The present research was performed with a primary objective to study the performance of different MnO₂ polymorphs for oxidative transformation of As(III). Five different MnO₂ polymorphs with distinct morphologies were hydrothermally synthesized by altering the precursor and synthesis condition. The As(III) oxidative reactivity of MnO₂ polymorphs, as confirmed by the kinetic experiment, were found to be in the order: γ-M2 > α-M2 > α-M1 > γ-M1 > β-M1. The study confirms that the surface Mn(III) content, crystallinity, reducibility, and synthesis temperature are the primary deciding factors that regulate the As(III) oxidative performance of MnO₂. In addition, 2-line ferrihydrite nanoparticles were synthesized from iron ore slime for removal of As(V). A new nanocomposite of 2-line ferrihydrite and γ-MnO₂ was prepared for the removal of both As(III) and As(V). The nanocomposite with high As(III) and As(V) removal affinity, excellent reusability and outstanding performance in groundwater environment demonstrate its potential as a front-line arsenic removal material.