ray spectroscopy, BET, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The experimental results showed that the oil adsorption capacity of BM–OH–BC can reach 2.75 g/g, which is 129% higher than that of unmodified biochar. Moreover, after five adsorption and desorption cycles, the adsorption rate of BM–OH–BC only decreased by 5.9%, which was because the oil adsorption of BM–OH–BC is mainly chemical adsorption. The adsorption processes involving single layer, heterogeneous phase and multilayer made BM–OH–BC have excellent adsorption properties. After biochar co-modification, the specific surface area of BM–OH–BC increased to 466 m2/g, the total pore volume increased to 0.245 cm3/g, the average pore size decreased to1.90 nm, and the number of hydrophobic non-polar functional groups increased, leading to the enhancement of its oil absorption capacity. This study provides a reference for the treatment of oil-contaminated water bodies.