Contamination from Fluorocarbon Films William A. Lonneman, Joseph J. Bufalini," Richard L. Kuntz, and Sarah A. Meeks U.S. Environmental Protection Agency, Environmental Sciences Research Laboratory, Research Triangle Park, North Carolina 277 11
Fluorinated ethylene-propylene copolymer (FEP) plastic is often used for handling air samples for analyses and in smog-chamber studies. However, this film was found to liberate high concentrations of contaminants such as fluorocarbons, solvents, antioxidants, and manufacturing residues. These contaminants could interfere with both hydrocarbon analyses and smog-chamber reactivity studies. Heat treatment of FEP plastic film a t 190 "C for 24 h minimized film outgassing but did not eliminate it. Less outgassing occurred with 2-mil film than with 5-mil film. However, bags prepared from 2-mil film were more fragile than those prepared from 5-mil film. Introduction Materials made of fluorinated hydrocarbon polymers, known as Teflon and Tedlar (registered trademarks of E. I. Du Pont de Nemours and Co., Wilmington, DE), are used extensively in air-pollution studies. Because of the inert nature of these materials, they seem ideal for use as sample containers and for collection and storage of both reactive and unreactive pollutants found in ambient air. For the same reasons, they are often used in smog chambers, as liners for metal surfaces or as collapsible bag-type chambers. During a 1974 field study, ambient-air samples were collected in Tedlar (polyvinyl fluoride, PVF) bags for totalhydrocarbon analysis ( 1 ) .When the air in these bags was analyzed, unusual characteristics of the hydrocarbons found suggested bag contamination. These studies showed that the total nonmethane hydrocarbon (TNMHC) increased rather than decreased during an irradiation experiment. Also, when the air samples were spiked with a reactive hydrocarbon, no effect was noted on ozone production. This contamination was found to account for 10-30% of the total nonmethane hydrocarbon response. In the same study, the Tedlar bags were used as smog chambers to determine the reactivity of ambient air. I t was later learned from the manufacturer that the solvent N , N -dimethylacetamide is used in the extrusion process to produce Tedlar film. Chromatographic retention time suggested that the most abundant peak represented this compound. It was suggested that most of the contamination peaks observed were due to the solvent and its impurities. Efforts to clean or condition these bags by heat treatment and solvent washing procedures were unsuccessful. Thus, it was concluded that Tedlar cannot be used as a smog-chamber material ( 11. Teflon (fluorinated ethylene-propylene copolymer, FEP) is another plastic often used in air-pollution studies. The manufacturing process for Teflon is quite different from that for Tedlar. Because no solvent is used in the extrusion process for Teflon, sample contamination from the film surface would seem much less likely than with Tedlar. These investigations included two distinct types of studies: storage studies and irradiation studies. The purpose of the storage studies was to find a sample container to replace the 2-mil Tedlar PVF bags reported to have serious outgassing contamination problems ( 1 ) . Teflon FEP, of 2- and 5-mil thicknesses, was selected because of its inertness and similarity to the Tedlar PVF material. The irradiation studies were to determine the effect of outgassing contamination on the formation of ozone when bags made of Teflon FEP film were used in photochemical smog-chamber studies.
Experimental Section Bag containers were prepared from 2- and 5-mil Teflon FEP Type A film. Bag sizes were 15-40 L for the storage studies and 100 L for the irradiation studies. The bags were prepared as follows. A folded sheet of Teflon FEP film was heat sealed on two sides with a water-cooled impulse heat sealer (Vertrod). A 5 X 5-cm piece of polyethylene tape was placed at the center of one of the outside surfaces of the bag. A 0.95-cm hole was made through the polyethylene tape and Teflon FEP film. A bag valve consisting of an O-ring adapter (Swagelok Co.) was placed in the hole and fitted with a 1.1-cm 20-thread Teflon nut. The remaining open side was heat sealed, and the bag was tested for leaks. The Teflon FEP film was heat treated by placing the bags sealed on three sides in a temperature-controlled oven at -190 "C and flushing the surfaces with a stream of prepurified nitrogen for periods of 24-52 h. The bags were then removed and fitted with valves, and the open sides were heat sealed. The bags were irradiated in a temperature-controlled 1.2 X 1.8 X 1.5-m chamber containing two rows of 7 1-m Westinghouse sunlamps (E,,, = 3100 A) and 11 1-m General Electric black lights (E,,, = 3600 A). The first-order dissociation constant Izl for NO2 served as the measure of the photon flux. This value was 0.30 min-l. Identification of the outgassing contamination was attempted by cryogenic preconcentration of the sample followed by gas-chromatographic analysis ( 2 , 3 ) . Irradiation studies were conducted for Teflon FEP bags filled with hydrocarbon (HC)-free air spiked with NO, and propylene-NO, mixtures. Clean air was produced by passing outside air through a high-temperature oxidizing catalyst and then through chemical filters to remove any NO, produced by the high-temperature treatment. Air thus treated usually contained less than 10 ppbC hydrocarbon, of which most was usually methane. Less than 1-2 ppb NO, was present (below instrument sensitivity). In the irradiation studies, 0 3 and NO, concentrations were measured with commercial chemiluminescent instruments. Results and Discussion
Storage Studies. Two patterns of outgassing contamination were observed during studies of clean-air storage in 5-mil Teflon F E P bags. The first pattern, observed from freshly prepared bags filled with HC-free clean air, was of large contamination peaks appearing within a few hours of storage. This contamination was as great as 4500 ppb of equivalent carbon response, as shown in Table I. More than 90% of this total carbon response was due to compounds that appeared at early retention times on the gas chromatographs (see Figure 1). The concentration of these compounds increased with storage time; however, as shown in Table I, the magnitude of this maximum differed from bag to bag. Because these peaks were quite large (with a single peak as large as 3000 ppbC response), attempts were made to identify these compounds through gas-chromatographic-massspectrometric (GC/MS) techniques. The largest peak was identified as perfluoropropylene, one of the two starting materials in the production of Teflon FEP. The other gas-chromatographic peaks shown in Figure 1are not the compounds listed since their elution times are not identical with the hydrocarbons. No definite structure could be assigned to the other compounds; however, the outgassing contaminations
This article not subject to U.S. Copyright. Published 1981 American Chemical Society
Volume 15, Number 1, January 1981
99
~~
~
Table 1. Total Contamination Levels Exceeding 1000 ppbC Response for Clean-Air Storage in Previously Unused 5-mil Teflon Bags date
storage t h e , h
storage temp, “C
61 18/75
24
10-35
concn, ppbC
2500
6r2or75 6/23/75 6I 28I 75
96 72 72
24 24 10-35
1300 4500 3000
HEXAFLUOROPROPE8
ISOBUTANE
I
ACETVI
I
RETENTION TIME
Figure 1. Gas chromatogram of a 500-cm3sample from a 5-mil Teflon bag filled with HC-free air and stored for 24 h. The column was 1.8 m X 0.32 cm stainless steel packed with 60-80 mesh silica gel. Retention times of some light HC compounds are shown.
Table II. Total Contamination (TOT) and Heavy Molecular Weight Fraction (HMWF) (as ppbC). Results for Clean-Air Storage in Untreated and HeatTreated 5-mil Teflon Bags 24 h storage tlme TOT HMWF
48-h storage ilme TOT HMWF
72-h storage tlme TOT HMWF
Preflushed but Not Heat Treated N
32.4 39.8 19.5
P
F
14.6
11.6
17.2
13.8
G H
14.2 12.4
10.7 9.1
17.1 15.3
12.3 12.1
0
23.3 38.7 15.5
147.3 173.7 82.9
109.4 149.4 68.6
235.7 216.2 182.0
45.3 54.9 34.8
27.2 23.4 22.0
18.3 15.1 16.0
Heat-Treated at 190 O C for 52 h
STORAGE TIME, hr
Figure 2. Total outgassing contamination concentration (ppbC) vs. storage time (h) for HC-free clean air in 2- and 5-mil Teflon bags. 100
Environmental Science & Technology
were probably fluorinated carbon compounds. This was evident in the mass-spectrometric ion fragmentation patterns of these peaks; observed m/e values were separated by values of 19 and 31, suggesting the loss of F and CF groups. When these same Teflon bags were emptied, flushed, and refilled with clean air, a second pattern of outgassing emerged. The contamination response was much smaller, especially for the peaks shown in Figure 1. The total contamination concentration for the storage of clean air in preflushed 5-mil Teflon bags is reported in Table 11. Apparently, the very high contamination concentration reported in Table I was due to the desorption of volatile fluorocarbon compounds entrapped on the Teflon film surface during the manufacturing process. On close inspection of the roll of Teflon FEP film, a “wetted” surface was seen when one layer of the roll was in contact with another layer. That this material was lightly bonded to the film was evident from the ease of its removal by just one or two clean-air flushings of the Teflon bag. The 2-mil Teflon FEP bags gave quite different results in the HC-free air-storage studies than did the 5-mil bags. None of the large volatile peaks observed for the 5-mil bags were seen for the 2-mil bags. The contamination buildup in the freshly prepared 2-mil bags was lower than that in preflushed 5-mil bags of the same size, for all storage periods (see Figure 2). The main difference between the GC results of air samples taken from 2- and 5-mil bags was in the more volatile region of the chromatograph; the 2-mil bags yielded primarily heavy molecular weight and more polar compounds. The HC-free air-storage studies of the 5-mil bags lasted up to 72 h (3 days), and the studies of the 2-mil bags lasted 360 h (15 days). As shown in Figure 2, the rate of contamination buildup in both the 2- and 5-mil Teflon bags appeared constant over the storage periods. However, as the studies with 5-mil bags lasted only 3 days, no definitive conclusions can be drawn. Further removal of contamination compounds was attempted through heat treatment and solvent washing of the Teflon FEP 5-mil film. Solvent washings with acetone followed by distilled water proved ineffective. Total contamination concentrations were of the same magnitude as those reported in Table I1 for clean-air storage in untreated 5-mil Teflon FEP bags. Furthermore, a larger peak area at the retention time for acetone was observed for bags washed with acetone, indicating a sorption memory on the Teflon material for this solvent. Since Teflon FEP is reported to be thermally stable to more than 200 O C , heat treatment of the film at 190 OC before bag fabrication was evaluated as a method to remove the contamination. The 2-mil Teflon FEP film was not included in these heat-treatment studies because the bag seal for the 2-mil containers frequently failed during preliminary storage studies, and heat treatment of the Teflon FEP film would make the sealing process even more difficult. The results for several storage studies of HC-free air in heat-treated 5-mil Teflon F E P bags are shown in Table 11. The heat treatment reduced contamination from the film by about an order of magnitude (as ppbC), compared with contamination from untreated bags. The relative rates of increase of total contamination concentration were similar for treated and untreated bags for each storage period. Two possible explanations for the increase in contamination during the second and third days of storage are (1)the release of contamination during permeation of water vapor from the outside environment into the bag and (2) the change in the surface-to-volume ratio of the bag. Permeation rates for small polar molecules such as water are reported to be quite large for Teflon film ( 4 ) , and the relative humidity of the laboratory was not controlled during these storage studies. The mechanism suggested here is that water permeated the film and released contamination trapped in the pores or adsorbed on the Teflon FEP film. It
Table 111. Total Contamination (TOT) and Heavy Molecular Weight Fraction (HMWF) (as ppbC). Results for Clean-Air Storage in Heat-Treated (190 "C) and Vacuum-Treated (25 torr) 5-mil Teflon Bags 24-h storage tlme
120-h storage time
2.2
4.3
11.6
0
0.4
1.6
1-h storage time
TOT HMWF
should be pointed out that the observed contamination did not enter the bag from the surrounding environment, because the air in the laboratory did not contain the major contaminating compounds. GC analysis showed the principal ambient-air peaks to be light molecular weight hydrocarbons, which were not major contaminants in the storage bags. Also, the total HC concentration in the surrounding air was lower than in the untreated bags. A second explanation for the increase in contamination could be the increases in the bag's surface-to-volume ratio after the first and second samples were taken for analysis; each analysis required a sample approximately one-tenth of the initial bag volume of 11 L. Some of the contamination peaks were grouped as a heavy molecular weight fraction (HMWF). These summations are listed in Table I1 as HMWF. All of the peaks included in the HMWF were resolved on the aromatics GC column. Because this column was coated with a polar liquid phase, these contamination compounds could have been either heavy molecular weight (Cg-CI?) nonpolar compounds or intermediate molecular weight (C&-C10)polar compounds. Of course, the compounds were probably fluorinated, and these assignments are, a t best, very approximate. The remaining compounds contributing to the total contamination burden were observed on the other two GC columns and included the c 2 - C ~molecular weight range. As seen in Table 11, heat treatment was most effective in removing the low molecular weight fraction and had the least effect on the HMWF. Heat-treatment times were selected arbitrarily. Table I1 shows the results of a 52-h heat treatment; in another study, a 24-h heat treatment produced similar reduction in outgassing contamination. No shorter heat-treatment time was evaluated. As another treatment condition, sheets of 5-mil Teflon FEP film were placed under a constant vacuum during the 190 "C heat treatment. The results are shown in Table 111. The vacuum-assisted heat treatment appeared to be much more effective than was the heat treatment alone, particularly for the HMWF. However, the film tended to lose its heat-sealing properties, so that most of the film thus treated could not be used to make a sturdy bag for long-term storage or repeated use. The heat treatment reduced but did not eliminate the contamination problem with Teflon FEP film. Some contamination peaks persisted, especially in the HMWF. Even though contaminant concentrations were low, efforts were made to identify some of these compounds through GC/MS. In this study, clean air was stored in 50-L untreated 5-mil Teflon F E P bags for several weeks. This increased the contamination level; however, no GC measurement of total contamination concentration was made. A preconcentration procedure used with the GC/MS, system permitted the trapping of a 20-L volume of sample-bag air and allowed the determination of peaks of 10-15 ppbC concentration. Although several peaks were observed on the GC/MS system, the only longer-retention-time peak conclusively identified was 2,6-di-tert-butyl-p-cresol, a common antioxidant added to organic polymers to minimize degradation. Teflon and Tedlar films are not the only fluorinated hy-
IRRADIATION TIME, min
Figure 3. Ozone concentration (ppb) during 6-h irradiation of HC-free clean air with 50 ppb NO in heat-treated 5-mil Teflon bags.
drocarbon polymer materials that contaminate air samples. When tetrafluoroethylene polymer (TFE) tubing was used in a field study of natural hydrocarbons, unusual compounds were observed that could not be identified as emissions from vegetation. Further investigation revealed outgassing from the T F E tubing. We later learned that in the manufacture of T F E tubing, a lubricating oil (Isovar-Shell)is used in the extrusion process (5).When baking the tubing for 24 h at 120 "C did not completely remove the lubricating oil, the TFE tubing was replaced in the field study with FEP tubing. No problems were observed with the FEP; however, the gas was in the tubing for only 18 s. Problems may arise if a gas is kept in the tubing for longer times. One should suspect at least low levels of outgassing contamination in any system that includes Teflon materials. The problem will be most serious for trace-level analyses. Irradiation Studies. Another equally important use of Teflon FEP films isto make smog chambers for air-pollution studies. These chambers are often used in determining the reactivity of organic compounds. This information is then used in air-quality simulation models for planning ozone-control strategies. To determine the effect of Teflon FEP film contamination on ozone formation, a series of bag irradiations were conducted. In these studies, NO was added to a 100-L heat-treated 5-mil Teflon FEP bag filled with HC-free clean air and irradiated for 360 min a t a controlled temperature of 25 f 2 "C. The bag was heat treated as described above for 24 h at 190 "C. The same bag was used for nine such irradiations. In five of these irradiations, the initial NO concentration was -50 ppb; for the other four, it ranged from 25 to 100 ppb. Ozone formation for the five 50-ppb NO runs is shown in Figure 3. In the early 50-ppb NO runs, the contamination produced significant levels of 0 3 during the 360-min irradiation period. In the first run (10/31/77), an 0 3 maximum of 91 ppb was observed at 190 min. During the second run (11/2/77), the 0 3 maximum decreased slightly to 86 ppb; but more importantly, the time to 0 3 maximum was delayed 100 min compared with the previous run. The next 50-ppb NO run was performed after the four irradiations with initial NO levels other than 50 ppb were completed. In this run (11/28/77), no 0 3 maximum was observed. At 360 min of irradiation, the 0 3 level had reached -40 ppb, and one could speculate from the shape of the 0 3 curve that a maximum might have occurred as late as 600-700 rnin if the irradiation had continued. The results shown in Figure 3 suggest that the irradiations aided in conditioning the Teflon FEP bag. This conclusion is supported by measurements of total contamination as ppbC made at the end of the 360-min irradiation periods. At the end of the first run, contamination was more than 300 ppbC. This level decreased to just over 100 ppbC a t the end of the 11/ 28/77 run. In both runs, 40% of the initial NO was measured Volume 15, Number 1, January 1981 101
as gas-phase NO, species at the end of the irradiation period. It appears that in the early runs, high concentrations of outgassing contamination were available to participate in 0 3 formation. Following irradiation treatment, lower levels of contamination were present, and consequently lower 0 3 concentrations were observed. It is apparent from the O3 profiles (Figure 3) that the more readily released contamination was principally responsible for 0 3 formation. As the Teflon FEP film was conditioned by irradiation, the amount of these materials remaining on the surfaces or in the pores of the film must have greatly decreased. Ozone surface reactions may also have contributed to the conditioning of the film. It is not clear why 0 3 decreased markedly after reaching a maximum in the first two 50-ppb NO runs. At that time in the runs, more than 50% of the NO, remained in the gas phase. Contamination levels had increased -40% since the start of the irradiation, and they continued to increase until the irradiation was terminated. These data agree with the observation above that the most reactive contaminants were released in the early stages of the irradiation. The rapid 03-loss rate may have been due to conditioning of active sites on the bag surface. A similar set of irradiations were performed with untreated 5-mil Teflon FEP bags. Although, in earlier runs with untreated bags, O3 maxima and total contamination levels at 360 min were higher than those observed with heat-treated bags, the patterns of 0 3 and total contamination level after five irradiations were similar. These results support the conclusion that these smog chambers required several irradiations for conditioning to minimize interference from outgassing and pacification of active surface sites. It has been assumed that bag emissions are not important when a highly reactive hydrocarbon is irradiated. This is based largely upon the observation of different reactivities for various hydrocarbons with the use of such plastic-bag smog chambers; if bag emissions were important, then most hydrocarbons would show very similar Os-forming potentials. However, this distinction would not apply at very low reactant concentrations, because the bag emissions could result in contaminant concentrations as high as those of the compounds to be studied. Thus, the reactivity of the bag emissions would overwhelm any reactivity from the minute sample under investigation. To test this hypothesis, we irradiated propylene a t 0.86 and 0.10 ppm with NO, a t 0.51 and 0.048 ppm, respectively. An untreated 5-mil Teflon FEP bag was used for these studies to maximize the possible interference from outgassing contamination. In connection with these two smog-chamber observations, a modeling effort was also undertaken, with the intent of first fitting the high-concentration propylene run with a model and then using the model to predict the results of the low-concentration run. If wall emanations were important, the simulated O3 level would reflect the magnitude of the interference of contamination with the irradiation results. An HC/NO, mole ratio of -2 was chosen because it is hydrocarbon poor; Le., in a propylene-NO, system, maximum 0 3 is observed at a ratio of 3:l HC/NO, (6). Thus, the experiments were performed at a hydrocarbon-poor concentration for maximum 0:jformation; and, if emissions from the Teflon were large and photochemically reactive, then the 0 3 level should have been relatively higher at the low propylene concentration than at the high propylene concentration, because of O3 generated from these emissions. The propylene modeling mechanism was similar to that used by Whitten et al. (7). In the model, the measured light intensity ( h l of 0.30 min-I) and the observed dark rate of decay for 0 3 (1.4 X min-l) were used. The photolytic rate con102
Environmental Science & Technology
li 0'
20
40
60
80
100
120
140
I60
110
200
220
240
260
2110
300 320 340 360
I R R A D I A T I O N T I M E . min
Figure 4. Experimental and simulated compound coricentrationsduring an irradiation ( k , = 0.3 min-') of air containing 0.86 ppm propylene and 0.51 ppm NO, (initial NO, = 0.48 ppb NO 4- 0.03 ppb NO2).The modeled runs are at 1.O ppm propylene and 0.50 ppm NO, (0.45 ppm NO f 0.05 ppm NO2).
__EXPERIMENTAL SlMULATEO
____
/ - - - - - - - -
0
20
40
60
110
100
120
160 Ill0 200 220 IRRADIATION TIME min
140
240
260
210
300
320 300 360
Figure 5. Experimental and simulated compound concentrationsduring an irradiation (kl = 0.3 min-I) of air containing 104 ppb propylene and 48 ppb NO, (initial NO, = 41 ppb NO 7 ppb NO2). Modeled runs are at 100 ppb propylene and 50 ppb NO, (45 ppb NO 5 ppb NO2).
+
+
stants for aldehydes were calculated by using the manufacturer's relative intensity distribution for the lights. However, the calculated photodissociation rates appeared too high and had to be adjusted downward. This was not unreasonable, because (1)there is uncertainty in the quantum yields for aldehydes and (2) the lights show an aging effect that is most influential in the short UV region of the spectrum. The computed and experimental profiles for the highpropylene-concentration run (using the lowered aldehyde photolysis rates for computation) are shown in Figure 4. The fit appeared reasonable for the NO2 concentration, although the model value peaked slightly earlier and higher. Although the 0 3 concentration likewise started to rise too early, its peak value was almost identical with the observed value, and the 0 3 maximum was considered to be the more important of these factors. Despite these small differences, however, the experimental and predicted results fit reasonably well. The same model was used to predict the low-propyleneconcentration results, which are shown in Figure 5 . Unfortunately, the results were the opposite of those predicted above: less 0 3 was observed than had been predicted. The reaction rates in the model were then altered, but to no avail. Decreasing aldehyde photolysis rates increased NO, loss, causing the predicted 0 3 level for the high-concentration run to be much lower than that observed. It is not clear why bag emission effects were not observed in the low-propylene system. As shown in Figure 3, 50 ppb NO, in a treated bag would still produce -40 ppb O3 at 360 min of irradiation. A possible explanation is that the bag was conditioned by the first high-concentration run. Also, the
emissions from the bags, as stated earlier, consisted mostly of less reactive compounds that did not produce high levels of ozone. The reactivity of hexafluoropropylene was not tested. However, it should be low since halogenation decreases both the photooxidation and ozonolysis rates of ethylene (8, 9). Another possible contributing factor is that there may not have been sufficient NO, to drive the photochemical production of ozone. The NO, remaining after 300 min of irradiation in the low-concentration propylene-NO, system was 20 ppb. However, the peroxyacetyl nitrate (PAN) at this time was 2 1 ppb. Thus, practically all of the NO, was in fact tied up as PAN. (NO2 was measured as NO by thermal degradation of NOz. This procedure also measured PAN as NOz, as PAN was also thermally degraded.) This would suggest that, even if the emissions from the Teflon were reactive, there was insufficient NO2 to produce a significant amount of ozone. With prolonged irradiation, NO2 would probably build up slightly through PAN decomposition. Ozone would possibly then build up because of the bag emissions. However, experiments were not performed to test this hypothesis. Conclusions Wall-contamination problems raise serious questions as to the utility of Teflon-film smog chambers in determining reactivities of hydrocarbons (IO). These studies performed with a fast-reactivity hydrocarbon such as propylene suggest that bag outgassing is not important. With very low-reactivity hydrocarbons (and organic compounds), the reactivity parameters obtained from these smog chambers are of questionable value. Plastic smog chambers are not reliable for use in multiday irradiations when low levels of pollutants are present. The use of large outdoor Teflon smog chambers may minimize the contamination problem because of their high S/V ratio. However, contamination effects are not easily tested in
such chambers since leaks are almost always present. Thus, contamination from ambient air can also occur. Heat treatment of Teflon FEP film was effective in reducing outgassing contamination. Air samples stored in treated containers appeared to be free of severe contamination, a t least for short periods. The heat-treatment process, however, weakened the sealing properties of the film, resulting in frequent bag failure. Acknowledgment Special gratitude is expressed to Dr. Marcia C. Dodge for performing the modeling exercises included in this investigation. Literature Cited
S.A. J . Enuiron. Sci. Health, Part A 1976,11, 121-30. (2) Lonneman, W. A. “Ozone and Hydrocarbon Measurements in Recent, Oxidant Transport Studies”; Proceedings of the International Conference on Photochemical Oxidant Pollution and Its Control; EPA-600/3-77-001a; U S . Environmental Protection Agency: Research Triangle Park, NC, 1977; p p 211-23. (3) Lonneman, W. A.; Kopczynski, S.L.; Darley, P. E.; Sutterfield, F. D. Enuiron. Sci. Technol. 1974,8, 229-36. (4) E. I. Du Pont de Nemours and Co. Inc., Wilmington, DE, Technical Information Bulletin, T-3E. (5) Ribbans, R., Du Pont de Nemours and Co., Inc., Wilmington, DE, personal communication, 1977. (6) Arnts, R. R.; Gay, B. W. Research Triangle Park, NC, 1979, U.S. Environmental Protection Agency Report EPA-600/3-79-081. (7) Whitten, G. Z.; Killus, 3.P.; Hogo, H. Research Triangle Park, NC, 1980, U.S. Environmental Protection Agency Final Report EPA-600/3-80-028a, Vol. 1. (8)Gay, B. W.; Hanst, P. L.; Bufalini, 3.J.; Noonan, R. C. Enuiron. Sci. Technol. 1976,10, 58-67. (9) Williamson, D. C.; Cvetanovic, R. J. J . Am. Chem. SOC.1968,9, 4248-52. (10) Bufalini, J. J.; Walter, T. A.; Bufalini, M. M. Enuiron. Sci. Technol. 1977,fl, 1181-5.
(1) Seila, R. L.; Lonneman, W. A.; Meeks,
Received for review M a y 19,1980. Accepted October 10,1980.
Possible Clay Concentration Effects on Soluble Phosphate Contents of Runoff Everett M. White Plant Science Department, South Dakota State University, Brookings, South Dakota 57007
Different amounts of clay (montmorillonite, bentonite, kaolinite, illite, or pyrophyllite) were equilibrated with different concentrations of P (PO,-P) to evaluate whether changing clay concentrations could alter the soluble P concentration in runoff. Bentonite and montmorillonite had decreasing P sorption as the clay concentration increased above 1.5 g/50 mL if exchangeable cations were mainly Na but not if they were mainly polyvalent cations. Na-rich highexchange-capacity clays in runoff would sorb P if precipitation dilutes runoff or release it if water evaporates. Thus, P transport processes in runoff could be different with montmorillonitic Na-rich soils and mine spoils than those for runoff from most landscaDes. Introduction The ratio of sediment to solution can change rapidly in runoff water flowing over a soil and subsequently into a stream or lake. Sediment in runoff is sorted during transport so that the average particle size usually decreases with distance. This sediment transports much of the potentially active P(P04) as adsorbed anions on the mineral surfaces. The effect of 0013-936X/81/0915-0103$01.OO/O
@ 1981 American Chemical Society
sediment concentration on the solution P-sorbed P equilibrium apparently has not been studied for different kinds of clay minerals. The equilibrium was studied at several clay concentrations with some of the more common clay minerals to determine whether P transport processes were affected by clay kind or concentrations in runoff. Literature Review Sorption studies have been used to evaluate the solid phase-solution equilibria for many soils and sediments. At high P concentrations, sorption may be limited by the repulsive forces of sorbed phosphate anions on a mineral such as gibbsite unless the negative charges from P anions or those causing cation exchange of the clay are neutralized by a cation. Ca cations neutralize the charges more effectively than Mg, K, or Na ( I ) . In natural soils, a neutral salt may displace exchangeable Ca into the solution to react with sorbed P and increase sorption. In some cases, P sorption may remove surface cations and free negative charges in the mineral surface ( 2 )which may reduce the total P sorption if the unneutralized negative charge of the mineral is very high. Barrow and Shaw ( 3 )suggested that desorption of previously sorbed Volume 15, Number 1, January 1981 103
