Flexible and free-standing pristine polypyrrole membranes with a nanotube structure for repeatable Cr(VI) ion removal.

• Pristine polypyrrole (PPy) membrane with nanotubes were prepared for Cr(VI) removal. • The flexibility and shape stability make PPy membranes easy for separation. • The maximum Cr (VI) adsorption capacity of PPy membrane was 250.31 mg/g. • The Cr (VI) adsorption capacity fell by only 8.2% after four cycles of regeneration. The effective removal of toxic Cr(VI) from wastewater remains an important environmental issue. The low adsorption capacity and the complex processes to separate the adsorbents from the treated wastewater are the main scientific and technological challenges. In this study, a flexible and free-standing pristine polypyrrole (PPy) membrane with a nanotube structure (PPy-N) was synthesized via template-assisted interfacial polymerization (TIP) for the selective removal of Cr(VI) through a simple separation and collection process. The superior Cr(VI) adsorption capacity as high as 250.31 mg/g was endowed by the unique structure of the PPy-N membrane. The Langmuir adsorption isotherm model was used to describe the adsorption behavior, with the adsorption kinetics followed a quasi-second-order kinetic model. After four adsorption-and–desorption cycles, the PPy-N membrane adsorption capacity fell by only 8.2%. The mechanism of the selective removal of Cr(VI) was proposed, including electrostatic adsorption, ion exchange and reduction. A combination of the highly attractive material properties, including mechanical processability, large surface area, superior Cr(VI) adsorption capacity and regeneration, makes the single-component PPy-N membrane a good candidate for removing Cr(VI) ions from wastewater. [ABSTRACT FROM AUTHOR]