Garrett McKay

Temperature dependence of the reaction between the hydroxyl radical and organic matter.

The temperature-dependent bimolecular rate constants for the reaction of the hydroxyl radical (HO(•)) with organic matter (OM) (k(OM-HO(•))) have been measured for three natural organic matter (NOM) isolates and three bulk effluent organic matter (EfOM) samples using electron pulse radiolysis and thiocyanate competition kinetics. The range of values for the room temperature k(OM-HO(•)) was 1.21-9.37 × 10(8) M(C)(-1)s(-1), with NOM isolates generally reacting slower than EfOM samples. The NOM isolates had an average apparent activation energy of 19.8 kJ mol(-1), while the EfOM samples had an average value slightly lower (14.3 kJ mol(-1)), although one NOM isolate (Elliot Soil Humic Acid, 29.9 kJ mol(-1)) was a factor of 2 times greater than other samples studied. These apparent activation energies are the first determined for OM and HO(•), and the Arrhenius plots obtained for NOM isolates (lowest R(2) > 0.993) suggest that no significant structural changes are occurring over the temperature range 8-41 °C. In contrast, the greater scatter (lowest R(2) > 0.903) observed for the EfOM samples suggests that some structural changes may be occurring. These results provide a deeper fundamental understanding of the reaction between OM and HO(•) and will be useful in quantifying HO(•) reactions in natural and engineered systems.

Temperature Dependence of the Photochemical Formation of Hydroxyl Radical from Dissolved Organic Matter

The temperature dependence of the photochemical production of the hydroxyl radical (•OH) from dissolved organic matter (DOM) was investigated by measuring the apparent temperature dependence of the quantum yield (Φa) for this process. Temperature dependent Φa values were analyzed using the Arrhenius equation. Apparent activation energies obtained for DOM isolates purchased from the International Humic Substances Society ranged from 16 to 34 kJ mol(-1). Addition of 40 units mL(-1) catalase, used to hinder the hydrogen peroxide (H2O2)-dependent pathway to •OH, did not impact the observed activation energy. However, an increase in activation energy was observed in lower molecular weight DOM obtained by size fractionation. We also measured the temperature dependence of p-benzoquionone photolysis as a model compound for DOM and observed no temperature dependence (slope p = 0.41) for the formation of phenol from oxidation of benzene (the •OH probe used), but a value of about 10 kJ mol(-1) for p-benzoquinone loss, which is consistent with formation of a quinone-water exciplex. These data provide insight into DOM photochemistry as well as provide parameters useful for modeling steady state •OH concentrations in natural systems.

Investigation of the Coupled Effects of Molecular Weight and Charge-Transfer Interactions on the Optical and Photochemical Properties of Dissolved Organic Matter

We studied the formation of photochemically produced reactive intermediates (RI) from dissolved organic matter (DOM). Specifically, we focused on the effects of variable molecular weight and chemical reduction on the optical properties of DOM (absorbance and fluorescence) and the formation of singlet oxygen ((1)O2), DOM triplet excited states ((3)DOM*), and the hydroxyl radical ((•)OH). The data are largely evaluated in terms of a charge-transfer (CT) model, but deficiencies in the model to explain the data are pointed out when evident. A total of two sets of samples were studied that were subjected to different treatments; the first set included secondary-treated wastewaters and a wastewater-impacted stream, and the second was a DOM isolate. Treatments included size fractionation and chemical reduction using sodium borohydride. Taken as a whole, the results demonstrate that decreasing molecular weight and borohydride reduction work in opposition regarding quantum efficiencies for (1)O2 and (3)DOM* production but in concert for fluorescence and (•)OH production. The optical and photochemical data provide evidence for a limited role of CT interactions occurring in lower-molecular-weight DOM molecules. In addition, the data suggest that the observed optical and photochemical properties of DOM are a result of multiple populations of chromophores and that their relative contribution is changed by molecular-weight fractionation and borohydride reduction.

Kinetic study of the reactions between chloramine disinfectants and hydrogen peroxide: temperature dependence and reaction mechanism.

The temperature-dependent kinetics for the reaction between hydrogen peroxide and chloramine water disinfectants (NH2Cl, NHCl2, and NCl3) have been determined using stopped flow-UV/Vis spectrophotometry. Rate constants for the mono- and dichloramine-peroxide reaction were on the order of 10(-2)M(-1)s(-1) and 10(-5)M(-1)s(-1), respectively. The reaction of trichloramine with peroxide was negligibly slow compared to its thermal and photolytically-induced decomposition. Arrhenius expressions of ln(kH2O2-NH2Cl)=(17.3±1.5)-(51500±3700)/RT and ln(kH2O2-NHCl2)=(18.2±1.9)-(75800±5100)/RT were obtained for the mono- and dichloramine peroxide reaction over the temperature ranges 11.4-37.9 and 35.0-55.0°C, respectively. Both monochloramine and hydrogen peroxide were first-order in the rate-limiting kinetic step and concomitant measurements made using a chloride ion selective electrode showed that the chloride was produced quantitatively. These data will aid water utilities in predicting chloramine concentrations (and thus disinfection potential) throughout the water distribution system.

Predicting Reactive Intermediate Quantum Yields from Dissolved Organic Matter Photolysis Using Optical Properties and Antioxidant Capacity

The antioxidant capacity and formation of photochemically produced reactive intermediates (RI) was studied for water samples collected from the Florida Everglades with different spatial (marsh versus estuarine) and temporal (wet versus dry season) characteristics. Measured RI included triplet excited states of dissolved organic matter (3DOM*), singlet oxygen (1O2), and the hydroxyl radical (•OH). Single and multiple linear regression modeling were performed using a broad range of extrinsic (to predict RI formation rates, RRI) and intrinsic (to predict RI quantum yields, ΦRI) parameters. Multiple linear regression models consistently led to better predictions of RRI and ΦRI for our data set but poor prediction of ΦRI for a previously published data set,1 probably because the predictors are intercorrelated (Pearsons r > 0.5). Single linear regression models were built with data compiled from previously published studies (n ≈ 120) in which E2:E3, S, and ΦRI values were measured, which revealed a high degree of similarity between RI-optical property relationships across DOM samples of diverse sources. This study reveals that •OH formation is, in general, decoupled from 3DOM* and 1O2 formation, providing supporting evidence that 3DOM* is not a •OH precursor. Finally, ΦRI for 1O2 and 3DOM* correlated negatively with antioxidant activity (a surrogate for electron donating capacity) for the collected samples, which is consistent with intramolecular oxidation of DOM moieties by 3DOM*.

The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics

The optical properties of dissolved organic matter influence chemical and biological processes in all aquatic ecosystems. Dissolved organic matter optical properties have been attributed to a charge-transfer model in which donor-acceptor complexes play a primary role. This model was evaluated by measuring the absorbance and fluorescence response of organic matter isolates to changes in solvent temperature, viscosity, and polarity, which affect the position and intensity of spectra for known donor-acceptor complexes of organic molecules. Absorbance and fluorescence spectral shape were largely unaffected by these changes, indicating that the distribution of absorbing and emitting species was unchanged. Overall, these results call into question the wide applicability of the charge-transfer model for explaining organic matter optical properties and suggest that future research should explore other models for dissolved organic matter photophysics.

Hydroxyl Radical Probes for the Comparison of Secondary Treated Wastewaters

The majority of the wastewater treatment facilities in the United States treat to secondary treatment, as required by the Clean Water Act. However, because of the growing list of chemical pollutants commonly detected in treated wastewaters, advanced oxidation processes (AOPs) are finding application as treatment options. AOPs are defined by the use of the hydroxyl radicals as the major oxidant. For such applications to be cost-effective and optimized, a full understanding of the radical chemistry must be achieved. This chemistry can be complex, with both reactivity and efficiency of radical reactions strongly dependent upon the contaminants and overall water quality. In this work, the degradation efficiencies of two probe molecules, caffeine and sulfamethoxazole, were studied in secondary treated wastewaters from Southern California and Northwest Indiana. These radical efficiencies showed significant differences; therefore, further solution modifications, kinetic radical scavenging, and production measurements were performed to elucidate the causes of these differences.

Ozone and chlorine reactions with dissolved organic matter - Assessment of oxidant-reactive moieties by optical measurements and the electron donating capacities

Oxidation processes are impacted by the type, concentration and reactivity of the dissolved organic matter (DOM). In this study, the reactions between various types of DOM (Suwannee River fulvic acid (SRFA), Nordic Reservoir NOM (NNOM) and Pony Lake fulvic acid (PLFA)) and two oxidants (ozone and chlorine) were studied in the pH range 2-9 by using a combination of optical measurements and electron donating capacities. The relationships between residual electron donating capacity (EDC) and residual absorbance showed a strong pH dependence for the ozone-DOM reactions with phenolic functional groups being the main reacting moieties. Relative EDC and absorbance abatements (UV254 or UV280) were similar at pH 2. At pH 7 or 9, the relative abatement of EDC was more pronounced than for absorbance, which could be explained by the formation of UV-absorbing products such as benzoquinone from the transformation of phenolic moieties. An increase in fluorescence abatement with increasing pH was also observed during ozonation. The increase in fluorescence quantum yields could not be attributed to formation of benzoquinone, but related to a faster abatement of phenolic moieties relative to fluorophores with low ozone reactivity. The overall •OH yields as a result of DOM-induced ozone consumption increased significantly with increasing pH, which could be related to the higher reactivity of phenolic moieties at higher pH. The •OH yields for SRFA and PLFA were proportional to the phenolic contents, whereas for NNOM, the •OH yield was about 30% higher. During chlorination of DOM at pH 7 an efficient relative EDC abatement was observed whereas the relative absorbance abatement was much less pronounced. This is due to the formation of chlorophenolic moieties, which exert a significant absorbance, and partly lose their electron donating capacity. Pre-ozonation of SRFA leads to a decrease of chloroform and haloacetic acid formation, however, only after a threshold of > ∼50% abatement of the EDC and under conditions which are not precursor limited. The decrease in chloroform and haloacetic acid formation after the threshold EDC abatement was proportional to the relative residual EDC.

Temperature Dependence of Dissolved Organic Matter Fluorescence

The temperature dependence of organic matter fluorescence apparent quantum yields (Φf) was measured for a diverse set of organic matter isolates (i.e., marine aquatic, microbial aquatic, terrestrial aquatic, and soil) in aqueous solution and for whole water samples to determine apparent activation energies ( Ea) for radiationless decay processes of the excited singlet state. Ea was calculated from temperature dependent Φf data obtained by steady-state methods using a simplified photophysical model and the Arrhenius equation. All aquatic-derived isolates, all whole water samples, and one soil-derived fulvic acid isolate exhibited temperature dependent Φf values, with Ea ranging from 5.4 to 8.4 kJ mol-1 at an excitation wavelength of 350 nm. Conversely, soil humic acid isolates exhibited little or no temperature dependence in Φf. Ea varied with excitation wavelength in most cases, typically exhibiting a decrease between 350 and 500 nm. The narrow range of Ea values observed for these samples when compared to literature Ea values for model fluorophores (∼5-30 kJ mol-1) points to a similar photophysical mechanism for singlet excited states nonradiative inactivation across organic matter isolates of diverse source and character. In addition, this approach to temperature dependent fluorescence analysis provides a fundamental, physical basis, in contrast to existing empirical relationships, for correcting online fluorescence sensors for temperature effects.

Response to Comment on The Case Against Charge Transfer Interactions in Dissolved Organic Matter Photophysics

Interactions in Dissolved Organic Matter Photophysics I response to our recent study regarding the role of chargetransfer (CT) interactions in dissolved organic matter (DOM) photophysics, Blough and Del Vecchio raise concerns about the use of solvent polarity, viscosity, and temperature to test for the prevalence of CT interactions in DOM. This topic is of great significance to environmental chemistry and engineering due to the ubiquity and reactivity of DOM in these systems. Blough and Del Vecchio argue against the types of potential donor−acceptor complexes occurring in DOM proposed in Figure 1 of McKay et al. (2018), which includes independent, covalently tethered, and conjugated donor−acceptor moieties. The counterargument asserts that the types of donor− acceptor complexes we propose play a limited role, suggesting instead that the model for polydopamine, as proposed by Dreyer and co-workers, also applies to DOM. In this model, structural units composing DOM would form static complexes as a result of H-bonding, π-stacking, and charge-transfer interactions. It is hypothesized that such donor−acceptor complexes in DOM are hindered kinetically and thermodynamically from dissociating into their respective independent moieties. Blough and Del Vecchio state that the dynamics of these static complexes would not be affected by temperature and solvent polarity in the same way as dynamic donor− acceptor complexes, offering an alternative explanation for the lack of change in spectral shape presented in our study. Further, the absence of solvatochromism reported in our study could potentially be due to solvent-protected donor−acceptor complexes. This model hypothesizes that this hydrophobic core is further stabilized by charged outer groups. Although we welcome the criticism and further discussion, we disagree with the presented counterarguments. First, the model for the primary structure of polydopamine proposed by Dreyer et al. is controversial within the literature. Specifically, a subsequent study argued that polydopamine monomers are covalently linked, as opposed to being held together by noncovalent interactions (e.g., chargetransfer). The merits of these individual studies are beyond the scope of this response, but it is noteworthy that there is debate over the structure of a material that is synthesized from a known monomer unit (i.e., dopamine), whereas the structural units in DOM are much more heterogeneous and poorly characterized. In addition, the isolation of donor and acceptor moieties from solvent is inconsistent with other lines of evidence used to support a CT model. For example, putative acceptor and donor moieties in the CT model, aromatic ketones/aldehydes and phenols/polyphenols/alkoxy phenols, respectively, exhibit aqueous phase photochemistry. If such CT complexes are protected from the solvent, they would also be expected to be protected from participating in bimolecular reactions in the aqueous phase. Furthermore, according to the CT model, reduction of DOM with sodium borohydride would result in DOM with substantially lower molecular weight due to disruption of these donor−acceptor interactions. This hypothesis conflicts with high-pressure size exclusion chromatography measurements published recently for DOM treated with sodium borohydride. In addition, it is difficult to reconcile how an anionic reductant, such as borohydride, but not neutral solvent molecules, could access such complexes in a static, anion-encircled hydrophobic microenvironment. Another remaining question is how solvents of such different polarity could have no effect on DOM spectra, when pH so readily affects the optical properties of DOM, as reported by Blough and Del Vecchio. In the referenced pH experiments, the CT model holds that increased pH deprotonates phenolic donors, which leads to increased charge-transfer excitation. However, for donor−acceptor pairs to be pH-sensitive, the donors would have to be solvent accessible, contrary to the proposed CT model. These observations demonstrate that the evidence presented in support of a CT model (i.e., reactivity with borohydride and pH-sensitive spectra) also contradicts the provision that donor−acceptor complexes reside in a solventinaccessible microenvironment. A final argument against solvent-inaccessible chromophores and fluorophores is that fluorescence quantum yield was significantly changed by solvent polarity at all excitation wavelengths measured (Figure 3c in manuscript), extending well into the visible wavelength range. This change in fluorescence intensity with solvent polarity across the DOM absorption spectrum indicates that these fluorophores are solvent-accessible. Lastly, Blough and Del Vecchio assert that the formation of particulates for soil humic acids in tetrahydrofuran supports the hypothesis that donor−acceptor moieties are in a solvent inaccessible hydrophobic microenvironment. The data presented in Figure 4 of our study contradicts this view. A solvatochromic shift in fluorescence spectra of >20 nm was observed for soil humic acids in acetonitrile and a soil fulvic acid in both acetonitrile and tetrahydrofuran at a range of excitation wavelengths. We hypothesized that this solvatochromism in the fluorescence spectra could be due to exciplex formation, but not ground state donor−acceptor complexes due to the lack of solvatochromism in the corresponding absorbance spectra. This result gives us confidence that solvent polarity would affect the dynamics of donor−acceptor complexes present in the other isolates investigated, if they were present. Furthermore, in the apolar organic solvent systems investigated, carboxylates would be protonated, removing a speculated stabilizing force, and thus encouraging complex dissociation. Lastly, the view that the low solubility of soil humic acids in tetrahydrofuran is evidence for a hydrophobic microenvironment does not fit our observation that these materials dissolved f irst and then, after 24 h, showed evidence of precipitates (Supporting Information section 3.4). The hypothesis that the stabilization by charged moieties controls