Molecular characterization of coastal seawater dissolved organic matter by ultrahigh-resolution mass spectrometry: a photochemical study of the Tokyo Bay, Japan

Abstract The coastal seawater in Tokyo Bay contains abundant sulfur (S)-containing DOM compounds because it receives municipal effluents from wastewater treatments. However, the effect of photohalogenation on the molecular composition of these coastal seawater DOM remains unknown. Herein, light irradiation experiments were combined with the Fourier transform ion cyclotron resonance mass spectrometry to investigate the transformation in the molecular composition of coastal seawater DOM during photohalogenation. In total, 3147 S-containing formulae were identified in the coastal seawater, accounting for 33% of the total number and 24% of the full intensity. Moreover, ~ 44% to ~ 67% of CHOS formulae with high molecular weight, aromaticity, and unsaturation were preferentially photodegraded via decarboxylation and desulfonation. Compared with the visible and UVA lights, UVC light exhibited much significant influences on the molecular composition of coastal seawater DOM. Only a few OBCs and OICs were generated during the photohalogenation, mainly via substitution and addition reactions, from CHO-class DOM belonging to lignin-like compounds. Additionally, the novel DBE-0.5O parameter was proposed as a better indicator of the unsaturation of the carbon skeleton than DBE-O. Our findings provided valuable information on the effect of photohalogenation on the molecular composition of the natural coastal DOM under natural sunlight irradiation. Graphical Abstract