Environ. Sci. Technol. 1907, 2 1 , 485-490
Photoproduction of Hydrated Electrons from Natural Organic Solutes in Aquatic Environments Rlchard G. Zepp'
Environmental Research Laboratory, U.S. Environmental Protection Agency, Athens, Georgia 30613 Andr6 M. Braun
Institut d e Chimie Physique, h o l e Polytechnique FBdQrale d e Lausanne, CH-1015 Lausanne, Switzerland Jurg Hoignd
Swiss Federal Institute for Water Resources and Water Pollution Control (EAWAG), CH-8600 Dubendorf, Switzerland Jerry A. Leenheer U.S. Geological Survey, Denver Federal Center, Lakewood, Colorado 80403
Laser flash photolysis was used to investigate the transients formed on absorption of 355-nm light by dissolved organic matter (DOM) from natural water bodies and from soil. Absorption spectra and quenching studies of the transients provided confirming evidence that hydrated electrons were formed by all of the DOM that were studied. The DOM from the Suwannee River in Georgia and from the Greifensee, a Swiss lake, exhibited great variability in light-absorbing properties. Despite this high variability in absorption coefficients, the primary quantum yields for electron ejection from the Greifensee and Suwannee DOM fell in a narrow range (0.005-0.008). Steady-state irradiations (355 nm) of the DOM with 2chloroethanol (0.02 M) present as an electron scavenger produced chloride ions with quantum yields that were about 2 orders of magnitude lower than the primary quantum yields. This result indicates that most of the photoejected electrons recombine with cations before escaping into bulk solution. Irradiations of DOM solutions under sunlight (April, latitude 34" N) photoproduced electrons at rates falling in the range of 0.2-0.4 pmol/ [(mg of DOC) h]. These results indicate that hydrated electrons can play a significant role in the environmental photoreduction of persistent, electronegative pollutants but may be relatively unimportant in the environmental production of hydrogen peroxide.
Introduction The natural organic solutes that occur in the hydrosphere, referred to in this paper as DOM, have been shown to photosensitize oxidations and other reactions of a variety of substrates (1). Most past studies of photoreactions sensitized by DOM have involved steady-state irradiations with monochromatic light or broad-band radiation, including sunlight. Through kinetic analyses and product studies, the identities and steady-state concentrations of short-lived intermediates such as singlet molecular oxygen and triplet states have been inferred (2-4). Recently, several studies have appeared in which the high-intensity radiation available from lasers was used in flash photolysis studies of aqueous solutions of humic substances (5, 6). In these laser studies, it was possible to directly observe the absorption spectra and the decay of transients that mediate sensitized photoprocesses. Both Fischer and co-workers (5) and Power and co-workers (6) have provided evidence that humic substances from soils and in natural waters produce hydrated electrons (e;J on absorption of near-ultraviolet light. These findings confirmed the earlier speculations of Swallow (7) that hydrated electrons may be formed on the absorption of light 0013-936X/87/0921-0485$01.50/0
by aromatic compounds in natural waters. Hydrated electrons have been shown to be photoproduced from polyhydroxy aromatic compounds (8, 9) and aromatic carboxylic acids (IO),which are known structural components of humic substances (11). The formation of hydrated electrons is of interest for several reasons. On the basis of voluminous kinetic data of radiation chemistry (12),it is well-known that hydrated electrons rapidly react with a variety of organic and inorganic species that contain electronegative atoms. Some examples of such reactions are shown in Figure 1. Moreover, hydrated electrons also react at a diffusioncontrolled rate with dioxygen and protons leading finally to the formation of hydrogen peroxide. Thus, as both Draper and Crosby (13,14) and Cooper and Zika (15)have pointed out, photoproductionof electrons from DOM could mediate formation of a part of the hydrogen peroxide that occurs in the sea and in freshwaters. In this study, we report quantitative results concerning the photoproduction of hydrated electrons from DOM of various origins, in particular those isolated from freshwater systems in Switzerland and the US. Included in the study is DOM from several well characterized sources. We present quenching results confirming that the transient that absorbs strongly in the 700-750-nm spectral region is indeed the hydrated electron and that its photoproduction can be induced by light of wavelengths found in underwater solar radiation. Results of electron scavenging studies involving steady-state irradiations indicate, however, that most of the photoejected electrons recombine with cations prior to diffusion into bulk solution.
Experimental Section Materials. Fulvic acid was obtained commercially from Contech ETC, Ottawa, Canada. Characteristics of this humic substance have been previously described (16). Organic solutes also were isolated from the Suwannee River at the point where it drains the Okefenokee Swamp, a large water body in the southern part of Georgia. The Suwannee DOM was isolated and fractionated according to its hydrophobicity and acid-base properties by adsorption chromatography, as described by Leenheer and Noyes (17,18). The symbols used for these fractions were as follows: SHPO-A, strong hydrophobic acids; HPO-A, weak hydrophobic acids; HPO-N, hydrophobic neutral fraction; HPI-A, hydrophilic acids. DOM also was obtained from the Greifensee, a highly eutrophic lake that is located in a glaciated valley of calcareous material near Zurich, Switzerland. A fraction of the DOM (fraction 1) that had molecular weights from
0 1987 American Chemical Society
Environ. Sci. Technol., Vol. 21, No. 5, 1987 485
- -
e&
+
ClCHzCHzOH
eiq
+
N,O
---+
2eiq
+
20,
2e&
+
2Hf
N,
+
20,
---+
+
C1-
2H.
.OH
+
TH2CH20H.
OH-
H+
A
+
HZOZ
02
HzOz
+
0,
Figure 1. Reactions of hydrated electrons (e;,). Second-order rate constants for these reactions are (72) as follows: k(eCICH,CH,OH) = 5.6 X lo* M-ls-'; k(e;, N20) = 6 X lo9 M"s-'; k(e;, 0,)= 1.6 X loio M-ls- I;k(e;, H+) = 2.3 X 10" M-Is-l.
+
+ +
+
1100 to 20000 was used (4,19). This fraction represented about half of the dissolved organic matter in the lake. It was isolated by gel permeation chromatography with phosphate-buffered water as eluent (19). Finally, a humic acid was obtained commercially from Fluka AG. The Fluka humic acid solution was prepared by extracting the commercial material (1g) with 1L of 0.1 M sodium hydroxide solution. The resulting mixture was gravity-filtered; then the filtrate was adjusted to pH 6 by adding HC1. Then, it was filtered again through a 0.45-wm membrane. The filtrate was diluted by the addition of phosphate buffer and used for the laser experiments. Nitrous oxide (Merck AG), trichloroacetic acid (h.p., ChemService), chloroacetic acid (h.p., ChemService), and N,N-dimethylaniline (h.p., ChemService) were of the purest grade available commercially and were used with no further purification. The 99% 2-chloroethanol was found to contain 0.001 mol of chloride ions/mol, a low content but unacceptably high for the steady-state irradiations with conversions to chloride of only 0.01 %, Therefore, the chloroethanol was purified by first taking it up into spectrograde ether and then by washing the ether solution sequentially with 1% aqueous sodium bicarbonate and with distilled water. After the ether layer was dried over anhydrous sodium sulfate, it was filtered and evaporated under high vacuum (0.1 mmHg). Attempted vacuum distillation of the residue resulted in re-formation of chloride through dehydrochlorination. Therefore, it was used with no further purification; typically the chloride content of the purified chloroethanol was less than 0.0001 mol/mol of alcohol. Combined gas chromatography/mass spectrometry confirmed that it was >99% pure. Procedures for Laser Experiments. The laser system used in the study has been described by Braun and coworkers (20,211. The intensity of the 355-nm light (third harmonic JK 2000 Nd/YAG laser) was monitored by a Laser Instrumentation joulemeter, Model 172. Typically the 15-11s pulse energy was about 5 mJ. The analyzing light was detected, after passage through a monochromator, by a SHS-100 diode detector (spectral range 650-1400 nm). The DOM samples were optically matched (absorbance of 0.1) and were buffered by 0.01 M phosphate at pH 6.2 with the exception of one experiment at pH 4.2, which used an acetate buffer. Optical densities of the transients in the 650-1300-nm region were determined for solutions that were deaerated by slowly bubbling argon through them for about 15 min. For the quenching studies, the samples containing trichloroacetate were outgassed by argon, or alternatively, the DOM solutions were saturated by nitrous oxide gas. Procedures for Steady-State Irradiations. Argonsaturated solutions were prepared that contained 0.02 pM 2-chloroethanoland either 0.0010 M N,N-dimethylaniline or the DOM (concentration of 3-32 mg of C/L) in distilled water buffered with phosphate (0.005 M). The solutions were irradiated at 20 "C with monochromatic light in a 486
Environ. Sci. Technol., Vol. 21, No. 5, 1987
0
4
8
12
16
20
Time [seconds]x106 Flgure 2. Optical density of transients at 720 nm as a function of time following flash photolysis (355nm) of a purified fraction of the humic substances from the Greifensee, a eutrophic lake near Zurich, Switzerland.
merry-go-round apparatus (313 nm, 366 nm) (22) or in a Schoeffel reaction chemistry system (310 nm, 355 nm). In the case of the Schoeffel system, the band-pass was set at 10 nm, and the light intensity was measured with a YSI Model 65A radiometer that was calibrated as described previously (3). On irradiation, the solutions produced chloride ions that were generally quantitated by ion-exchange chromatography. Analyses were performed by use of a Micromeritics high-pressure liquid chromatograph equipped with Alltech Absorbosphere SAX 8s stationary phase and aqueous 0.01 M phosphorous acid and 0.001 M potassium nitrate as mobile phase. The chloride was detected by a Dionex electrochemical detector with a silver electrode and a silver/silver chloride reference electrode. The sensitivity limit was 2 pmol/L. Irradiation doses were limited to keep the total conversion to chloride less than 0.1%. Under these conditions, the chloride formation was linear with increasing irradiation time. Dark controls were determined in all cases, and it was found that the hydrolysis of chloroethanol to chloride ions was negligible during the irradiation period. Solutions were not held for analysis for more than 1 day following irradiation. As a cross-check, the final chloride concentrations also were determined, in some cases with an Orion Model 701A pH meter equipped with an Orion Model 94-17A chloride electrode (sensitivity limit of 10 pmol/L). In these experiments, 0.2% of the chloroethanol was converted to chloride. The kinetic data were treated statistically with a program written by N. L. Wolfe, U S . EPA. Typically, standard deviations of the rate constants in triplicate experiments were 515% of the'mean value. Steady-state irradiations in sunlight were conducted at Athens, GA (latitude 34" N), with the quartz reaction tubes submerged in water that was maintained at 20 f 2 "C by periodic addition of ice. Results and Discussion
Transient Decay Kinetics and Spectra. Under the conditions used in these flash kinetic studies at 355 nm, only transients with lifetimes longer than 500 ns were detected. Flash photolysis of the various optically matched, deaerated solutions of DOM produced transients that absorbed weakly in the spectral region from 650 to at least 1050 nm. In all cases the decay of the optical density exhibited at least bimodal behavior, as illustrated
Table I. Comparison of Absorption Coefficients with Quantum Yields (355 nm) for the Production of Hydrated Electrons in Laser Flash Photolysis of Selected Natural Organic SolutesR
6-
103 x
5-
source of natural organic solute 4-
9 0 Q
Greifensee, Switzerland F1 fraction Suwannee River/ Okefenokee Swamp, Georgia SHPO-A fraction WHPO-A fraction HPO-N fraction HPI-A fraction Contech fulvic acid Fluka humic-acid
32-
0 1
600
I
I
700
800
do
1 1100
Id00
Wavelength,. nm Flgure 3. Absorption spectrum of a 1.5-ps transient formed on laser flash photolysis of an aqeuous solution of a strong, hydrophobic acid fraction of the humic substances isolated from the Suwannee River where it drains the Okefenokee Swamp in Georgia.
0
4
8
12
16
Time [ secondslx 1O6
20
0
4
8
12
16
20
Time [seconds]x106
Flgure 4. Influence of various hydrated electron quenchers on optical density of 720-nm transients formed on flash photolysis of the strong, hydrophobic fraction of Suwannee DOM: (A) deaerated aqueous solution at pH 6.2 with no added quencher; (B) nitrous oxide saturated aqueous solution, deaerated aqueous solution with 0.0062 M trichloroacetate present, or deaerated aqueous solution buffered at pH 4.2 with 0.02M acetate buffer (all the same wthin experimental error).
by a plot of transient optical density vs. time for a solution of a fraction of the DOM from the Greifensee (Figure 2). A short-lived transient with a lifetime of about 1.5 ps was observed, and much longer lived transients with lifetimes of 40 ps or longer also were detected. These results are consistent with those obtained in laser flash photolysis studies of other DOM (5, 6, 21). The absorption spectrum of the short-lived transient observed in solutions of the strong hydrophobic acid fraction (SHPO-A) of the Suwannee DOM (Figure 3) exhibited a broad maximum in the 700-750-nm spectral region. This spectrum is very similar to that of the hydrated electron as observed in radiolysis experiments (23). Quenching Studies. In all the DOM solutions studied, saturation of the solution with nitrous oxide completely quenched the short-lived transient without affecting the long-lived transient(s) in the 650-1050-nm region. More detailed studies in the case of the Suwannee SHPO-A fraction and the soil-derived Contech fulvic acid showed that the short-lived transient could also be completely quenched by 6.2 mM trichloroacetate ion (at pH 6.2) and by M protons (Figure 4). Nitrous oxide, trichloroacetate, and protons are all known to react with hydrated electrons at a diffusion-controlled rate (12). These quenching results are consistent with those reported with
[DOC], kDOM,L quantum mg/Lb mg-'m-lC yield 32
0.72
7.6 k 0.2
12 16 32 21 4.4 4.6
1.9 1.4 0.72 1.1 5.2
5.6 f 1.0 6.2 f 0.6 5.2 f 1.0 4.6 k 1.0 4.0 f 1.0 1.7 k 0.2
5.0
n p H 6.2, 25 "C; absorbance = 0.1 in all cases. bConcentration of dissolved organic carbon. Absorption coefficient (355 nm), defined bv ea l.
DOM from other sources (5, 6) and, coupled with the spectrum shown in Figure 3, strongly indicate that the short-lived transient is the hydrated electron. The longlived transients are likely to be cations derived from the electron ejection and/or long-lived triplet states of the DOM. Quantum Efficiencies for Hydrated Electron Photoproduction. On the basis of the data obtained in these experiments coupled with the known molar absorptivity of the hydrated electron [molar absorptivity of 18500 L/(mol cm) at 715 nm (23)],quantum efficiencies were estimated for the primary photoproductionof the hydrated electron from the various sources of DOM. Table I compares the absorption coefficients of the DOM at 355 nm with the primary quantum efficiencies for electron photoejection at this wavelength. The absorption coefficients hD,-,M are defined by 2.3034855 (1) = [DOC11 where A,,, is the absorbance of a DOM solution with a concentration denoted by [DOC] (mg of C/L) in a cell of path length 1 (m). Absorption coefficients of the DOM from the Suwannee River and the Greifensee exhibited nearly a 3-fold increase from weakest to strongest absorbing fraction. The soil extracts, Contech fulvic acid and Fluka humic acid, absorbed light much more strongly. In contrast to the absorption coefficients, most of the primary quantum yields in Table I are remarkably similar. (Results were not so tightly clustered in the steady-state irradiations, as discussed below.) The Fluka humic acid was an outlier. This similarity in efficiencies for DOM from widely separated water bodies is reminiscent of earlier findings (2-4) that quantum yields for production of singlet molecular oxygen from various humic substances fall into a narrow range. Steady-State Irradiations. Other studies were conducted to examine the electron ejection process by using steady-state irradiations with light intensities more characteristic of sunlight. To quantify the rates of hydrated electron photoproduction, we followed the reduction of 2-chloroethanol (0.02 M) to chloride ions in aqueous solutions of the DOM (Figure l). Under our conditions, the production of chloride from 2-chloroethanol through direct photolysis and dark processes was negligible. To check the validity of this method under our conditions, hydrated electrons were produced through the photolysis of N,N-dimethylaniline (DMA). Our results for Environ. Sci. Technol., Vol. 21, No. 5, 1987
487
Table 11. Quantum Yields for Chloride Photoproduction with Steady-State Irradiations of Various Natural Organic Solutes in Deaerated Aqueous Solutions of 2-Chloroethanol (0.02 M) at 20 O C conditions
wavelength, nm
quantum yield for chloride production
pH 8.0 pH 8.0 (+12 mg of DOC/L, Suwannee River, SHPO-A)
310 310
0.027 f 0.007" 0.028 f 0.007
pH 4.0 pH 8.0 pH 6.2 pH 6.2 (+0.05 M ethanol) pH 6.2 pH 6.2
355 355 355 355
(2.5 f 0.3) X (2.6 f 0.3) X (2.2 f 0.3) X
355 355
(2.8 f 0.4) X (1.2 f 0.2) x 10-4
pH 6.2 pH 6.2
355 355
electron source N,N-dimethylaniline (0.0010 M) N,N-dimethylaniline (0.0010 M) Suwannee River (Georgia), SHPO-A fraction 12 mg of DOC/L
100 mg of DOC/L Greifensee (Switzerland), F1 fraction (27 mg of DOC/L) Contech fulvic acid (12 mg of DOC/L) Fluka humic acid (11mg of DOC/L)
(1.7 f 0.3) X
(2.3 f 0.3) X 1 8 X lo4
" Literature value is 0.04 (24) a t unspecified temperature. the quantum efficiency of electron photoejection from DMA at 310 nm, with or without Suwannee River DOM present, agreed with the previously published value (0.04) of Kohler and co-workers (24) (Table 11). Results of steady-state irradiations of DOM from the Greifensee and the Suwannee River, the Contech fulvic acid, and the Fluka humic acid are compared with the DMA results in Table 11. The results lead to several significant conclusions: (1)The quantum yields determined in these experimenta were generally 2 orders of magnitude lower than the primary quantum yields for electron ejection found in the laser experiments (Table I). (2) The quantum yield was found to be independent of the concentration of the Suwannee DOM up to 100 mg of C/L. (3) Ethanol, an excellent hydroxyl radical scavenger (25)) had no effect on the quantum efficiency for the Suwannee DOM, thus excluding the possible involvement of this oxidizing species in the chloride production. (4) As in the laser studies, the Fluka humic acid was found to be considerably less reactive than the DOM from natural waters. (5) Electron photoejection from the Suwannee DOM showed little pH effect. The remarkable difference in quantum yields between the laser studies and steady-state irradiations is difficult to explain. Figure 5 illustrates two possible processes that can account for the difference: biphotonic processes or charge recombination within DOM aggregates. In this figure, H and H* represent the DOM in its ground state and excited state, respectively, Ht represents the cation resulting from the electron photoejection, and e- represents the electron in the DOM aggregate prior to escape into bulk solution. The symbols for DOM are used in a generic sense to represent the various DOM chromophores that contribute to electron photoproduction. H' represents recombination products. It is possible that most of the electrons produced by laser photolysis resulted from biphotonic processes that do not occur with lower intensity light. Such processes involve the consecutive absorption of two photons of light by the chromophore followed by ionization to produce an electron (Figure 5). Other studies have shown that such biphotonic processes are important in the laser flash photolysis of organic chemicals such as naphthol (26). In the case of laser photolysis of DOM, however, recent studies by Fischer and co-workers (5)and by Power and co-workers (6)have shown that electron production is linearly related 488
Environ. Sci. Technol., Vol. 21, No. 5, 1987
Monophotonic H
+ light
-
H'
---+
H?
+
H?
-k e-
e-
Biphotonic
H'
+ light
H?
+
H'
e-
H' H?
+
H?
eiq
+
e&
Figure 5. Potentially significant processes in the photoproduction of hydrated electrons from natural organic solutes.
to laser pulse energy. These results indicate that biphotonic processes are not likely to be important with the pulse energies used in the experiments reported here. Another possible explanation involves light intensity effects on the escape of ejected electrons from aggregates of the DOM. Charge recombination within the DOM aggregates (shown as a bar in Figure 5) competes with escape of the electron into bulk solution. Recombination could involve the return of the electron to the atom where it was ejected, or it could involve reaction with other functional groups on the cation. Possibly, the higher concentrations of electrons produced in the laser pulse may cause localized saturation of reducible functional groups within the aggregate, thus permitting more efficient escape of the electrons. Additional studies are required to clarify this point. Environmental Considerations
To estimate values for the sunlight-induced production of hydrated electrons, deaerated solutions (pH 6.2) of the Greifensee and the Suwannee River (SHPO-A) DOM containing sufficient 2-chloroethanol to scavenge all photoejected electrons were exposed to sunlight at 20 "C during mid-April at Athens, GA (latitude 34" N). Mean values for the electron photoproduction ratep k, in these solutions, during daylight and normalized to DOC, were 0.056 f 0.013 pmol of e&/[(mg of DOC) h] for the Greifensee DOM and 0.018 f 0.002 pmol of e&/ [(mg of DOC) h] for the Suwannee River DOM. By comparison, a p nitroanisole actinometer (27) that was simultaneously exposed photoreacted with a rate constant of 0.14 h-l. This actinometer rate constant is within 20% of the mean annual value for the central U S . (latitude 40" N) (28) and the mid-July value for central Europe (latitude 50" N). It should be kept in mind that the production rates may be
susceptible to pH effects, that these rates apply to full sunlight exposure near the surface of a water body, and that the rates are dependent on season and latitude. Hydrogen Peroxide Production. As mentioned above, reactions involving hydrated electrons with dioxygen or with protons can lead to formation of hydrogen peroxide. Assuming nearly quantitative conversion of electrons to hydrogen peroxide, about 0.01-0.03 pmol of H202/ [ (mg of DOC) h] could result via this pathway. By comparison, Cooper and Zika (15) have reported sunlight-induced hydrogen peroxide accumulation rates, normalized to DOC, of 0.2-0.3 pmol of H202/[ (mg of DOC) h] in natural water samples having DOC values in the same range as those of the samples used in this study. This comparison suggests that photochemical processes other than electron photoejection are mainly responsible for mediating the peroxide production in these waters. This conclusion has been reinforced by recent studies of Sturzenegger and Hoign6 (29),wha have shown that high concentrations of electron scavengers (chloroform, nitrate, and nitrous oxide) did not retard the photoproduction of hydrogen peroxide in water from the Greifensee. Further research would be required to firmly establish the generality of this conclusion, however. Pollutant Reductiohs. Many pollutants react very rapidly with hydrated electrons. In this section equations are presented that can be used to estimate rate constants for such photoreductions under environmental conditions. Derivations of these equations are discussed in more detail in the supplementary material (see paragraph at end of paper regarding supplementary material). With conditions normally encountered in a water body, the concentration of natural or man-made chemicals is usually much lower than the concentration of 0,. Hydrated electrons react with O2at a diffusion-controlled rate [k3= 1.9 X 1O1O M-l s-' (12)].This reaction consumes most of the e-aqthat is photoproduced in freshwaters, although scavenging by protons or nitrate can be significant in some water bodies. The steady-state concentration of hydrated electrons is defined by [e&],, = keTe[HI
(2)
where 7, is the lifetime in seconds of the hydrated electron, [HI is the concentration of DOC in milligrams per liter, and k,[H] is the photoproduction rate in moles per liter per second. With O2 as the dominant electron scavenger, 7, = l/k3[O2]. At 20 "C, [O,] = 2.8 X M in air-saturated water, so the lifetime of the hydrated electron re is about 2 X s (5,6). In such a system the reduction of a trace organic chemical P is described by the first-order rate expression -d[Pl/dt = k[e,l,,[Pl
= kJP1
(3)
where k is the second-order rate constant for diffusive reaction of a hydrated electron with the chemical (M-l s-l) and k, is the first-order rate constant for reaction of P. It should be noted that this equation does not describe possible electron photoreductions involving substrates that are complexed or bound by the DOM. Such "static" photoreductions could be important for very hydrophobic nonionic pollutants as well as for cations. For example, Fischer and co-workers ( 5 ) have shown that the cationic herbicide paraquat is rapidly photoreduced by humic substances via such a static process. In the case of the Swiss lake Greifensee, the DOC is about 4 mg/L, and the computed photoproduction rate k,[H) is 0.2 pM h-l (or 5.6 X M 8-l) from sunlightirradiated DOM near the surface during July. By sub-
Table 111. Computed Values for Reductions of Selected Organic Chemicals by Photoejected Hydrated Electrons in Sunlight compound trichloroacetate carbon tetrachloride chloroform trichloroehtylene methyl iodide nitrobenzene
k, M-' s-la 2.1 x 3.1 X 3.0 X 1.9 x 1.7 X 2.8 X
1010
10" 10" 1010
10" 1O'O
k,, h-' 9.4 x 1.4 x 1.3 x 8.5 x 7.6 X 1.3 x
10-4 10-3 10-3 10-4 10-3
a Second-order rate constant for reaction of solvated electron 1.2 with compound (12). bComputed with eq 3, assuming [e;,],, X M, the average value estimated for a Swiss lake (Greifensee) durine Julv. near the water surface.
stitution in eq 2, it can be seen that the average during daylight is 1.2 x M. With eq 3 and values of k from the literature (12),first-order rate constants were computed for several pollutants (Table 111). These rate constants correspond to near-surface half-lives on the order of 1-2 months. Whether such reductions are significant depends on the magnitude of the rates of competing processes such as volatilization, waterborne export, and biodegradation, as well as light attenuation effects in a water body. Conclusions Results presented here provide additional evidence that absorption of near-ultraviolet light induces the ejection of electrons from natural organic solutes in water bodies. As originally postulated by Fischer and co-workers (5),electron photoejection appears to be a general photochemical process exhibited by DOM. Under the conditions used in the laser studies, primary quantum yields for electron ejection at 355 nm (pH 6.2, 25 "C) were clustered in the 0.005-0.008 range for the DOM from natural waters. The efficiency for Fluka humic acid, a commercial substance that has been widely used as a model for aquatic DOM, was significantly lower (0.002). Steady-state irradiations, however, indicated that quantum yields of scavengable electrons in bulk solution are approximately 2 orders of magnitude lower than the primary quantum yields found id the laser experiments. This difference is attributed to less efficient recombination of electrons with cations in the case of the laser experiments. On the basis of steady-state irradiations in sunlight, it is estimated that hydrated electrons are produced at a rate of about 0.04 pmol/[(mg of DOC) h] near the surface of water bodies. This rate of production of electrons corresponds to first-order rate constants for reductions of electron-capturing pollutants that range up to about 0.001-0.002 h-l (half-lives of 1-2 months). These estimates do not apply to pollutants that may be sorbed on the DOM however. Comparison of this rate of electron photoejection to previously published values for hydrogen peroxide photoproduction indicates that the hydrated electron plays only a minor role in forming H202. Acknowledgments We thank LeRoy Ritmiller for his technical assistance in the steady-state experiments, F. Fuchs for the isolation and fractionation of the Greifensee DOM, and M. Gratzel for the opportunity to use the laser photolysis equipment. Supplementary Material Available Derivations of the kinetic equations that describe steady-state photoreactions involving hydrated electrons (3 pages) will appear Environ. Sci. Technol., Vol. 21, No. 5, 1987
489
Environ. Sci. Technol. 1987, 21, 490-494
following these pages in the microfilm edition of this volume of the journal. Photocopiesof the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduction, negatives) may be obtained from Microforms Office, American Chemical Society, 1155 16th St., N.W., Washington, DC 20036. Full bibliographic citation (journal,title of article, authors names, inclusive pagination, volume number, and issue number) and prepayment, check or money order for $10.00 for photocopy ($12.00 foreign) or $10.00 for microfiche ($11.00 foreign),are required. Registry No. Cl(CH2)20H,107-07-3; C,H5N(CH3),,121-69-7; CCl,COZH, 76-03-9; CCl,, 56-23-5; CHC13,67-66-3; ClCH=CClZ, 79-01-6; CH31,74-88-4; C6H5N0,,98-95-3; HzOz,7722-84-1.
F.; Gjessing, E. T., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; pp 107-125. (12) Anbar, M.; Bambenek, M.; Ross, A. B. Natl. Stand. Ref. Natl. Bur. Stand.) 1973, NSRDS-NBS 43. Data Ser. (US., (13) Draper, W. M.; Crosby, D. G. J. Agric. Food Chem. 1983, 31, 734-737. (14)
Draper, W. M.; Crosby, D. G. Arch. Enuiron. Contam.
(15)
Cooper, W.; Zika,'R. G. Science (Washington,D.C.) 1983,
(16) (17)
220, 711-712. Gamble, D. S. Can. J. Chem. 1970,48, 2662-2669. Leenheer, J. A.; Noyes, T. I. U S . Geol. Surv. Water-Supply Pap. 1984, No. 2230.
Toxicol. 1983. 12. 121-126.
Leenheer, J. A.; Brown, P. A.; Noyes, T. I.; Aiken, G., submitted for publication in Enuiron. Sci. Technol. (19) Fuchs, F.; Raue, B. Vom Wasser 1981, 57, 95-106. (20) Jacques, P.; Braun, A. M. Helv. Chim. Acta 1981, 64, (18)
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Received for review August 25,1986. Accepted February 5,1987. Financial support from the President o f the Swiss Federal Institutes of Technology is acknowledged. Mention of trade names or commercial products does not constitute endorsement by the U S . Environmental Protection Agency.
Trace Element Partitioning during the Retorting of Julia Creek Oil Shale John H. Patterson,* Leslie S. Dale, and James F. Chapman CSIRO, Division of Energy Chemistry, Lucas Heights Research Laboratories, Menai, N.S.W., 2234 Australia
A bulk sample of oil shale from the Julia Creek deposit in Queensland was retorted under Fischer assay conditions at temperatures ranging from 250 to 550 "C. The distributions of the trace elements detected in the shale oil and retort water were determined a t each temperature. Oil distillation commenced at 300 "C and was essentially complete at 500 "C. A number of trace elements were progressively mobilized with increasing retort temperature up to 450 "C. The following trace elements partitioned mainly to the oil: vanadium, arsenic, selenium, iron, nickel, titanium, copper, cobalt, and aluminum. Elements that also partitioned to the retort waters included arsenic, selenium, chlorine, and bromine. Element mobilization is considered to be caused by the volatilization of organometallic compounds, sulfide minerals, and sodium halides present in the oil shale. The results have important implications for shale oil refining and for the disposal of retort waters. Introduction
Oil shales often contain relatively high concentrations 490
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of some trace elements that may pose occupational health and environmental pollution problems during processing. The mobilization of trace elements and their distribution during oil shale retorting are therefore important in processing studies, and a number of partitioning studies have been carried out on the retorting of Green River Formation (2-4) and Australian (5) oil shales. The Julia Creek deposit in Queensland, Australia, contains a number of trace elements that are potentially hazardous to the environment and to occupational health. These elements include arsenic, selenium, molybdenum, cadmium, thallium, and uranium (6);those of interest to shale oil refining include vanadium, nickel, iron, and arsenic (5). Preliminary trace element partitioning studies (5) have revealed significant mobilization of arsenic and selenium during retorting under Fischer assay conditions. Subsequently, a definitive study was undertaken of the geochemistry and mineralogical residences of a comprehensive range of trace elements in this oil shale (6). This work, and the availability of a more representative composite sample, provided a favorable opportunity to extend
0013-936X/87/0921-0490$01.50/0
0 1987 American Chemical Society
