One-electron oxidant-induced transformation of dissolved organic matter: Optical and antioxidation properties and molecules.

Dissolved organic matter (DOM) is a major sink of radicals in advanced oxidation processes (AOPs) and the radical-induced DOM transformation influences the subsequent water treatment processes or receiving waters. In this study, we quantified and compared DOM transformation by tracking the changes of dissolved organic carbon (DOC), UVA254, and electron donating capacity (EDC) as functions of four one-electron oxidants (SO4•-, Cl2•-, Br2•-, and CO3•-) exposures as well as the changes of functional groups and molecule distribution. SO4•- had the highest DOC reduction while Cl2•- had the highest EDC reduction, which could be due to their preferential reaction pathways of decarboxylation and converting phenols to quinones, respectively. Br2•- and CO3•- induced less changes in DOC, UVA254, and EDC than SO4•- and Cl2•-. Additionally, DOM enriched with high aromatic contents tended to have higher DOC, UVA254, and EDC reductions. Decreases in hydroxyl and carboxyl groups and increases in carbonyl groups were observed in these four types of radicals treated DOM using Fourier transform infrared spectroscopy. High resolution mass spectrometry using FTICR-MS showed that one-electron oxidants preferred to attack unsaturated carbon skeletons and transformed into molecules featuring high saturation and low aromaticity. Moreover, SO4•- was inclined to decrease oxidation state of carbon and O/C of DOM due to its strong decarboxylation capacity. This study highlights the distinct DOM transformation by four one-electron oxidants and provides comprehensive insights into the reactions of one-electron oxidants with DOM.

One-Electron Oxidant-Induced Transformations of Aromatic Alcohol to Ketone Moieties in Dissolved Organic Matter Increase Trichloromethane Formation.

Radicals in advanced oxidation processes (AOPs) degrade micropollutants during water and wastewater treatment, but the transformation of dissolved organic matter (DOM) may be equally important. Ketone moieties in DOM are known disinfection byproduct precursors, but ketones themselves are intermediates produced during AOPs. We found that aromatic alcohols in DOM underwent transformation to ketones by one-electron oxidants (using SO4•- as a representative), and the formed ketones significantly increased trichloromethane (CHCl3) formation potential (FP) upon subsequent chlorination. CHCl3-FPs from aromatic ketones (Ar-CO-CH3, average of 22 mol/mol) were 6-24 times of CHCl3-FPs from aromatic alcohols (Ar-CH(OH)-CH3, average of 0.85 mol/mol). At a typical SO4•- exposure of 7.0 × 10-12 M·s, CHCl3-FPs from aromatic alcohol transformation increased by 24.8%-112% with an average increase of 53.4%. Notably, SO4•- oxidation of aliphatic alcohols resulted in minute changes in CHCl3-FPs due to their low reactivities with SO4•- (∼107 M-1 s-1). Other one-electron oxidants (Cl2•-, Br2•-,and CO3•-) are present in AOPs and also lead to aromatic alcohol-ketone transformations similar to SO4•-. This study highlights that subtle changes in DOM physicochemical properties due to one-electron oxidants can greatly affect the reactivity with free chlorine and the formation of chlorinated byproducts.

Kinetics and Transformations of Diverse Dissolved Organic Matter Fractions with Sulfate Radicals.

Dissolved organic matter (DOM) scavenges sulfate radicals (SO4•-), and SO4•--induced DOM transformations influence disinfection byproduct (DBP) formation when chlorination follows advanced oxidation processes (AOPs) used for pollutant destruction during water and wastewater treatment. Competition kinetics experiments and transient kinetics experiments were conducted in the presence of 19 DOM fractions. Second-order reaction rate constants for DOM reactions with SO4•- (kDOM,SO4•-) ranged from (6.38 ± 0.53) × 106 M-1 s-1 to (3.68 ± 0.34) × 107 MC-1 s-1. kDOM,SO4•- correlated with specific absorbance at 254 nm (SUVA254) (R2 = 0.78) or total antioxidant capacity (R2 = 0.78), suggesting that DOM with more aromatics and antioxidative moieties reacted faster with SO4•-. SO4•- exposure activated DBP precursors and increased carbonaceous DBP (C-DBP) yields (e.g., trichloromethane, chloral hydrate, and 1,1,1-trichloropropanone) in humic acid and fulvic acid DOM fractions despite the great reduction in their organic carbon, chromophores, and fluorophores. Conversely, SO4•--induced reactions reduced nitrogenous DBP yields (e.g., dichloroacetonitrile and trichloronitromethane) in wastewater effluent organic matter and algal organic matter without forming more C-DBP precursors. DBP formation as a function of SO4•- exposure (concentration × time) provides guidance on optimization strategies for SO4•--based AOPs in realistic water matrices.