A complete picture of the phosphorene oxide (PhosO) sorption properties for the simultaneous removal of inorganic As(III) and As(V) pollutants from water has been developed using first-principles calculations. Calculated adsorption energies, competitive adsorption with co-existing species, energy decomposition analyses (ALMO-EDA), implicitly/explicitly solvated geometries, and adsorption free energies provide deep insights into the adsorption mechanism as well as the origin of the strong selectivity sorption ability. The PhosO nanoadsorbents establish inner-sphere surface complexes with arsenicals even under competition with water molecules. These proposed structures also show a strong affinity with the highly mobile As(III), where energy saving is achieved by avoiding the pre oxidation process to convert As(III) into As(V) as requested in related materials. Results show that electrostatic driving forces govern the adsorption of neutral arsenicals, while the interplay between electrostatic and polarization phenomena drives the uptake of anionic arsenicals. By computing the adsorption strength as a function of the oxidation degree, the optimum adsorption efficiency is reached with a 25% in the content of oxidizing groups. In this oxidation degree, the strong repulsive surface charge at high pH turns the PhosO nanoadsorbents convenient to recycle via simple treatment with alkaline eluents. Finally, the adsorption ability remains thermodynamically allowed in a wide range of ambient temperatures (enthalpically governed reaction). Conceptually understanding the sorption properties of phosphorene-oxide-based materials towards arsenic pollutants provides a useful framework for future water treatment technologies.