Adsorption is a popular water treatment technique that encounters several challenges in practical applications, such as rapidly achieved adsorption equilibrium and selective recovery. In this work, a quaternary-ammonium-salt modified chitosan adsorbent with porous structure (porous CS-CTA) was obtained conveniently by removing embedded inorganic nanoparticles from a previously well-prepared chitosan-based magnetic composite adsorbent (CS-CTA-MCM). Compared with CS-CTA-MCM and CS-CTA without porous structures, porous CS-CTA shows clearly improved and rapid adsorption of both methyl orange and chromium [Cr(VI)] from water because of its larger specific surface area. It also presents significantly enhanced selective adsorption of methyl orange over Cr(VI) in their aqueous mixture at neutral and alkaline conditions, an outcome that is ascribed to the stronger affinity of methyl orange to porous CS-CTA compared with that of Cr(VI). The porous microstructure of CS-CTA results in its higher adsorption efficiency in the removal of those two pollutants in comparison with the other adsorbents.