Solar irradiation-induced photochemical processing of dissolved organic matter in reclaimed water

Abstract

The photochemical fate of various organic components of reclaimed water DOM was comprehensively investigated using diverse spectroscopic characterization coupled with mathematical analysis methods. Findings show reclaimed water DOM to be susceptible to solar irradiation-induced photodegradation, causing notable reduction of UV254 absorbance, fluorescence intensity, and average molecular weight of DOM in reclaimed water. A multiple exponential kinetic model for DOM photodegradation demonstrates humic-like constituents were more susceptive to solar irradiation, whereas the protein-like constituents showed tolerance. FRI analysis show photodegradation rate constants of DOM factions under sunlight irradiation fall in the sequence of humic-like > fulvic-like > soluble microbial byproduct-like > tryptophan-like > tyrosine-like fractions. Upon solar irradiation, the photodegradation kinetics of three PARAFAC-extracted fluorophores varied in the order of microbial humic-like > terrestrial humic-like > tryptophan-like components. This order of fraction decrease is further evidenced by synchronous fluorescence spectra (SFS) integrated with 2DCOS analysis. Different photoreactivities and photosensitivity to visible (Vis), Vis + UVA, Vis + UVA + UVB by PARAFAC-derived components demonstrated that UV irradiation, but not visible light, drive DOM photodegradation. UVB and UVA played predominant roles in the photodegradation of tryptophan-like and terrestrial humic-like constituents, respectively. The photodegradation of microbial humic-like constituents was attributed to the combined actions of UVB and UVA irradiation. Furthermore, varied ROS quenching/enhancing experiments revealed that the photodegradation of tryptophan-like components might be ascribed to indirect photolysis involving 3DOM*. The photodegradation of terrestrial/microbial humic-like DOM constituents was associated with direct photolysis. The comprehensive illustration of sunlight irradiation-induced photochemical transformation of DOM in reclaimed water during storage contributes to understanding the fate of DOM and associated environmental processes impacted by solar irradiation.