Effects of humic acids with different polarities on the photocatalytic activity of nano-TiO2 at environment relevant concentration.

Large volume production and application of nano-TiO2 make it inevitably release to natural waters and its environmental behaviors would be affected by natural organic matters. In this study, the mechanisms of humic acid (HA) affecting the photocatalytic performance of nano-TiO2 were elucidated by using three HA fractions from the same source but with different polarities. Bulk HA was fractionated on a silica gel column to get three fractions with polarity increasing in the order of FA, FB and FC. FA was fulvic acid-like while FB and FC were humic acid-like. All the three fractions (at 0.1 mg/L) promoted the generation of hydroxyl radicals (OHs) by nano-TiO2, and thus in turn facilitated the photocatalytic degradation of bispheol A (BPA). FA and FC displayed a stronger promotion effect than FB and the bulk HA. Online in situ flow cell ATR-FTIR and XPS analyses indicated that HA fractions could form charge-transfer complex with nano-TiO2 surface through the phenolic hydroxyl and carboxylic groups, which favored the separation of photogenerated electron-hole pairs. Through step methylation experiments, it was verified that the phenolic hydroxyl and carboxylic groups of HA fractions played important roles in promoting the photocatalytic performance of nano-TiO2, and the effect of carboxylic group was more significant than the phenolic hydroxyl group.