Interpreting pH-Dependent Differential UV/VIS Absorbance Spectra to Characterize Carboxylic and Phenolic Chromophores in Natural Organic Matter

Natural organic matter (NOM) is critical for the biogeochemical cycles of energy and many elements in terrestrial and aquatic ecosystems, and protonation-active functional groups in NOM molecules, notably carboxylic and phenolic groups often mediate these critical environmental functions. Molecular heterogeneity, polydispersity and dynamic behavior of NOM complicate achieving an unambiguous description of its molecular properties and reactivity. This study demonstrates that differential ultraviolet-visible (UV/VIS) absorbance spectra (DAS) of NOM acquired at varying pH values exhibit several distinct features associated with the deprotonation of NOM molecules, independent of the environmental provenance of NOM (e.g., surface water, seawater, sediment, and wastewater). The protonation-active functionalities that contribute to the Gaussian distribution bands present in the DAS were identified here by comparing characteristic properties of the bands with the stoichiometries of NOM molecules ascertained by Ultrahigh-Resolution Fourier-Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS). The protonation-active individual chromophores universally present in NOM molecules were identified by a genetic molecular network analysis. The observed DAS features were closely modeled via superimposing DAS spectra of 51 individual protonation-active chromophores. Molecular orbital theory was applied to further interpret the deprotonation of these chromophores, their molecular structure, electron distribution, and electron transitions measured using DAS. The high sensitivity and easy implementation of the DAS approach allows using it as a powerful tool to quantify the molecular properties and reactivity of NOM at environmental concentrations.