Literature Cited 6 00
500
400
-
6
m
a
5
I
300
rn V
200
IOC
0
+5
1 (2)
AB DB
WHITE
DRUM
CRAPPIE
Figure 4. Determination of cadmium in samples of fish flesh. See caption to Figure 3 for an explanation of symbols
feeder. Although not shown here, there was some correlation between size, within a given species, and cadmium content per unit weight. This relationship is reasonable if the uptake rate exceeds the rate of‘ excretion; an accumulation occurs and the trace element content increases with age and size. The EPA limit for cadmium in fish used for human consumption is 2 ppm. Only one of the 113 fish was above this limit, the value being 2.26 ppm for a channel catfish. These data indicate that there is little problem with cadmium pollution in Kentucky and Barkley Lakes in western Kentucky and Tennessee. Standard additions studies on all types of samples gave recoveries in the range of 95 to 102% and no interferences, analyte loss, or contamination problems were encountered for either nickel or catdmium. The extraction-atomic absorption techniques employed have adequate precision and sensitivity for determination of the trace elements investigated in a variety of environmental samples. Acknowledgment
The assistance of Kenneth B. Jolly and Kenneth J. Frazer in performing some of the analyses is appreciated.
(I) Robinson, J. W., Anal. Chim. Acta, 23,479 (1960). (2) Allan, J. E., Spectrochim. Acta, 17,467 (1961). (3) Sachdev, S. L., Robinson, J. W., West, P. W., Spectrochim. Acta, 37,156 (1967). (4) Lerner, L. A., Rusanov, A. A., Nedler, V. V., 2. Anal. Khim., 26, 1967 (1971); Chem. Abstr., 7 6 , 5 3 9 7 2 ~(1971). ( 5 ) Takeuchi, T., Suzuki, M., Yanagisawa, M., Anal. Chim. Acta, 36, 258 (1966). ( 6 ) Montford, B., Can. Spectrosc., 13,126 (1968); Chem. Abstr., 70, 63916 (1969). ( 7 ) Belcher, R., Dagnall, R. M., West, T. S., Talanta, 11, 1257 (1964). (8) Sachdev, S. L., West, P. W., Anal. Chim. Acta, 44,301 (1969). (9) Hannaker, P., Hughes, T. C., Anal. Chem., 49,1485 (1977). (10) Kuz-Min, N. M., Vlasor, V. S., Krasil’Shchik, V. Z., Zauod. Lab., 4 3 , l (1977); Chem. Abstr., 86,164698 (1977). (11) Winefordner, J. D., Vickers, T. J., Anal. Chem., 46, 192R (1974). (12) Hieftje, G. M., Copeland, T . R., de Olivares, D. R., Anal. Chem., 48, 142R (1976). (13) Hieftje, G. M., Copeland, T. R., Anal. Chem., 50, 300R (1978). (14) Stary, J., “The Solvent Extraction of Metal Chelates”, MacMillan, New York, 1964, Chapters 5 and 6, p p 51-195. (15) Morrison, G. H., Freiser, H., “Solvent Extraction in Analytical Chemistry”, Wiley, New York, 1965, Chapter 13, p p 189-247. (16) Chambers, J. C., McClellan, B. E., Anal. Chem., 48, 2061 ( 1976). (17) Yanagisawa, M., Suzuki, M., Takeuchi, T., Anal. Chim. Acta, 43,500 (1968). (18) Yamagata, N., Shigematsu, I., Koshu Eiseiin Kenkyu Hokoku, 19. 1 (1970): Chem. Abstr.. 74. 14597811 (1971). (19) Schroeder, H. A., Nason, A. P., Tipton, H. I., Balassa, J. J., J . Chronic Dis., 20,179 (1967); Chem. Abstr., 6 7 , 3 0 3 0 4 ~(1967). (20) Friberp.. L.. Piscator. M.. Nordberg.. G.. “Cadmium in the Environrne;;’, CRC Press, Cleveland, 1571, p 80. (21) Szadkowski. D.. Schaller. K. H.. Lehnert. G.. Klin. Chem. Klin. Biochem., 7,551 (1969); Chem. Abstr., 71, 110736t (1969). (22) Prinde, B. H., Hissong, D. E., Katz, E. L., Mulawka, S. T., J . Sanit. Eng. Diu., 94, 455 (1968); Chem. Abstr., 69, 50128t (1968). (23) “Water Quality Network, Annual Compilation of Data”, U S . Public Health Service, Washington, D.C., 1959. (24) Bowen, H. J. M., “Trace Elements in Biochemistry”, Academic Press, New York, 1966, p 195. (25) Schroeder, H., Air Qual. Monogr., No. 70,14 (1972). Received for reuieu: July 3, 1979. Accepted April 1,1980. The authors are grateful to the Murray State Uniuersity Committee on Institutional Studies and Research for financial support of this work. Presented in part before the Fifth International Conference on Atomic Spectroscopy in Melbourne, Australia, August 1975, and before the 175th National Meeting ofthe American Chemical Society, Anaheim, Calif., March 1978.
Lead( 11)-Fulvic Acid Complexes. Conditional Stability Constants, Solubility, and Implications for Lead(l1) Mobility Robert A. Saar and James H. Weber* Department of Cheimistry, Parsons Hall, University of New Hampshire, Durham, N.H. 03824
The lead(I1) in soils and in water bodies comes from the breakdown of parent rock and from artificial sources such as industrial emissions, leaded gasoline, and lead-bearing insecticides applied to farm lands. Whatever the source, lead(I1) can be toxic to plants and animals ( I ) . Therefore, it is important to understand how lead(I1) moves in the environment so that we may prevent dangerously high concentrations of it from entering the food chain. Several factors c’ontrol lead(I1) movement. Inorganic ions such as phosphate and carbonate may control lead(I1) mobility. Clay minerals in soils are involved because they have 0013-936X/80/0914-0877$01 .OO/O
cation exchange capacity. Soils and water bodies contain organic matter that also has cation exchange capacity and often, more specifically, the ability to chelate divalent metal ions. A large part of this organic matter is humic substances ( 2 ) ,of which fulvic acid (FA) is the acid-soluble portion. The p H of the soil also influences metal ion mobility (3-5) by affecting the exchange and chelation properties of minerals and humic substances (6, 7). Furthermore, a change in p H can alter the solubility of inorganic compounds containing heavy-metal ions (8). Several groups have examined Pb2+complexation by humic
@ 1980 American Chemical Society
Volume 14, Number 7, July 1980
877
We compare the complexation of lead(I1) by fulvic acids derived from a podzol soil (SFA) and from river water (WFA). In 0.1 M electrolyte, complexation by SFA is stronger than it is by WFA. From pH 4.0 to 6.0, the logarithm of 1:l lead(I1)SFA conditional stability constants increases from 4.0 to 6.3. The corresponding constants for lead(I1)-WFA in the range pH 4.5 to 6.0 increase from 3.7 to 5.1. Lead(I1) forms insoluble precipitates at low lead(II)/fulvic acid mole ratios. This limits
the range of titration data that can be used for calculating the solution-phase lead(I1)-fulvate conditional stability constants. It may also limit the mobility of lead(I1) in soils that are amended with sludges rich in organic matter and lead(I1). However, precipitation is not likely to occur in river and lake waters where lead(I1) and fulvic acid concentrations are typically very low.
substances (6, 9-16). A few of these reports describe Pb2+ complexation by fulvic acid derived from soils (6,9,13), and complexation by aquatic organic matter (13, 15),although this organic matter was not explicitly described as water-derived fulvic acid. Our group has isolated and characterized fulvic acids from both river water (WFA) and from a podzol soil (SFA); we can now compare the Pb2+ complexation properties of these two organic-matter samples. Furthermore, we studied the binding reaction at four pH values, a larger number than appears in other single studies. Although the earlier papers taken together cover many pH values, it is nearly impossible to quantify the important pH effect because the various groups use different humic materials, different data treatment (method of calculating stability constant), and different experimental procedures. In addition to determining Pb-WFA and Pb-SFA conditional stability constants at four pH values, we studied the solubility of these complexes and the conditions required to precipitate lead fulvate. We report the effect such precipitates have on removal of hydrated Pb2+from solution and how such precipitation limits the range of data applicable to solutionphase stability constant calculations.
sample with one part of a methanol-formaldehyde mixture (20), but this is not acceptable for our aqueous system. Instead, we obtained reproducible electrode response by bubbling nitrogen through the sample and against the electrode surface for several minutes before starting a titration and all during titration. We also allowed the fulvic acid solution to equilibrate with the electrode surface for 30 min before addition of the first aliquot of Pb2+ titrant, which was as concentrated as possible to limit the change in total FA solution volume (and hence FA concentration). The fulvic acid solution caused a rapid shift in the electrode potential, a shift that remained constant until the electrode surface was repolished. Because of this FA effect, we calibrated the electrode after each titration, when the electrode surface was conditioned by the fulvic acid. Because the electrode response M Pb2+, we computer-fitted a flattened below about polynomial to the calibration curve for calculation of [Pb2+]. To check solution scattering, we prepared sets of solutions containing FA and Pb2+.Each set had a specific pH and FA type and concentration, but each member of a set had a different total metal ion to total FA molar concentration ratio (CP&'FA). The fluorescence spectrophotometer measured the amount of scattered light upon excitation and analysis at 400 nm. Calculations. Our conditional stability constants apply only to the solution-phase complexation of Pb2+by fulvic acid. Because precipitation of lead fulvate occurs a t low Cpb/CFA, generally below 1.0, we feel that Pb-FA (1:l)and Pb-(FA)2 (1:2) complexes represent the type of binding that occurs. Also, the literature contains evidence for these types of complexes (11). We employed a calculation scheme developed by Buffle and co-workers ( 1 3 ) that yields conditional stability constants for these two types of complexes. We developed a Fortran program to minimize the function represented by Equation 14 in ref 13. The results are conditional stability constants for 1:l and 1:2 complexes. We used previously determined number-average molecular weights (19) for the calculation.
Experimental Materials. Earlier papers describe the isolation procedures for our soil-derived fulvic acid (17)and for our water-derived fulvic acid (18);the characterization of these materials also appears in the literature (18, 19). The electrolyte for all experiments and reagents was 0.1 M KNOB,made from J. T. Baker reagent grade crystals dissolved in double deionized water. We prepared Pb2+ titrant by mixing Fisher SO-L-21 1000-ppm certified atomic absorption standard with electrolyte. FA solutions, also prepared in electrolyte, had a conor 2 X M based on the numbercentration of 5 x average molecular weights of 644 for SFA and 626 for WFA (19).During titrations, we adjusted the pH with reagent grade KOH and "03, diluted as necessary in 0.1 M KNOB. Apparatus. We measured free lead(I1) concentration, [Pb2+],with an Orion 94-82 ion-selective electrode, and pH with an Orion 910100 AglAgCl glass electrode. These two electrodes shared a common reference: a Princeton Applied Research (PAR) K77 saturated calomel reference, isolated from the test solution by a PAR K65 reference electrode bridge tube with a Vycor tip. We performed all titrations into a PAR 9301 water-jacketed cell, and monitored [Pb2+]and pH simultaneously with two Orion 701 pH/mv meters. A P.M. Tamson T9 water bath controlled the experimental temperature (25 f 0.2 "C), and a magnetic stirrer ensured a homogeneous solution. To remove oxygen from the system, we purged the titration cell with nitrogen gas passed through a vanadium(II1) chloride oxygen scrubber. We performed scattering experiments with a Perkin-Elmer 204 fluorescence spectrophotometer. Procedures. The surface of the lead(I1) ion-selective electrode is subject to air oxidation and to coating by surface-active materials such as our fulvic acids. The oxidation problem can be overcome by mixing one part of aqueous 878
Environmental Science & Technology
Results and Discussion As is the case with other metal ions (2), the conditional stability constant for Pb2+binding to SFA and WFA increases with increasing pH. Constants for Pb-SFA and Pb-WFA complexes a t several pH values appear in Table I. Included th'ere are values for PI, which corresponds to the quotient [Pb-FA]/[Pb2+] [FA], where [FA] is the uncomplexed fulvic acid concentration, and 0 2 , which corresponds to [Pb(FA)2]/[Pb2+] [FAI2. or 2 X M) does The FA concentration (either 5 X not appear to influence the lead-FA conditional stability constant. Lead(I1) is like copper(I1) in this respect. Cadmium(II), however, differs from both lead(I1) and copper(I1) in that it binds to our fulvic acids one to two orders of magnitude more weakly, and fulvic acid concentration does affect the Cd-FA stability constant (21). The data for the stability constant calculations came from the early part of our titrations, the part before any lead fulvate
Table 1. Pb-Fulvic Acid Conditional Stability Constants Pb-SFA
Pb-WFA
PH
log 81 a
log 82
4.0 4.5 5.0 6.0
4.0 4.3 4.9 6.3
9.1 9.5 10.1
" p i = [Pb-FA]/[Pb2+][FA].
bP2
log
81 a
pn 6.0
2.0
log 82
3.7 4.7 5.1
8.8 9.3 10.1
1.5
PH
CPbICSFA'
4.0 4.5 5.0 6.0
1.2 0.95 0.8 0.90
M CPbIcWFA
2.1 1.4 1.2 0.90
[FA] = 2 X CPbICSFA
0.55 0.50 0.50 0.65
0.0
Pb-SFA
Pb-(SHA)Z Pb-AHA Pb-(peat HA)2 Pb-(SHA)* Pb-SFA Pb-WHA Pb-WHA Pb-(WHA)2 Pb-WFA Pb-(WFA)2 Pb-river water Pb-SFA Pb-SFA Pb-(SFA)*
3.5 5.0 3.0 5.0 3.0 5.0 6.8 6.8 f f
3.0 5.0 6.7 6.8 6.8 6.0 6.0 7.3 3.0 3.0
stability constant b
3.1 6.1 2.6c 4.1 2.7 4.0d 8.35 4- 0.30 (pH - 5) 6.5 e 5.3 = 7.0 7 .O 2.6 4.1 6.0 5.5 10.4 5.1 9.7 3.95 3.64 3.21 5.58
I
1 I
I
I I
5
-log [Pb2+]
Table 111. Pb-Humic Matter Conditional Stability Constants
Pb-SFA
pn 4.0
CPbICWFA
0.45 0.5 0.55 0.90
CPb= total lead(l1) concentration; C S F = ~ total soil-derived fulvic acid concentration. CwFA= total water-derived fulvic acid concentration.
PH
0.5 '.O::
M
a
type of complex a
pH 5.0
p n 4.5
Table II. Scattering Thresholds for Pb-FA Complexes [FA] = 5 X
s
P
= [Pb-(FA),]/[Pb2'][FA]'.
Figure 1. Formation curves for lead(l1) complexes of podzol soil derived M, T = 25 O C , and the supporting fulvic acid. [SFA] = 5 X electrolyte is 0.1 M KN03. 5 is (C,- [Pb2+])/GA, where C, is total metal ion concentration, [Pb2+] is free metal ion concentration, and GAis the total fulvic acid concentration. Precipitation of solid Pb-SFA aggregates occurred before the end of these titrations. so the entire curves do not represent solution-phase equilibria
2.01
ref
9 14
P
I
10 6
rl
12 13
-.-
24 15
,
6.5
I
6:O
5.5
510 4.5 -log [Pb'q
410
5
Figure 2. Formation curves for lead(l1) complexes of river water derived fulvic acid. [WFA] = 5 X M, T = 25 O C , and the supporting electrolyte is 0.1 M KNO3. The variables are defined in the caption for Figure 1. As with Pb-SFA, Pb-WFA aggregates formed during the later parts of these titrations
16
Abbreviations: SFA, soil-derived fulvic acid; SHA, soilderived humic acid: AHA, Aidrich humic acid: peat HA, peatderived humic acid; WHA. waterderived humic acid; WFA, water-derived fulvic acid. For complexes of the form Pb-L (where L is any ligand in the table), the stability constant is log 61 = log ([PbL]/[Pb2+][L]);for Pb--(L)2type complexes, the stability constant is log P2 = log ([Pb-(L)2]/[Pb2+][LIZ). Continuous variation method. Ion exchange method. e Log K = 6.5 for the strong class of sites and log K = 5.3 for the weak class as determined by Scatchard-type calculations. 'Average of experiments at pH 4 and 5. a
precipitation began, thus assuring a solution-phase-only system. We used the results of our scattering experiments to determine the highest C P ~ / C F A we could reach before the M FA soonset of lead fulvate precipitation. For a 5 X lution, for example, we could include titration data up to about Cpb/CFA = 0.9 in the Buffle-type calculation of stability constant. However, for a 2 X M FA solution, we often could not use data points having Cpb/CFA greater than 0.4 or 0.5 for the stability constant determination. Table I1 contains
scattering thresholds (the point of solid Pb-FA aggregate formation) for several pH values and for two FA concentrations. Much more Cu2+ and Cd2+ is required to precipitate our fulvic acids. For example, the scattering threshold a t p H 4.5 for 5 X M WFA is C C u f C W F A = 4 and C C ~ / C W F=A 20. Table I11 contains a selection of stability constants measured in other laboratories for Pb2+complexation by various types of humic materials. Our values are not at odds with much of this earlier work, although comparisons from group to group are hard to make, as noted above. Our data show that when all variables are controlled, FA derived from soil binds Pb2+ somewhat better than does FA derived from freshwater. The functional group analysis of our SFA and WFA (18) provides a reason for this difference. The total acidity is 13.4 mequiv/g for SFA and 10.6 mequiv/g for WFA. These values are the sum of the concentrations of carboxyl and phenolic OH groups, groups considered to be the reason for fulvic acid's ability to bind metal ions (7). Both the formation curves (Figure 1for Pb-SFA and Figure 2 for Pb-WFA) and the stability constants (Table I) show the Volume 14, Number 7, July 1980
879
greater Pb2+-binding ability of SFA compared to WFA. However, a typical freshwater lake or river has an ionic strength much lower than that of our experiments (0.1 M). In the environment, there is less cation competition for WFA complexing sites, so that the lead(I1)-WFA conditional stability constants in a natural water will be higher than those listed in Table I. Knowing that Pb2+ binds strongly to fulvic acid much as Cu2+does, does not fully explain the behavior of Pb2+-fulvate complexes. We have observed that when precipitation begins, there is increased removal of Pb2+ from solution ( 2 2 ) .This extra loss of free Pb2+may be caused by adsorption onto the solid precipitates or entrapment within developing aggregates. The work of Hassett ( 2 3 ) shows that plant uptake of Pb2+ correlates best with how saturated a soil is with Pb2+ (the amount of Pb2+ in the soil compared to the soil’s maximum Pb2+ holding capacity) rather than with the amount of Pb2+ in the soil. The large conditional stability constants for Pbfulvate complexes indicate the potential for much lead(I1) in a soil or lake sediment to be bound to fulvic acid. Generally, the larger the excess of metal ion, the weaker the sites that the metal ion is bound to, and, hence, the more available the metal ion is for plant uptake. The same kind of Pb2+complexation likely occurs for other fractions of naturally occurring organic matter, such as humic acid (10,13).While much Pb2+in a soil may be immobilized by inorganic ions and clay minerals, the ions may well enter the solution phase before it is available to plants. Dissolved organic matter can exert its control over Pb2+ availability at this point. A complication arises, however, when Pb-organic matter aggregates form and become insoluble. Such precipitates may render some of the Pb2+biologically unavailable. Therefore, we conclude that lead fulvate precipitation joins lead(I1) precipitation by inorganic species and solution-phase ion exchange and chelation as determiners of lead(I1) mobility in soils where locally high lead(I1) and organic matter concentrations may exist. Such a situation could arise in soils used to renovate sewage sludge. Sludges commonly have high concentrations of both organic matter and heavy metals, including lead(I1). Lead(I1) fulvate precipitation is not likely to be a factor, however, in freshwaters, where lead(I1) con-
centrations rarely exceed 2.4 X ( I ) and where humic substance concentrations typically are to M using an average molecular weight of 1000 ( 2 ) .These lead(II)/organic matter ratios are much too low for precipitation to occur.
Literature Cited (1) . , Lovering. T. G.. Ed.. “Lead in the Environment”. Geol. Suru. Prof.
Pap. 197& No. 957. ’ 12) Reuter. J. H.: Perdue, E. M. Geochim. Cosmochim. Acta 1977, 41, 325-334. (3) Hassett, J. J. Commun. Soil Sci. Plant Anal. 1974,5, 499-505. (4) MacLean, A. J.; Halstead, R. L.; Finn, B. J. Can. J . Soil Sci. 1969, 49, 327-334. (5) Jackson, K. S.; Skippen, G. B. J . Geochem. Explor. 1978, 20, 117-138. (6) Guy, R. D.; Chakrabarti, C. L. Can. J . Chem. 1976, 54, 26002611. (7) Gamble, D. S.; Schnitzer, M. In “Trace Metals and Metd-Organic Interactions in Natural Waters”; Singer, P. C., Ed.; Ann Arbor Science: Ann Arbor, 1973; pp 265-302. (8) Hem, J. D. In “Lead in the Environment”. Geol. Suru. Prof. Pap. 1976, NO. 957, pp 5-11. (9) Schnitzer, M.; Skinner, S. I. M. Soil Sci. 1967,103, 247-252. (10) Takamatsu, T.; Yoshida, T. Soil Sci. 1978,225, 377-386. (11) Stevenson, F. J. Soil Sci. SOC.Am. J . 1976,40, 665-672. (12) Stevenson, F. J. Soil Sci. 1977,123, 10-17. (13) Buffle, J.; Greter, F.-L.; Haerdi, W. Anal. Chem. 1977, 49, 216-222. (14) Schnitzer, M.; Hansen, E. H. Soil. Sci. 1970,109, 333-340. (15) Ramamoorthy, S.; Kushner, D. J. J . Fish. Res. Board Can. 1975, 32, 1755-1766. (16) Ramamoorthy, S.; Manning, P. G. J . Inorg. Nucl. Chem. 1974, 36, 695-698. (17) Schnitzer, M.; Skinner, S. I. M. Soil Sci. 1968,105, 392-396. (18) Weber, J. H.; Wilson, S. A. Water Res. 1975,9, 1079-1084. ( l 9 ) Wilson, S. A,; Weber, J. H. Chem. Geol. 1977,29, 285-293. (20) “Analytical Methods Guide”, 9th ed.; Orion Research: Cambridge, Mass., 1978. (21) Saar, R. A.; Weber, J. H. Can. J. Chem. 1979,57, 1263-1268. (22) Saar, R. A.; Weber, J. H. Geochim. Cosmochim. Acta 1980, in
press. (23) Hassett, J. J. Commun. Soil. Sei. Plant Anal. 1976, 7, 189195. (24) Buffle, J.; Greter, F.-L. J. Electroanal. Chem. 1979,101, 231251.
Received for review October 25, 1979. Accepted April 8, 1980. We thank the National Science Foundation for their partial support of this work through Grants OCE 77-08390 and OCE 79-10571.
NOTES
Determination of Phenolics in Coal Gasifier Condensate by High-Performance Liquid Chromatography with Low-Wavelength Ultraviolet Detection Charles M. Sparacino” and Douglas J. Minick Research Triangle Institute, P.O. Box 12194, Research Triangle Park, N.C. 27709
Methodology has been developed for the rapid determination of levels of phenol, cresols, and xylenols in coal gasifier condensates. Aliquots of condensate are injected directly onto a reverse-phase HPLC system, and the effluent is monitored by UV absorbance at 215 nm. At this wavelength, cresol isomers and xylenol isomers exhibit similar extinction coefficients, and can thus be analyzed via single calibration curves. An additional curve is employed for phenol determination. Validation was achieved by blinds analysis. 880
Environmental Science & Technology
The increasing emphasis on coal-based synthetic fuel production poses several potentially severe pollution problems. Since water is utilized or produced in all gasification processes, analytical methodology for aqueous media is required to monitor levels of hazardous materials. The organic contaminants present in greatest amounts in such waters are phenol and its alkylated homologues ( I , 2 ) ;these compounds are toxic to most organisms. Phenolic materials in water are most commonly determined by the 4-aminoantipyrine method ( 3 ) . This colorimetric
0013-936X/80/0914-0880$01 .OO/O @ 1980 American Chemical Society
