Article pubs.acs.org/est
Photochemical Production of Singlet Oxygen from Dissolved Organic Matter in Ice Alexis Fede and Amanda M. Grannas* Department of Chemistry, Villanova University, 800 Lancaster Avenue, Villanova, Pennsylvania 19085, United States S Supporting Information *
ABSTRACT: Dissolved natural organic matter (DOM) is a ubiquitous component of natural waters and an important photosensitizer. A variety of reactive oxygen species (ROS) are known to be produced from DOM photochemistry, including singlet oxygen, 1O2. Recently, it has been determined that humic-like substances and unknown organic chromophores are significant contributors to sunlight absorption in snowpack; however, DOM photochemistry in snow/ice has received little attention in the literature. We recently showed that DOM plays an important role in indirect photolysis processes in ice, producing ROS and leading to the efficient photodegradation of a probe hydrophobic organic pollutant, aldrin.1 ROS scavenger experiments indicated that 1O2 played a significant role in the indirect photodegradation of aldrin. Here we quantitatively examine 1O2 photochemically produced from DOM in frozen and liquid aqueous solutions. Steady-state 1O2 production is enhanced up to nearly 1000 times in frozen DOM samples compared to liquid samples. 1O2 production is dependent on the concentration of DOM, but the nature of the DOM source (terrestrial vs microbial) does not have a significant effect on 1O2 production in liquid or frozen samples, with different source types producing similar steady-state concentrations of 1O2. The temperature of frozen samples also has a significant effect on steady-state 1O2 production in the range of 228−262 K, with colder samples producing more steady-state 1O2. The large enhancement in 1O2 in frozen samples suggests that it may play a significant role in the photochemical processes that occur in snow and ice, and DOM could be a significant, but to date poorly understood, oxidant source in snow and ice.
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INTRODUCTION
receive further attention regarding their potential photochemical reactivity in frozen systems. As ice forms from freezing water, pure water crystallizes first, and solutes are excluded from the bulk and become concentrated at the surface. Solutes may also become trapped in micropocketsinclusions within an ice crystalof highly concentrated solution or incorporated into grain boundaries (the surface between two ice crystals), veins (the linear intersections of grain boundaries), and nodes (the junction of veins) of ice crystals. This process leads to a chemically enriched, potentially liquid-like environment in these distinct regions within snow and ice. This process of solute enrichment during freezing is known as the freeze-concentration effect (reviewed in greater detail in ref 11). This effect can enhance the chemical reactivity in ice compared to liquid solutions. In addition, chemical reaction rates in snow and ice are dependent on factors such as temperature, solute content, and aqueous solubility of reactants, all of which impact the nature and reactivity of these liquid-like regions.12−14
Dissolved organic matter (DOM) is a heterogeneous mixture resulting from the breakdown of bacterial, algal, and plant organic material that is ubiquitous to surface waters and can significantly impact aquatic photochemistry processes.2−5 Irradiation of chromophoric DOM (CDOM) results in the production of reactive intermediates, known as reactive oxygen species (ROS).2,6−8 ROS are oxygen-containing species with strong oxidizing abilities, including for example singlet oxygen (1O2), hydroxyl radical (OH), hydrogen peroxide (H2O2), and superoxide (O2•‑). The photochemical processing of DOM has been well studied in the aquatic chemistry literature, but less in known about the photochemistry that can occur in snow and ice. DOM is a ubiquitous component of environmental snow and ice, which means that the photochemical reactions observed in aquatic systems may also occur in snow and ice. However, knowledge of the processes observed in aquatic systems cannot simply be extrapolated to frozen systems because freezing causes snow and ice to exhibit different properties than liquid water. Recent studies show that in some locations humic-like substances (HULIS) and unknown chromophores mainly contribute to the absorption of sunlight in snow and ice.9,10 These uncharacterized, likely organic, chromophores need to © XXXX American Chemical Society
Received: July 24, 2015 Revised: October 7, 2015 Accepted: October 13, 2015
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DOI: 10.1021/acs.est.5b03600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
Figure 1. Production of 1O2 from irradiation of DOM-containing solutions and various deactivation processes controlling the steady-state concentration of 1O2 in natural waters.
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Recent work1 provided evidence that DOM plays an important role in indirect photolysis processes in ice via production of ROS. In that study, ROS produced from DOM photochemistry in ice led to the efficient photodegradation of a probe hydrophobic pollutant molecule, aldrin. DOM-mediated aldrin loss rates were enhanced by up to a factor of 56 in ice compared to liquid water, proposed to be caused by a freezeconcentration effect. All DOM source types studied, regardless of origin, were able to mediate pollutant loss, pointing to the potential ubiquity of DOM in indirect photochemistry in environmental ices. While not the focus of that work, preliminary scavenging experiments pointed to the importance of singlet oxygen in the photochemical degradation of aldrin that was observed. Singlet oxygen is formed in natural waters by energy transfer from excited triplet state DOM (3DOM) to molecular oxygen. Production and deactivation pathways for 1O2 are outlined in Figure 1. The pathway that determines 1O2 lifetime in natural waters (approximately 4 μs) is typically quenching by the solvent (water).15 Typical steady-state concentrations of singlet oxygen found in natural waters has ranged from 10−14 to 10−12 M.15 In this study we quantitatively investigate 1O2 production in snow and ice samples by measuring the rate of degradation of furfuryl alcohol (FFA), a selective 1O2 scavenger, in the presence of irradiated DOM. FFA is used as a probe for singlet oxygen because it is highly selective for singlet oxygen and does not undergo reactions with other ROS species generated in solution such as OH radical, hydrogen peroxide, superoxide, or triplet state DOM.16,17 Loss of FFA is directly proportional to the amount of 1O2 produced in the sample. Work done by Bower and Anastasio18 using Rose Bengal as a 1O2 sensitizer reported a 10 000-fold rate enhancement in FFA loss in ice samples compared to liquid water samples, an indication that photochemical production of 1O2 from natural organic matter may also be enhanced in ice compared to liquid water. Our goals for this study were to determine (1) how the rate of degradation of FFA in frozen aqueous DOM samples compares to that of liquid aqueous DOM samples, (2) how DOM concentration affects 1O2 production, (3) how DOM source impacts 1O2 production, and (4) how temperature and solute content (which influence the liquid-like regions in/on ice) affect 1O2 production in frozen DOM samples. The DOM sources investigated include Suwannee River fulvic acid (terrestrial DOM source), Suwannee River humic acid (terrestrial DOM source), Pony Lake fulvic acid (microbial DOM source), and organic material isolated from the coastal Arctic snowpack in Barrow, Alaska (potential mixed DOM source of both marine and terrestrial origins).
MATERIALS AND METHODS Materials. Acetonitrile was purchased from Fisher Scientific. Furfuryl alcohol, 2-nitrobenzaldehyde and hydrochloric acid were purchased from Sigma-Aldrich. Pony Lake fulvic acid (PLFA), Suwannee River fulvic acid (SRFA), and Suwannee River humic acid (SRHA) were purchased from the International Humic Substances Society. All reagents were used as received. Solution Preparation. DOM stock solutions were prepared from isolated humic substances from the various sources. A solution of 25 mg DOM in 25 mL ultrapure water was prepared from solid SRFA, SRHA, and PLFA. Standard solutions of furfuryl alcohol (FFA) were prepared in ultrapure water. For each experiment, a solution of a certain DOM concentration with 5 μM of FFA was prepared in a 250 mL flask and diluted with ultrapure water (Millipore, DirectQ, 18 MΩ·cm). All of the solutions were wrapped in foil and stored in dark conditions to avoid any possible FFA decay by light prior to the start of the experiment. DOM stock solutions were stored in a refrigerator and all other solutions were stored in a dark room temperature cabinet. Isolation of Barrow, Alaska Snow DOM. The DOM present in Barrow, Alaska snow could originate from a variety of distinct sources, including local vegetation inputs, marine inputs, as well as long-range transport. Some recent work19 has attempted to better characterize the organic materials in Barrow snow, however a full accounting of its sources and composition are not available at this time. To investigate the chemistry of this unique “mixed” source, DOM was isolated from melted Barrow, AK snow via C18 solid phase extraction (47 mm, Empore disks, 3M) in October 2011. Snowmelt was acidified on-site to pH 2 with trace metal grade HCl. Approximately 3−5 L of snowmelt was passed through an individual C18 disk. A total snowmelt volume of approximately 200 L was extracted on 40 separate C18 disks. Disks were returned to Villanova University, eluted with methanol (eluates from multiple disks were combined into one fraction), and freeze-dried to obtain solid material for use in laboratory experiments. Because not enough material was isolated to allow for accurate mass measurement on an analytical balance, the solid material was reconstituted into 25 mL of ultrapure water to serve as the stock DOM solution. Total organic carbon (TOC) analysis of the aqueous solution was conducted for quantitative determination of the total carbon content of this stock solution, allowing us to make final diluted solutions of known organic carbon concentration. An appropriate quantity of stock DOM solution was added to the FFA solution to result in the desired DOC concentration, using either the isolated Barrow snow DOM solution, or commercially available IHSS standards dissolved in Milli-Q water. Samples were prepared by transferring 750 μL of the DOM solution spiked with 5 μM FFA into borosilicate ampules or 1 B
DOI: 10.1021/acs.est.5b03600 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
of FFA. Little degradation was observed in dark controls (
