Removal of chromium(VI) and naphthalenesulfonate from textile wastewater by photocatalysis combining ionic exchange membrane processes

A novel technology of nitrogen-doped TiO2 with visible light photocatalysis combining electrodialysis was proposed for simultaneous oxidization of naphthalenesulfonate (NS)/reduction of hexavalent chromium (Cr(VI)) by electron–hole pairs. N-TiO2 nanocrystal was prepared by calcinating the mixture of Degussa P-25 TiO2 and NH4Cl at temperatures of 400 °C under airtight condition, and the application of cation-exchange membrane was used to enhance the efficiency for prevention of the reoxidation of reductive chromium with the electron–hole pairs. The results showed, when pH is lower than 6.8, the electrostatic attractions were happened for the positive charge Ti-OH2+, negatively charged Cr(VI) and NS, resulting in higher efficiency. Only 20.2% of Cr(VI) was removed when NS was absence for pH 3 with N-TiO2 of 1.0 g/L, but complete Cr(VI) removal was observed for initial NS concentration up to 50 mg/L, indicating NS plays the role of hole scavenger in the system. The generation of sulfate (as sulfur) is equivalent to the degradation of NS (as sulfur), representing the yield of sulfate is an excellent indicator for NS removal. Moreover, incomplete NS decomposition contributes to the occurrence of intermediates, including propenoic acid, isocrotonic acid, formic acid, and formaldehyde, as identified by GC/MS analysis.