40 CFR 795.70 – Indirect photolysis screening test: Sunlight photolysis in waters containing dissolved humic substances
(a) Introduction. (1) Chemicals dissolved in natural waters are subject to two types of photoreaction. In the first case, the chemical of interest absorbs sunlight directly and is transformed to products when unstable excited states of the molecule decompose. In the second case, reaction of dissolved chemical is the result of chemical or electronic excitation transfer from light-absorbing humic species in the natural water. In contrast to direct photolysis, this photoreaction is governed initially by the spectroscopic properties of the natural water.
(2) In general, both indirect and direct processes can proceed simultaneously. Under favorable conditions the measurement of a photoreaction rate constant in sunlight (K
(3) In pure water only, direct photoreaction is possible, although hydrolysis, biotransformation, sorption, and volatilization also can decrease the concentration of a test chemical. By measuring k
(4) Two protocols have been written that measure k
(5) This protocol provides a cost effective test method for measuring k
(6) To correct for variations in solar irradiance during the reaction period, an actinometer is simultaneously insolated. From these data, an indirect photoreaction rate constant is calculated that is applicable to clear-sky, near-surface, conditions in fresh water bodies.
(7) In contrast to k
(8) The value of k
(9) This protocol consists of three separate phases that should be completed in the following order: In Phase 1, SHW is prepared and adjusted; in Phase 2, the test chemical is irradiated in SHW and pure water (PW) to obtain approximate sunlight photoreaction rate constants and to determine whether direct and indirect photoprocesses are important; in Phase 3, the test chemical is again irradiated in PW and SHW. To correct for photobleaching of SHW and also solar irradiance variations, tubes containing SHW and actinometer solutions are exposed simultaneously. From these data k
(b) Phase 1—Preparation and standardization of synthetic natural water—(1) Approach. (i) Recent studies have demonstrated that natural waters can promote the indirect (or sensitized) photoreaction of dissolved organic chemicals. This reactivity is imparted by dissolved organic material (DOM) in the form of humic substances. These materials absorb sunlight and produce reactive intermediates that include singlet oxygen (
(ii) The indirect photoreactivity of a chemical in a natural water will depend on its response to these reactive intermediates, and possibly others yet unknown, as well as the ability of the water to generate such species. This latter feature will vary from water-to-water in an unpredictable way, judged by the complexity of the situation.
(iii) The approach to standardizing a test for indirect photoreactivity is to use a synthetic humic water (SHW) prepared by water-extracting commercial humic material. This material is inexpensive, and available to any laboratory, in contrast to a specific natural water. The SHW can be diluted to a dissolved organic carbon (DOC) content and uv-visible absorbance typical of most surface fresh waters.
(iv) In recent studies it has been found that the reactivity of SHW mixtures depends on pH, and also the history of sunlight exposure (Mill et al. (1983) under paragraph (f)(11) of this section). The SHW solutions initially photobleach with a time-dependent rate constant. As such, an SHW test system has been designed that is buffered to maintain pH and is pre-aged in sunlight to produce, subsequently, a predictable bleaching behavior.
(v) The purpose of Phase 1 is to prepare, pre-age, and dilute SHW to a standard mixture under defined, reproducible conditions.
(2) Procedure. (i) Twenty grams of Aldrich humic acid are added to a clean 2-liter Pyrex Erlenmeyer flask. The flask is filled with 2 liters of 0.1 percent NaOH solution. A stir bar is added to the flask, the flask is capped, and the solution is stirred for 1 hour at room temperature. At the end of this time the dark brown supernatant is decanted off and either filtered through coarse filter paper or centrifuged and then filtered through 0.4)m microfilter. The pH is adjusted to 7.0 with dilute H
(ii) Pre-aging is accomplished by exposing the concentrated solution in the 2-liter flask to direct sunlight for 4 days in early spring or late fall; 3 days in late spring, summer, or early fall. At this time the absorbance of the solution is measured at 370 nm, and a dilution factor is calculated to decrease the absorbance to 0.50 in a 1 cm path length cell. If necessary, the pH is re-adjusted to 7.0. Finally, the mixture is brought to exact dilution with a precalculated volume of reagent-grade water to give a final absorbance of 0.500 in a 1-cm path length cell at 370 nm. It is tightly capped and refrigerated.
(iii) This mixture is SHW stock solution. Before use it is diluted 10-fold with 0.010 M phosphate buffer to produce a pH 7.0 mixture with an absorbance of 5.00 × 10
(3) Rationale. The foregoing procedure is designed to produce a standard humic-containing solution that is pH controlled, and sufficiently aged that its photobleaching first-order rate constant is not time dependent. It has been demonstrated that after 7 days of winter sunlight exposure, SHW solutions photobleached with a nearly constant rate constant (Mill et al. (1983) under paragraph (f)(11) of this section).
(c) Phase 2—Screening test—(1) Introduction and purpose. (i) Phase 2 measurements provide approximate solar photolysis rate constants and half-lives of test chemicals in PW and SHW. If the photoreaction rate in SHW is significantly larger than in PW (factor of >2X) then the test chemical is subject to indirect photoreaction and Phase 3 is necessary. Phase 2 data are needed for more accurate Phase 3 measurements, which require parallel solar irradiation of actinometer and test chemical solutions. The actinometer composition is adjusted according to the results of Phase 2 for each chemical, to equalize as much as possible photoreaction rate constants of chemical in SHW and actinometer.
(ii) In Phase 2, sunlight photoreaction rate constants are measured in round tubes containing SHW and then mathematically corrected to a flat water surface geometry. These rate constants are not corrected to clear-sky conditions.
(2) Procedure. (i) Solutions of test chemicals should be prepared using sterile, air-saturated, 0.010 M, pH 7.0 phosphate buffer and reagent-grade (or purer) chemicals.
(ii) This solution should be mixed 9.00:1.00 by volume with PW or SHW stock solution to provide working solutions. In the case of SHW, it gives a ten-fold dilution of SHW stock solution. Six mL aliquots of each working solution should then be transferred to separate 12 × 100 mm quartz tubes with screw tops and tightly sealed with Mininert valves.
(iii) The sample tubes are mounted in a photolysis rack with the tops facing geographically north and inclined 30° from the horizontal. The rack should be placed outdoors over a black background in a location free of shadows and excessive reflection.
(iv) Reaction progress should be measured with an analytical technique that provides a precision of at least ±5 percent. High pressure liquid chromatography (HPLC) or gas chromatograph (GC) have proven to be the most general and precise analytical techniques.
(v) Sample and control solution concentrations are calculated by averaging analytical measurements for each solution. Control solutions should be analyzed at least twice at zero time and at other times to determine whether any loss of chemical in controls or samples has occurred by some adventitious process during the experiment.
(vi) Whenever possible the following procedures should be completed in clear, warm, weather so that solutions will photolyze more quickly and not freeze.
(A) Starting at noon on day zero, expose to sunlight 24 sample tubes mounted on the rack described above. Tape 24 foil-wrapped controls to the bottom of the rack.
(B) Analyze two sample tubes and two unexposed controls in PW and SHW for chemical at 24 hours. Calculate the round tube photolysis rate constants (k
(C) If less than 20 percent conversion occurs in SHW in 1 day, repeat the procedure for SHW and PW at 2 days, 4 days, 8 days, or 16 days, or until 20 percent conversion is reached. Do not extend the experiment past 16 days. If less than 20 percent photoreaction occurs in SHW at the end of 16 days the chemical is “photoinert”. Phase 3 is not applicable.
(D) If more than 80 percent photoreaction occurs at the end of day 1 in SHW, repeat the experiment with eight each of the remaining foil-wrapped PW and SHW controls. Divide these sets into four sample tubes each, leaving four foil-wrapped controls taped to the bottom of the rack.
(1) Expose tubes of chemical in SHW and PW to sunlight starting at 0900 hours and remove one tube and one control at 1, 2, 4, and 8 hours. Analyze all tubes the next day.
(2) Extimate (k
(3) The rate constants (k
(4) Once (k
(vii) Since the rate of photolysis in tubes is faster than the rate in natural water bodies, values of near-surface photolysis rate constants in natural and pure water bodies, k
(3) Criteria for Phase 2. (i) If no loss of chemical is found in dark control solutions compared with the analysis in tubes at zero time (within experimental error), any loss of chemical in sunlight is assumed to be due to photolysis, and the procedure provides a valid estimate of k
(ii) Rate constants determined by the Phase 2 protocol depend upon latitude, season, and weather conditions. Note that (k
(4) Rationale. The Phase 2 protocol is a simple procedure for evaluating direct and indirect sunlight photolysis rate constants of a chemical at a specific time of year and latitude. It provides a rough rate constant for the chemical in SHW that is necessary for Phase 3 testing. By comparison with the direct photoreaction rate constant, it can be seen whether the chemical is subject to indirect photoreaction and whether Phase 3 tests are necessary.
(5) Scope and limitations. (i) Phase 2 testing separates test chemicals into three convenient categories: “Photolabile”, “photoinert”, and those chemicals having sunlight half-lives in round tubes in the range of 1 hour to 50 days. Chemicals in the first two categories fall outside the practical limits of the test, and cannot be used in Phase 3. All other chemicals are suitable for Phase 3 testing.
(ii) The test procedure is simple and inexpensive, but does require that the chemical dissolve in water at sufficient concentrations to be measured by some analytical technique but not have appreciable absorbance in the range 290 to 825 nm. Phase 2 tests should be done during a clear-sky period to obtain the best results. Testing will be less accurate for chemicals with half-lives of less than 1 day because dramatic fluctuations in sunlight intensity can arise from transient weather conditions and the difficulty of assigning equivalent reaction times. Normal diurnal variations also affect the photolysis rate constant. Phase 3 tests should be started as soon as possible after the Phase 2 tests to ensure that the (k
(6) Illustrative Example. (i) Chemical A was dissolved in 0.010 M pH 7.0 buffer. The solution was filtered through a 0.2 )m filter, air saturated, and analyzed. It contained 1.7 × 10
(ii) The SHW solution of A was photolyzed in sealed quartz tubes (12 × 100 mm) in the fall season starting on October 1. At the end of 1 and 2 days, respectively, the concentration of A was found to be 1.13 × 10
(iii) The tube photolysis rate constant of chemical A was calculated from Equation 2 under paragraph (c)(2)(vi)(B) of this section. The first time point at day 1 was used because the fraction of A remaining was in the range 20 to 80 percent:
(iv) From this value, k
(v) From measurements in pure water, k
(d) Phase 3—Indirect photoreaction with actinometer: Calculation of k
(i) The purpose of Phase 3 is to measure k
(ii) In the case (k
(iii) The actinometer used is the p-nitroacetophenone-pyridine (PNAP/PYR) system developed by Dulin and Mill (1982) under paragraph (f)(5) of this section and is used in two EPA test guidelines (USEPA (1984) under paragraphs (f) (14) and (15) of this section). By varying the pyridine concentration, the PNAP photolysis half-life can be adjusted over a range of several hours to several weeks. The starting PNAP concentration is held constant.
(iv) SHW is subject to photobleaching that decreases its ability to promote indirect photolysis based on its ability to absorb sunlight. This effect will be significant when the test period exceeds a few days. To correct for photobleaching, tubes containing SHW are irradiated in action to the other tubes above.
(v) At any time, the loss of test chemical is given by Equation 8 assuming actinometric correction to constant light flux:
(vi) The indirect photolysis rate constant, k
(vii) To evaluate k
Table 1—Day Averaged Rate Constant (k
Latitude | Season | |||
---|---|---|---|---|
Spring | Summer | Fall | Winter | |
20° N | 515 | 551 | 409 | 327 |
30° N | 483 | 551 | 333 | 232 |
40° N | 431 | 532 | 245 | 139 |
50° N | 362 | 496 | 154 | 6 |
1 k
2 For use in Equation 15 under paragraph (d)(2)(i) of this section.
(viii) To obtain k
(ix) Then, (k
(x) Finally, k
(2) Procedure. (i) Using the test chemical photoreaction rate constant in round tubes, (k
(A) The variable k
(B) The variable k
(ii) Once [PYR] is determined, an actinometer solution is prepared by adding 1.00 mL of 1.0 × 10
(iii) The following solutions should be prepared and individually added in 6.00 mL aliquots to 12/100 mm quartz sample tubes; 8 tubes should be filled with each solution:
(A) PNAP/PYR actinometer solution.
(B) Test chemical in pH 7.0, 0.010 M phosphate buffer.
(C) Test chemcial in pH 7.0, 0.010 M phosphate buffer/SHW.
(D) pH 7.0, 0.010 M phosphate buffer/SHW. Four tubes of each set are wrapped in foil and used as controls.
(iv) The tubes are placed in the photolysis rack (Phase 2, Procedure) at 0900 hours on day zero, with the controls taped to the bottom of the rack. One tube of each composition is removed, along with their respective controls, according to a schedule found in Table 2, which categorizes sampling times on the basis of (k
Table 2—Category and Sampling Procedure for Test and Actinometry Solutions
Category | k | Sampling procedure |
---|---|---|
A | 5.5 J K | Sample at 0, 1, 2, 4, and 8h. |
B | 0.69>k | Sample at 0, 1, 2, 4, and 8d. |
C | 0.17>k | Sample at 0, 4, 8, 16, and 32d. |
(v) The tubes containing PNAP, test chemical, and their controls are analyzed for residual concentrations soon after the end of the experiment. PNAP is conveniently analyzed by HPLC, using a 30 cm C
(vi) If controls are well-behaved and show no significant loss of chemical or absorbance change, then k
(vii) According to Equation 12 under paragraph (d)(1)(vii) of this section, plot the quantities Pn(A
(viii) Then, using Equation 13a under paragraph (d)(1)(vii) of this section, determine the slope (S3) by least squares linear regression. Under the assumptions of the protocol, S3 is equal to (k
(ix) From Equation 18
(x) The indirect photoreaction rate constant, k
(xi) The rate constant k
(xii) Then, (k
(xiii) Finally, k
(3) Criteria for Phase 3. As in Phase 2, Phase 3 tests are assumed valid if the dark controls are well behaved and show no significant loss of chemical. In such a case, loss of test chemical in irradiated samples is due to photoreaction.
(4) Rationale. Simultaneous irradiation of a test chemical and actinometer provide a means of evaluating sunlight intensities during the reaction period. Parallel irradiation of SHW solutions allows evaluation of the extent of photobleaching and loss of sensitizing ability of the natural water.
(5) Scope and limitations of Phase 3 protocol. Test chemicals that are classified as having half-lives in SHW in the range of 1 hour to 50 days in Phase 2 listing are suitable for use in Phase 3 testing. Such chemicals have photoreaction half-lives in a range accommodated by the PNAP/PYR actinometry in sunlight and also accommodate the persistence of SHW in sunlight.
(6) Illustrative example. (i) From Phase 2 testing, under paragraph (c)(6)(iii) of this section, chemical A was found to have a photolysis rate constant, (k
(ii) The actinometer solution was made up by adding a volume of pyridine (1.95 mL) calculated from equation 16 under paragraph (d)(2)(ii) of this section to a 1 liter volumetric flask containing 1.00 mL of 1.00 × 10
(A) Chemical A (1.53 × 10
(B) Chemical A (1.53 × 10
(C) SHW standard solution diluted with water 0.90 to 1.00 to match solution A.
(D) PNAP/PYR actinometer solution. Ten additional foil-wrapped controls of each mixture were taped to the bottom of the rack.
(iii) The test chemical had been placed in category B, Table 2 under the paragraph (d)(2)(iv) of this section, on the basis of its Phase 2 rate constant under paragraph (c) of this section. Accordingly, two tubes of each irradiated solution and two tubes of each blank solution were removed at 0, 1, 2, 4, and 8 days at 1,200 hours. The averaged analytical results obtained at the end of the experiment are shown in the following Table 3.
Table 3—Chemical Analytical Results for Illustrative Example, Phase 3
Day | 105[C]SHW, M | 105[C]W, M | ASHW | 105 [PNAP], M |
---|---|---|---|---|
0 | 1.53 | 1.53 | 0.0500 | 1.00 |
1 | 1.03 | 1.40 | 0.0470 | 0.810 |
2 | 0.760 | 1.30 | 0.0440 | 0.690 |
4 | 0.300 | 1.01 | 0.0370 | 0.380 |
8 | 0.130 | 0.800 | 0.0320 | 0.220 |
(A) From these items the functions Pn(C
Table 4—Photoreaction Function for Illustrative Examples, Phase 3, Derived From Table 3
Day | Pn(C | Pn(C | 1-(A | Pn(Ao | Pn(C |
---|---|---|---|---|---|
0 | 0 | 0 | 0 | 0 | 0 |
1 | 0.396 | 0.0888 | 0.0600 | 0.0618 | 0.211 |
2 | 0.700 | 0.163 | 0.120 | 0.128 | 0.371 |
4 | 1.629 | 0.415 | 0.260 | 0.301 | 0.968 |
8 | 2.465 | 0.648 | 0.360 | 0.446 | 1.514 |
(B) Slope S1 = (k
(C) Slope S2 = (k/k
(D) Using the data in columns 3 and 6 in Table 4 under paragraph (d)(6)(iii)(A) of this section, slope S3 was calculated by regression from Equation 13a under paragraph (d)(1)(viii) of this section and was found to be 0.428 with correlation coefficient equal to 0.99997.
(E) Using Equation 18 under paragraph (d)(2)(ix) of this section, k
(F) The values of S1, S2, and k
(G) The rate constant k
(H) The sum of k
(I) Since k
(e) Data and reporting—(1) Test conditions—(i) Specific analytical and recovery procedures. (A) Provide a detailed description or reference for the analytical procedures used, including the calibration data and precision.
(B) If extraction methods were used to separate the solute from the aqueous solution, provide a description of the extraction method as well as the recovery data.
(ii) Other test conditions. (A) Report the site and latitude where the photolysis experiments were carried out.
(B) Report the dates of photolysis, weather conditions, times of exposure, and the duration of exposure.
(C) If acetonitrile was used to solubilize the test chemical, report the volume percent.
(D) If a significant loss of test chemical occurred in the control solutions for pure water and SHW, indicate the causes and how they were eliminated or minimized.
(2) Test data report—(i) Phase 2 Screening Test under paragraph (c) of this section. (A) Report the initial molar concentration of test chemical, C
(B) Report the molar concentration of test chemical, C
(C) Report the molar concentration of test chemical for each replicate control sample and the mean value for each time point.
(D) Report the values of (k
(E) If small losses of test chemical were observed in SHW and pure water, report a first-order rate constant loss, (k
(F) Report the value of R calculated from Equation 4 under paragraph (c)(2)(vi)(D)(4) of this section.
(G) Report the values of k
(ii) Phase 3—Indirect photoreaction with actinometer. (A) Report the initial molar concentration of test chemical, C
(B) Report the initial absorbance A
(C) Report the initial molar concentration of PNAP of each replicate and the mean value in the actinometer. Report the concentration of pyridine used in the actinometer which was obtained from Equation 15 under paragraph (d)(2)(i) of this section.
(D) Report the time and date the photolysis experiments were started, the time and date the experiments were completed, and the elapsed photolysis time in days.
(E) For each time point t, report the separate values of the absorbance of the SHW solution, and the mean values.
(F) For each time point for the controls, report the separate values of the molar concentrations of test chemical in pure water and SHW, and the absorbance of the SHW solution, and the mean values.
(G) Tabulate and report the following data: t, [C]
(H) From the data in (G), tabulate and report the following data: t, Pn(C
(I) From the linear regression analysis of the appropriate data in step (H) in Equation 17 under paragraph (d)(2)(vi) of this section, report the slope S1 and the correlation coefficient.
(J) From the linear regression analysis of the appropriate data in step (H) in Equation 12 under paragraph (d)(1)(vii) of this section, report the slope S2 and the correlation coefficient.
(K) From the linear regression analysis of the appropriate data in step (H) in Equation 13a under paragraph (d)(1)(viii) of this section, report the slope S3 and the correlation coefficient.
(L) If loss of chemical was observed during photolysis in pure water and SHW, then report the data Pn(C
(M) Report the value of the actinometer rate constant obtained from Equation 18 under paragraph (d)(2)(ix) of this section.
(N) Report the value of k
(O) Report the value of k
(P) Report the value of (k
(Q) Report the half-life, t
(f) References. For additional background information on this test guideline the following references should be consulted.
(1) Cooper W.J., Zika R.G. “Photochemical formation of hydrogen peroxide in surface and ground waters exposed to sunlight.” Science, 220:711. (1983).
(2) Draper W.M., Crosby D.G. “The photochemical generation of hydrogen peroxide in natural waters.” Archives of Environmental Contamination and Toxicology, 12:121. (1983).
(3) Draper, W.M. and Crosby D.G. “Solar photooxidation of pesticides in dilute hydrogen peroxide.” Journal of Agricultural and Food Chemistry, 32:231. (1984).
(4) Draper W.M., Crosby D.G. “Hydrogen peroxide and hydroxyl radical: Intermediates in indirect photolysis reactions in water.” Journal of Agricultural and Food Chemistry, 29:699. (1981).
(5) Dulin D., Mill T. “Development and evaluation of sunlight actinometers.” Environmental Science and Technology, 6:815. (1982).
(6) Haag H.R., Hoigne J., Gassman E., Braun A.M. “Singlet oxygen in surface waters—Part I; Furfuryl alcohol as a trapping agent.” Chemosphere, 13:631. (1984).
(7) Haag W.R., Hoigne J., Gassman E., Braun A.M. “Singlet oxygen in surface waters—Part II: Quantum yields of its production by some natural humic materials as a function of wavelength.” Chemosphere, 13:641. (1984).
(8) Mill T., Winterle J.S., Fischer A., Tse D., Mabey W.R., Drossman H., Liu A., Davenport J.E. Toxic substances process data generation and protocol development. Work assignment 12, test standard development. “Section 3. Indirect photolysis.” Draft final report. EPA Contract No. 68-03-2981. Environmental Research Laboratory, Office of Research and Development, EPA, Athens, GA, and Office of Pollution Prevention and Toxics, EPA, Washington, DC. (1984).
(9) Mill T., Mabey W.R., Bomberger D.C., Chou T.W., Hendry D.G., Smith J.H. “Laboratory protocols for evaluating the fate of organic chemicals in air and water. Chapter 3. Photolysis in water. Chapter 4. Oxidation in water.” EPA 600/3-82-022. Environmental Research Laboratory, Office of Research and Development, EPA, Athens, GA. (1981).
(10) Mill T., Mabey W.R., Winterle J.S., Davenport J.E., Barich V.P., Dulin D.E., Tse D.S., Lee G. “Design and validation of screening and detailed methods for environmental processes. Appendix C. Lower-tier direct photolysis protocol.” Draft final report. EPA Contract No. 68-01-6325. Office of Pollution Prevention and Toxics, EPA, Washington, DC. (1982).
(11) Mill T., Davenport J.E., Winterle J.S., Mabey W.R., Dossman H., Tse D., Liu A. Toxic substances process data generation and protocol development. Work assignment 12. “Appendix B. Upper-tier protocol for direct photolysis in water.” Draft final report. EPA Contract No. 68-03-2981. Environmental Research Laboratory, Office of Research and Development, EPA, Athens, GA, and Office of Pollution Prevention and Toxics, EPA, Washington, DC. (July 1983).
(12) Winterle J.S., Mill T. Toxic substances process data generation and protocol development. Work assignment 18. “Indirect photoreaction protocol.” Draft EPA special report. EPA Contract No. 68-03-2981. Environmental Research Laboratory, Office of Research and Development, EPA, Athens, GA and Office of Pollution Prevention and Toxics, EPA, Washington, DC. (1985).
(13) Mill T., Hendry D.G., Richardson H. “Free radical oxidants in natural waters.” Science, 207:886. (1980).
(14) U.S. Environmental Protection Agency (USEPA), Office of Pollution Prevention and Toxics (OPPT). “Chemical fate test guidelines. Test guideline (CG, CS-6000). Photolysis in aqueous solution.” EPA-560/6-84-003. NTIS publication PB-84-233287. (1984).
(15) USEPA, OPPT. “Chemical fate test guidelines. Test guildeline (CG, CS-6010). Laboratory determination of the direct photolysis reaction quantum yield in aqueous solution and sunlight photolysis.” EPA-560/6-84-003. NTIS publication PB-84-233287. (1984).
(16) Wolff C.J.M., Halmans M.T.H., Van der Heijde H.B. “The formation of singlet oxygen in surface waters.” Chemosphere, 10:59. (1981).
(17) Zepp R.G., Baughman G.L., Schlotzhauer P.F. “Comparison of photochemical behavior of various humic substances in water: I. Sunlight induced reactions of aquatic pollutants photosensitized by humic substances.” Chemosphere, 10:109. (1981).
(18) Zepp R.G., Baughman G.L., Schlozhauer P.F. “Comparison of photochemical behavior of various humic substances in water: II. Photosensitized oxygenations.” Chemosphere, 10:119. (1981).
(19) Zepp R.G., Cline D.M. “Rates of direct photolysis in aquatic environments.” Environmental Science and Technology, 11:359. (1977).
(20) Zepp, R.G., Wolfe N.L., Baughman G.L., Hollis R.C. “Singlet oxygen in natural waters.” Nature, 267:421. (1977).
(21) Zepp R.G., Schlotzhauer P.F., Merritt S.R. “Photosensitized transformations involving electronic energy transfer in natural waters: role of humic substances.” Environmental Science and Technology, 19:74. (1985).