Photosensitizing and Inhibitory Effects of Ozonated Dissolved Organic Matter on Triplet-Induced Contaminant Transformation

Abstract

Dissolved organic matter (DOM) is both a promoter and an inhibitor of triplet-induced organic contaminant oxidation. This dual role was systematically investigated through photochemical experiments with three types of DOM of terrestrial and aquatic origins that were preoxidized to varying extents by ozonation. The inhibitory effect of DOM was assessed by determining the 4-carboxybenzophenone photosensitized transformation rate constants of two sulfonamide antibiotics (sulfamethoxazole and sulfadiazine) in the presence of untreated or preoxidized DOM. The inhibitory effect decreased with the increasing extent of DOM preoxidation, and it was correlated to the loss of phenolic antioxidant moieties, as quantified electrochemically, and to the loss of DOM ultraviolet absorbance. The triplet photosensitizing ability of preoxidized DOM was determined using the conversion of the probe compound 2,4,6-trimethylphenol (TMP), which is unaffected by DOM inhibition effects. The DOM photosensitized transformation rate constants of TMP decreased with increasing DOM preoxidation and were correlated to the concomitant loss of chromophores (i.e., photosensitizing moieties). The combined effects of DOM preoxidation on the inhibiting and photosensitizing properties were assessed by phototransformation experiments of the sulfonamides in DOM-containing solutions. At low extents of DOM preoxidation, the sulfonamide phototransformation rate constants remained either unchanged or slightly increased, indicating that the removal of antioxidant moieties had larger effects than the loss of photosensitizing moieties. At higher extents of DOM preoxidation, transformation rates declined, mainly reflecting the destruction of photosensitizing moieties.

Probe Compounds to Assess the Photochemical Activity of Dissolved Organic Matter

“…32 Among the most reactive phenols (with respect to photosensitization by CDOM), TMP and 3,4-dimethoxyphenol were proposed as PCs for 3 CDOM * . 63 TMP has later been used in various studies, 18,39,60,80,[88][89][90][91][92] mainly at low concentrations suitable to quantify [ 3 CDOM * ]ss (see equations 4 and 5) and in some cases 447 also at higher concentrations to quench 3 CDOM * . 39, 60,88 Compared to other organic contaminants susceptible to transformation by 3 CDOM * , TMP not only has a very high 449 reactivity, but is also unaffected by the adverse inhibition side-effect ascribed to antioxidant 450 moieties of CDOM.…”

Laser flash photolysis study of the photoinduced oxidation of 4-(dimethylamino)benzonitrile (DMABN)

“…Various pieces of evidence have been presented to support this hypothesis. 7,14,[18][19][20] The effects of DOM on the overall phototransformation rates of substituted anilines are now fairly well understood. However, very little is known about the nature and fate of the intermediates produced after the first oxidation step.…”

Triplet State Formation of Chromophoric Dissolved Organic Matter in Atmospheric Aerosols: Characteristics and Implications

“…3 CDOM*, as a reactive intermediate (McNeill & Canonicab, 2016;Wenk et al, 2015), is characterized by instability, complex composition and low concentration.…”

“…Compared with other chemical probes, TMP contains methyl-substituted groups that can be used as electron donors for 3 CDOM* reactions; thus, TMP has a higher reactivity (Canonica & Freiburghaus, 2001). Some substances in the environment can inhibit the reaction between the probes and 3 CDOM*, but TMP is not easily affected by these substances (Canonica & Laubscher, 2008;Wenk et al, 2015). Therefore, TMP is suitable as a probe to describe the characteristics of 3 CDOM*.…”

“…The types and abilities of CDOM to form 3 CDOM* may be different, which requires us to analyze both the types and compositions of CDOM. Excitation-emission matrices spectra (EEMs) are a direct method for the characterization of CDOM, allowing identification of the chromophore types by applying parallel factor analysis (PARAFAC), such as humic-like substances (HULIS), quinones, phenols and amino acids (Korak et al, 2014;Ma et al, 2010;McKnight et al, 2001;Rosario-Ortiz & Canonica, 2016;Wenk et al, 2015). Therefore, the EEM method is expected to be used in the structure-activity relationship analysis between the CDOM types and 3 CDOM* formation.…”