Effect of dissociation constant (pKa) of natural organic matter on photo-generation of reactive oxygen species (ROS)

[Display omitted] •Initially established the correlation between pKa and ROS generation.•The dissociation degree of NOM has a certain effect on ROS production.•DHB2,5, DHB3,4 and DHB3,5 have different abilities to produce ROS.•DHB2,5, DHB3,4 and GUA produce ROS positively correlated with their dissociation. Natural organic matter (NOM) contains a large number of aromatic ketones and quinones, which can produce reactive oxygen species (ROS), such as 1O2 and HO·, through photosensitization. These ROS play important roles in the attenuation of organic pollutants in sunlit surface water. The NOM components fractionated with XAD-8 resin at different pH influence the photo-generation of ROS, which may be related to the degree of dissociation of functional groups. In this study, probe chemicals were used to investigate the effect of dissociation degree (pKa) of NOM model compounds on ROS production under 254 nm UV irradiation. Experimental results showed that six model compounds with similar molecular weights and structures but different pKa values have different ROS production abilities in a pH 7.0 reaction system. The ability for DHB2,5 (2.97), DHB3,4 (4.48), and GUA (9.99) to photo-generate 1O2 increased with increasing dissociation. The ability for DHB3,4 and GUA to photo-generate HO· followed the same trend. The correlation between NOM dissociation constant and ROS generation is preliminary established, which explains the influence of functional groups. Our results may provide some basic information for understanding the correlation between NOM structural features and ROS generation.