Anal. Chem. 1980, 52, 1515-1518
(5) Rodriquez, F.; Kulakowski, R. A.; Clark, 0. K. I n d . Eng. Chem., Prod.
Res. Dev. 1966, 5 , 118-121. Ross, J. H.: Shank, R. L. Adv. Chem. Ser. 1973, No. 125, 108-116. Mirabella, F. M.; Johnson, J. F.; Barrall, E . M. Am. Lab. 1975 (IO), 65-74. (8) Polym. Sci. 1975, 19, . . Dawkins. J. V.: Hemming. M. J . ADD/. .. 3107-3118. (9) Bartick, E. G. J . Chromatogr. Sci. 1979, 17, 336-339. (IO) Ettre, L. S. J . Chromatogr. Sci. 1978, 16, 396-417. (11) Shafer, K. H.; Lucas, S. V.; Jakobsen. R. J J . Chromatogr. Sci. 1979, 17. 464-470. (12) Vidrine, D. W. J . Chromatogr. Sci. 1979, 17, 477-482. (13) Kuekl, D.; Griffiths, P. R. J . Chromatogr. Sci. 1979, 17, 471-476. (14) Vidrine, D. W.; Mattson, D. R. Appl. Spectrosc. 1978, 32, 502-506.
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(15) Bockrath, B. C.;Schroeder, K. T.; Steffgen, F. W . Anal. Chem. 1979, 51. 1168-1172. (16) FarcaskT M. Fuel 1977, 56, 9-14. (17) Hausler, D. W.; Hellgeth, J. W.; Taylor, L. T.;Borst, J.: Cooley, W . 8. Fuel, in press. (18) Ghss, T.E.;Dorn, H. C.; Taylor, L. T.;Manheim. A ; Sleevi, P. S. Anal. Chem. 1980, 82, 1135.
RECEIVED for review December 10,1979. Resubmitted March 28, 1980. Accepted May 8, 1980. T h e financial assistance of Department of Energy Grant EF-77-G-01-2696 and the Commonwealth of Virginia is gratefully appreciated.
Voltammetric Methods for Determination of Metal Binding by Fulvic Acid S. A. Wilson,’ T. C. Huth,* R. E. Arndt, and R. K. Skogerboe” Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
The use of anodic stripping voltammetry (ASV) and differential pulse polarography (DPP) for the measurement of the concentrations of aquo ions in the presence of fulvic acid, and the subsequent use of these data for estimation of the metal-fuivic acid Conditional stability constants, have been evaluated. The results of such measurements for Cd, Cu, Ni, Pb, and Zn, combined with those reported by others, are generally indicative of the adsorption of fuivic acld on the mercury electrodes used, accompanied by the probable formation of metal complexes with the adsorbed fuivic acid so that the stability constants estimated do not appear to be strictly representative of the degree of complexation occurring in the bulk solution. Comparisons are made with stability constants measured by others using other methods which clrcumstantialiy support this conclusion. Therefore, it is suggested that the use of ASV and DPP for studying metal binding by fuivic acid be carefully evaluated for each metal of interest.
T h e potential importance of fulvic acid (FA) as a complexing agent for metal ions in natural waters has been emphasized in recent reviews by Gamble and Schnitzer (1) and by Reuter and Perdue (2). The ability of naturally occurring fulvic acids to complex metal ions, thereby changing their effective chemical forms and potential interactions with other entities associated with aquatic systems, has led to the development of numerous research efforts focusing on defining the characteristics of fulvic acid and the determination of the conditional equilibrium constants which would permit the thermodynamic modeling of metal complexation. Several analytical approaches have been used as means of differentiating between the various chemical forms of trace metals in aqueous systems (speciation) so that the metal-FA complexation chemistry can be investigated. Some of these have been discussed in the reviews cited above ( I , 2 ) . The most Present address: U.S. Geological Survey, 5293 Ward Road, Arvada, Colo. 80002. ‘Present address: Department of Chemistry, University of Arizona, Tucson, Ariz. 85724. 0003-2700/80/0352-1515$01 .OO/O
widely used methods include potentiometric titrations (3-6), dialysis (7), ion selective electrode (ISE) techniques (8-12), amperometric titrations (I3-18), and voltammetric techniques (9,19-23). The reports by some authors (19,20,24,25) that amperometric titrations may be subject to large errors when compounds are present which adsorb on the electrode, coupled with the fact that voltammetric techniques allow measurements without major modifications of the composition of the test solutions, seems to have led many investigators to rely on the voltammetric techniques (22, 23). T h e use of polarographic or anodic stripping voltammetric methods for measuring the binding of metals by organic ligands usually involves titration of the ligand with metal ion, or vice versa, under conditions held constant. If the redox potentials of the metal ions shift according to the ion-to-ligand concentration ratio, the investigator may use the shifts to determine stability constants and coordination numbers of the complexes by the Lingane method (26,27). Alternatively, reductions in the limiting currents of the metal ions in the presence of increasing amounts of ligand may be used t o estimate the stability constants. In general, the latter requires that the complexed metal ions should not be electroactive and should not contribute to the faradaic current measured at the potentials characteristic of the aquo ions. Complexes which do contribute to the faradaic current a t the relevant potentials may be operationally defined as electrochemically “labile”. Similarly, the reduction of the aquo ions a t the working electrode, as in anodic stripping voltammetry (ASV), should not result in a shift in the aquo ion-metal complex equilibrium in favor of complex dissociation. Complexes which do undergo dissociation during the measurement period may be defined as kinetically “labile”. The degree of dissociation, and its concomitant effect on the estimation of the aquo ion concentrations and the stability constants, will accordingly depend on the time scale of the electrochemical measurement. Finally, the presence of the ligand should not affect the diffusion coefficients of the aauo ions in solution nor should the ligand adsorb onto the woriing electrode where it may change the ratesof the redox reactions being monitored 01 it may complex metal ions a t the electrode surface. Failure to satisfy these essential criteria may lead C 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
t o rather large errors in the estimation of "free" metal ion concentrations and the binding constants of interest. These problems have been discussed elsewhere (22, 23, 27). T h e presence of a n electroactive metal-ligand complex, the occurrence of complex dissociation during the voltammetric measurement, and/or the adsorption of a ligand species on the electrode may often be experimentally detected by shifts in t h e peak current potentials and/or half-widths relative to those observed in the absence of the ligand. Thus, the voltammetric methods often provide information indicative of t h e possibility of interference effects being operative. Although the voltammetric methods offer sensitivity that is sufficient to allow measurements at metal ion concentrations generally representative of natural waters, few investigators have reported on their use in the evaluation of metal binding by FA. T h e most comprehensive studies have been reported by Buffle and associates (22, 23) who evaluated the complexation of P b by FA. T h e present investigation has extended t h a t evaluation t o Cd, Cu, Ni, P b , and Zn using differential pulse anodic stripping voltammet,ry (DPASV) and differential pulse polarography (DPP). Although the measurements have been used t o estimate conditional stabilit,y constants for the above metals, the present results strongly imply that adsorption of FA on the Hg electrode surface and the complexation of a t least Cu, Ni, and Pb by the sorbed FA causes problems t h a t tend t,o negate t,he validity of these measurement methods. This conclusion is supported for P h by t h e investigations of Ruffle and associates (22, 23).
EXPERIMENTAL Reagents. The fulvic acid (FA) used in the present studies was isolated from the Bh horizon of a New Hampshire podzol soil. This is the same soil used as a source of FA by others (9, 28--30) and the isolation procedure described elsewhere (28 -30) was followed exactly. Therefore, the number average molecular weight of 644 measured by Wilson and Weber (28-30) was accepted as representative of the FA used herein. Titration of a solution of this material with standard KOH solution while monitoring both the p H and the specific conductance indicated two types of ionizable acid groups having pK values of 2.48 and 4.98. These agree well with those reported by Wilson and Weber (28-30). A stock solution of 5.0 X M FA was prepared by dissolving the dry material in water that was doubly distilled over KMnO,. Dilutions of this stock solution were prepared just prior to use by addition of an appropriate volume of the same water. A stock KNOBsolution was prepared from 99.9997~pure K2COydissolved in hot, dilute HN03, doubly distilled from quartz, follcrwed by pH adjustment with a base solution prepared from recrystallized KOH. Stock metal ion solutions (1000 ,ug/mI,) were prepared by dissolving 99.999+% pure metals or metal oxides in HN03and dilution to volume to maintain a pH < 2 for storage. Test solutions of metal ions were prepared by dilution of these stock solutions to obtain final solutions of appropriate metal concentrations and being 0.1 M in K N 0 3 a t the test pH; all pH adjustments were made with high purity H N 0 3 and KOH solutions. Apparatus. All voltammetric measurements were made with a Princeton Applied Research (model 174) Polarographic analyzer and recorded with a Houston Omnigraph 2000 XY recorder. A platinum wire counter electrode and a Ag/AgCl reference electrode were used in cells that were arranged to allow removal of dissolved oxygen with prepurified nitrogen prior to analysis and nitrogen blanketing of the solution cell during measurements. All measurements were made a t 26 "C using magnetic stirring at a constant rate. One set of measurements also used an Orion, model 94-48A, Cd2+ion selective electrode (ISE) and an Orion, model 701A, digital ionalyzer. All pH measurements were made with the latter unit coupled to a combination electrode that was regularly calibrated against pH 4 and 7 buffers. Procedures. ASV, DPP, and ISE measurements were performed by titration of metal ion solutions of known concentrations with FA solutions at selected pH values. The volumes of the metal ion solutions were typically 50 or 100 mI, so that successive additions of microliter quantities (measured with Eppendorf
pipets) of FA and H N 0 3 or KOH for pH adjustments did not significantly change the total volume or the total metal concentrations. After each aliquot of FA was added with stirring and the pH adjustment completed to maintain the pH within h0.05 unit of the test value, the solution was purged with nitrogen for 2 -3 min prior to the measurements. All measurements were made in duplicate or triplicate and quantitated by reference to Calibration curves run in parallel on standard metal ion solutions under the same conditions without FA present. ASV measurements were made by electrodeposition a t a potential of -1.5 V vs. Ag/AgCl into a hanging mercury drop for 2 min with stirring. Following a 30-s rest period, a differential pulse stripping scan was carried out in a quiescent solution. The conditions used were: 0 . 5 s pulse repetition time, 5 mV/sec scan rate, 50-mV pulse amplitude, 7-ms pulse duration, 1.5-nis current sampling duration, and 0.67-ms current measuring circuit time constant. Instrument sensitivity settings were selected to acquire peak currents that were half to full scale in the absence of FA. DPP measurements were made by scanning from 4 . 2 to -1.5 V (vs. Ag/AgCl) at 2 or 5 mV/s with a modulation amplitude of 25 mV and a drop time of 1 s. Again, instrument sensitivity settings were selected to give peak currents that were nearly full scale in the absence of FA. The peak potentials, shapes, and half-widths were monitored carefully through out all DPP and ASV measurements. Calculations. When the addition of FA resulted in peak potential shifts, complex coordination numbers and the conditional stability constants were estimated by the method of Lingane (26, 27). Estimations were also made based on reductions in the peak currents observed using the procedures described elsewhere ( 9 ) . Two factors germane to these calculations should be noted. First, some measurements were carried out at pH values for which the formation of metal-hydroxy complexes should have been important. However, the evaluation of binding by FA was always made in reference to calibration curves run at the same pH values in the absence of FA. Therefore, the presence of hydroxy complexes was at least partially compensated for and the measurements should primarily reflect M-FA complexation. Second, for those cases in which the formation of MzFA complexes was postulated, the concentration of metal was always in molar excess of that for FA. Furthermore, estimation of free FA concentrations assuming a 1:l model usually resulted in the estimation of negative values when the mole fraction of metal present exceeded approximately 0.7.
RESULTS AND DISCUSSION T h e present experiments relied on the use of ASV and/or D P P as means of measuring conditional stability constants for the binding of metal ions by fulvic acid. Thus, comparisons of the results are possible in several instances. For convenience, the results obtained via each technique are discussed individually below. ASV Results. These measurements were carried out for several metals using a 0.1 M KNO, solution as supporting electrolyte and metal ion concentrations approximately representative of those found in natural water systems, Le., 9.2 x M for Cd, 2.9 X lo-' M for P b , M for Zn, 5.4 x and 9.4 x lo-' M for Cu. Two general effects on the stripping peaks for zinc due t o the addition of increasing amounts of FA were observed. First, the initial additions caused reasonably consistent increases, rather than decreases, in the peak current. At higher FA concentrations, however, the Zn peak current was diminished. In addition, the hydrogen reduction wave shifted anodic with increasing FA, thereby causing shifts in the base line even though the p H was maintained constant a t 5.00 f 0.05 throughout the titration. T h e peak enhancement and the hydrogen reduction wave shift imply t h a t FA adsorbs onto the Hg electrode, perhaps causing significant p H shifts a t the solution-electrode interface or perhaps forming a Zn -ligand species a t the surface that is oxidized a t the same potential as Zn during the stripping cycle. Since the stripping potential and t h e peak half-width remained constant throughout t h e range of FA concentrations studied, neither
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
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Effects of fulvic acid on t h e determination of Cd, TI, and Pb by ASV. Concentration of FA given below each potential scan
Figure 1.
K
f 5yA
Effects of fulvic acid on t h e determination of Cu by ASV. FA concentrations given above each peak Figure 2.
of these possibilities may be proved conclusively. Typical results obtained for Cd and P b are illustrated in Figure 1. Thallium(1) was added to these solutions at 2.9 x M to evaluate its utility as a n internal standard (31). Because of the similarity of Tl(1)to alkali metal ions, it would be less likely to be complexed by FA. These measurements indicated that the peak currents for Cd and P b were diminished by the addition of FA, that the Cd peak potential and half-width were unaffected, and that the peak potential of P b shifted cathodically such that it merged with that for T1 (see Figure 1). These results are suggestive of the formation of a n electroactive Pb-FA complex which might be amenable to analysis by the Lingane method (26) for the estimation of t h e stability constant. A typical example of the effects of FA on the determination of copper by ASV is shown in Figure 2. The results show that its presence causes an apparent decrease in the peak current; the appearance of a subsidiary peak approximately 12-15 mV anodic of the Cu2+/Cuopeak which becomes more prominent than the latter at higher FA levels; and a cathodic shift in the Hg wave that affects the base line quite prominently. Although the overlap in the two peaks tends to obscure the issue, it did not appear that there were significant shifts in the Cu2+/Cu0oxidation potential. These results are also suggestive of the formation of a n electroactive Cu-FA complex as postulated by Bresnihan et al. (9) or of adsorption of FA on the Hg surface thereby affecting its oxidation potential perhaps by affecting the local p H a t the solution-electrode interface. As Crow (27) has pointed out, when the reduction of a metal ion occurs reversibly with or without a complexing agent present, the shifts observed are typically cathodic as noted for P b (Figure 1). When the potential shifts anodic, as observed for the subsidiary Cu peak in Figure 2, it typically indicates that reduction of the complexed ion occurs more readily and reversibly than that of the aquo ions (27). The present observations, i.e., the probable presence of electroactive Cu and P b complexes and the possible adsorption of FA on the mercury electrode, tend to cast doubt on the
MOLAR RATIO
FA Pb
Effects of fulvic acid on the peak potential ( 0 )and the peak half-width ( A ) in the determination of lead by DPP Figure 3.
validity of the use of ASV for accurate measurements of metal- fulvic acid binding constants T o obtain further assessments germane t o this possibility, measurements using D P P were carried out. DPP Results. These measurements were carried out a t metal ion concentrations that were nominally M to achieve adequate sensitivity relative to that obtained via ASV. The results obtained closely parallelcd those reported above for the ASV measurements. For zinc, the hydrogen reduction again shifted anodic with increasing amounts of FA even though the pH was maintained a t 5.00 h 0.05, and the Zn potential and half-width remained constant at all levels of FA studied. However, the increased peak current noted with ASV for the lower FA concentrations was not observed with DPP; the peak current decreased regularly with increasing FA levels. These observations imply that, if FA adsorption on the Hg was responsible for the effects observed by ASV, it did not occur in the time scale of the Hg drops used in the D P P experiments (1.0 s). The DPP behavior for Cd also paralleled those discussed for the ASV measurements. The peak current decreased with FA concentration and the peak potential and half-width remained constant. Typical measurements on lead indicated that the peak current was reduced by the addition of FA with concomitant increases in the half-widths which reached a constant value as shown in Figure 3. Further, the peak potential shifted cathodically by 25 mV over the range of FA concentrations studied. Thus, the D P P results support the previous ASV results suggesting the presence of an electroactive Pb-FA complex. T h e general effects of FA on Cu illustrated for ASV measurements in Figure 2 were also observed via D P P measurements. Again, the presence of the FA caused cathodic shifts in the Hg oxidation wave and the appearance of a secondary peak about 15 mV anodic of the Cu2+/Cuopotential. In contrast to the ASV results, the addition of FA first caused increases in the D P P peak currents followed by decreases at higher FA concentrations. Thus, these observations strongly imply that FA was adsorbed on the Hg surface in sufficient quantities to shift the complexation equilibria even in the 1-s time period of the Hg drops. The effects of FA on D P P measurements of Ni at 1.7 X 10 M were also examined. Changing the molar concentration ratio of FA/Ni from 0 to 10 resulted in doubling of the peak half-width, an 80-mV anodic shift in the peak potential, and reductions in the peak current. These effects were, however, rather small a t FA/Ni concentration ratios below 4 and increased sharply thereafter to attain plateaus at ratios above 10. These results implied the occurrence of the binding of
'
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A N A L Y T I C A L CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
7 MOLE FRACTION FULVIC ACID
7
\o
'1 L
0.n
0.4
m.6
o.a
MOLE FRACTION Ni
Figure 4. Effects o f solution composition on the degree of N i complexation by F A
Ni in a two-step process which is supported by the results summarized in Figure 4. T h e data presented indicate the formation of a 1:l complex when the mole fraction of Ni was ?0.5, attainment of an equilibrium status a t a mole fraction of -0.5, a n d the occurrence of a second complexation step consistent with the formation of a NizFA species when Ni was present in excess. T h e same data trends were observed for Cu reactions with FA in t h e present work a n d by Bresnihan e t al. (9). They suggested t h a t the first complexation step caused a conformational change in the FA molecule which then permitted it t o accept a second metal ion. Further, their ASV measurements indicated the presence of more Cu(I1) than obtained via ion selective electrode measurements, leading to the conclusion t h a t there may have been electroactive CuFA species present as suggested herein. These observations, in combination with others discussed above, emphasize t h a t the use of voltammetric methods for evaluation of metal complexation by fulvic acid must be considered with caution. Estimation of Conditional Stability Constants. Measurements via ASV and D P P are often used to estimate constants for metal binding reactions. When such estimates are based on changes in the currents measured, their validity depends on the assumption of the formation of electroinactive, inert complexes. This assumption is clearly invalid when shifts in peak potentials are observed as for P b and Ni in this work. Estimates of stability constants based on potential shifts rely on the assumption of the reversible reduction of totally labile complexes. Peak broadening and/or peak splitting, as noted for Cu, P b , and Ni herein, indicates the general unreliability of this assumption. Either of the above estimation approaches also requires t h a t secondary reactions be of minimal importance. Buffle e t al. (22, 23) and Brezonik e t al. (24) have demonstrated that secondary reactions, and effects associated with the adsorption of organics including FA on mercury, affect both the peak currents and potentials observed. Indeed,
Buffle and Greter (23) presented convincing evidence for the complexation of Pb(I1) by FA adsorbed on the electrode, thereby prominently influencing the polarographic measurements. The results summarized herein are generally indicative of the importance of such processes for Cd, Cu, Pb, Ni, and Zn when fulvic acid is the ligand species. Therefore, the use of voltammetric measurements t o distinguish between free and complexed forms of these ions is of questionable utility for fulvic acid systems. This conclusion has been supported indirectly by calculation of stability constants from the present data using the peak shift (26,27) and peak current reduction (9,27) methods and comparison of the values obtained with those determined by ISE (9,11,20,33,34), ion exchange (32) and/or continuous variation measurements (32). The present estimates for Cd agreed with literature values within 10-20%; those for Cu, Pb, Ni, and Zn tended to be higher than literature values by 1to 3 orders of magnitude. Such discrepancies generally reflect the magnitude of the FA adsorption and surface complexation effects and tend to emphasize the dangers inherent in the use of voltammetric methods for characterizing metal binding by fulvic acid and/or other organic ligands.
LITERATURE CITED (1) Gamble, D. S.;Schnitzer, M. The Chemistry of Fulvic Acid and its Reactions with Metal Ions, in "Trace Metals and Metamrganic Interactlons in Natural Waters", Singer, P. C., Ed.; Ann Arbor Science Publishers: Ann Arbor, Mich., 1973. (2) Reuter, J. H.; Perdue, E. M. Geochim. Cosmochim. Acta 1977, 47, 325-334. (3) Stevenson, F. J. Soil Sci. 1977, 723,10-17. (4) Sposito, G.; HoRzclaw. K. M. SoilSci. SOC.Am. J . 1979, 43,47-51. (5) Sposito, G.; Holtzclaw, K. M.; LeVesque-Madore, C. S. Soil Sci. SOC. Am. J . 1976, 42, 600-607. (6) Stevenson, F. J.; Krastanov, S. A,: Ardakoni, M. S. Geoderma 1973, 9 , 129-141. (7) Guy, R. D.; Chakrabarti, C. L. Can. J . Chem. 1976, 54, 2600-2611. (8) Buffle, J.; Greter, F. L.; Haerdi. W. Anal. Cbem. 1977, 49,216-222. (9) Bresnihan, W. T.; Grant, C. L.; Weber, J. H. Anal. Chem. 1978, 5 0 , 1675-1679. (10) Cheam. V. Can. J . Soil Sci. 1973, 53,377-382. ( 1 1 ) Brady, B.; Pagenkopf, G. K. Can. J . Chem. 1978, 56, 2331-2336. (12) Whitworth, C.; Pagenkopf, G. K. J . Inorg. Nucl. Chem. 1979, 47, 317-321. (13) Chau, Y. K.; Gichter, R.; Lumm-Shue-Chan, K. J . Fish. Res. Board Can., 1974, 37, 1515-1519. (14) Mancy, K. H. Progr. Water Techno/. 1973, 3 , 63-72. (15) Hanck, K. W.; Dlllard. J. W. Anal. Cbim. Acta 1977, 89, 329-340. (16) Chau, Y. K.; Lun-Shue-Chan, K. Water Res. 1974, 8. 383-388. (17) Shuman, M. S.;Woodward, G. P. Anal. Cbem. 1973, 45,2032-2035. (18) O'Shea, T. A,; Mancy, K. H. Anal. Chem. 1978, 48, 1603-1607. (19) Davison, W.; Whitfield, M. J. Electroanal. Cbem. 1977, 75, 763-770. (20) Buffle, J.; Greter, F. L.; Nembrlnl, P. J.; Haerdi, W. Fresenius' 2.Anal. Cbem. 1976, 282, 339-345. (21) Nurnberg, H. W.; Vaienta, P.;Mort, L.; Raspor, B.; Sipos, L. Fresenius' 2 . Anal. Chem. 1976, 282, 357-361. (22) Greter, F. L.; Buffle, J.; Haerdi, W. J. Electroanal. Chem. 1979, 707, 21 1-229. (23) Buffle,J.; Greter, F. L. J . Elecfroanal. Chem. 1979, 707,231-251. (24) Brezonik, P. L.; Brauner, P.; Stumm. W. Water Res. 1976, 70,605-612. (25) Siegerman, H.; O'Dom, G. Am. Lab. 1972, (4), 59-68. (26) Lingane, J. J. Chem. Rev. 1941, 29, 1-35. (27) Crow, D. R. "Pobrography of Metal Complexes"; Academic Press: New York, 1969; pp 56-64. (28) Weber, J. H.; Wilson, S. A. Water Res. 1975, 9 , 1079-1084. (29) Wilson, S.A.; Weber, J. H. Chem. Geol. 1977, 79,285-293. (30) Wilson, S.A., Weber, J. H. Anal. Lett. 1977, 10,75-84. (31) Copehnd, T. R.; Osteryoung, R. A.; Skogerbce, R. K. Anal. Chem. 1974. 46, 2093-2097. (32) Schnitzer, M.; Hansen. E. H. Soil. Sci. 1970, 709,333-340. Weber, J. H.; Saar, R. Can. J. Chem. 1979, 5 7 , 1263-1268. (33) (34) van den Berg, C. M. G.; Kramer. J. R. Anal. Chlm. Acta, 1979, 706, 113-120.
RECEIVED for review January 18,1980. Accepted May 12,1980. Research supported by EPA Grant No. R805-183-01.
