Temporal Dynamics and Intermediate Product Formation in DOM Phototransformation Revealed by Liquid Chromatography Ultrahigh-Resolution Mass Spectrometry

The complex composition of dissolved organic matter (DOM) has been extensively studied by modern high-resolution analytical methods. However, DOM reactivity is still enigmatic due to a lack of experimental data with sufficiently high temporal resolution to resolve the intrinsic dynamics within DOM. Likewise, extensive isomeric overlap prevents studying transformation of DOM components with respect to their chemical properties, e.g., molecular polarity. Online ultrahigh-performance liquid chromatography with ultrahigh-resolution mass spectrometry (UHPLC-UHRMS) increases the resolution of isomeric DOM composition across a wide range of polarity. We performed a TiO₂-aided photo-irradiation experiment with wastewater treatment plant effluent with high temporal sampling resolution (8 time points, 5 h irradiation). Besides new products (<10%) and removed components (25-60%), intermediate products (<i>IntP</i>) were also found, representing 20-60% of components within distinct polarity fractions. The reaction time to reach the peak magnitude maximum was positively related to the H/C ratio of <i>IntP</i>. About 35% of the DOM components showed different reactivities for different polarity fractions. If applied to experiments in the future, our approach offers new perspectives for biogeochemical interpretation and provides important information for drinking water processing or wastewater treatment with respect to potential toxic <i>IntP</i>.