Conventional adsorbents for decontaminating arsenic exhibit low efficacy for the removal of arsenite (As(III)). This study aims to develop a robust As adsorbent from natural magnetite (M0) via a facile ball milling process, and evaluate their performance for decontaminating As(III) and As(V) in water and soil systems. The ball milling process decreased the particle size and crystallinity of M0, resulting in pronounced As removal by the ball-milled magnetite (Mm). Ball milling under air facilitated the formation of Fe-OH and Fe-COOH functional groups on Mm interface, contributing to effective elimination of As(III) and As(V) via hydrogen bonding and complexation mechanisms. Synergistic oxidation effects of hydroxyl and carboxyl groups, and reactive oxygen species (O2·-, and ·OH) on the transformation of As(III) to As(V) during the adsorption were proposed to explain the enhanced As(III) removal by Mm. A short-term soil incubation experiment indicated that the addition of Mm (10 wt%) induced a decrease in the concentration of exchangeable As by 30.25%, and facilitated the transformation of water-soluble As into residual fraction. Ball milling thus is considered as an eco-friendly (chemical-free) and inexpensive (scalable, one-stage process) method for upgrading the performance of natural magnetite towards remediating As, particularly for tackling the highly mobile As(III).