Environ. Sci. Technol. 2007, 41, 1601-1607
Production of Hydrated Electrons from Photoionization of Dissolved Organic Matter in Natural Waters
from known. One of the important initial steps is thought to be the ionization of CDOM to produce hydrated electrons (eaq) (9)
W E I W A N G , * ,†,‡ O L I V E R C . Z A F I R I O U , † IU-YAM CHAN,‡ RICHARD G. ZEPP,§ AND NEIL V. BLOUGH| Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, Department of Chemistry, Brandeis University, Waltham, Massachusetts 02454, Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, Georgia 30613, and Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742
The product, eaq, is a short-lived strong reductant that feeds further reactions with potential consequences for the cycling of CDOM, organic carbon, ocean chemistry, biology, optics and optical properties (10), and remote sensing, as well as bioavailability of organic molecules (11-14). It also has a possible role in intrahumic dechlorination of mirex, a pesticide, illustrating its potential to affect the fate of organic pollutants (15, 16). A number of efforts have been made in the last two decades to study reaction 1 and to obtain the apparent quantum yield (AQY) of eaq. Two alternative approaches have been employed: the laser flash photolysis (LFP) (17-20) and the steady-state scavenger (S-SS) methods (20, 21). In the steadystate scavenger method, samples are “spiked” with a eaq specific scavenger and irradiated continuously with a lowintensity, broadband light source for hours to days. The eaq generated are converted by the scavenger to stable, measurable end products. In laser flash photolysis, samples are irradiated with short, intense laser pulses. A second light source is used to monitor the transient absorption arising from the intermediate photochemical products. The difference in the transient absorption signal amplitudes with and without added eaq scavenger gives the yield of eaq. All previous results have consistently showed that the AQYs obtained with LFP were higher than those from S-SS methods. In the only known direct comparison (20) of these two methods, Zepp and co-workers measured the efficiency of eaq formation at 355 nm in organic solutes isolated from the Suwannee River (Georgia) and the Greifensee (a lake in Switzerland), as well as commercial fulvic and humic acids. The resulting quantum yields from flash photolysis were more than 100 times larger than those from the steady-state method. Later, Thomas-Smith and Blough (21) used a highly sensitive but different S-SS method on various humic and fulvic acids, and water samples from rivers and estuaries. Their results were in the same vicinity as the previous S-SS measurements, indicating that different scavengers gave consistent results. Zepp (20) and Thomas-Smith and Blough (21) both pointed out that the high AQYs from LFP measurements might possibly result from biphotonic processes due to the high intensity of the UV excitation light. If so, because natural sunlight intensities are such that multiphotonic ionization is highly unlikely to occur, the high yields of eaq from the LFP experiments would not reflect the production of eaq in natural waters. On the contrary, the light source used in S-SS methods is very similar to sunlight (intensity more than 108 times smaller than pulsed lasers), so that provided the capturing eaq with scavengers is efficient, the AQYs from the S-SS methods are valid. If the results from both methods are correct and comparable, then the lower values from the S-SS method may result from eaq formed within colloidal structures inaccessible to scavenger methods, where eaq remain optically detectable. In this work, we have refined the LFP approach by building a new laser flash photolysis apparatus. Compared with the conventional LFP apparatus, there are two main differences. (1) To minimize multiphoton effects, the UV excitation light is defocused to keep the intensity low. Additionally, to
Under UV irradiation, an important primary photochemical reaction of colored dissolved organic matter (CDOM) is ). The electron ejection to produce hydrated electrons (eaq efficiency of this process has been studied in both fresh water and seawater samples with both steady-state scavenger (S-SS) and time-resolved laser flash photolysis (LFP) methods. However, the apparent quantum yields (AQYs) of eaq for the same samples using the two methods differ by as much as a factor of 100, necessitating a closer reexamination of how the process is measured. We developed a highly sensitive multipass LFP apparatus that allows detection of transient species at very low and variable UV irradiation intensities. Under single-photon conditions, from Laurentian fulvic acid we measured the AQY of eaq as 1.3 × 10-4, and set the upper limit for other CDOM samples at 6 × 10-5, bringing the LFP results into agreement with those from S-SS methods. We also examined the ionization at elevated irradiation intensities and clearly demonstrated that multiphoton ionization occurs at intensities well below those usually used in LFP experiments, but well above those likely to occur at the earth’s surface. This multiphoton ionization is probably the cause of the high AQYs reported by earlier LFP work. In addition, we also observed in real time other photochemical reactions, such as triplet quenching and bleaching, in the single photon regime.
Introduction Colored dissolved organic matter (CDOM) in terrestrial and coastal waters consists of biologically refractory organic polymers with a large molecular weight range that are derived from the decomposition of plant materials (1). Upon absorption of light, CDOM undergoes photochemical reactions which yield, among other products, CO, CO2, and H2O2 (2-8). However, the mechanisms of these reactions are far * Corresponding author phone: (508) 289-3222; fax: (508) 4572164; e-mail:
[email protected]. † Woods Hole Oceanographic Institution. ‡ Brandeis University. § U.S. Environmental Protection Agency. | University of Maryland. 10.1021/es061069v CCC: $37.00 Published on Web 02/02/2007
2007 American Chemical Society
CDOM + hν f CDOM‚+ + eaq
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(1)
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FIGURE 1. Schematic of the apparatus. The overall setup is shown as a top view on the left. The components inside the oval are shown in the enlargement on the right, which is a side view along the direction of the UV beam. The two movable cylindrical lenses permit spreading or focusing the UV beam across the front of the cell in both horizontal and vertical directions, thus varying UV intensity. The shading on the sample cell indicates the areas illuminated at high (black) and low (gray) intensities. evaluate multiphoton processes, the intensity can be adjusted upward continuously, approaching the same level as in the previous LFP work. (2) To achieve high detection sensitivity, the path length of the probe light through the samples for measuring transient absorption is very long. With this approach, if the photochemical processes are kept in the monophotonic regime, we should be able to directly observe the initial photochemical reactions that occur under natural sunlight conditions.
Experimental Section Instrument. A schematic of the laser flash photolysis apparatus is shown in Figure 1. The UV source for the excitation was a nanosecond pulsed Lambda Physik XeCl excimer laser (308 nm, pulse width 10 ns). For detection, a continuous-wave Helium-Neon (HeNe) laser (Spectra Physics, 633 nm) was used because hydrated electrons have a strong, broad absorption peak in the visible to near-IR region, centered at 720 nm (22, 23). To obtain low and variable UV intensities, instead of simply focusing UV on the sample, we used two cylindrical lenses that were mounted on an optical rail. By moving the lenses with respect to the quartz sample cell, we were able to expand or focus the UV beam to the desired sizes in both vertical and horizontal directions, thus varying the UV intensity. During the experiments, the vertical spread of the UV beam was normally fixed. The intensity could also be adjusted by controlling the excimer laser output, and/or by using an UV attenuator. The lower bound of the intensity during the pulse was less than 50 kW/cm2, or 0.5 mJ/cm2 per pulse, in contrast to previous experiments that used intensities on the order of 100 mJ/cm2 per pulse or higher. To obtain a very long path length, the probe beam was reflected multiple times by a coated mirror (to minimize loss of light upon each reflection) on each end of the sample cell. The absorption path length was up to about 230 cm. The exiting probe beam from the sample cell was passed through an aperture (2 3 mm in diameter), an UV and a long-pass Schott filter, focused through the entrance slit of a monochromator, and detected with a photomultiplier tube (Hamamatsu R928). The signal was recorded with a LeCroy digital averaging oscilloscope, and stored on a PC. The sample cell was large compared to the 1-cm cuvettes normally used in LFP experiments: a 35 cm × 1 cm × 4 cm rectangular quartz tube with wedge windows (to prevent interference) glued on both ends. The large side wall (35 cm × 1 cm) faced the expanded UV beam; the probe laser light passed along the long axis of the cell. Inline with the cell were a micropump for circulating the sample solutions, a filter to remove occasional large particles (diminishing 1602
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scattering), and a sample reservoir with a frit at the bottom where gases for purging the samples were introduced. During each measurement, samples were continuously circulated at about 10 mL/s and purged with various gases (N2O, O2, and N2) with known eaq scavenging rates in solution. Samples. Soil CDOM samples were Laurentian fulvic acid (FA) and Laurentian humic acid (HA) purchased from Fredriks Research Products. Suwannee River fulvic acid (SRFA) was the same sample used by Thomas-Smith and Blough (21), originally obtained from the International Humic Substances Society. These humic substances were dissolved in a borate buffer solution (0.01 M, pH ∼8.2) and filtered (0.2 µm) prior to use. Freshwater samples, filtered (0.22 µm) immediately after collection, were from the Satilla and Altamaha Rivers (Georgia). Both are relatively unpolluted and have substantial inputs of humic material from floodplain swamps and coastal plains (24-27). Satilla river samples were concentrated by ultrafiltration to obtain fractions of molecular weight greater than 10 kD (SAT10kD) and greater than 1 kD (SAT1kD). Altamaha water was concentrated using low-temperature, liquid-phase evaporation, during which sample temperature ranged between 0 and 40 °C. All of the samples were adjusted by dilution so that the absorption at 308 nm was about 0.1/cm. The exact absorption of each batch of sample was measured using a UV-VIS spectrophotometer (HP8452A) just before the measurement. To evaluate any changes due to the UV exposure, absorption spectra were also taken for Laurentian FA, SAT1kD, and SAT10kD immediately following the experiment. Potassium ferrocyanide solutions were used for actinometry throughout the experiments. The quantum yield of eaq from its ionization is 0.104 at 313 nm (28). N,N-dimethylaniline was used once for inter-comparison, with its reported quantum yield of eaq at 308 nm being 0.08 (29). Our result was about 10% higher (0.089), referenced to ferrocyanide. Sodium nitrite (NaNO2) was used as a “blank absorber” to investigate and minimize heating effects (see below). Because its photodecomposition products, NO, OH, and OH-, do not absorb at 633 nm, any signal observed in NaNO2 solution at 633 nm was from physical effects, such as heating. Transient Absorption Signals. To obtain the amount of eaq produced, we measured the transient peak amplitude difference between samples saturated with nitrogen and eaq scavenger (N2O), normalized to an absorbance at 308 nm of 9 -1 0.1/cm. The reaction rate of N2O with eaq is 9 × 10 L mol s-1, so that in N2O-saturated samples, eare scavenged at aq a rate of 2.1 × 108 s-1, and do not contribute to the transient absorption signal. The decay curve after the peak was not used in the quantitative analysis of the eaq yield. It contains
FIGURE 2. Transient absorption signal of eaq from ionization of ferrocyanide in water. The signal (solid line) is quenched by saturating the solution with N2O (dashed line). The inset shows that the wavelength dependence of the transient signal amplitude is consistent with the spectrum of the eaq absorption coefficient. The instrument response function is also shown (thin solid line near time ) 0). This set of data was taken using 50 Ω input impedance.
multiple components including eaq and other transient species such as triplet state CDOM (3CDOM), so that its e-folding time depends on the loss pathways and concentrations of the species involved, neither of which are well-known for any given sample. To verify our approach, the transient absorption of the actinometer, ferrocyanide, was measured at 476, 488, and 514 nm with a tunable Ar ion laser, and 633 nm with the HeNe laser. The amplitude of the signal followed the absorption coefficient spectrum of hydrated electrons as expected (Figure 2 and inset). With our instrument (using 500 Ω input impedance), transient absorption signals peaked at about 300-400 ns after the laser pulse, although the solvation of the electrons, once ejected from their parent molecules, is very fast compared with the time scale of this experiment (30). The signals with and without the scavenger were always compared at the peak. For all water samples studied, the major part of the transient absorption at 633 nm was not quenchable with N2O. We measured each sample under N2 and N2O saturated conditions separately. The signals from 4000 pulses of the UV laser (2 Hz repetition rate) were averaged. At this light dosage, the photobleaching of the sample (total volume about 350 mL), saturated with either N2 or N2O, was very small (
