Article pubs.acs.org/est
Role of Dissolved Organic Matter in Ice Photochemistry Amanda M. Grannas,* Lisa P. Pagano,‡ Brittany C. Pierce,‡ Rachel Bobby, and Alexis Fede Department of Chemistry, Villanova University, Villanova, Pennsylvania 19085, United States S Supporting Information *
ABSTRACT: In this study, we provide evidence that dissolved organic matter (DOM) plays an important role in indirect photolysis processes in ice, producing reactive oxygen species (ROS) and leading to the efficient photodegradation of a probe hydrophobic organic pollutant, aldrin. Rates of DOM-mediated aldrin loss are between 2 and 56 times faster in ice than in liquid water (depending on DOM source and concentration), likely due to a freeze-concentration effect that occurs when the water freezes, providing a mechanism to concentrate reactive components into smaller, liquid-like regions within or on the ice. Rates of DOM-mediated aldrin loss are also temperature dependent, with higher rates of loss as temperature decreases. This also illustrates the importance of the freeze-concentration effect in altering reaction kinetics for processes occurring in environmental ices. All DOM source types studied were able to mediate aldrin loss, including commercially available fulvic and humic acids and an authentic Arctic snow DOM sample isolated by solid phase extraction, indicating the ubiquity of DOM in indirect photochemistry in environmental ices.
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INTRODUCTION Dissolved organic matter (DOM), derived from the decay of plants and microorganisms, is the dominant UV-light absorbing constituent in natural waters. Absorption of UV light by the aromatic, chromophoric fraction of DOM (CDOM) leads to the production of reactive species such as hydroxyl radical (OH), singlet oxygen (1O2), peroxy radicals (RO2), and excited state (triplet state) DOM, among others.1,2 Thus, the photochemical properties of DOM can impact the fate of pollutants,3−5 metal speciation,6,7 and biological activity in sunlit waters.8 Aquatic DOM photochemistry also plays a role in pollutant fate in Arctic surface waters.9 DOM is likely an important chromophore contributing to light absorption and reactive oxygen species (ROS) formation in snow and ice.10−16 Recent work from Barrow, Alaska11 reports that the chromophores most commonly measured in polar snow (hydrogen peroxide (H2O2), nitrate (NO3−), and nitrite (NO2−)) on average accounted for less than 1% of sunlight absorption in the various samples investigated (snow, ice, and frost flowers). Light absorption was dominated by unidentified “residual” species, hypothesized to be organic compounds. The residual absorption spectra (after subtracting the contributions from peroxide, nitrate, and nitrite) were similar to the spectrum of CDOM from aquatic systems. Further, light absorption coefficients for frost flowers (ice crystals that grow over refreezing leads in the ocean) were 40 times larger than values from nearby terrestrial snow samples, consistent with the enrichment of dissolved species that occurs during formation of brine and frost flowers from seawater.17,18 Because DOM may play an important role in light absorption in snowpack, it is likely involved in the production of ROS and subsequent photochemistry processes. To date, no studies have © 2014 American Chemical Society
probed the photoreactivity of natural DOM in ice or snow, despite its ubiquity and likely importance as a chromophore in these systems. Although there are similarities between sources, light absorption properties, and possibly ROS production between aquatic DOM and snow/ice DOM, there are likely critical differences in the photochemical processes due to the physical properties of snow/ice that may influence the fate of ROS once produced. For example, a liquid-like (quasi-liquid or disordered) layer exists on the surface of pure ice and in the presence of sufficient solutes, a liquid-like layer (LLL) or quasibrine layer (QBL) is also found on and within ambient snow crystals and ice at temperatures relevant to polar conditions. The LLL has been demonstrated to influence reaction kinetics and mechanisms in ice.19−25 As ice forms from freezing water, pure water crystallizes first, and solutes are excluded from the bulk and concentrated in liquid-like regions in and on the ice.26 This will lead to a chemically “enriched” liquid-like region at the surface, grain boundaries, and interstitial pores of snow and ice. Potential reactants are highly concentrated in these regions compared to what would be measured in the bulk sample,27−31 which may influence photochemical processes and ROS fate in snow and ice. Despite the challenges associated with reproducing the reactivity of these liquid-like regions in a laboratory setting, recent work has begun to probe photochemistry in the LLL.19,32−37 For example, Bower and Anastasio38 illustrated a 10 000-fold enhancement in singlet oxygen production from Received: Revised: Accepted: Published: 10725
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environmentally relevant probe of DOM reactivity, and how that reactivity may change under different conditions. We do not intend to infer environmental aldrin degradation kinetics from these results. However, it has been argued by Guzman et al. that although natural and laboratory ices may differ in many ways, the important component is the “molecular domains sensed by the solutes themselves, rather than the macroscopic/ mesoscopic ice textures and morphologies perceived by observers...”.39 They argue that the differences in concentrations of the bulk solution (lab vs environmental samples) are incidental to ice photochemistry processes because the local concentrations in the microscopic ice domains where the chemistry that occurs is, in both cases, greatly elevated relative to the bulk (melted) solute concentration. An appropriate amount of different types of DOM was added to the equilibrated aldrin solution from a stock aqueous DOM solution. The aldrin with DOM solution equilibrated overnight and samples for irradiation were prepared by transferring 500 μL of the solution into precombusted (overnight at 450 °C) borosilicate ampules, which were then flame-sealed. Samples for ice experiments were frozen overnight in a temperaturecontrolled freezer at −15 °C. Samples for liquid experiments were stored overnight at room temperature in the dark prior to use. A home-built irradiation apparatus was used for sample irradiation. The apparatus used Q-Panel UV 340 lamps to simulate the profile of natural sunlight (described previously in ref 19). An Eppley TUVR radiometer was used to measure any variations in light intensity over the course of all experiments (18.2 MΩ·cm, 100 000 in the liquid-like or quasi-brine regions of ice (assuming complete exclusion from the bulk ice matrix), it is not necessarily the case that the kinetics of a given reaction will change by the same factor. The change in reaction rate will be dependent on a number of factors including the order of reaction, the temperature dependence of the reaction, changes in light absorption properties upon concentration of solutes into these regions (e.g., enhanced light screening effects), possible precipitation of solutes at high concentrations, and the change in sources/sinks for any reactive intermediates being produced. For example, the production of reactive oxygen species will depend on both the condensed phase concentration of DOM (the source of ROS) as well as the efficiency of quenching or reaction of ROS with a species other than aldrin (e.g., liquid water, or the DOM itself). It is difficult to isolate each of these variables in ice irradiation experiments in order to quantify the relative importance of each; however it
confidence interval (CI)), indicating there is no aldrin reactivity dependence on DOM concentration in liquid conditions. The slopes of the other three types of DOM investigated are statistically greater than zero, indicating there is a concentration dependence; however, the slopes are not statistically significantly different from one another under liquid conditions at the 90 or 95% confidence level (indicating that in liquid conditions these DOM types exhibit similar concentration dependence behavior with respect to DOM-mediated aldrin loss). Interestingly, this behavior changes upon freezing. In ice conditions, the slopes of the concentration dependence data shown in Figure 1 differ significantly from one another (95% CI). The order of concentration dependence (i.e., greatest to least slope) is in the order of SRFA > PLFA > Barrow Snow DOM > SRHA. Interestingly, this is the same trend as the relative difference in slopes between ice and liquid experiments of a given DOM type (i.e., SRFA showed the greatest change in slope when comparing liquid to ice conditions whereas SRHA showed the smallest change). This could be evidence of the freeze concentration of DOM into the liquid-like regions of ice, which, in turn, impacts the fate of photogenerated intermediates such as singlet oxygen and makes the degradation of aldrin under frozen conditions more favorable. The greatest 10729
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Figure 4. Relative change in the observed photochemical aldrin loss rate constant as a function of the relative change in the solute concentration factor (F) in ice. The values of k and F for the different temperatures investigated are plotted relative to the value of k and F at the warmest temperature (T = 268). As temperature decreases the FT/FT=268 K ratio increases.
aldrin and DOM). To quantitatively investigate this effect, we calculated the factor by which solutes were concentrated (F) at the different ice temperatures. Figure 4 illustrates the relative change in the rate constant as a function of the relative change in the solute concentration factor (F) for the four different DOM sources investigated. The slopes are statistically indistinguishable at the 95% confidence interval for SRHA, SRFA, and Barrow snow DOM, indicating that the freeze concentration effect may in fact be the controlling variable impacting kobs for these DOM types. However, the PLFA experimental slope is statistically significantly different from the other three DOM types at the 95% confidence interval. It should be noted that the calculation of freeze concentration factors assumes that all solutes are equally and completely excluded to liquid-like layers and does not take into account any effects such as precipitation of the solutes (which could occur at high solute concentrations likely to occur in liquid microenvironments within the ice), or incorporation of solutes within the ice crystal matrix. Because we rely on monitoring the photochemical loss of a hydrophobic probe as a proxy for ROS generation from the DOM, the partitioning of the probe molecule (aldrin) to the DOM will also influence the efficiency with which it reacts with ROS (in addition to the absolute amount of ROS generated per unit DOM). In the case of singlet oxygen chemistry, it has been shown that hydrophobic molecules bound to DOM encounter much higher concentrations of singlet oxygen produced photochemically by irradiated DOM42,43 than molecules in the bulk aqueous solution. Using comparative studies of hydrophilic versus hydrophobic singlet oxygen scavengers, singlet oxygen reactivities that were enhanced by greater than a factor of 100 were observed in these hydrophobic “microheterogenous” environments within the DOM as compared to the bulk aqueous solution.42 Thus, a steep concentration gradient exists between the singlet oxygen found within/near DOM macromolecules and the bulk aqueous phase. It is unclear how these environments might be impacted in frozen samples (i.e., within liquid-like regions). Grandbois et al.43 reported that their hydrophobic singlet oxygen probe, TPMA (2-[1-(3-tertbutyldimethylsiloxy)phenyl-1-methoxymethylene]tricyclo[3.3.1.1]decane), showed complete lack of binding to SRFA, but observed KOC values of 4.7 for SRHA and 5.0 for PLFA. Their experimentally determined singlet oxygen concentrations
is clear that the ability of DOM to mediate aldrin loss is enhanced in the ice matrix for this experimental approach. We next examined the impact of temperature on DOMmediated aldrin loss in ice. The volume of the liquid-like layer present in/on an ice sample depends both on the temperature of the sample and the concentration of solutes present. As temperature increases (up to the melting point), the volume of the liquid-like layer in/on ice also increases. As the concentration of solutes in the liquid-like layer increases, the volume of this region in/on ice will increase, due to a localized freezing-point depression effect. The fraction of water molecules present as a “liquid-like fraction” of the sample can be calculated as a function of solute concentration and temperature.40 This liquid-like fraction (ϕ) is calculated by eq 1, where ϕ represents the fraction of H2O molecules present in the liquid-like layer, m(H2O) is the molecular weight of water, R is the ideal gas constant, Tf is the freezing temperature of pure water, T is the experimental temperature, ΔHf is the melting enthalpy of water, and CT is the total molal concentration of solutes in the completely unfrozen solution. Here, solutes include 20 μg/L aldrin and 1 mg/L DOM. It is necessary to assume an average molecular weight of DOM in order to calculate the molal concentration, and here we have used 450 g/mol as an average molecular weight, based on the typical distribution of molecular components measured in SRFA and PLFA using Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS).41 Using this approach, both temperature and solute concentration can be accounted for in a calculation of the liquid-like fraction. If it is assumed that all solutes will be concentrated into the liquid-like fraction of the ice sample, then the solute concentration factor (F) can be calculated as F = 1/ ϕ.38 φ (T ) ≅
m H2ORTf ⎛ T ⎞ 0 ⎟C T ⎜ 1000ΔHf0 ⎝ T − Tf ⎠
(1)
Figure 3 illustrates the increase in the observed rate constant for aldrin loss at lower ice temperatures (i.e., smaller liquid-like volume). For each type of DOM, the rate constant increases linearly with decreasing temperature. This would be expected if the primary driver of the changing rate is the concentration of DOM and aldrin into smaller liquid-like volumes within the ice sample (e.g., a linearly increasing apparent concentration of 10730
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Figure 5. Comparison of observed rate constant of aldrin photochemical loss for liquid and ice experiments in the presence of various ROS scavengers. Error bars represent the 95% confidence interval, calculated from statistical regression analysis of the linearized degradation plots. An asterisk indicates experiments where the rate constant in the presence of scavenger is statistically significantly less than the corresponding nonscavenger experiment, at the 95% confidence level. Suwannee River Humic Acid, SRHA; Suwannee River Fulvic Acid, SRFA; Pony Lake Fulvic Acid, PLFA; Barrow Snow, C18 solid phase extracts from melted Barrow, AK snow.
aldrin loss in the presence of SRHA. Addition of benzoic acid did not statistically significantly impact the aldrin degradation rate in liquid samples for any of the other types of DOM investigated. In ice samples, the addition of benzoic acid did slow aldrin degradation kinetics in the presence of the Barrow snow extracted DOM (where no impact was observed in liquid samples). This indicates that the mechanisms of OH production vary in ice samples compared to liquid, with OH production more efficient (or sinks less effective) in ice. The comparison of C18 extracted Barrow snow DOM to the commercially available IHSS standards should be done with caution, however, due to the different isolation methods used. The potential role of 1O2 was investigated via competitive scavenging experiments using both hydrophilic (furfuryl alcohol, 10 μM) and hydrophobic (β-carotene, 54 nM) singlet oxygen scavengers. The addition of furfuryl alcohol is expected to preferentially scavenge 1O2 available in the bulk aqueous solution (or liquid-like region in ice), whereas the addition of βcarotene is expected to preferentially scavenge 1O2 available within the DOM or at the DOM surface (e.g., “microheterogeneous environments” as described by Latch and McNeill42). This experimental approach allowed us to assess the relative importance of singlet oxygen reaction within the bulk aqueous/liquid-like phase as compared to within microheterogenous environments at/within DOM macromolecules, as discussed above. Observed rate constants for aldrin photochemical loss in liquid and ice in the presence and absence of both furfuryl alcohol and β-carotene are summarized in Figure 5 and Table S1, Supporting Information. In both liquid and ice phase experiments, furfuryl alcohol addition statistically significantly (95% CI) inhibited photochemical degradation of aldrin for all DOM sources tested, indicating the importance of bulk aqueous phase singlet oxygen reaction with aldrin. Addition of β-carotene statistically significantly (95% CI) inhibited photochemical degradation of aldrin for all DOM sources tested except frozen SRFA (although the relatively large error associated with the kobs for the ice plus scavenger experiment may mask a true inhibitory effect). In both
(in liquid environments) for SRHA were 1900 fM for the “intrahumic” region and 8.6 fM for the bulk aqueous phase at room temperature. Singlet oxygen concentrations for PLFA were 2700 fM for the intrahumic region and 1.7 fM for the bulk aqueous phase at room temperature. In comparing aldrin reactivity in the presence of SRHA, SRFA, and PLFA in ice, it is interesting to note that the aldrin reactivity shows the greatest relative increase as a function of the relative increase in the solute concentration factor (F) in ice for PLFA, and the least for SRFA. Assuming aldrin is more poorly bound by SRFA relative to PLFA, these results are consistent with the Grandbois et al. model of hydrophobic probe molecule binding to the DOM, with subsequent reaction with singlet oxygen (or other ROS species) produced at/within the DOM macromolecules,43 which are likely further concentrated/aggregated within liquid-like regions of frozen samples. Thus, concentration of the DOM and aldrin into liquid-like regions results in a relatively greater increase in singlet oxygen reaction with aldrin for PLFA (where aldrin binding is greater) than for SRFA (where aldrin may be more poorly bound). Investigating the Role of ROS. We next examined the potential role of ROS via competitive scavenging experiments. These experiments reveal that multiple phototransformation pathways exist in our system. The importance of photogenerated OH in mediating aldrin transformation was determined by the addition of a competing OH scavenger (benzoic acid at 50 ppm, 4 × 10−4 M). Observed rate constants for aldrin photochemical loss in liquid and ice in the presence and absence of benzoic acid are summarized in Figure 5, and numerical values are available in Table S1, Supporting Information. In liquid phase experiments, benzoic acid addition statistically significantly (95% CI) inhibited photochemical degradation of aldrin only in the presence of SRHA, highlighting the importance of an OH-mediated pathway of aldrin degradation for this DOM source. The overall photochemical loss of aldrin in the presence of SRHA was slowed with addition of benzoic acid, but not entirely quenched. Therefore, other reactive species play a role in photochemical 10731
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logistical support in the field in Barrow, Alaska; and to Rose Cory for helpful discussions.
hydrophobic and hydrophilic scavenger addition experiments, the kinetics were slowed to a greater degree in ice samples than in liquid, consistent with an enhanced production of singlet oxygen upon freezing of the samples.38 By comparing the relative effect of hydrophobic versus hydrophilic 1O2 scavenger, we can also determine the importance of 1O2 reactions occurring within the bulk aqueous phase of the sample (or liquid-like regions of the frozen sample) versus reactions occurring at/in DOM microenvironments/surface. In liquid samples, hydrophilic scavenging (a measure of the importance of bulk aqueous phase 1O 2 reaction) is greater than hydrophobic scavenging (a measure of the importance of 1O2 reaction within DOM “microenvironments”) for SRHA and Barrow snow DOM. In ice samples, hydrophilic scavenging is greater than hydrophobic scavenging in SRHA, SRFA and Barrow snow DOM. Hydrophobic 1O2 scavenging dominates for PLFA in both liquid and ice conditions, consistent with the hydrophobic probe (aldrin) being the most strongly associated with this DOM type. Environmental Significance. The above findings add further support to the idea that many types of chemical processes are enhanced in frozen aqueous systems, and chemical reactions/kinetics can be greatly enhanced in environmental snow and ice samples. The role of natural dissolved organic matter photochemistry, known to be an important chromophore in snow and ice, has for the first time been investigated in ice under environmentally relevant conditions. Organic matter from a variety of source types (terrestrial, microbial, and DOM isolated from an authentic Arctic snow sample) was able to efficiently mediate the photochemical loss of a probe hydrophobic molecule and the degradation was enhanced in ice relative to liquid water solutions. Both OH radical and singlet oxygen are important reactive oxygen species produced during the irradiation of DOM in the ice phase, although the relative importance of these is related to the DOM source. Isolation and characterization of authentic Arctic snow/ice phase DOM samples will be necessary in order to obtain sufficient quantities of material for full characterization of the DOM and additional studies of its photochemical properties across a wider range of environmentally relevant conditions.
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(1) Blough, N. V.; Zepp. R. G. Reactive oxygen species in natural waters. In Active Oxygen: Reactive Oxygen Species in Chemistry, Foote, C. S., Valentine, J. S., Greenberg, A., Liebman, J. F., Eds.; Chapman and Hill: New York, 1995; pp 280−332. (2) Vaughan, P. P.; Blough, N. V. Photochemical formation of hydroxyl radical by constituents of natural waters. Environ. Sci. Technol. 1998, 32, 2947−2953. (3) Gerecke, A. C.; Canonica, S.; Mller, S. R.; Wirz, J. Quantification of dissolved natural organic matter (DOM) mediated phototransformation of phenylurea herbicides in lakes. Environ. Sci. Technol. 2001, 35, 3915−3923. (4) Miller, P. L.; Chin, Y. P. Indirect photolysis promoted by natural and engineered wetland water constituents: Processes leading to alachlor degradation. Environ. Sci. Technol. 2005, 39, 4454−4462. (5) Halladja, S.; 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. (6) Buschmann, J.; Canonica, S.; Sigg, L. Photoinduced oxidation of antimony(III) in the presence of humic acid. Environ. Sci. Technol. 2005, 39, 5335−5341. (7) Meunier, L.; Laubscher, H. U.; Hug, S. J.; Sulzberger, B. Effects of size and origin of natural dissolved organic matter compounds on the redox cycling of iron in sunlit surface waters. Aquat. Sci. 2005, 67, 292−307. (8) Cory, R. M.; Crump, B. C.; Dobkowski, J. A.; Kling, G. A. Surface exposure to sunlight stimulates CO2 release from permafrost soil carbon in the Arctic. Proc. Nat. Acad. Sci. U. S. A. 2013, 110, 3429− 3434. (9) Grannas, A. M.; Cory, R. M.; Miller, P. L.; Chin, Y. P.; McKnight, D. M. Role of dissolved organic matter in Arctic surface waters in the photolysis of hexachlorobenzene and lindane. J. Geophys. Res.: Biogeosci. 2012, 117, G01003, doi: 10.1029/2010JG001518. (10) Anastasio, C.; Galbavy, E.; Hutterli, M.; Burkhart, J.; Friel, D. K. Photoformation of hydroxyl radical on snow grains at Summit, Greenland. Atmos. Environ. 2007, 41, 5110−5121. (11) Beine, H.; Anastasio, C.; Esposito, G.; Patten, K.; Wilkening, E.; Domine, F.; Voisin, D.; Barret, M.; Houdier, S.; Hall, S. Soluble, lightabsorbing species in snow at Barrow, Alaska. J. Geophys. Res.: Atmos. 2011, 116, doi: 10.1029/2011JD016181. (12) Beine, H.; Anastasio, C.; Domine, F.; Douglas, T. A.; Barret, M.; France, J. L.; King, M. D.; Hall, S. R.; Ullmann, K. Soluble chromophores in marine snow, seawater, sea ice and frost flowers near Barrow, Alaska. J. Geophys. Res.: Atmos. 2012, 117, doi:10.1029/ 2011JD016650. (13) Fisher, F. N.; King, M. D.; Lee-Taylor, J. Extinction of UVvisible radiation in wet midlatitude (maritime) snow: Implications for increased NOx emission. J. Geophys. Res.: Atmos. 2005, 110, doi: 10.1029/2005JD005963. (14) France, J.; King, M.; Lee-Taylor, J. Hydroxyl (OH) radical production rates in snowpacks from photolysis of hydrogen peroxide (H2O2) and nitrate (NO3−). Atmos. Environ. 2007, 41, 5502−5509. (15) Galbavy, E.; Anastasio, C.; Lefer, B.; Hall, S. Light penetration in the snowpack at Summit, Greenland: Part 1: Nitrite and hydrogen peroxide photolysis. Atmos. Environ. 2007, 41, 5077−5090. (16) Voisin, D.; Jaffrezo, J.-L.; Houdier, S.; Barret, M.; Cozic, J.; King, M. D.; France, J. L.; Reay, H. J.; Grannas, A. M.; Kos, G.; Ariya, P. A.; Beine, H. J.; Domine, F. Carbonaceous species and humic like substances (HULIS) in Arctic snowpack during OASIS field campaign in Barrow. J. Geophys. Res.: Atmos. 2012, 117, doi:10.1029/ 2011JD016612. (17) Alvarez-Aviles, L.; Simpson, W. R.; Douglas, T. A.; Sturm, M.; Perovich, D.; Domine, F. Frost flower chemical composition during growth and its implications for aerosol production and bromine
ASSOCIATED CONTENT
S Supporting Information *
Table of observed rate constants with and without various chemical scavengers added to both liquid and ice samples. This material is available free of charge via the Internet at http:// pubs.acs.org/.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*Amanda M. Grannas. E-mail:
[email protected]. Author Contributions ‡
These authors contributed equally. The paper was written through contributions of all authors. All authors have given approval to the final version of the paper.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by National Science Foundation Grant ATM-0547435. Thanks also to UMIAQ for providing 10732
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