The inhibitory mechanism of humic acids on photocatalytic generation of reactive oxygen species by TiO2 depends on the crystalline phase

•Humic acids (HA) exerted stronger inhibition of photocatalytic activity of anatase than rutile.•HA adsorption hindered differentially OH generation mechanisms by anatase and rutile.•HA hindered OH produced via O2 reduction (anatase) more than via H2O oxidation (rutile)•Decreased OH production was offset by HA increasing O2−generation over rutile.•Crystal-phase-dependent effects were reproducible with HA from different sources. To enable the practical use of TiO2 for photocatalytic water treatment, it is important to understand how dissolved natural organic matter affects its photocatalytic activity and the mechanisms involved in generating reactive oxygen species (ROS). In this study, we systematically investigated the inhibitory effects of humic acids (HA) on 4-chlorophenol (4-CP) degradation using two common TiO2 crystalline phases (anatase and rutile) individually. HA strongly hindered the photocatalytic activity of anatase, with an Ra (the ratio of the first-order rate constant for 4-CP degradation in the absence vs. presence of 30 mg/L HA) of 2.30 (±0.13). This ratio was significantly higher than that of rutile (Rr = 1.21 ± 0.08), which was less susceptible to this inhibitory effect and even exhibited improved photocatalytic activity at HA concentrations below 20 mg/L. Similar trends were observed for various HA sources, corroborating the crystalline-phase-dependent effect of HA. Adsorption experiments, Fourier transform infrared spectroscopy, and photoelectrochemical analyses suggested that the adsorption mechanisms and hole-scavenging effect of HA on the two TiO2 polymorphs did not differ. Importantly, scavenger, probe, and electron spin resonance experiments revealed that the difference in inhibitory effects of HA originate from the distinct ROS generation mechanisms for the two polymorphs. The oxygen reduction pathway for OH generation over anatase was hindered by surface-adsorbed HA, while the water oxidation pathway for OH generation over rutile was less affected. Furthermore, surface-adsorbed HA boosted O2− generation on rutile, increasing 4-CP degradation efficiency. This mechanistic insight into NOM-TiO2 interactions informs materials selection and strategies for higher TiO2 photocatalytic performance in different water matrices.