Impact of Halides on the Photoproduction of Reactive Intermediates

Nov 12, 2013 - Understanding the impact of specific quenchers on DOM could be a key to ... Andrew C. Maizel and Christina K. Remucal .... Photo-reacti...
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Impact of Halides on the Photoproduction of Reactive Intermediates from Organic Matter Caitlin M. Glover and Fernando L. Rosario-Ortiz* Department of Civil, Environmental and Architectural Engineering, 428 UCB, University of Colorado, Boulder, Colorado 80309, United States S Supporting Information *

ABSTRACT: The excitation of dissolved organic matter (DOM) from sunlight produces a range of reactive intermediates, including triplet-excited state dissolved organic matter (3DOM*), hydroxyl radical (HO•), and singlet oxygen (1O2). These intermediates are important for the inactivation of pathogens and for the degradation of trace organic contaminants (OC) within natural and engineered systems. However, halides found in the background matrix can alter the photoproduction rates by promoting or quenching the formation of these intermediates. Apparent quantum yields (Φa) for 1O2, HO•, and steady state 3DOM* concentrations photoproduced from DOM isolates were determined with varying concentrations of chloride and bromide. Fluorescence quantum yields were measured as well to probe the photophysics of the system. The maximum fluorescence quantum yield (ΦF) decreased with the addition of halides, representing a quenching of the excited singlet state of DOM. In contrast, the steady state concentrations for 3DOM* were enhanced, suggesting intersystem crossing from the singlet state to the triplet state was increased by the presence of halides. The Φa for 1O2 was increased with the addition of halides, which was expected following the 3 DOM* results because the mechanism for 1O2 production occurs through the inactivation of 3DOM* by dissolved oxygen. Although HO• production would be expected to follow 1O2, the opposite trend was seen, which suggests the formation of HO• does not occur through the same precursor. Understanding the impact of specific quenchers on DOM could be a key to understanding the true formation potential for reactive intermediates and is especially important in estuaries and wastewater impacted aquatic systems.

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lowest singlet excited state.8 An alternative pathway is for the DOM* to undergo intersystem crossing (ISC) to 3DOM*, which is a spin-forbidden transition and gives 3DOM* a longer lifetime.8 3 DOM* is an important precursor for 1O2 and HO•, but it is also able to oxidize OC and undergo energy or electron transfers.5,6,9,10 The 1O2 formation pathway is an energy transfer (physical quenching) from 3DOM* to dissolved oxygen.11,12 1O2 is a selective oxidant for the degradation of OC and can also play a role in the treatment of bacteria in natural systems. 2,9 The proposed mechanism for HO • formation from DOM is less clear. Research suggests that 3 DOM* is capable of producing a hydroxylating species through the oxidation of water and is typically portrayed as undergoing a hydrogen abstraction from water to produce HO•.13−15 However, there is still debate as to whether the

INTRODUCTION In recent years, there has been growing concern regarding the seemingly ubiquitous presence of organic contaminants (OC) in natural systems, due to their potential to harm the environment and human health.1 OC include pesticides, personal care products, and pharmaceuticals. In order to understand their fate, the natural photolytic processes (direct and sensitized) for degradation must be completely understood. Sensitized photolysis, which involves the creation of reactive intermediates, has been shown to be a highly effective type of OC mitigation.2,3 The efficiency of sensitized photolysis depends on the reactivity of OC with reactive intermediates, including singlet oxygen (1O2), hydroxyl radical (HO•), and triplet excited dissolved organic matter (3DOM*).2,4 The formation of these oxidants is impacted by the photophysical behavior of specific DOM chromophores, whose photochemical behavior has been modeled by aromatic ketones.5−8 Upon excitation from the DOM ground singlet state, the excited singlet state (1DOM*) can undergo nonradiative relaxation processes, such as internal conversion (IC) and vibrational relaxation (VR).8 IC and VR are the precursors to fluorescence, which is a radiative process that occurs from the © 2013 American Chemical Society

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Received: Revised: Accepted: Published: 13949

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species is a free HO•, low-level hydroxylating species, hydroxylating complex, or a combination.16 The reactions of OC by sensitized photolysis are significantly altered as they are transported from freshwater systems downstream into the ocean as a result of changes in background constituents, such as halides.5,6,10,17 Halides first appear in significant concentrations following anthropogenic inputs from wastewater treatment plants and increase within estuary systems out to seawater.18 Their appearance in aquatic systems is important as they can have a significant impact on the composition and photochemical activity of DOM. A study by al Housari et al. showed that the production of HO•, 1O2, and 3 DOM* in natural waters was shown to increase from freshwater to seawater, but the pH, ionic strength, halide concentrations, and DOM characteristics were not consistent across the waters.19 In contrast, when pH and ionic strength are kept constant halides have been shown to act as scavengers of reactive intermediates with the reactivity of Br− being much greater than Cl−. The scavenging effect of HO• is dictated by the trapping of HO• as HOCl−• and HOBr−•, which occurs at kforward = 4.3 × 109 M−1 s−1 and kreverse = 6.1 ± 0.8 × 109 M−1 s−1 for chloride and kforward = 1.1 × 1010 M−1 s−1 and kreverse = 3.3 × 107 M−1 s−1 for bromide.20 With these high reverse reaction rates a significant majority of the HO• will be reformed, unless the HOX−• species is rapidly consumed, as would occur in an acidic system.20 Liao et al. found that at pH 7 the formation of HO• from hydrogen peroxide was only scavenged by ∼1% at 500 mM chloride, a concentration similar to seawater, and this scavenging decreased the degradation of butyl chloride by ∼9%.21 Bromide radicals would form at a significantly higher rate, but the scavenging effect would be dependent upon the other scavengers present in the system. Halide radicals (X•, X2•−) can also form through an electron transfer reaction from 3DOM*.20,22 In natural waters, a number of halide radical reactions are possible with DOM or other low molecular weight compounds, creating chlorinated or brominated byproducts.20,23 Furthermore halide radicals can enhance the photobleaching of DOM by ∼40% with halide concentrations similar to those in seawater.24 Photobleaching destroys the chromophores necessary for 3DOM* formation, which leads to a loss of sensitized photolysis pathways.17 Recently the specific effect of halides and ionic strength on 3DOM* was investigated by Parker et al., who found that high ionic strength (regardless of the halide) slowed the intraorganic matter electron transfer pathways and resulted in longer triplet lifetimes. They also found that the steady state values of 3 DOM* doubled at seawater ionic strength as compared to freshwater systems, which was caused by a lower decay rate constant of 3DOM* and enhanced energy transfer interactions.25 While many studies have examined the interaction of halides and reactive intermediates produced from DOM, limited information is available on how fundamental photophysical processes are altered. By exposing several DOM isolates to simulated sunlight, we hope to expand the information currently available on the impact of specific halides (chloride and bromide) on reactive intermediates. The photochemical production of HO• in the presence of halides will be compared against 1O2 production, fluorescence, and 3DOM* data.

Article

MATERIALS AND METHODS

DOM Samples. Four well-characterized DOM isolates from the International Humic Substances Society were used in this study, including Suwannee River humic acid (SRHA), fulvic acid (SRFA), and natural organic matter (SRNOM) as well as Pony Lake fulvic acid (PLFA). The SRFA and PLFA are assumed to be representative end points of organic matter characteristics, as being derived from terrestrially or microbially dominated sources, respectively. As bromide and chloride alter the ionic strength of solution, sodium perchlorate was used to maintain a consistent ionic strength in each standard solution prepared. To maintain the pH at near neutral conditions (pH ≈ 7), a 5 mM phosphate buffer was utilized. Since halides are known scavengers of HO• in aquatic systems the pH of 7 was designed to push the reaction away from chloride and bromide radicals and to generate the optimal amount of HO•.21,26 Sodium chloride (EMD OmniPur, Darmstadt, Germany, Iodine = 0.0005%) and sodium bromide (Fisher Scientific, NH, USA Chloride = 0.07%) were added to samples at concentration ranges of 0.05−500 mM for chloride and 0.001−0.8 mM for bromide representative of the concentrations naturally found as a freshwater system moves into an estuary. Solar Exposures. An Oriel SOL1A solar simulator was used for these experiments. The system was equipped with a 1000 W xenon lamp accompanied by an Air Mass 1.5 filter. The system was designed to mimic the atmospheric filtration with a solar intensity of approximately 71 W m−2 from 290 to 400 nm, which is approximately double that of the sun. The irradiance was measured using an Ocean Optics spectrophotometer and is compared against solar output in Supporting Information Figure S1. The samples were kept in headspace-free Pyrex vials, which prevented contamination, and chilled to 25 °C ± 2 °C throughout the experiments. Apparent quantum yield (Φa) values for the solar simulator experiments were calculated from 290 to 400 nm because of the relative insignificance of the sum of Φa for wavelengths >400 nm observed elsewhere.15,27 Photobleaching of the DOM isolates over the solar simulator exposures was not observed. Reactive Intermediate Measurements (HO•, 1O2, and 3 DOM*). The HO• formation rate (RHO•) was measured with the hydroxylation of benzene (99.8% Alpha Aesar, MA, USA) to phenol, which has an average yield of 0.85.15,26 DOM isolate samples were spiked with 3 mM benzene and exposed to the solar simulator for four hours, during which four time points were collected. Dark controls and time zero samples were collected, but no detectable phenol was produced. Previous work by Dong et al. (2012) has shown that the scavenging potential of DOM as compared to benzene is 2 orders of magnitude smaller; therefore, benzene is the main scavenger in the system.26,28 The presence of bromide scavenges HO• at a rate of (3.0 ± 0.3) × 109 M−1 s−1,29 whereas benzene reacts at a rate of 7.8 × 109 M−1 s−1.30 Bromide would scavenge half of the HO• at a concentration of 7.8 mM, but the highest concentration studied was 0.8 mM. To eliminate the formation of HO• from the photo-Fenton pathway, bovine liver catalase (Sigma-Aldrich, MO, USA) was added at a concentration of 20 unit/mL to quench the formation of hydrogen peroxide. Φa values for HO• were calculated from the RHO• and specific rate of light absorption, as shown in eqs S1 and S2 in the Supporting Information. The 1O2 probe, furfuryl alcohol (FFA) (TCI America, OR, USA), was spiked into samples at an initial concentration of 13950

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22.5 μM, and the concentration was followed as it degraded over the exposure time. Methanol (ACS grade, Honeywell, NJ, USA) was added in these 1O2 experiments to prevent HO• from further degrading the FFA and interfering with the calculation.27,31 The spiked DOM isolate samples were exposed for two hours to the solar simulator and FFA degradation was determined at four half-hour intervals. Time zero controls were used to determine the initial FFA concentration. Steady state concentrations of 1O2 were calculated using the established second-order reaction rate constant of 1.2 × 108 M−1 s−1.12 To calculate the Φa for 1O2 Supporting Information eq S3 was utilized, which incorporates the known 1O2 steady state concentration and the specific rate of light absorption. Trimethylphenol or TMP (Alpha Aeser, USA) was spiked into samples at 1.5 mM as a probe for 3DOM*. Steady state concentrations were calculated assuming an average secondorder rate constant of ∼3.0 × 109 M−1 s−1.6 Although this rate constant was measured by Canonica et al. for aromatic ketones reacting with phenol compounds it is used here as a constant in order to compare the different 3DOM* produced. We believe this approach is acceptable because it is assumed that the isolates will have a similar amount of variation in the half-life and energy of their triplets.4,32 The interference from 1O2 was assumed to be negligible because TMP reacts with 1O2 at a reaction rate of 6.3 × 107 M−1 s−1,33 which is over 100 times lower than the average triplet reaction rate. Furthermore, the interference of 1O2 was shown to be negligible when using TMP as a probe with humic substances by Aguer et al.34 Another potential interference could come from the halides themselves, which are capable of quenching 3DOM*.22 Jammoul et al. determined the rates for an excited triplet state of a model benzophenone with chloride (7.3 ± 2.1) × 105 M−1 s−1 and bromide (3.5 ± 0.3) × 108 M−1 s−1 at pH = 5.5.22 Using the values from Jammoul et al., despite the pH difference, only bromide concentrations >2 mM could compete as a scavenger. Phenol, FFA, and TMP were measured using an Agilent 1200 series high performance liquid chromatograph (HPLC) with a C18 XDB column and online ultraviolet (UV) detection. The HPLC-UV methods have been published previously and are available online in the Agilent Chromatography library.35 Photochemical experiments were run in triplicate or duplicate and the error bars presented in the figures below represent the propagated error. Fluorescence. To complete the fluorescence quenching experiments a Horiba-Yvon Fluoromax-4 was employed. Routine lamp, Raman, and cuvette checks were completed daily. The instrument included an excitation range of 260−450 nm with data collected every 10 nm and an emission range of 450−550 nm with data collected every 2 nm. The bandpass width for both excitation and emission was set to 5 nm, and the integration time was set to 0.25 s. All samples were diluted to below a maxima of 0.1 absorption over the wavelength range used, Raman normalized, and corrected for inner filter effects using Matlab scripts similar to those in the work of Murphy et al..31,36 To calculate the quantum yield (ΦF) of fluorescence, eq 1 was used. Quinine sulfate (QS) acidified with 0.1 N H2SO4 was used as the standard. ΦF for each DOM sample was calculated by incorporating the wavelength specific absorption coefficient for both the DOM sample and QS and the sum of the fluorescence spectra (IDOM) for the emission wavelength range stated above. The maximum value of ΦF over the excitation range was determined and reported. Previous papers

have used this method with SRHA and SRFA, but it should be equally effective with the other two isolates, SRNOM and PLFA.37−39 ΦF(λEx ) =

∫ IDOM(λEx , λEm)dλEm ′ (λEx ) aDOM

×

′ (λEx ) aQS

∫ IQS(λEx , λEm)dλEm

× ΦQS

(1)

The calculation is effective because quinine sulfate has a uniform quantum yield over the wavelength range used (Φ = 0.54).36,37 The fluorescence spectra were run in triplicate, and the error bars in the figures below represent the propagated error. Stern−Volmer Quenching. To model the quenching dynamics produced by halides on DOM isolates, a Stern− Volmer equation was used. Φ0 = 1 + KSV[Q ] Φ

KSV = τ0kq

(2)

Equation 2 models a linear quenching system where [Q] is the quencher concentration, KSV is the Stern−Volmer constant, τ0 is the natural fluorescence lifetime (without quenchers), and kq is the second order quenching rate constant. The derivations for these equations can be found in the work of Geddes.8,37

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RESULTS AND DISCUSSION Impact of Halides on Fluorescence. Fluorescence is a photophysical process that occurs from the lowest excited singlet state of a molecule within 10−9 to 10−7 s of being excited.8 This process has a quantum yield associated with it and a vast majority of compounds retain a specific quantum yield throughout the entire wavelength range, a photochemical principle referred to as Kasha’s rule.8,38 However, DOM ΦF changes over the range of wavelengths from 340 to 400 nm. This deviation from Kasha’s rule could potentially be explained by the intramolecular interactions between chromophores within the DOM, as was reported by del Vecchio et al.,38 but could also be indicative of individual chromophores fluorescing. To analyze the DOM ΦF values, the maxima for the four DOM isolates were chosen, as is shown in Figure 1. SRFA, SRHA, and SRNOM isolates each have a maximum ΦF around 350 nm, and the PLFA samples are shifted down to around 355 nm. The ΦF values of reported here agree with the SRFA maximum of approximately 0.012 and the SRHA maximum of approximately 0.0045 calculated by del Vecchio et al.38,40

Figure 1. Comparison of the fluorescence Φ values for the wavelengths that surround the fluorescence maximum (ΦF) for the four organic matter isolates (SRHA, SRFA, SRNOM, PLFA). 13951

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The maximum ΦF values reported were subsequently plotted using Stern−Volmer quenching dynamics with the addition of chloride and bromide, as shown in Figure 2 and Supporting

Table 1. Stern−Volmer Constants for Fluorescence of Chloride and Bromide Calculated from the First Five Concentration Data Points for the Four Organic Matter Isolatesa fluorescence Stern−Volmer constant (m M−1) organic matter isolate

chloride

SRHA SRFA SRNOM PLFA

0.11 ± 0.04 0.038 ± 0.02 0.059 ± 0.08 0.16 ± 0.08

bromide 0.62 0.95 0.75 1.00

± ± ± ±

0.02 0.05 0.03 0.03

a

The DOM isolates (5 mg/L) were dissolved in a buffered solution where the ionic strength was kept constant. The error values represent one standard deviation from the triplicate data set.

between the quenching constants and different chemical parameters, such as aromatic/aliphatic composition, did not yield any significant correlation. Furthermore, correlation to other parameters such as fluorescence index and E2:E3 ratio (ratio absorbance at 254 nm to absorbance at 365) were not significant. E2:E3 ratio has proven useful with the quantum yields for 1O2 because it has the ability to act as a surrogate for the level of charge transfer interactions within the organic matter.42,43 Furthermore, E2:E3 has been used to model photobleaching of DOM, the source of DOM, molecular weight, and aromaticity of organic matter samples.44−47 A theory has been proposed where charge transfer effects between acceptor moieties (quinones and aromatic ketones) and donor moieties (poly hydroxylated aromatics, phenols, or indoles) are responsible for the long-range absorbance.38,48 Fluorescence would then be also impacted by these interactions and would explain the emission spectra range and ΦF. This model helps explain why specific moieties are expected to contribute to fluorescence, while others may not, which could explain differences in fluorescence between DOM samples. However, it is difficult to understand the observed differences in fluorescence quenching. The other constant in this system is the lifetime of the excited state. The fluorescence lifetimes of different DOM samples have been measured and fall within 0.5−9 ns.37,49 It is likely that a combination of differences in chromophores and fluorophores, coupled with lifetimes (which are impacted by the fluorophores present) are contributing to the observed differences in the samples. The observed differences in Stern−Volmer constants for chloride and bromide suggest the impact of an external heavy atom effect. The external heavy atom effect increases the population of excited triplets by increasing the levels of intersystem crossing (ISC) and forming a charge-transfer complex.8,37 This process occurs when an element with a high atomic number passes. As a result the electrons accelerate within orbitals due to the higher electrostatic attraction to the high positive charge in the nucleus of the heavy atom or high atomic element atom. The electron acceleration increases the magnetic moment and therefore spin−orbit coupling. The magnitude of this effect occurs at a ratio of 1:17.5 for NaCl to NaBr, which is similar to the average (of the four isolates) ratio observed here of 1:12 for NaCl to NaBr.8,50 Previous studies have shown that aromatic hydrocarbons generally have very low spin flip tendencies in comparison with carbonyl groups.8,51,52 Each of the four DOM isolates responds differently to the addition of halides, which would suggest that the population of fluorescence emitting chromophores varies based on the concentration of carbonyl and aromatic hydrocarbons within

Figure 2. Stern−Volmer plots (Φ0/Φ) comparing the quenching effect of chloride on the four DOM isolates (SRHA, SRFA, SRNOM, and PLFA) for the Φa of fluorescence (top) and HO•(bottom). The error bars represented the propagated error of one standard deviation calculated from triplicate or duplicate experiments.

Information Figure S2. Linear quenching dynamics are followed until reaching concentrations greater than 10 mM for chloride and 0.4 mM bromide. This initial linear trend suggests a single pool of chromophores, which are being quenched to varying degrees depending on the isolate origin. However, once the isolates reach high concentrations, the quenching effect is maximized and essentially stops even with additional halides. The SRFA and SRNOM behaved similarly in terms of the quenching, but the PLFA and SRHA were more efficiently quenched. At the highest chloride concentration of 500 mM, the reduction in quantum yield was ∼84% for PLFA and ∼86% for SRHA, whereas the SRFA was quenched by ∼70% and the SRNOM by ∼65%. The fact that the fluorescence differs between isolates is well-known, what is interesting is how more efficient the quenching is for PLFA and SRHA. In Table 1 the Stern−Volmer constants (calculated using eq 2 and the first five data points) are presented and each isolate has a similar magnitude for chloride and bromide. The Stern− Volmer constants represent a combination of the lifetime and second-order deactivation of the excited state. The constants for chloride are 0.11 for SRHA, 0.038 for SRFA, 0.059 for SRNOM, and 0.16 for PLFA. The constants increase upon the addition of bromide, changing to 0.62 for SRHA, 0.95 for SRFA, 0.75 for SRNOM, and 1.00 for PLFA. Once again, PLFA has a higher deactivation. The IHSS isolates have been well characterized, and their chemical composition is found in online databases.38,41 An evaluation of any potential correlation 13952

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each sample. Figure 3 highlights the trends that can be seen between SUVA254 and the Stern−Volmer constants (represent-

Figure 3. Trends between Stern−Volmer constants and SUVA254, a spectroscopic parameter. Stern−Volmer constants were calculated using the linear quenching seen for ΦF.

ing the susceptibility to quenching) calculated for bromide and chloride. SUVA254 is a measure of the UV-absorbance normalized to the organic carbon content, and it is a useful parameter in this situation because halides do not absorb an appreciable amount of light at 254 nm and therefore do not interfere with the measurement. SUVA254 correlates well with the level of aromaticity of a sample.53 In Figure 3, the percentage of aromatic groups increases, and the amount of quenching decreases. This trend is supported by the idea that the aromatic hydrocarbons have lower spin flip tendencies, and this leads to a lower level of quenching. This explanation is supported by the differences observed in the generation of 3 DOM* and 1O2 as described below. However, the focus of this correlation was merely to suggest that the use of spectroscopic properties could be useful for applying this data in natural systems applications. Formation of 3DOM* and 1O2 in the Presence of Halides. The effective reduction in ΦF indicates that fluorescence is not the only important deactivation mechanism of 1DOM* as halides are added. With the quenching results from fluorescence, 3DOM* are assumed to perform in the opposite manner with an increase in the number of triplets produced with respect to halide concentration. The measured results are shown in Figures 4 (chloride) and S2 (bromide). Both show an enhancement trend for the steady state concentration of 3DOM* and consequently for the Φa of 1O2. As a result of this enhancement, the Stern−Volmer equations for quenching cannot be used. An alternative derivation of the Stern−Volmer for this process cannot be determined due to the unknown yield of ISC from singlet states to triplet states and other details on the photophysics of the system. Previous studies have examined the yield of ISC for two fulvic acids, which they found to range depending on pH of the solution and the ionic strength of the solution.51,52 Without chloride the yield for the fulvic acids is approximately 0.6 for a near neutral solution, but with the addition of 1 M KCl, the yield increases to approximately 0.91.37,52 This presents an optimal level of enhancement for the fulvic acid of approximately 50%. 3 DOM* steady state concentrations at the five lowest chloride concentrations were enhanced in a nonlinear fashion. The SR isolates behaved in a similar manner and were enhanced approximately 34−46% from the control, but PLFA did not exhibit as significant of an effect as it was enhanced by

Figure 4. Stern−Volmer plots (Φ0/Φ) comparing the enhancing effect of chloride on DOM isolates (SRHA, SRFA, SRNOM, and PLFA) for the steady state concentration of 3DOM* and the apparent quantum yield of 1O2. The error bars represented the propagated error of one standard deviation calculated from triplicate or duplicate experiments.

22%. At the highest chloride concentrations, the SR isolates were almost to the proposed maximum level of enhancement ∼50%. Once again PLFA lags behind and is essentially stagnant for the three highest chloride concentrations at around ∼22%. Supporting Information Figure S3 shows the impact of bromide and all of the OM isolates behave similarly, being increased by ∼50%. Although the magnitude of the change is similar, the bromide concentrations are much lower and therefore once again we attribute this to an external heavy atom effect. The stagnation at the highest concentrations could be a result of the scavenging competition by the halides themselves.22 Parker et al. measured the formation of triplets from SRNOM using two different probe molecules (TMP and sorbate) in the presence of ionic strength controls and varying levels of halides.25 They observed a similar increase in the steady state concentration under a simulated seawater matrix and attributed it to the aggregation of OM under high ionic strength while using a sorbate probe. This aggregation process allows for increased intramolecular interactions and therefore protects the 3DOM* that form. Using the TMP probe, they found that ionic strength increases reduced the formation of triplets.25 However, in our experiments, the ionic strength is controlled and halide specific effects are seen. The completion of the 1O2 experiments confirmed the behavior seen with the 3DOM* and resulted in an increase in the Φa. Chloride had an increased level of enhancement of 1O2 as compared to 3DOM*. The SRHA and SRFA rose to 40% of the control Φa, whereas the SRNOM and PLFA both were at 80%. The heavy atom effect was once again suspected by the 13953

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effect of bromide on 1O2, as shown in Supporting Information Figure S3. Despite the low concentration of bromide, it was a significantly more efficient promoter of the Φa. The halide enhancement on the 3DOM* was lower than that seen for 1O2. 1 O2 is formed through the physical quenching of excited states through an energy transfer from triplets with energy greater than 94.3 kJ/mol, whereas to transform TMP an electron transfer occurs. This difference could account for the different levels of enhancement seen. A similar type of leveling-off or stagnation occurred with the 1O2 results and this could once again indicate an interference from the scavenging of 3DOM* by halides. Once again the general trend of the SR isolate samples behaving differently than the PLFA isolate was seen, and this can be attributed back to the impact of halides on certain photochemically active regions. Impact of Halides on HO•. The formation of HO• has been proposed to stem from a triplet excited state.13 Experiments to determine the photophysical mechanism behind this process found that specific 3DOM* contain the appropriate amount of energy required to abstract a hydrogen from water. This mechanism has also been observed for small organic molecules in aquatic systems.13,15,54 There is the possibility that a fraction of the HO• produced is not a free radical, but instead a low-level hydroxylating species. These alternatives to free HO• have been proposed due to the comparison of probe compounds with varying energetics and from examining different reaction conditions.15,54 From previous research on the formation of HO•, we would expect it to follow the trend seen with 3DOM* and 1O2. However, Figure 2 shows the Φa for HO• following the quenching pathway similar to fluorescence. Even at the highest concentrations of chloride, the SR isolates continued being quenched, which was contrasted against the PLFA sample that leveled off once the concentration was above 10 mM. Once again the addition of bromide shows a similar level of quenching, but at a significantly reduced concentration. These HO• results differ from the fluorescence quenching because the data points fit a nonlinear model. However, this behavior would suggest that the data follows a similar excited state pathway to fluorescence as opposed to the accepted triplet state. The energetics required to abstract a hydrogen from water are on the edge of the limitations for the energy of DOM triplets, which have been shown to be around 170−180 kJ/mol or >250 kJ/mol.4,55 The singlet state of DOM would have a higher level of energy to contribute to the abstraction of water, despite its shorter lifetime.8 Environmental Significance. Throughout this paper, we have shown that within environmentally relevant concentrations, chloride and bromide can provide significant quenching or promotion for RIs. This is an important consideration when monitoring and mapping the degradation of OC through indirect photolysis in estuary regions or heavily wastewater impacted streams. Past studies of the effect of halides on the degradation of OC have shown mixed results. Several compounds (ibpuprofen, ketoprofen, 17α-ethinylestradiol56) showed no difference in degradation. While other OC (triclosan,57 carbofuran,58 carbamazepine59) had increased degradation, which was attributed to the formation of halide radicals. When Grebel et al. controlled for ionic strength they found that halides decreased the degradation of 17β-estradiol and attributed it to the photobleaching of DOM by halides, leading to a reduced concentration of 3DOM*.17 Although our study adds to the body of literature on the impact of halides on

the photochemical formation of RIs, the complexity of natural systems and changing background parameters leaves additional research to be completed on these aquatic systems.

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ASSOCIATED CONTENT

S Supporting Information *

Three figures and one table. In addition, it contains details for the Materials and Methods section. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 303-4927607. Fax: 303-492-7317. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation through the Graduate Research Fellowship (GRFP no. 115198 for C.M.G.) and CBET (Project no. 1235288).

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