Article pubs.acs.org/est
Controls on the Redox Potential of Rainwater Joan D. Willey,*,† Katherine M. Mullaugh,§ Robert J. Kieber,† G. Brooks Avery, Jr.,† and Ralph N. Mead† †
Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, North Carolina 28403-5932, United States § Department of Chemistry, Elon University, Elon, North Carolina 27244, United States ABSTRACT: Hydrogen peroxide acting as a reductant affects the redox potential of rainwater collected at the Bermuda Atlantic Time Series Station, the South Island of New Zealand, the contiguous USA, and the primary study site in Wilmington, NC. Analytical measurements of both halves of redox couples for dissolved iron, mercury, and the nitrate−nitrite−ammonium system can predict the rainwater redox potential measured directly by a platinum electrode. Measurements of these redox couples along with the pH in rain yields pe− between 8 and 11; the half reaction for hydrogen peroxide acting as a reductant using typical rainwater conditions of 15 μM H2O2 at pH 4.7 gives pe− = 9.12, where pe− = negative log of the activity of hydrated electrons. Of the six rainwater redox systems investigated, only manganese speciation appeared to be controlled by molecular oxygen (pe− = 15.90). Copper redox speciation was consistent with superoxide acting as a reductant (pe− = 2.7). The concentration of H2O2 in precipitation has more than doubled over the preceding decade due to a decrease in SO2 emissions, which suggests the redox chemistry of rainwater is dynamic and changing, potentially altering the speciation of many organic compounds and trace metals in atmospheric waters.
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INTRODUCTION The redox potential of rainwater controls trace metal speciation, which in turn controls solubility, toxicity, bioavailability, and transport mechanisms of many important trace metals in the environment. Redox potential also affects the composition and oxidation state of many different organic compounds in precipitation. In addition, the abundance of certain reactive radicals is highly dependent upon the atmospheric redox potential. Identification of reactions that set the redox potential or pe− (negative log of the activity of free electrons) of rainwater is therefore a question of fundamental importance to the chemistry of the atmosphere. Redox reactions are difficult to study experimentally because hydrated electrons exist for a very short time; therefore, pe− can be thought of as a tendency for electron transfer rather than a discrete activity or concentration.1 To date very little is known about what species are most important in controlling rainwater pe−. Oxidants and reductants in the troposphere that can potentially affect redox potential and hence trace metal speciation include molecular oxygen, hydrogen peroxide, ozone, and the radicals superoxide, hydroperoxyl, and hydroxyl.2 Molecular oxygen and hydrogen peroxide are likely candidates because they occur in tens to hundreds of micromolar concentrations in rainwater and persist for days. Ozone is not likely to be important because it has a low atmospheric partial pressure (40 natm) and low water solubility (Henry’s Law constant 9.4 × 10−3 M atm−1) resulting in very low rainwater concentrations (≤1 nM).3 Other possible oxidants and reductants that should be considered include © 2012 American Chemical Society
hydroxyl and superoxide radicals which, although low in concentration, are highly reactive in atmospheric waters.3−5 The goal of the current study is to utilize six half reactions (Table 1) to investigate the mechanisms controlling the redox potential of rainwater. We have measured analyte concentrations of several redox couples and compared the calculated pe− obtained from these couples with the predicted values (Table 1) and with independent platinum electrode measurements of redox potential. A similar approach was used in a more limited study by Hofmann et al.6 who measured redox speciation of iron and several other trace metals along with the redox potential in two filtered rain samples collected in Germany in February 1989. They determined redox potentials of 385 and 310 mV using a platinum electrode with an average pe− of 5.9. Deutsch et al.7 analyzed seven rain and two snow samples collected in Germany during the autumn and winter of 1993 and reported redox potential values between 528 and 665 mV, with an average pe− of 9.8. The focus of their study was analytical method development, so they did not attempt to interpret these low millivolt readings. Other studies of various natural waters have also yielded interesting redox results. Liss et al.8 measured the nitrite− nitrate and iodide−iodate redox couples in seawater and calculated a pe−of 10.5, much lower than predicted by the oxygen water couple (12.6 at pH 8) and more in line with Received: Revised: Accepted: Published: 13103
June 25, 2012 November 15, 2012 November 20, 2012 November 20, 2012 dx.doi.org/10.1021/es302569j | Environ. Sci. Technol. 2012, 46, 13103−13111
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were determined spectrophotometrically using a modification of the ferrozine method.15 Fe(II) was measured after addition of ferrozine while the sum of Fe(III) and Fe(II) was measured after reduction by hydroxylamine hydrochloride for 1 h followed by complexation with ferrozine. The resulting difference provided the concentration of Fe(III). Prior to 2001, a 10 cm cell was used after which a 1 or 5 m liquid waveguide capillary cell was utilized corresponding to a detection limit (dl) improved from 10 nM (relative standard deviation (rsd) 4%) to 0.4 nM (rsd 0.1 pM) with the highest concentration observed of 0.3 pM.28 Mercury in its gaseous, elemental phase (Hg0) is the predominant form of mercury in the atmosphere,53,54 but it is not very water-soluble. Dissolved gaseous mercury has an equilibrium value of 0.02 pM (assuming a gas-phase concentration in the northern hemisphere of 10 pmol m−3 55 and a Henry’s Law constant of 9.3 × 10−2 M atm−1 at 15 °C), based on scavenging of Hg0 from the gas phase. Lin and Pehkonen54 calculated a similar equilibrium value estimate, ranging from 0.013 to 0.053 pM. In the current rainwater study, a range of concentrations from 0.02 pM to 0.3 pM was used. The speciation of Hg(II) depends upon
NO3−(aq) + 2H+(aq) + 2e−(aq) ⇌ NO2−(aq) + H 2O(l)
log K = 28.30
(19)
NO2−(aq) + 8H+(aq) + 6e−(aq) ⇌ NH4 +(aq) + 2H 2O(l)
log K = 90.84
(20)
NO3−(aq) + 10H+(aq) + 8e−(aq) ⇌ NH4 +(aq) + 3H 2O(l)
log K = 119.2
(21)
Equation 21 is the overall reaction occurring in the two steps described by eqs 19 and 20. National Atmospheric Deposition Program (NADP, http://nadp.sws.uiuc.edu/) data were used to calculate pe− values to compare with data for Wilmington, NC over similar time periods. NADP data do not include nitrite so only the overall nitrate−ammonium couple (eq 21) could be considered. Each of these half reactions, using measured 13108
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the developed world is moving toward a less anthropogenically impacted and more natural composition. Increases in rainwater hydrogen peroxide concentration will continue to alter the redox chemistry of rainwater for years to come (eq 3, Table 1), including the speciation of trace metals, concentration and molecular composition of organic compounds and occurrence of radicals in atmospheric waters.
concentrations and pH values, is consistent with reaction with the hydrogen peroxide−oxygen redox couple, with calculated pe− values close to 9 (Table 4). The older rainwater data from Leeds, UK, even though much higher in concentration, yields similar pe− values (Table 4).56,57 These points are plotted in Figure 1b and, like the iron data, lie close to the line representing H2O2 acting as a reductant (eq 3 in Table 1). The comparison between NADP and Wilmington rainwater data indicates that the Wilmington data are consistent with national averages. Using the nitrate−ammonium redox couple (eq 21), the overall reaction is
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*Phone: 910-962-3459; fax: 910-962-3013; e-mail: willeyj@ uncw.edu.
NO3−(aq) + 2H+(aq) + 4H 2O2 (aq) +
⇌ NH4 (aq) + 4O2 (g) + 3H 2O(l)
AUTHOR INFORMATION
Corresponding Author
Notes
The authors declare no competing financial interest.
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(22)
ACKNOWLEDGMENTS The rainwater research program at UNCW has been continuously supported by a variety of NSF Atmospheric Chemistry grants, the most recent of which are AGS 0646153 and AGS 1003078. The authors gratefully acknowledge the NOAA Air Resources Laboratory (ARL) for the data from the HYSPLIT transport and dispersion model. Event-based sampling and timely analysis of rainwater required by this project would not have been possible without the contributions of undergraduate students, master’s students, and postdoctoral fellows. Angela Carroll and Briana Rice performed the experiment presented in Figure 2. The authors appreciate the thoughtful comments received from four anonymous reviewers.
where nitrate is the oxidant and hydrogen peroxide the reductant. Redox Potential Measurements. Redox potential measurements of 11 rainwater samples during the summer of 2010 had an average of 637 mV (±28 mV), which corresponds to a pe− of 10.8 (pe− = 16.9E, where E is measured in V). This is very close to the value of 10.1 obtained by measuring the iron couple and calculating pe− using eq 9 during the same time period (Table 2). The measured pe− value is several orders of magnitude lower than that calculated for equilibrium with the oxygen−water redox couple (eq 1), which predicts 941 mV (pe− = 15.91) for pH 4.70 rain in contact with air. The millivolt reading did not correlate with H + (aq) or H 2 O 2 (aq) concentrations in these rain samples (p > 0.1), indicating that the electrode was not responding to these parameters directly. Two earlier studies of rainwater in Germany using a platinum electrode measurement also found lower millivolt readings than predicted by the oxygen−water couple and more in line with hydrogen peroxide as a reductant.6,7 Implications. The combination of experimental results for iron (Figure 2), experimental work with chromium,9 analytical rainwater data (Tables 2−4) and direct electrode measurements in this study and others6,7 indicate that the redox chemistry of rainwater is not dominated by molecular oxygen even though rainwater is in close contact with the atmosphere for time periods of hours. These results suggest that the redox potential of rainwater is poised by hydrogen peroxide acting as a reductant. This has direct and significant implications for the chemistry of atmospheric waters because a recent study demonstrated that the concentration of hydrogen peroxide in Wilmington, NC precipitation has more than doubled during the preceding decade due to a decrease of SO2 emissions.58 Fe(II) and Fe(III) concentrations and stability in Wilmington, NC rainwater have also changed dramatically during this same time period.18 Hydrogen ion concentrations decreased by approximately half between 1985 and 2005 in Wilmington rainwater59 although in more recent years it appears to have leveled off at approximately 20 μM, which is approximately twice the background concentration estimated for this region.60,61 Dissolved organic carbon decreased between 1995 and 2005 59 and now appears to have leveled off at approximately 60 μM, half the concentration in the early 1990s. Formic and acetic acid concentrations also decreased by more than half in rainwater during the previous decade.61 These changes reflect better emission control from power plants and motor vehicles, including lower emissions of volatile organic compounds in the USA. Therefore, even though hydrogen peroxide concentrations are increasing, rainwater in
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