In this study, 2D magnetic composites based on Fe3O4 and scallion sheathings biochar were designed and prepared via simple hydrothermal method for removal of As(III) in aqueous solutions. The effects of acid/base pretreatment of biochar on the physicochemical properties and As(III) adsorption performance of the composites were investigated. The mechanism was discussed by XPS study and sorption isotherm models. The results revealed that oxygen content on the surface of the acid/base pretreated biochar decreased, which directly reduced the Fe3O4 content loaded on the biochar. The removal efficiency of As(III) by the composites based on original biochar, HCl-pretreated biochar and NaOH-pretreated biochar could reach 93.19%, 92.84% and 79.61%, respectively. XPS, adsorption kinetics, and isothermal adsorption model studies have shown that As(III) adsorption by these three composites is a chemical process with the maximum adsorption capacity of 39.47, 31.37 and 28.54 mg/g, respectively. In addition, the magnetic biochar composites still showed good reusability after 5 times of reuse. This study shows that the magnetic biochar composites possessed excellent performance to remove As(III) from solutions, especially the composites based on original biochar, indicating it is a promising adsorbent for arsenic removal from contaminated water. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.