Environ. Sci, Technol. 1982, 16, 609-613
ronmental Hygiene, Delft, The Netherlands, 1979. (13) Rajendran, N. “Theoretical Investigation of Inlet Characteristics for Personal Aerosol Samplers”; Contract No. 210-78-0092, IIT Research Institute, Chicago, IL, 1979. (14) Vitols, V. J. Air Pollut. Control Assoc. 1966, 16, 79-84. (15) Badzioch, S. Brit. J. App. Phys. 1959, 10, 26-32. (16) Zenker, P. Staub-Reinhalt. Luft (English) 1971,31, No. 6, 30-36. (17) Selden, M. G. J. Air Pollut. Control Assoc. 1977, 27, 235-236. (18) Liu, B. Y. H.; Pui, D. Y. H. Atmos. Environ. 1981, 15, 589-600, (19) Ogden, T. L.; Birkett, J. L. Ann. Occup. Hyg. 1978, 21, 41-50. (20) Ogden, T. L.; Wood J. D. Ann. Occup. Hyg. 1975, 17, 187-195. (21) Steen, B. Ph.D. Thesis; Chalmers University of Technology, Goteborg, Sweden, 1975. (22) Sehmel, G. A. Ann. Occup. Hyg. 1967,10,73-82. (23) Sehmel, G. A. Am. Ind. Hyg. Assoc. J. 1970531,758-771. (24) Raynor, G. S. Am. Ind. Hyg. Assoc. J. 1970,31,294-304. (25) Breslin, J. A,; Stein, R. L. Am. Ind. Hyg. Assoc. J. 1975, 36,576-583, (26) Pattenden, N. J.; Wiffen, R. D. Atmos. Environ. 1977,11, 677-681. (27) Wedding, J. B.; McFarland, A. R.; Cermak, J. E. Environ. Sci. Technol. 1977, 11, 387-390. (28) Wedding, J. B.; Weigand, M.; John, W.; Wall, S. Environ. Sci. Technol. 1980,14,1367-1370. (29) Vincent, J. H.; Gibson, H. Atmos. Environ. 1981, 15, 703-712. (30) Tufto, P. Ph.D. Thesis, University of Cincinnati, Cincinnati, OH, 1981. (31) Use of HEPA filters in this arrangement wm recommended by J. B. Wedding, Civil Engineering Department, Colorado State University, Fort Collins, CO. (32) Berglund, R. N.; Liu, B. Y. H. Enuiron. Sci. Technol. 1973, 7,147-153. (33) Raabe, 0. G.; Newton, G. J. “Development of Techniques for Generating Monodisperse Aerosols with the Fulwyler Droplet Generator”; Report LF-43, Lovelace Foundation, Albuquerque, NM, 1970; pp 13-17.
Results and Discussion Figure 3 exemplifies our method by showing measured sampling efficiencies for a thin-walled tube operated at 30” downward from the flow direction. Oleic acid particles from 5 to 40 pm in diameter were used as test aerosols. The relative sampling efficiency of the inlet for various velocity ratios and 0 = 30” is shown in the upper graph. The transmission efficiency for isokinetic flow is shown in the middle graph. The overall sampling efficiency is the product of the two efficiencies and is shown in the lower graph. The low degree of data scatter, exemplified in Figure 3 and noted in all our experiments to date (30),as well as the large body of data that one can obtain for a specific inlet in relatively short time, will hopefully facilitate the development of analytical models for the prediction of the overall sampling efficiency. Such a model should include aspiration of particles to the face of the inlet, bounce of particles from the edge of the inlet into or away from the inlet, impaction onto the inner wall of the inlet just past the inlet face, gravitational settling in the inlet, and turbulent or laminar deposition inside the inlet. Literature Cited (1) Davies, C. N. Staub-Reinhalt. Luft (English) 1968, 28, NO. 6 , 1-9. (2) Davies, C. N.; Subari, M. “Inertia Effects in Sampling Aerosols”; Proc. Advances in Particle Sampling and Measurement, EPA Report 600/7-79-065; February 1979; pp 1-29. (3) Jayasekera, P. N.; Davies, C. N. J. Aerosol Sci. 1980,11, 535-547. (4) Fuchs, N. A. Atmos. Environ. 1975, 9, 697-707. (5) Belyaev, S. P.; Levin, L. M. J.Aerosol Sci. 1972,3,127-140. (6) Belyaev, S. P.; Levin, L. M. J. Aerosol Sci. 1974,5,325-338. (7) Durham, M. D.; Lundgren, D. A. J. Aerosol Sci. 1980,11, 179-188. (8) Agarwal, J. K.; Liu, B. Y. H. Am. Ind. Hyg. Assoc. J. 1980, 41, 191-197. (9) Bien, C. T.; Corn, M. Am. Ind. Hyg. Assoc. J. 1971, 32, 453-456. (10) Zebel, G. In “Recent Developments in Aerosol Science”; Shaw, D. T., Ed.; Wiley: New York, 1978; pp 167-185. (11) Kaslow, D. E.; Emrich, R. J. “Particle Sampling Efficiencies for an Aspirating Blunt Thick-Walled Tube in Calm Air”; Technical Report No. 25, Department of Physics, Lehigh University, Bethlehem, PA, 1974. (12) ter Kuile, W. M. “Comparable Dust Sampling a t the Workplace”; Report F 1699; Research Institute for Envi-
Received for Review August 6,1981. Revised nanwcript received March 11,1982. Accepted April 19,1982. This work was supported by the U.S. National Institute for Occupational Safety and Health under Grant No. OH 00774. P.A.T. was financially supported by the Norwegian Institute of Technology and the Royal Norwegian Council for Scientific and Industrial Research. Some assistance was provided by funds from Center Grant USPHS ES 00159. W e thank A . Fodor and J.Svetlik for their technical assistance.
Retention of Boron by Coal Ash Gordon K. Pagenkopf * and Joan M. Connolly Department of Chemistry, Montana State university, Bozeman, Montana 597 17
rn Adsorption of boron by the hydrous oxides of aluminum,
iron, and silicon appears to be the major process that controls the net release of boron from fly ash when the ash is leached at loading rates of greater than 25 g of ash/L. Coprecipitation of borate species with the hydrous oxides also contributes to the retention. The amount of coal ash obtained from the combustion of a selected but representative group of United States coals ranges from 4% to 22% ( I ) . The major components of the ash include the alkali and alkaline earth oxides, 00 13-936X/82/09 16-0609$01.25/0
main-group oxides of aluminum and silicon, and iron oxide ( I , 2). The trace components are many ( I , 3 ) ,with some of the elements being enriched (3). In general, the highest concentrations of trace and toxic species are found to be associated with the smallest particles (3, 4). One of the trace components in coal ash of special concern is boron. The amount of boron found in coal ash varies from 5 to 200 mg/kg, depending upon the mine site (5). Of the total boron present in the coal as much as 71% may be lost to the atmosphere upon combustion (6),and of that found in the ash a sizable fraction (>50%) is readily
@ 1982 American Chemical Society
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water soluble (7, 8). Environmental concerns involving boron are primarily associated with the quality of irrigation waters. Boron is an essential micronutrient (9);however, when concentrations exceed 2 mg/L, sizable plant toxicity may be observed (10). The release of boron from coal ash to leachate waters is dependent upon contact time, pH of the leachate water, ash particle size, and ratio of ash to leachate water (8, 11-13). The last factor is the subject of this report. It is observed that the rate of boron released decreases as the ash to leachate water is increased (8). There are at least three possible physical-chemical reactions that can influence the release of boron from coal ash. These are (1) adsorption of boron by hydrous oxides, (2) solubility of metal borates, and (3) formation of surface coatings that seal the boron within the ash particles. Experiments have been designed to assess the relative contribution of each.
Experimental Section The coal ash used in this study was an upper ash obtained from the Boundary Dam Power Station near Estevan, Saskatchewan, Canada. All utilized ash passed through 325 mesh screen (particle size less than 45 pm). Total analyses for major component cations provided 60.3 mg of Na/g, 3.9 mg of K/g, 64.8mg of Ca/g, and 23.9 mg of Mg/g. For the experiments that investigated the adsorption of boron by conditioned ash, the ash was conditioned by washing three times with distilled water at a loading of 10 g of ash/L. This removed more than 90% of the soluble boron present initially in the ash. The isothermal adsorption experiments utilized ash as received. For the experiments where the ash was coated with aluminum hydroxide, the procedure was as follows: 10 g of ash was added to 100 mL of 1000 mg/L of A13+solution, and the pH was rapidly adjusted to 11with NaOH. The slurry was allowed to sit for 5 days in the presence of N2, filtered through a 0.45-pm filter, and air-dried. The filtrate was analyzed for boron to maintain mass balance. Adsorption experiments were initiated by adding the desired amount of boric acid solution to an ash-water slurry. In all cases doubly distilled water was utilized and 0.001 M tris(hydroxymethy1)aminomethane was utilized to maintain constant pH. The pH values were checked regularly, and if small changes were observed the pH was adjusted to the predetermined value with NaOH or HC1. For the isothermal adsorption experiment a slurry of 12 g of ash/L was prepared and an aliquot withdrawn for boron analysis. The slurry was then placed in a 40 "C oven. Loss of water was monitored by weight loss, and at convenient intervals aliquots were withdrawn for boron analysis. The experiments that investigated coprecipitation of boron with iron or aluminum and a soil support involved mixing the reagents and slowly adjusting the pH to 8.6 with dilute NaOH. Aliquots were then withdrawn and analyzed. Boron was determined spectrophotometrically by using the curcumin method (14) with color development at 75.0 "C. Utilization of this temperature speeds color development and provides a standard curve reproducibility of f l % in the 0.5 mg of B/L range. Metals were determined by atomic absorption; pH was measured by using glass SCE electrodes and a Radiometer M-26 pH meter. Reagent grade chemicals were utilized in all cases.
Results The release of boron from fly ash to leachate water is found to be dependent upon a variety of conditions (7,8, 610
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4oo 300
4 0
40
eo
120
160
mg/ I B a d d e d
Flgure 1. Adsorption of boron by conditioned fly ash, pH 8.6, 0.55 g of ash, total volume 10.0 mL: a contact time of 9 days (0)and a contact time of 1 day (*).
11,13). One of the more important is the ratio of ash to leachate water. For example, leaching at a loading of 1g of ash/L of distilled water resulted in the release of more than 90% of the water-soluble boron present in the ash. When the ash to leachate water ratio is increased to 50 g of ash/L, approximately 40% of the boron is released; when the ratio is increased to 100 g of ash/L, less than 30% is released. Contact of western coal fly ash with water causes a rapid rise in solution pH. This results from the hydrolysis of the basic oxides such as CaO and Na20. As the pH of the solution rises, the hydration of the other oxides such as A1203, Fe203,and Si02is facilitated. The solubility of these materials greatly increases when the pH exceeds 11,and in addition the surface of the particles at this pH may assume a negative charge through a reaction of the type =%OH + OH-- ~ s - 0 -+ H2O (1)
where ES represents a surface element. Charge neutralization of the surface may be accomplished through a reduction in pH with concomitant protonation or adsorption of a cation such as Na+ or Ca2+. The adsorption of borate and boric acid by a variety of hydrous oxide surfaces has been reported (11,15-24). The adsorption of boron by conditioned fly ash is shown in Figure 1. From Figure 1it is apparent that more boron adsorbs with increased contact time, and an adsorption maximum is being approached. It is possible that conditioning causes an alteration of the surface characteristics of freshly hydrated fly ash. To test this possibility, we conducted isothermal adsorption experiments whereby the ash to leachate ratio was increased through the constant-temperature (40"C) evaporation of water. The results are shown in Figure 2. If all of the soluble boron in the ash were being released, the solution concentration would increase in a linear manner as the loading rate increases. The slope should be consistent with 0.5 mg of soluble boron per gram of ash. This is demonstrated by curve a in Figure 2. What is observed is a retention of boron by the ash, and this is shown by curves b and c in Figure 2. When this experiment is conducted the other way, by keeping the volume constant and increasing the amount of ash, boron is also retained. This is shown in Figure 2 by curve d. These observations indicate that a competition exists between the boron release and retention processes. The net release of boron as a function of ash loading is shown in Figure 3. These data points were derived from the data shown in Figure 2, curves b and c, by using the observed solution concentrations and calculating the amount of boron released by 1g of ash. When the loading rate is less than 25 g of ash/L, boron release is dominant, whereas at greater loading rates retention dominates.
~
Table I. Coprecipitation of Boron with Iron and Aluminum: Total Volume 100.0 mL; 5.5 g of Bozeman Silt Loam Support; pH 8.6 B Al Fe added, added, added, Bobsd, Bppt, mg mg mg mg/L mg 0.00 0.04 2.50 22.5 0.25 0.96 2.54 104 15.8 2.41 9.9 19.9 0.52
.
-
rn
E
m
IO
0 40
0
00 g
120
160
ash/ I
Figure 2. Retention of boron by fly ash as a function of the ash/ leachate ratio: (a) expected boron concentration if none was retained, (b) and (c) boron retained in isothermal evaporation experiments, initial concentration 12 g/L, and (d) boron retalned in batch leaching experiments. 500
I 1
m
n \
400:
-1
\
\
-I
o \
m
l
41
\
2t
\
0
O L 5 6
.
1
\ 0
\ m
7
8
9
1
0
1
\
1
PH
Flgure 5. Precipitation of boron by aluminum hydroxide as a function of pH.
i
l o o l l I /:
0
l o0 00 Y y l 0
40
g
80 ash11
120
160
Figure 3. Net release of boron from 1 g of ash as a function of ash to leachate ratio. 500
m
zoo
0
1
i
I
Reports in the literature indicate that borate (boric acid) forms an insoluble species with aluminum, and thus it is possible that isomorphic precipitation could provide a route for the transfer of soluble boron to the solid phase. This degree of precipitation of borate by iron and aluminum in the presence of a well-characterized soil is shown in Table I. Precipitation was achieved by first preparing the soil suspension, adding boron, iron or aluminum, and adjusting the pH with NaOH. The data for the precipitation of boron by aluminum hydroxide as a function of pH are shown in Figure 5. For these solutions sufficient A13+, boron, and sodium hydroxide were mixed together to provide [All, = 1.0 g/L, [BIT = 25.5 mg/L, and the desired pH value. After 96 h the solutions were analyzed for boron content. The boron precipitated is the difference between that found in solution and the total present initially.
Discussion A precise description of the surfaces of fly ash particles once they have contacted water is probably not possible; however, there are sufficient studies to provide an indication of what could be present. Contact with water results in rapid hydrolysis of the basic oxides, CaO, MgO, Na20, and K20. This results in a rapid rise in pH, and a majority of the solubilized materials may be attributed to these salts. Concurrent with the dissolution of these oxides is the partial solubilization of the oxides of iron, aluminum, and silicon. Since extreme pH values are needed to completely dissolve these metals, the concentrations of these metals in solution are not expected to be high; however, this does not preclude a net migration of these hydrated metal oxides to the surface layers of the ash particles. In addition, the presence of these species will provide the particles with a net negative charge. An increase in pH also facilitates the hydrolysis of boron oxides. Solution speciation of boron in fly ash leachate waters will be Environ. Sci. Technol., Vol. 16, No. 9, 1982
611
dominated by B(OH)4- and H,BO, since the polymeric species do not form until the concentration of total boron is near 0.6 M (25). Thus, the physical-chemical reactions that regulate the concentration of boron in solution will primarily involve these two species. Adsorption of boron by layer silicates increases as the pH increases, reaching a maximum near pH 9 (19). The adsorption capacity of these soil materials is small when compared to that observed when the soils are coated with aluminum and iron oxides (20, 21). For example, the amount of boron adsorbed by kalonite increases from approximately 80 to 525 pg of B/g at pH 9 when the soil is coated with 9.3% A1203 (21). The coal ash utilized in this study was observed to retain approximately 400 pg of B/g (8). The data shown in Figure 1 indicate that between 50 and 225 pg of B can be adsorbed per gram of conditioned fly ash. The role of conditioning is difficult to assess, but it is possible that the hydrous oxide surface initially established “aged” and, as a consequence, exhibits a decreased adsorption capacity. This behavior has been observed in studies of boron adsorption by freshly precip’ltated Al(OH),(s) (18). The magnitude of the decreased adsorption capacity is sizable. For example, when the concentration of boron is 5 mg/L, the Al(OH),(s) adsorption capacity decreases from 2.4 to 0.4 mg of B/g as the equlibration period is increased from 20 min to 7 days. The data in Figure 2, curves b and c, indicate that the fly ash is capable of retaining a net fraction of the total soluble boron that is available. When the loading ratio is 75 g of ash/L or greater, maximum retention capacity per gram has been obtained and thus the limiting slopes for curves b and c. Comparison of curves b and c with curve a provides graphic illustration of how effectively the boron is retained by the ash as the ash to water ratio increases. Curve d presents the data obtained in batch leaching experiments. It is evident that similar behavior is observed in both types of experiments; however, the adsorption process in the isothermal evaporation experiment is not as pronounced. Comparison of the data at an ash to leachate ratio of 60 g/L indicates that 325 pg of boron are retained per gram of ash for the batch experiment, and from curves b and c one calculates 233 and 267 pg/g, respectively. These are not the maximum retention values, but they do indicate the relative contribution. These values are on the low side when compared to values ranging from 320 to 2038 pg/g observed for volcanic soils (17). For these soils there is a significant correlation between the amount of boron adsorbed and the amount of aluminosilicate, “allphane”, present in the soils. The adsorption increases from 525 of boron by kalonite coated with A1203 to 925 pglg as the percent A1,0, increases from 9.3% to 16.7% (21). The fly ash retention values are higher than those observed for noncoated soils (18, 19). The rate of boron released from the ash as a function of the ash to leachate ratio is shown in Figure 3. Competition between two processes, leaching and adsorption, is indicated with adsorption becoming dominant at the higher loading rates. In coal ash disposal systems where the moisture content will be in the 20-40% range, one predicts that adsorption will dominate, and thus many pore volumes of water will have to flow through the system to leach out the soluble boron. The release of boron from aluminum coated and noncoated ash is presented in Figure 4. The parallel nature of the curves indicates that coating of the particles does not significantly retard the release of boron from the ash material. 612
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Coprecipitation of boron with aluminum and iron hydroxides could provide a convenient route for the transfer of boron from the solution phase to solid phase. There are sufficient similarities between the species to propose isomorphic substitution of B(OH), for Al(OH),, Fe(OH),, or Si(OH)4 in the hydrous oxide network. The degree to which boron can be precipitated by aluminum hydroxide is shown in Figure 5. The decrease in precipitation effectiveness as the pH increases parallels the decrease in adsorption capacity of Al,O, (21). Values listed in Table I indicate that iron is a more effective boron precipitation agent than aluminum; however, this phenomenon is certainly surface area related, and thus these experiments are probably not comparable. The amount of boron removed per gram of hydroxide is much greater than that observed in the adsorption experiments. For example, at pH 8.6, aluminum can precipitate 9230 pg of B/g of Al, which is at least a factor of 4 greater than that observed in the adsorption experiments (15, 18, 21).
Summary Adsorption of boron by the aluminosilicate matrix appears to be the major process for the retention of boron by coal ash particles. Hydrous oxide coatings do not appear to retard the release of boron, and coprecipitation of boron with hydrous oxides appears to assist adsorption in the retention of boron by hydrated fly ash.
Literature Cited Furr, A. K.; Parkinson, T. F.; Hinricks, R. A.; Vancampen, D. R.; Bache, C. A.; Gutenmann, W. H.; St. John, L. E., Jr.; Pakkala, I. S.; Lisk, D. J. Enuiron. Sci. Technol. 1977, 11, 1194. Green, J. B.; Manahan, S. E. Anal. Chem. 1978,50,1975. Coles, 0. G.; Ragaini, R. C.; Ondov, J. M.; Fisher, G. L.; Silberman, D.; Prentice, B. A. Enuiron. Sci. Technol. 1979, 13, 455. Smith, R. 0.; Campbell, J. A.; Nielson, K. K. Enuiron. Sci. Tecnol. 1979, 13, 553. Ruch, R. R.; Gluskoter, H. J.; Shimp, N. F. Environmental Geology Notes, No. 72, Ill. St. Geol. Survey, August 1974. Gladney, E. S.; Wangen, L. E.; Curtis, D. B.; Jurney, E. T. Enuiron. Sci. Technol. 1978. 12. 1084. Cox, J. A.; Lundquist, G. L.: Przyjazny, A.; Schmulbach, C. D.; Enuiron. Sci. Technol. 1978, 12, 722. Halligan, A. S.; Pagenkopf, G. K. Enuiron. Sci. Technol. 1980, 14, 995. Donald, C. M.; Prescott, J. A. In “Trace Elements in Soil-Plant-Animal Systems”; Nichols, D. J. D., Egan, A. R., Eds.; Academic Press: New York, 1975; pp 7-37. Wilcox, L. V. “Determining the Quality of Irrigation Water”; Agricultural Information Bulletin, No. 197, 1958; pp 1-6. Talbot, R. W.; Anderson, M. A,; Andren, A. W. Enuiron. Sci. Technol. 1978, 12, 1056. Dressen, D. R.; Gladney, E. S.; Owens, J. W.; Perkins, B. L.; Wienke, C. L.; Wangen, L. E. Enuiron. Sci. Technol. 1977,11, 1017. Elseewi, A. E.; Page, A. L.; Grimm, S. R. J. Enuiron. Qual. 1980, 9, 424. “Standard Methods for the Examination of Water and Wastewater”, 13th ed.; American Public Health Association: Washington, D.C., 1971. Choi, W. W.; Chen, K. Y. Environ. Sci. Technol. 1979,13, 189. McPhail, M.; Page, A. L.; Bingham, F. T. Soil Sci. SOC.Am. Proc. 1972, 36, i l 0 . Schalscha, E. B.; Bingham, F. T.; Galindo, G. G.; Galvan, H. P. Soil Sci. 1973, 116, 70. Hatcher, J. T.; Bower, C. A.; Clark, M. Soil Sci. 1967,104, 422. Sims, J. R.; Bingham, F. T. Soil Sci. SOC.Am. Proc. 1967, 31, 728.
Environ. Sci. Technol. 1982, 16, 613-616
Sims, J. R.; Bingham, F. T. Soil Sei. SOC.Am. Proc. 1968, 32, 364. Sims, J. R.; Bingham, F. T. Soil Sci. SOC.Am. Proc. 1968, 32, 369. Keren, R.; Gast, R. G.; Bar-Yosef, B. Soil Sei. SOC.Am. J . 1981, 45, 45. Keren, R.; Gast, R. G. Soil Sei. SOC.Am. J . 1981,45, 478. Mezuman, U.; Keren, R. Soil Sei. SOC.Am. J . 1981,45,722.
(25) Adams,R. M., Ed. “Boron, Metallo-Boron Compounds and Boranes”; Interscience: New York, 1964.
Received for review June 30,1981. Revised manuscript received April 9,1982. Accepted May 3, 1982. Appreciation is extended to the United States Environmental Protection Agency for partial support of this project through Grant R-805935.
Fluorescence Polarization Studies of Perylene-Fulvic Acid Bindingt Paul M. Roemelt* and W. Rudolf Seltz”
Department of Chemistry, University of New Hampshire, Durham, New Hampshire 03824
w When fulvic acid is added to perylene solutions, perylene fluorescence becomes more polarized. By measuring perylene polarization as a function of added fulvic acid, we have determined an equilibrium constant of 1.5 X lo6 for the binding of perylene to fulvic acid in 75% glycerol. In 40% glycerol perylene forms aggregates. From fluorescence polarization and scattering data, we show that the addition of fulvic acid causes perylene aggregates to disperse into smaller units. However, fulvic acid does not completely solubilize perylene in 40% glycerol, even when present in large excess. The interaction of perylene and fulvic acid in water is thought to be similar to the behavior in 40% glycerol. The interaction of perylene and fulvic acid is the same within experimental error at pH 2,7, and 11in both 40% and 75% glycerol. This study illustrates how fluorescence polarization may be used to follow the interaction of natural organic substances with organic pollutants. Fluorescence polarization is a particularly attractive method for this type of study because it can distinguish free and bound organic substances without a separation.
Introduction Humic substances have been the subject of considerable interest because they can interact with pollutants in soil and water. In particular, the nature and strength of metal complexes with humic substances have been studied because the formation of these complexes may influence the transport and fate of heavy metals. Humics can also interact with organics. The adsorption of s-triazines onto soil humic matter has been widely studied because soil humics modify the herbicidal activity of s-triazines (1-9). Other compounds that have been shown to interact with humic substances include DDT (10-12), bipyridilium herbicides (13-14), PCBs (15),carbaryl and parathion (16, 17))cationic dyes (18))2,4-D and picloran (19),and linuron and malathion (20). Various methods have been used to study the interaction of organic compounds with humics. The most common approach has been to add a known amount of the organic compound to a suspension of undissolved humics. After equilibration, the suspended material is removed by centrifugation, and the amount or organic compound remaining in the supernatant is measured. Free and bound organic compound have also been distinguished by gel chromatography (14) and solvent extraction (2). Most Contribution No. UNH-SG-JR-137. Knolls Atomic Power laboratory, General Electric Co., Schenectady, NY 12301
* Present address:
0013-936X/82/0916-0613$01.25/0
studies have measured adsorption isotherms although it has been shown that dissolved humics substantially increase the solubility of otherwise highly insoluble organic pollutants (10). The interaction of dissolved humics with organic pollutants is a matter of considerable importance. By significantly enhancing the solubility of organic pollutants, humics may modify their transport as well as influence degradation processes. This interaction is, however, quite difficult to study. Methods involving a physical separation of free and bound organic compound are both tedious and highly subject to error due to adsorption of the organic compound to container surfaces. The primary purpose of the work reported here is to demonstrate the use of fluorescence polarization to study the interaction of organic pollutants with humic substances. Fluorescence polarization offers two major advantages for this type of study. It greatly reduces the probability of error because it can distinguish free and bound organic compound without a separation. Also, because of the sensitivity of fluorescence, it is possible to make measurements at the low concentration ranges that occur naturally. Fluorescence polarization has been used to distinguish adsorbed and dissolved polynuclear aromatic hydrocarbons (21) and to study the conformation of soil fulvic acid (22). However, it has not otherwise been applied to environmental chemistry even though it is widely used in the study of proteins and synthetic polymers. The principles of fluorescence polarization measurements are reviewed elsewhere (23-25). A fluorophor is excited with polarized light. Fluorescence intensities are measured in the planes parallel and perpendicular to the excitation radiation, I and I, , respectively. The polarization, p , is calculatedl from these values:
In the absence of depolarizing processes the value of p depends on the orientation of the moments ofdhe transitions involved in excitation and emission. Values range from l/z to -1/3. Under the conditions of this study fluorescence is depolarized by molecular rotation during the lifetime of the excited state, causing a decrease in the measured value of p . The extent of rotational depolarization depends on several factors, including temperature, viscosity, fluorescence lifetime, and molecular size and shape. When a fluorophor is bound to another molecule, its effective size increases and it rotates more slowly. Thus, the polarization value for the bound fluorophor is higher than for the free
0 1982 American Chemical Society
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