Removal of perfluorooctanoic acid by MWCNT-modified carbon-doped titanium dioxide in a peroxymonosulfate/simulated sunlight system

Abstract

Perfluorooctanoic acid (PFOA) is widely used as an industrial surfactant and has been detected in various aquatic systems. Herein, a series of multiwalled carbon nanotube (MWCNT)-modified carbon-doped TiO2 catalysts (xCNT/C-TiO2) were prepared through a one-step calcination process and applied as adsorptive photocatalysts. Characterization and density functional theory (DFT) calculation results revealed that carbon doping introduced impurity energy levels between the band structures of TiO2. Moreover, the MWCNTs acted as photosensitizers that expanded the wavelength absorption range. The xCNT/C-TiO2 composites achieved efficient adsorption through multiple pathways (metal-ligand interaction, electrostatic attraction, π-anion interaction, and hydrophobic adsorption), and the adsorbed PFOA was degraded by the simulated sunlight/xCNT/C-TiO2/peroxymonosulfate (PMS) systems. The 2CNT/C-TiO2 sample exhibited the highest PFOA removal efficiency and degraded more than 90 % of the organic contaminant (2 mg/L, defluorination rate of 39.2 %) in 3.5 h. Furthermore, PFOA could be removed during repeated usage of the catalyst system without chemically regenerating the photocatalyst. The enhanced adsorption and photocatalytic removal by xCNT/C-TiO2 were due to the following reasons: (i) the MWCNTs in the composites provided a different adsorption pathway than the pre-existing monodentate mode and introduced more hydroxyl groups that could adsorb PFOA; (ii) the carbon doping in the TiO2 lattice reduced the band gap, leading to the more efficient utilization of visible light; (iii) MWCNTs inhibited the recombination of photogenerated charge carriers caused by doping, and this also promoted the production of active species and the direct oxidation of PFOA by photo-induced holes. Finally, enhanced monodentate adsorption mode and photodegradation mechanism were proposed based on experimental results and DFT calculations.