Microheterogeneous Distribution of Hydroxyl Radicals in Illuminated Dissolved Organic Matter Solutions

Abstract

Hydroxyl radicals ( • OH) are important reactive species that are photochemically generated through solar irradiation of chromophoric dissolved organic matter (CDOM) in surface waters. However, the spatial distribution within the complex three-dimensional structure of CDOM has not been examined. In this study, we used a series of hydrophobic chlorinated paraffins as chemical probes to elucidate the microheterogeneous distribution of • OH in illuminated CDOM solutions. The steady-state concentration of • OH inside the CDOM microphase is 210 ± 31-fold higher than the concentration in the aqueous phase. Our results suggest that the most photochemically generated • OH are confined into the CDOM microphase. Thus, illuminated CDOM behaves as a natural microreactor for • OH-based oxidations. By including intra-CDOM • OH, the quantum yield of • OH for CDOM solutions was estimated to be 2.2 ± 0.5 × 10 −3 , which is 2 orders of magnitude greater than previously thought. The elevated concentrations of photogenerated • OH within the CDOM microphase may improve the understanding of hydrophobic pollutant degradation in aqueous environments. Moreover, our results also suggest that • OH oxidation may play more important roles in the phototransformation of CDOM than previously expected.

Effects of dissolved organic matter characteristics on the photosensitized degradation of pharmaceuticals in wastewater treatment wetlands

“…These pharmaceuticals are photosensitive to various PPRIs and their individual polarity also inuences the photodegradation rates. 121,122 Such a drastic enhancement of photodegradation rates emphasizes the impact of DOM polarity on contaminant photochemical fate. 122,123 Quenching experiments (1% IPA in DOM samples) with three "hydrophobic DOM" wetland site samples and two "hydrophilic DOM" wetland site samples show that cOH is a major contributor to the photodegradation rates of AMX, ATL, and EE2, especially in samples with hydrophobic DOM.…”

“…121,122 Such a drastic enhancement of photodegradation rates emphasizes the impact of DOM polarity on contaminant photochemical fate. 122,123 Quenching experiments (1% IPA in DOM samples) with three "hydrophobic DOM" wetland site samples and two "hydrophilic DOM" wetland site samples show that cOH is a major contributor to the photodegradation rates of AMX, ATL, and EE2, especially in samples with hydrophobic DOM. In the presence of a cOH quencher, the photodegradation rates of these pharmaceuticals decrease, thereby distinguishing the contribution of cOH mediated degradation from total observed photodegradation (Fig.…”

“…This shi suggests that the importance of the cOH pathway increases with increasing hydrophobicity of DOM. 122 Estimates of the total concentration of cOH species participating in photodegradation of AMX, ATL, and EE2 are calculated using bimolecular rate constants of the respective pharmaceuticals and presented in Table S7. † For AMX and ATL, the apparent steady state concentration of cOH species ([cOH] ss,app ) during DOM sample irradiation averages around 5 Â 10 À16 M for "hydrophilic DOM" and 3 Â 10 À15 M for "hydrophobic DOM".…”

“…It could be that hydrophobic interactions between EE2 and DOM drive the cOH reaction rates to elevated levels and provide an untapped source of cOH species that are residing in the DOM structure. 121,122,125 As indicated by their low octanol-water partition coefficients (log K ow ), AMX (log K ow ¼ 0.87) and ATL (log K ow ¼ 0.16) have much less hydrophobicity than EE2 (log K ow ¼ 3.67). Based on its higher hydrophobicity, EE2 is suggested to bind closely with DOM, as an effect getting more access to The contributions of PPRIs other than cOH to photodegradation rates of AMX and EE2 are also higher for samples with "hydrophobic DOM" than "hydrophilic DOM" (Fig.…”

Interaction Mechanisms and Application of Ozone Micro/Nanobubbles and Nanoparticles: A Review and Perspective

“…In recent years, advanced oxidation methods with green and efficient characteristics have attracted the attention of researchers in the field of water treatment [1,2]. Advanced oxidation methods in the process of water treatment, through the input of light energy, electrical energy and other external energy and O 3 , H 2 O 2 and other substances [3], via a series of physical processes and chemical reactions, generate hydroxyl radicals (•OH) and peroxyradical (O 2 • − ) active radicals with strong oxidability [4], and the generated hydroxyl radicals can react quickly with electron sites that are rich in organic matter in water, which triggers complex free radical chain reactions, thereby resulting in the degradation and removal of organic matter [5].…”