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 (IntP) 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 IntP. 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 IntP.