pH-tunable surface charge of chitosan/graphene oxide composite adsorbent for efficient removal of multiple pollutants from water

• Chitosan/graphene oxide composite adsorbent (CS–GO) was prepared by self-assembly.
• CS–GO shows pH-tunable surface charge and morphology.
• CS–GO can be efficient removal of various pollutants with opposite charges.
• CS–GO bears high selective adsorption of MB over Cu(II) at low pH in their mixture.
• Diverse adsorption mechanisms are involved in removal of various pollutants.

A versatile composite adsorbent combination of chitosan and graphene oxide (CS–GO) has been prepared by self-assembly method. This well-obtained adsorbent has amphoteric characteristics that show pH-tunable surface charge and morphology. These vital features result in its flexible and tunable adsorption performance. CS–GO is highly efficient in removing multiple types of pollutants from water, including various dyes and metal ions with different charge properties: cationic [methylene blue (MB) and Cu(II)] and anionic species [methyl orange (MO) and Cr(VI)]. Multiple mechanisms, including electrostatic interaction, π–π stacking, and chelating effects, are involved in the adsorption of the four pollutants for their different structural characteristics and charge properties. Both pollutants can be efficiently removed at suitable pH conditions in MB/Cu(II) and MO/Cr(VI) binary systems. Moreover, CS–GO shows strongly preferential adsorption of MB over Cu(II) at lower pH levels, whereas very slight selectivity of MO over Cr(VI). According to the adsorption kinetics results, the equilibrium time of CS–GO for adsorption of each pollutant is less than 10.0 min, which is sufficiently fast and satisfactory for practical use. After saturated adsorption, CS–GO can be efficiently regenerated and reused with little uptake loss. Therefore, CS–GO is a versatile adsorbent that efficiently removes various pollutants from water.

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