Synergy of photocatalysis and adsorption for simultaneous removal of Cr(VI) and Cr(III) with TiO2 and titanate nanotubes

•Outstanding synergy of photocatalysis and adsorption for removal Cr was achieved.•Over 50% reaction time reduced for one-step system compared to two-step method.•Transfer of reduced Cr from TiO2 to TNTs promoted release of photocatalytic sites.•Cr(III) entered into TNTs' layer enhanced photocatalytic activity of TNTs.•Nanomaterials performed satisfactory synergy after easy regeneration.

An one-step efficient simultaneous removal of Cr(VI) and Cr(III) was achieved with mixture of TiO2 and titanate nanotubes (TNTs). Unlike the conventional two-step Cr removal with a first photocatalytic reduction of Cr(VI) and a subsequent adsorption of Cr(III), the proposed single process significantly reduced reaction time (over 50%). The synergy of photocatalysis and adsorption played an important role in enhancing Cr removal process. The synergetic mechanism was interpreted and indirectly confirmed with H2O2 variation during photocatalysis. The instant transfer of the reduced Cr from TiO2 surface to TNTs interlayer greatly promoted the release of photocatalytic sites of TiO2, which in turn considerably enhanced photocatalytic activity of TNTs by inhibiting electron–hole pairs recombination. The optimum condition for the whole process was at pH 5. Adsorption of Cr(III) was primarily in the interlayer of TNTs at pH ≤ 5. However, higher pH would lead to precipitation of Cr(OH)3 onto TNTs as observed by X-ray photoelectron spectroscopy (XPS). Addition of Ca2+ could promoted photocatalysis owing to its ionic bridging function and form of ≡TiOH+-Cr(VI)–Ca2+-Cr(VI) linkages, while SO42− only slightly inhibited photo-reduction of Cr(VI), indicating good synergy of photocatalysis and adsorption even at high ionic strength of electrolyte. Besides, the desorbed TNTs could be easily regenerated by remedying the damaged tubular structure and reused for Cr removal with excellent performance. The outstanding synergetic effects with essential explanation of the mechanism make this study not only fundamentally important but also potentially practical applicable.

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