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The natural lifetimes of triplet dissolved organic matter (3DOM*) were determined by an O2 saturation kinetics study of singlet oxygen quantum yields ...
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Lifetimes of Triplet Dissolved Natural Organic Matter (DOM) and the Effect of NaBH4 Reduction on Singlet Oxygen Quantum Yields: Implications for DOM Photophysics Charles M. Sharpless* Department of Chemistry, University of Mary Washington, Fredericksburg, Virginia 22401, United States

Environ. Sci. Technol. 2012, 46 (7), 3912−3920 S Supporting Information *

ABSTRACT: The natural lifetimes of triplet dissolved organic matter (3DOM*) were determined by an O2 saturation kinetics study of singlet oxygen quantum yields (Φ1O2) in buffered D2O. At least two distinct 3DOM* pools are present, and the observed lifetime range (∼20 to 80 μs) leads to a dependence of Φ1O2 on O2 concentrations between 29 and 290 μM. Thus, steady-state 1O2 concentrations will depend on [O2] in natural waters. The lifetimes are essentially identical for DOM samples of different origins and do not vary with excitation wavelength. However, Φ1O2 varies greatly between samples and decreases with excitation wavelength. These data strongly suggest that 3DOM* quantum yields decrease with excitation wavelength, which gives rise to the Φ1O2 variation. Borohydride reduction of several samples in both D2O and H2O lowers the absorbance and 1O2 production rates, but it does not alter Φ1O2. This is consistent with a model in which 1O2 sensitizing chromophores are borohydride reducible groups in DOM, such as aromatic ketones. Interpreted in the framework of a charge transfer (CT) model for DOM optical properties, the collective data suggest a model in which electron acceptor moieties are important 1O2 sensitizers and where CT interactions of these moieties disrupt their ability to produce 1O2.

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INTRODUCTION Dissolved natural organic matter (DOM) photochemistry often controls reactive oxygen species’ concentrations in surface waters. Sunlight absorption by DOM produces singlet oxygen (1O2), superoxide (O2−) and its dismutation product, H2O2, and hydroxyl radical (·OH) as well as solvated electrons (e−(aq)).1 In many cases, this photochemistry is either suspected or known to derive from triplet excited state DOM, 3DOM*. For example, 1O2 forms by energy transfer to molecular O2 from 3DOM* produced via intersystem crossing: 1

DOM + hν → 1DOM*

1

(1)

3

(2)

DOM* → DOM*

3

Several researchers have tried to elucidate the photophysical properties of 3DOM*. Such properties include energy, excited state lifetime, and quantum yield. In a seminal work, Zepp et al. used DOM sensitized 1,3-pentadiene photoisomerization to estimate that approximately half of 3DOM* has energies greater than 250 kJ mol−1,7 a value supported by phosphorescence maxima near 500 nm for peat humic and fulvic acids immobilized in polyvinyl alcohol films.8 However, magnetic circular dichroism results suggest a value near 180 kJ mol−1.9 Regarding the 3DOM* lifetime, there is also some uncertainty. Zepp et al. used kinetic arguments to suggest that the 3DOM* lifetime in air-saturated solution is about 2 μs, but they did not estimate the natural lifetime.7 Interestingly, other researchers have found μs lifetimes for spectroscopic transients that are quenched by O2 and appear to be triplets, but definitive identification of the states is lacking.10,11 There is also lingering uncertainty about 3DOM* quantum yields (ΦT). For example, Bruccoleri et al. used photoacoustic spectroscopy to measure the quantum yield of species having lifetimes >2 μs as approximately 50 to 90%.9 They argued that these were all

DOM* + O2 → 1DOM + 1O2

(3)

Additionally, 3DOM* is itself a potent oxidant of many aquatic contaminants, C:2−4 3

DOM* + C → ·DOM− + ·C+

(4) Received: Revised: Accepted: Published:

While the chemical identity of the triplets is not wholly clear, Canonica and co-workers have shown that they behave in a very similar manner to aromatic ketones.4−6 © 2012 American Chemical Society

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triplets and thus that 3DOM* is the major photophysical product. However, recent results suggest that a substantial portion of light absorption by DOM (≥50%) leads to excited state charge-transfer (CT) complexes.12 Although CT states could potentially form triplets, we have shown that 1O2 yields correlate negatively with CT absorbance, which suggests that states produced in this manner are not major precursors to 3 DOM*.13 This raises the possibility that much of the signal detected by the photoacoustic technique is from states other than reactive 3DOM*, such as very low energy triplets, or cation or peroxyl radicals, which may comprise the bulk of longlived (>100 μs) photooxidants.14 This study was conducted to probe the chemical nature of 3 DOM* precursors to 1O2 and to explore the molecular basis for variations of 1O2 quantum yields (Φ1O2) between DOM samples. One focus was on determining whether Φ1O2 is controlled by triplet quantum yields or triplet lifetimes in order to address the mechanism underlying our previously reported observation that Φ1O2 correlates negatively with CT absorbance in DOM.13 Here, CT absorbance is quantified by the E2/E3 ratio (A254/A365), which reflects the extent of absorbance tailing from the far- to near-UV. This tailing has been ascribed to transitions involving electron transfer between donor and acceptor moieties in DOM.12 Putative donors include hydroxyaromatic structures while acceptors could be aromatic ketones and quinones. Given that such acceptor moieties are postulated to be an important 3DOM* source,4−6 it is of interest to examine whether the relationship between E2/E3 and Φ1O2 is due to reduced triplet yields in DOM displaying higher CT absorbance or if a large CT character in the absorption reflects the presence of CT pathways for 3DOM* deactivation. Another focus of this study was on examining whether the effects of borohydride reduction on Φ1O2 are consistent with aromatic ketones as the source of 3DOM* precursors to 1O2. Although there is some evidence that at least a portion of oxidizing triplets are also capable of producing 1 O2,15 the overlap between these two triplets pools has not been well studied and remains poorly delineated. Several DOM samples were used to investigate the dependence of Φ1O2 on dissolved O2 over a range of environmentally relevant concentrations. Via a saturation kinetics analysis the natural lifetime of 3DOM* (τT) was determined at three excitation wavelengths from 310 to 415 nm. The lifetimes show virtually no dependence on DOM origins, pH, or wavelength, even though Φ1O2 varies greatly. This indicates that 1O2 production rates are controlled by ΦT rather than triplet lifetimes. The results also support the hypothesis that CT absorption is associated with lower yields of 3 DOM*.13 Chemical reduction with NaBH4 did not significantly alter Φ1O2 in any of the samples investigated, which can be most simply explained by positing that 3DOM* precursors to 1O2 are aromatic ketones and related structures that act as electron acceptors in CT absorbance. Furthermore, the similar behavior of the samples with regard to reduction taken together with the similarity of the triplet lifetimes suggests a potential common structural basis for 1O2 production. Finally, a synthesis of the results is presented as a refinement of a previously proposed model for DOM photophysics.13

(FFA), which was distilled on a regular basis. Deuterium oxide (99.8% D, Cambridge Isotope Laboratories) was the solvent for most experiments. When water was used, it had a resistivity of 18 MΩ cm and was supplied by an in-house system (Nanopure). To adjust the pD of D2O solutions, 1 M DCl (stock 37%) or NaOD (stock 30%) in D2O were used (pD = -log{D3O+}). In water, 1 M HCl and NaOH were used to adjust the pH. DOM Samples and Stock Solutions. Suwannee River Natural OM (SROM) and Suwannee River Fulvic Acid (SRFA) were purchased from the IHSS (http://www.ihss.gatech.edu). An XAD-8 isolate from Pony Lake in Antarctica (PLXAD) was provided by Dr. Ryan Fimmen at Ohio State University. Another XAD-8 isolate was taken from the Rappahannock River in March, 2011 by the Fredericksburg City dock (RRXAD). This was isolated as described previously13 and then drying the ultrafiltration retentate under N2 at 30 °C. The solid was dissolved in buffered D2O to prepare a stock solution. Phosphate buffered D2O (or H2O) containing 0.1 M KCl as a background electrolyte was prepared by dissolving equimolar amounts of NaH2PO4 and Na2HPO4 in 0.1 M KCl to obtain 10 mM total phosphate. Stock DOM (1.00 g/L) was prepared by stirring the solid overnight in phosphate buffered D2O (or H2O) to dissolve, followed by syringe filtration to 0.45 μm and storage at 4 °C. Experimental Solutions. Experimental solutions were prepared by diluting stock DOM with phosphate buffer, adjusting pD or pH to the desired value, and storing overnight at room temperature. The following day, the pD or pH was checked and adjusted if necessary. Sufficient 25 mM FFA was added to a final concentration of 25 μM, and experiments were initiated. DOM concentrations were chosen to obtain an absorption of about 0.3 cm−1 at 365 nm and were in the range of 55 mg/L (SRFA) to 75 mg/L (SROM and PLXAD). Analytical Methods. Absorbance spectra were collected in 1 cm cuvettes at 1 nm resolution. Oxygen concentrations were calculated using Henry’s Law (see Photochemical Procedures). Frequently, calculated values were checked with a dissolved O2 probe (Vernier), which always agreed above the probe quantitation limit (∼ 60 μM). FFA concentrations were measured by HPLC as described previously.13 The pD in D2O solutions was determined with a glass pH electrode calibrated with pH standard buffers. Water was removed from the electrode prior to measurements in D2O by shaking, gently dabbing with a Kimwipe, and rinsing in D2O. Measured pH was converted to pD by the equation, pD = pHmeas + 0.41.16 Photochemical Procedures. The photochemical apparatus is described briefly here. For more details, see the Supporting Information (SI). The light source was a 450 W Xe lamp whose output was filtered to remove heat and select an experimental wavelength. An iris shaped the beam to evenly irradiate approximately 1 cm2 each of two adjacent 13 × 100 mm test tubes. Photon fluxes were determined via ferrioxalate actinometry (SI). The light intensity was checked at the start and end of each experiment with a radiometer (International Light, IL1700, SED005 detector) whose readings were correlated with the actinometry results. Readings were taken at the same position as the center and edges of the test tube faces to verify the beam intensity and spatial quality. Duplicate 3.2 mL samples of experimental solution were placed into the tubes along with stir bars. The tubes were mounted, and, with stirring and the lamp shutter closed, the

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MATERIALS AND METHODS Reagents and Chemicals. All chemicals were reagent or analytical grade and used as received excepting furfuryl alcohol 4467

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solvents.3,4 This indicates similar 3DOM* steady-state concentrations, and thus similar ΦT and τT. Furthermore, with 1O2 intermediacy, rate enhancements in D2O typically match those calculated based on the longer 1O2 lifetime22,23 indicating no change in Φ1O2 and thus ΦT and τT. To verify these conclusions, experimental tests were also conducted. One piece of evidence suggesting similar photophysics in the two solvents is presented in Figure 1, which

samples were sparged for 30 min through cork stoppers with mixtures of UHP N2 and either air or O2 delivered with a dual stage rotameter at a total flow rate of 180 mL/min (i.e., 90 mL/ min per tube). The volumetric % O2 or air was determined at the beginning and end of each experiment using a bubble flow meter. Equilibrium O2 concentrations were calculated from the % O2 and its Henry’s Law constant, 1.39 × 10−3 M/atm in D2O.17 During irradiations, the headspace was continuously purged with the gas mixture. At various times, aliquots of solution (80 μL) were removed via a sampling tube placed in the cork stopper and analyzed for FFA. Calculation of Φ1O2. Rate constants for FFA loss (k′) were used to determine 1O2 quantum yields (Φ1O2) according to previously described procedures.13 For calculations in D2O, 8.3 × 107 M−1 s−1 was used as the FFA-1O2 reaction rate constant18 and 1.7 × 104 s−1 as the rate constant for solvent quenching of 1 O2, ks.19 Lamp spectra and representative rates of light absorption by DOM are provided as SI. Borohydride Reduction. Reductions were carried out using a previously reported procedure with slight modifications.12 Approximately 10 mL of solution at pD or pH 8.5 were sparged with N2 for 30 min. Then, NaBH4 (analytical grade) was added at a ratio of 30 mg per mg DOM. The samples were stirred and sparged continuously with N2 for 3 h. The pD (or pH) was lowered to approximately 5.0 followed by 1 h of air sparging to quench residual BH4−. The pD (or pH) was then adjusted, and the samples were stored overnight at room temperature before conducting experiments. Using NaBH4 adds H2O to the D2O solutions, but this could not have significantly altered the solvent quenching rate of 1O2 (see SI).

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RESULTS AND DISCUSSION Choice and Suitability of D2O Solvent. Quantitative use of FFA as a 1O2 probe requires following the kinetics over at least one-half-life to precisely determine k′. Preliminary experiments in H2O at low O2 (∼ 30 μM) required extended irradiation. Even with solvent saturated gas mixtures, a significant decrease in sample volume occurred, introducing substantial error into the data analysis. Therefore, the decision was made to work in D2O, increasing the 1O2 lifetime roughly 10-fold19 and decreasing experimental time. Despite the fact that D2O has been used in previous research on DOM photochemistry, there have been no experiments to directly investigate whether the change in solvent from H2O to D2O alters DOM photophysics. However, with some dyes that have substantial CT character in their excited states, drastic changes can occur. For example, Lucifer Yellow deactivates rapidly in water due to excited-state proton transfer (ESPT) from solvent, but switching to D2O removes this pathway, leading to enhanced singlet lifetimes and triplet yields.20 Given the expected CT nature of DOM excited states and the relative frequency with which D 2O is used to investigate its photochemistry, it is important to establish whether there are solvent-dependent photophysical effects. Based on previous work, it was not anticipated that either 3 DOM* quantum yields (ΦT) or lifetimes (τT) would change by switching solvents. Deuterium oxide is often used as a test for 1O2 intermediacy, due to the extended lifetime of 1O2 in this solvent. If no enhancement of reaction rates is observed, it can be taken to imply the intermediacy of some other species. Importantly, when 3DOM* is the intermediate, little or no difference is observed between reaction rates in the two

Figure 1. (a) Absorption spectra for DOM samples in D2O. Sample order in legend is the order (from top to bottom) at 254 nm. (b) Φ1O2 (370 nm, 21% O2) for samples in D2O plotted with previously reported correlation for samples in H2O.13 D2O samples not included in regression.

shows DOM absorbance spectra in buffered D2O along with their E2/E3 ratios. It also shows that Φ1O2 in D2O follows our previously observed correlation with E2/E3 established in H2O.13 Thus, DOM has very similar optical and photochemical behavior in both solvents. Also, approximate 3DOM* lifetimes determined in preliminary experiments with H2O were in the range of 10−40 μs, which, despite the volume changes and low FFA conversion rates, are consistent with values in D2O (below). In addition to these tests, fluorescence spectra and lifetimes for SROM and SRFA were measured (for results and experimental details, see SI). As shown in SI Figure S5, fluorescence features for these samples are virtually identical in both solvents at pH 7.0 and pD 7.0. There is an approximately 30% enhancement in peak intensities in D2O, which could suggest ESPT deactivation of 1DOM* in H2O, but this enhancement could simply be due to the well-known increase of apparent pKa for acid groups in D2O.16 This would make the DOM behave in D2O as though the pH were lower, a change which is known to increase DOM fluorescence.21 Indeed, fluorescence (singlet) lifetimes measured with 370 nm excitation are virtually identical in both solvents (SI Figure 4468

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S6 and Table S2), which argues strongly against ESPT as an important deactivation route. Thus, it appears that there is negligible difference in the photophysics induced in the two solvents. O2 Dependence of Φ1O2. The O2 dependence of Φ1O2 can be derived from steady-state kinetics (details in SI) assuming three decay routes for 3DOM*, which leads to eq 8: 3

DOM* → 1DOM3 k isc

(5)

3

DOM* + O2 → 1DOM + O2 kq

(6)

3

DOM* + O2 → 1DOM + 1O2 kET

(7)

Φ1O2 =

kET ΦT[O2 ] 3

k isc + (kET + kq)[O2 ]

(8)

Here, kisc is the first order rate constant for spontaneous decay of the triplet to the ground state; note that the natural lifetime of 3DOM* is the inverse of 3kisc. The rate constants kq and kET are second order rate constants for deactivation of 3 DOM* by O2, the former for physical quenching and the latter for energy transfer producing 1O2. For simplicity of the data analysis, it is assumed that the rate constants in eq 8 represent the behavior of all DOM triplets. In reality, due to the heterogeneity of DOM resulting from varied structural elements in varied microenvironments, a weighted average summation should apply. Indeed, as shown below, there is deviation from this equation at low [O2] that demonstrates 3 DOM* lifetime heterogeneity. The O2 saturation predicted by eq 8 is demonstrated in Figure 2(a), which shows Φ1O2 (370 nm) from 30 μM to 1.4 mM O2. Consistent with previous reports,7 Φ1O2 is essentially the same at 290 μM (air saturated) and 1.4 mM (O2 saturated). Thus, even in air saturated solution the majority of 3DOM* follows reaction 6 or 7. However, as the O2 concentration drops below 290 μM, Φ1O2 becomes a function of [O2]. This behavior is significant, because it shows that Φ1O2 varies with dissolved O2 in the environmentally relevant range of 30−300 μM. Additionally, this variation can be analyzed to determine 3 DOM* lifetimes. Natural Lifetimes of 3DOM*. Rearranging eq 8 into linear form allows a determination of τT (τT = 3kisc−1) from the ratio of the intercept to slope (eq 10): 3

⎛ 1 ⎞ ⎛ kET + kq ⎞ ⎛ 3k isc ⎞⎛ 1 ⎞ ⎟⎜ ⎟+⎜ ⎜ ⎟=⎜ ⎟ ⎝ Φ1O2 ⎠ ⎝ kETΦT ⎠ ⎝ kETΦT ⎠⎝ [O2 ] ⎠

⎛ kET + kq ⎞ ⎟ = (kET + kq)τT ⎜ 3 ⎝ k isc ⎠

Figure 2. (a) Φ1O2 (percent) as a function of [O2] at 370 nm for all samples with duplicate standard deviations indicated; (b) Inverse plots used to determine τT with regression through first four points shown: (○) SROM pD 8.5; (●) SROM pD 6.0; (△) SRFA pD 7.0; (◇) RRXAD pD 7.0; (□) PLXAD pD 7.0.

triplets, kq is typically near diffusion controlled and lies in the range of 108 to 109 M−1 s−1.24 Even for triplets with very efficient energy transfer, the sum of kET and kq cannot exceed this value by much, so it is a sufficient estimate of the sum. Zepp et al. previously used 2 × 109 M−1 s−1,7 which is adopted here as a maximum possible value, leading to minimum possible values for τT. Table 1 presents the results; all data and regressions are provided as SI. Two lifetimes are reported: τshort, representing the lifetime of shorter-lived 3DOM* as calculated from the regressions to the four highest O2 concentrations, and τlong, which was calculated from the line defined by the two Table 1. 3DOM* Lifetimes (μs) Determined by O2 Saturation Kinetics (τshort is calculated from the four highest O2 concentrations, and τlong is calculated from the two lowest O2 concentrations assuming (kq + kET) = 2 × 109 M−1 s−1. Standard deviations (in parentheses) were propagated from the standard error of regression for the slopes and intercepts.)

(9)

(10)

Thus, if kET and kq are known or can be reliably estimated, τT can be determined. Figure 2(b) shows the linearized data and regressions involving the first four points (four highest [O2]). Down to about 50 μM, the data is quite linear. Below that, the influence of longer lived 3DOM* becomes apparent. The data at low [O2] are not a result of FFA reacting with DOM excited states or other species: 1O2 intermediacy was verified by the addition of 5 mM N3−, which halted the reaction. The curvature thus reflects 3DOM* heterogeneity. To determine τT (eq 10), kET and kq must be estimated because neither is known for 3DOM*. For well characterized

310 nm sample SROM pD 6.0 SROM pD 8.5 SRFA pD 7.0 PLXAD pD 7.0 RRXAD pD 7.0

τshort 20 19 19 22 c

(2) (2) (2) (3)

370 nm

τlonga b 48 85 42 c

τshort 22 22 21 19 29

(2) (1) (2) (1) (2)

415 nm

τlonga 55 65 44 47 41

τshort 21 24 26 24 c

(3) (4) (3) (5)

τlonga 65 80 47 27 c

a No standard deviation for τlong as it is calculated from a two point regression. bExperiment at 29 μM O2 not performed. cExperiment not performed.

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lower Φ1O2 could reflect a decrease in kET. The present data do not favor one of these possibilities over the other. The data in Figure 1(a) show that CT absorbance (as judged by the E2/E3 ratio) is characteristic of samples with either or both a low ΦT or kET. Indeed, the former explanation seems a likely cause of decreasing Φ1O2 with increasing λex: excitation into CT bands appears to be an insignificant route to 3DOM*. A physical basis for this can be found in the model for DOM optical properties advanced by Blough and co-workers, who propose that CT interactions between phenolic electron donors (D) and aromatic ketone and/or quinone electron acceptors (A) produce a significant portion of the DOM absorbance in the UV-vis region.12,29 Given that aromatic ketones and quinones, putative A species, are good 1O2 sensitizers,24,30 their involvement in CT complexes or exciplexes could either prevent the formation of triplets or cause rapid triplet deactivation and lower Φ1O2. Indeed, the former hypothesis is supported by consideration of the pD effect on the optical and photochemical properties of DOM. As shown in Figure 1(a) for SROM, increasing pD decreases the E2/E3 ratio. Increased acid dissociation in DOM at higher pD thus appears to increase the extent of CT absorbance. This phenomenon is consistent with phenolic D moieties in the CT model, as phenols are more readily oxidized (i.e., better electron donors) at high pH.31 Concomitant with the E2/E3 decrease, Φ1O2 decreases, but τT is unaffected (Figure 3). The invariant τT indicates that the chemical identity of the 3DOM* is unaltered, which suggests that it should produce 1O2 with the same efficiency regardless of pD. Thus, the lower Φ1O2 indicates that 3DOM* is produced less efficiently at high pD, consistent with the reported pH trend for ΦT measured photoacoustically.9 Effect of NaBH4 Reduction on Φ1O2. Reduction of DOM with NaBH4, selective for carbonyls, substantially decreases CT absorbance,12 which has been attributed to reduction of A moieties that decreases absorbance by both independent A groups and CT complexes, via destruction of D−A interactions.12 To further investigate how CT absorbance is related to 1O2 production, Φ1O2 was measured before and after NaBH4 reduction for samples in both D2O and H2O (Table 2).

lowest O2 concentrations. In all cases, the value of τshort is approximately 20 μs, while τlong ranges from about 50−80 μs. Two important results stand out: τshort (and likely τlong, given its approximate nature) is virtually the same for all samples, and τT (short and long) is independent of excitation wavelength (λex). The similarity of the lifetimes is interesting given some of the known differences between the samples. For example, Pony Lake FA (the XAD-8 fraction studied here is similar in nature to the FA) has much lower aromaticity and carbonyl content than SRFA. 25 Thus, it might be expected that the chromophores would differ substantially. However, as both the triplet lifetimes and the data in Figure 1 show, the photochemistry of all the samples has clear underlying similarities. In fact, the biogeochemical origins of chromophoric DOM do provide reason to suspect the involvement of similar chemical moieties. DOM derived from ligneous material (e.g., SRFA), contains polyphenols and related oxidation products, such as aromatic ketones and quinones.26,27 In contrast, the chromophoric portion of microbial DOM (e.g., Pony Lake) is suspected to form via abiotic polymerization of extracellular quinones, reduced quinones, and probably flavonoids.28 Thus, chromophoric terrigenous and microbial DOM probably differ most in the proportions of phenol, quinone, and aromatic ketone moieties, which raises the possibility that a common structural basis underlies these materials’ photochemistry. Within an individual sample, the constancy of τT with λex suggests that the same chromophores are excited at every wavelength. In this context, it is interesting to note the decrease in Φ1O2 with λex (Figure 3), which would not occur with a

Table 2. 1O2 Quantum Yields for Air Saturated Native and Borohydride Reduced DOM Samples at 370 nm sample SROM SRFA SROM SRFA NAOMa

Figure 3. Φ1O2 (filled symbols) and τshort (open symbols) versus excitation wavelength: (○) SROM pD 8.5; (◇) SROM pD 6.0; (△) SRFA pD 7.0; (□) PLXAD pD 7.0. a

constant τT (3kisc−1) unless either ΦT or the 1O2 production efficiency (kET/[kET + kq]) were decreasing (eq 8). For a given group of chromophores, the 1O2 production efficiency should remain constant. Thus, it seems probable that the λex dependence of Φ1O2 reflects a decrease in ΦT as λex moves into the visible range where CT absorption begins to dominate. This hypothesis is consistent with the idea that CT absorption is not an effective triplet production mechanism.13 A similar situation arises in considering the variability of different samples’ Φ1O2 despite the close similarity in their τT values (Figure 3). Again, this could reflect a decrease in either ΦT or the 1O2 production efficiency. If similar chromophores serve as sensitizers in the different samples, then the variation in Φ1O2 could be attributed to different ΦT values. Alternatively,

solvent H2O, H2O, D2O, D2O, H2O,

pH 7.0 pH 7.0 pD 7.0 pD 7.0 pH 6.8

Φ1O2 native (%) 1.4 1.8 2.3 2.6 2.7

± ± ± ± ±

Φ1O2 reduced (%)

0.1 0.1 0.2 0.3 0.4

1.4 1.8 2.3 2.5 3.2

± ± ± ± ±

0.1 0.1 0.1 0.2 0.6

Nordic aquatic organic matter (obtained from IHSS).

It is noted here that FFA detects only 1O2 that has escaped from the DOM microenvironment into the aqueous phase. Within the DOM phase, 1O2 is generated at a rate determined by Φ1O2 and the light intensity, and it is lost by diffusion and electronic relaxation.32 Anything that alters the rates of 1O2 loss within the DOM microenvironment would appear here as a change in Φ1O2. It seems unlikely that borohydride reduction alters the diffusion rate, which Grandbois et al. have noted is the dominant means of 1O2 loss from the DOM phase.32 However, reduction could enhance the rate of charge-transfer induced quenching of 1O2 by DOM.24 This would manifest as a decrease in the measured Φ1O2, which is not observed here. 4470

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fluorescence quantum yields and emission maxima observed in NaBH4 reduced samples,12 which still display evidence of CT interactions. Revised Model of DOM Photophysics. Based on the preceding considerations, Scheme I is suggested as a refinement

Therefore, in the following discussion, it is assumed that reduction does not alter the rates of 1O2 loss within the DOM microenvironment. Consistent with previous reports, reduction produced extensive absorbance loss throughout the UV-vis, an increase in E2/E3, and blue shifts as well as increases in fluorescence (SI), all of which is consistent with the destruction of CT interactions as discussed by Ma et al.12 Surprisingly though, Φ1O2 did not change significantly, regardless of sample or solvent (Table 2). Interestingly, the same phenomenon was very recently reported for quantum yields of TMP oxidation by several DOM samples,33 which suggests a high degree of overlap between the pools of 3DOM* that act as oxidants and as 1O2 precursors. The invariance of Φ1O2 is not easily explained, although there are two possible scenarios: (i) The same types of sensitizers are present before and after reduction, but reduction decreases their concentration in direct proportion to the absorbance so that their fractional contribution to the rate of light absorption remains constant. (ii) Two pools of sensitizers exist, one reducible by NaBH4 and one not, and by a coincidence of the relative sensitizing efficiencies of the two pools, the 1 O 2 production rate after reduction is lower by almost the same percentage as the rate of light absorption. Scenario (i) requires that the great majority of the absorbance both before and after reduction be due to sensitizers and species whose concentration is directly proportional to that of the sensitizers. Again, the CT model provides a physical basis for this. If the sensitizers are A-type moieties (e.g., aromatic ketones) engaged in both direct and CT absorbance, reduction would decrease both types of absorbance in direct proportion to the concentration of A. The model also requires some A moieties to be inaccessible to BH4− since it assumes a single pool of sensitizers, some of which are not reduced despite conditions that cause maximal absorbance changes.12,34 This hypothesis, whose possibility is suggested by the electrostatic repulsion expected between BH4− and DOM at high pH, requires further investigation to examine its plausibility. Scenario (ii) requires a pool of sensitizers that are not reduced by NaBH4. A reasonable candidate group could be quinones, which, although they are reduced by NaBH4, would reoxidize upon extended aeration, as in these experiments. They would also account for only a small fraction of the absorbance after reduction, because their absorption coefficients are low to moderate in the 300−400 nm region12 and they are present at low concentrations in DOM (roughly 5% by mass; see SI). However, there would need to be extremely fortuitous coincidences in the relative concentrations, absorption coefficients, and 1O2 quantum yields of the quinones and the sensitizers destroyed by reduction for Φ1O2 to remain unchanged. Thus, this scenario seems rather implausible. Both scenarios assume that 1O2 producing triplets are A moieties because it is unlikely that they are D moieties. Such species would not be reduced by NaBH4, so after reduction they would constitute a substantially larger fraction of the absorbance. This would increase Φ1O2, which is not observed. Finally, it is noteworthy that both scenarios generate postreduction conditions where D and A moieties still coexist, which produces CT absorbance even after reduction. This is consistent with the observed wavelength dependence of

Scheme I. Proposed Photophysical Modela

F = fluorescence; i.c. = internal conversion; i.s.c. = intersystem crossing.

a

to our previous model of DOM photophysics.13 A central feature of this scheme is the depiction of chromophores as a collection of moieties loosely classified as either D-type (phenolic) or A-type (aromatic ketone or quinone) that exist in close proximity within the DOM microenvironment (D + A). This leads to three possible excitation pathways: to local excited states of D or A (eqs 11 and 12) or to excited CT states (eq 13); here, eq 13 represents both direct CT absorbance and the formation of exciplexes, which is more explicitly shown in Scheme I. All these states may relax to the ground state via internal and external conversion or fluorescence. Also, energy transfer can populate A* (eq 14). (D + A) + hν → 1(D* + A)

(11)

(D + A) + hν → 1(D + A*)

(12)

(D + A) + hν → 1(D/A−)

(13)

1

(D* + A) → 1(D + A*)

(14)

As noted previously, this model is consistent with higher molecular weight material displaying more CT absorbance, as larger polymers promote more D−A interactions.29 Interestingly, it also appears that higher molecular weight DOM contains a larger proportion of A and D moieties relative to DOM of lower molecular weight: a recent report shows that electron accepting capacities are greater in terrigenous than in aquatic humic substances,35 while another demonstrates that terrigenous DOM is a better antioxidant,36 which could reflect higher concentrations of donors or a lower oxidation potential. Thus, CT absorbance is associated with larger material containing increased amounts of A and D moieties or simply more easily oxidized D moieties. Another important aspect of Scheme I is that only 3A* leads to 3DOM* and 1O2. In the Scheme, 3A* is represented as 3(D + A*) to illustrate its presence within the microenvironment and the overall electronic state of the DOM, but this is not meant to imply a triplet complex between A and D. There are two reasons for assuming that only A (and not D) serves as the sensitizer. For one, both the negative relationship between 4471

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Φ1O2 and E2/E3 and the apparent decrease of ΦT with λex (i.e., with excitation further into the CT band) suggest that CT states are not 3DOM* precursors. Also, if D is identified with hydroxyphenols, then the observation by Aguer et al. that synthetic polymers of catechols and hydroxybenzoic acid do not produce 1O237 suggests that D* states are also not 3DOM* precursors. This leaves acceptors (e.g., aromatic ketones, quinones, and related structures) as the source of 3DOM*, structural assignments consistent with its oxidizing properties and ability to produce reactive oxygen species such as 1O2 and O2−.4−6,30 Future work with additional types of DOM, synthetic copolymers containing well-defined proportions of D and A groups, and experiments with modified humic substances or lignin could provide additional insight into the moieties responsible for 3DOM* production and help explain why 1O2 yields are unaffected by NaBH4.

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(8) Mazhul, V. M.; Ivashkevich, L. S.; Shcherbin, D. G.; Pavlovskaya, N. A.; Naumova, G. V.; Ovchinnikova, T. F. Luminescence properties of humic substances. J. Appl. Spectrosc. 1997, 64, 503−508. (9) Bruccoleri, A.; Pant, B. C.; Sharma, D. K.; Langford, C. H. Evaluation of primary photoproduct quantum yields in fulvic acid. Environ. Sci. Technol. 1993, 27, 889−894. (10) Frimmel, F. H.; Bauer, H.; Putzlen, J.; Murasecco, P.; Braun, A. M. Laser flash photolysis of dissolved aquatic humic material and the sensitized production of singlet oxygen. Environ. Sci. Technol. 1987, 21, 541−545. (11) Fischer, A. M.; Kliger, D. S.; Winterlea, J. S.; Mill, T. Direct observation of phototransients in natural waters. Chemosphere 1985, 14, 1299−1306. (12) Ma, J.; Del Vecchio, R.; Golanoski, K. S.; Boyle, E. S.; Blough, N. V. Optical properties of humic substances and CDOM: effects of borohydride reduction. Environ. Sci. Technol. 2010, 44, 5395−5402. (13) Dalrymple, R. M.; Carfagno, A. K.; Sharpless, C. M. Correlations between dissolved organic matter optical properties and quantum yields of singlet oxygen and hydrogen peroxide. Environ. Sci. Technol. 2010, 44, 5824−5829. (14) Canonica, S.; Hoigné, J. Enhanced oxidation of methoxy phenols at micromolar concentration photosensitized by dissolved natural organic material. Chemosphere 1995, 30, 2365−2374. (15) Halladja, S.; ter Halle, A.; Aguer, J.-P.; Boulkamh, A.; Richard, C. Inhibition of humic substances mediated photooxygenation of furfuryl alcohol by 2,4,6-trimethylphenol. Evidence for reactivity of the phenol with humic triplet excited states. Environ. Sci. Technol. 2007, 41, 6066− 6073. (16) Covington, A. K.; Paabo, M.; Robinson, R. A.; Bates, R. G. Use of the glass electrode in deuterium oxide and the relation between the standardized pD (paD) scale and the operational pH in heavy water. Anal. Chem. 1968, 40, 700−706. (17) Scharlin, P.; Battino, R. Solubility of 13 nonpolar gases in deuterium oxide at 15 - 45 °C and 101.325 kPa. Thermodynamics of transfer of nonpolar gases from H2O to D2O. J. Solution Chem. 1992, 21, 67−91. (18) Latch, D. E.; Stender, B. L.; Packer, J. L.; Arnold, W. A.; McNeill, K. Photochemical fate of pharmaceuticals in the environment: cimetidine and ranitidine. Environ. Sci. Technol. 2003, 37, 3342− 3350. (19) Average of all reported values in Wilkinson, F.; Helman, W. P.; Ross, A. B. Rate constants for the decay and reactions of the lowest electronically excited state of molecular oxygen in solution. An expanded and revised collection. J. Phys. Chem. Ref. Data 1995, 24, 663−1021. (20) Fürstenberg, A; Vauthey, E. Excited-state dynamics of the fluorescent probe Lucifer Yellow in liquid solutions and in heterogeneous media. Photochem. Photobiol. Sci. 2005, 4, 260−267. (21) Hemmingsen, S. L.; McGown, L. B. Phase-resolved fluorescence spectral and lifetime characterization of commercial humic substances. Appl. Spectrosc. 1997, 51, 921−929. (22) Boreen, A. L.; Edhlund, B. L.; Cotner, J. B.; McNeill, K. Indirect photodegradation of the dissolved free amino acids: the contribution of singlet oxygen and the differential reactivity of DOM from various sources. Environ. Sci. Technol. 2008, 42, 5492−5498. (23) Haag, W. R.; Hoigné, J. Singlet oxygen in surface waters. 3. Photochemical formation and steady-state concentrations in various types of waters. Environ. Sci. Technol. 1986, 20, 341−348. (24) Schweitzer, C.; Schmidt, R. Physical mechanisms of generation and deactivation of singlet oxygen. Chem. Rev. 2003, 103, 1685−1757. (25) Thorn K. A.; Folan D. W.; MacCarthy P. Characterization of the International Humic Substances Society Standard and Reference Fulvic and Humic Acids by Solution State Carbon-13 (13C) and Hydrogen-1 (1H) Nuclear Magnetic Resonance Spectrometry, Water-Resources Investigations Report 89-4196; U.S. Geological Survey: Denver, CO, 1989. (26) Hernes, P. J.; Benner, R. Photochemical and microbial degradation of dissolved lignin phenols: implications for the fate of

ASSOCIATED CONTENT

* Supporting Information S

Photochemical apparatus, ferrioxalate actinometry, calculated rates of light absorption, determination of optical path length, calculated effect of NaBH4 on ks in D2O, fluorescence spectra and lifetimes in H2O and D2O, kinetic model, complete data for τT analysis, absorbance and fluorescence spectra for native and reduced DOM, estimate of quinone concentrations in DOM. 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]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Gwathmey Chemistry Award (Virginia Academy of Science) and a Jepson Fellowship (University of Mary Washington). I also thank Neil Blough and Silvio Canonica for helpful discussions and comments.

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REFERENCES

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