The goal was to remove arsenate species in the presence of competitive anions by coupling of liquid-phase polymer-based retention, LPR, a procedure based on the selective As(V) adsorption properties of cationic water-soluble polymers, with an electro-catalytic oxidation process (EO) of As(III) into its more easily removable As(V). The electro-catalytic oxidation of As(III) to As(V) was performed with an organic supporting electrolyte, poly[3-(methacryloylamine)propyl)]trimethyl ammonium chloride, P(ClMPTA), which is recognized as an efficient reagent in removing divalent arsenate species. The bulk electro-catalytic conversion of As(III) to As(V) was carried out with a Pt-gauze electrode, and the resulting mixtures were introduced into a LPR cell to remove the As(V)-polymer adducts. Using P(ClMPTA) and ammonium salts at a 20:1 polymer:As(III) molar ratio at pH 8, complete (100%) Arsenic retention was achieved. For binary mixtures of Arsenic with competitive anions (e.g., SO(4) (2-), HPO(4) (2-), NO(3) (-), and NO(2) (-)), the retention profile varied in the range 100-70%. In addition, the As(V) retention efficiency was found to be directly related to the consumed charge in the mol ratio As(III) in solution with competitive anionic species.