Fdvic acid: modifier of metal-ion chemistry This class of compounds, derived from the decay of plants and animals, is being studied for its role in the transport and toxicity of metal ions in soil and water Robert A. Saar Geraghty & Miller, Inc. Syosset, N.Y. 11791
James H. Weber Chemistry Department University of New Hampshire Durham, N.H. 03824 Living biological systems are well ordered by the input of energy throughout any organism’s lifetime. The result is that organisms are composed of a fairly unchanging set of biological molecules, such as fatty acids, proteins, nucleic acids, and hormones, many of which are known to the last atom. Their reactivities can be reproduced in the laboratory. In contrast, the extreme ordering effects of energy are not present in an organism’s wastes, or in its body after death. As degradation progresses, an increasing variety of organic structures can form. Many of the resulting compounds are not fully characterized or named. Degradation compounds can be categorized as biopolymers, which are predominantly polysaccharides and polypeptides, and as geopolymers (humic substances), which are random polymers of a variety of biological monomers ( I ) . Fulvic acid is the most hydrophilic of several classes of geopolymers (Figure 1); it is soluble at both high and low pH. Fulvic acid molecules, with atomic masses ranging from a few hundred to
thousands of atomic units, have a wide variety of aromatic and aliphatic structures bearing many oxygen-containing functional groups, particularly -COOH and -OH (2). These functional groups, which can be protonated and deprotonated in the pH range common in natural waters (pH 3-9). enable fulvic acid to behave as a polyelectrolyte (3). The inability to characterize completely all fulvic acid structures does not prevent substantial progress in understanding the properties of these materials. Although particular mole-
cules may show up in one batch of fulvic acid and not in another, the overall chemical properties of the two batches will be remarkably similar. Thus, fulvic acid can be investigated and described in terms of its group properties. Fulvic acid research does not fall neatly into the traditional disciplines of analytical, physical, organic, or biological chemistry, and yet it draws information and techniques from each. The vocabulary of traditional disciplines is used in fulvic acid work (for example, “functional groups” and
‘IGURE 1
Originof humicsubstances and relationshipsamong them
eopolymem umic Substances
Feature articles in ES&T hoot by-lines. represent rhe uiews o/thc outhors, and ore edited by the Washington sta//. Ifyou are intermed in contributing an article, contact the managing editor. 510A
Emiron. Sci. Techmi., VOI. 16. NO. 9. 1962
0013-936X/82/091&0510A$01.25/0
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1982 American Chemical Society
FIGURE 2
Fulvic acid interactions with metal ions
l e e s t species,ofthoseshown, for complexation. NO*:FUMS acta ana mmai.ion (IWCIBII are arranged with thoae prevalent B1 IDYpH nmrthe lop and I h o s prevalentSI high pH near the bltom.
“conditional stability constants”), but the terms often take on new shades of meaning.
Fulvic acid and metal ions An important property of fulvic acid is its ability to form complexes with metal ions. Many of the oxygen-containing functional groups, particularly carboxylic and phenolic moieties ( 4 , 5 ) , associate with metal ions, notably the alkaline earths (commonly Ca and Mg), and transition metals (for example, Cu, Fe, Cd, Zn, V, and Ni). Whereas monovalent cations like Na and K can form weak electrostatic bonds with single anionic groups on fulvic acid, divalent metal ions may be complexed at two adjacent anionic sites, forming a chelate ring, an association that is generally much stronger than that formed by complexation through a single site. Many variables affect the strength of association between fulvicacid and metal ions. Figure 2 gives a greatly simplified view of the factors involved. The hydrogen ion concentration determines which forms of the fulvic acid and metal ions are prevalent; different forms of these species have different tendencies to enter a complex. The mast “eligible” species are indicated in the figure. Another way to look at the effect of pH on complexation is to consider that H+ competes with metal ions for anionic binding sites on fulvic acid, and OH- competes with fulvic acid for the cationic metal ion. As the pH is raised, fulvic acid becomes more available for complexation, and the metal ion becomes less available. An intermediate
pH mast favors complexation between fulvic acid and metal ions. Why be concerned with the interaction between fulvic acid and metal ions? There are many possible answers, of which three are offered here. First, many researchers have focused on the different biological availability or toxicities of complexed and uncomplexed metal ions (6-10). Metal ions such as Cu2+and Cd2+ are known to be I stoxic to aquatic organisms when they are part of complexes with fulvic acid or other ligands than when they are not complexed. Since it is easy to analyze for the total concentrafion of metal ions, one is tempted tocorrelate toxicity and other properties with this total. However, it is evident that in toxicity studies, complexed and hydrated species should be considered separately, as if they were different metal ions ( / I ) . Second, aquatic fulvic acid, as well as other fractions of dissolved and adsorbed organic matter, can alter the geochemical mobility of metal ions (5. 12-14). Dissolved organic matter can release metal ions that had been adsorbed on sediments, and organic matter adsorbed to sediments can sequester metal ions that are in solution. The stability constants for complexes between fulvic acid and metal ions like Cu2+ and Pb2+ are high, so that the fulvic acid can alter the metal-ion equilibria. Even the partitioning of cadmium, which is not complexed as strongly, is influenced by humic materials (15, 16). Buffering capacity refers most often in chemistry to the regulation of hydrogen-ion concentration. But, there
are other kinds of buffers: For example, fulvic acid in natural water systems is a metal-ion buffer. As with all buffers, its capacity (in this case, the ability to complex metal ions) is limited. This limit defines what is called the complexing capacity of a water sample. Third, fulvic acid is important because it may change the ability of water treatment processes to remove metal ions. In a study of alum reactivity with aqueous Cu2+, Cd2+, and Zn2+,fulvic acid increased the fraction of metal ions removed (17). Since alum coagulation can be important for removal of trace metals from a water supply, the changes in removal efficiency caused by dissolved organic matter must be known.
Analyzing for complexes A complete speciation scheme such as the one presented by Florence and Batley ( I / ) includes metal ions that are free (fully hydrated) and those associated with various substances, including fulvic acid, in both the solid and solution phases. Of primary interest is the solution phase which, by convention, includes all materials that pass through a 0.45-fim filter. Many experimental factors can alter the results of speciation studies involving fulvic acid and metal ions. Although the complexing properties of fulvic acid vary somewhat from sample to sample, the similarities are great enough so that findings in the world’s literature overlap substantially (13). Furthermore, fulvic acid derived from soils (SFA) has largely the same metal complexing properties as fulvic acid Envlron. SCLTeChml.. Vd. 10, No. 9,1982
6111
that influence tulvl -metal ion compiexatl nstant derived from water (WFA). Many of the experiments described here employed SFA because it is easier to obtain than WFA. Side-by-side studies of the two substances showed that the information obtained for SFA applies to WFA as well. The many variables listed in Table 1 can contribute to widely differing results for systems with fulvic acid and metal ions, which should otherwise be similar. The method of extraction can alter the fraction of organic matter that is obtained, or if the method is harsh, chemically change the fulvic acid. The concentration of fulvic acid appears to affect speciation, particularl for weakly bound metals like CdY+ (18). Ionic strength indicates the concentration of monovalent cations when a salt such as potassium nitrate is used. Ions like potassium can compete (if in
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Envlron. Sci. Tschnol.. Vol. 16, No. 9. 1982
high enough concentration) with divalent cations for complexation sites on fulvic acid molecules (19). Temperature is important because of its effect on the free energy of complexation. Hydrogen-ion concentration (pH), as indicated earlier, dictates the predominant forms of fulvic acid and metal ions and is, therefore, a key variable.
The method of speciation analysis can profoundly influence the results, because the various methods measure different aspects of the system and operate under different conditions. Finally, after all experimentation is done, the resulting data can be manipulated in different ways. Separation and nonseparation techniques are the two major types of analysis applied to speciation problems; commonly used methods are listed in Table 2. This table also shows the applicability, advantages, and disadvantages of the various methods. Separation of free and complexed metal ions can be done by chromatography, or with membranes that exclude the metal-ion complexes. Chromatographic techniques include liquid chromatography by size exclusion. Ultrafiltration and dialysis use membranes with small pores.
Nonseparation techniques that distinguish between free and c o m plexed metal ions in situ include voltammetry and potentiometry. Fluorescence, also a nonseparation technique, measures the concentration of free ligand. Separation techniques. These methods have two major pitfalls: adsorption of species on membranes or chromatographic materials and the possibility of shifting equilibria. Although the losses of metal ions ( 2 0 ) and organic matter ( 2 1 ) have been studied, the adsorption problem for membranes and chromatographic materials is nevertheless generally ignored. The adsorption of metal ions on purified dialysis membranes is apparently minimal in solutionsof high ionic strength (22, 23). The complexation equilibrium probably shifts during chromatographic separations. Figura
Can use mnmodified
Not particularly applicable for stability constant calculations
Faster man equilibrium dielysk no Ionic strength a metallon resMctions No metal-ion
Absorption; pos:
natural water
reSmctions; cnn
incomplete -ration; s+,-. UF cell needed
Possible adsorption
and contamination
use unrodified nabral watef
by membrane: possible incomplete
88mDIes
separation
Provides information on types Of canplexes Rapid titration
Only EPR-active metals; high
detection limit
Few metal Ions; possible adsaption on electrade: h@ detection limit
No ionic strenglh or metal-ion
reslrictions V w low metal-ion
cuvxihlion can be measued An altemstive view
of complexation; WY I
sensitive:
can use unmodified natual waters
ExhWlWcareneedsd
during titration
Few metal Ions; adsaption; possible equilibium shin Appllcable to few
meatal ions
and McDuffie ( 2 4 ) describe this problem in their use of Chelex chelating ion-exchange resin to separate various metal ions from natural water samples. The chief advantage of separation techniques is found in the wide range of metal ions that can be measured, generally by means of atomic absorption or inductively coupled plasma spectrophotometry. The nonseparation methods, except for hydrogen-ion potentiometry (25). are each applicable to only a few metal ions. Chromatography and ultraflltration. Two recent papers on reversedphase liquid chromatography demonstrate that some Cu2+ in swamp ( 2 6 ) and estuarine (27) watersjs associated with organic fractions. Gel filtration chromatography, which retards free metal ions, demonstrates that iron occurs in complexes in a variety of natural water samples ( 2 8 ) and that free and complexed metal ions exist in solutions of isolated humic and fulvic acids ( 2 9 ) . Ultrafiltration studies demonstrate that a variety of metal ions, including Cd2+, Pb2+, Cu2+, and iron, are retained on membranes with pores between approximately 0.4 pm and 0.02 pm (30). Since that is the “dissolved” range, the results suggest that the metal ions are complexed to aquatic organic matter. A major difficulty in interpreting ultrafiltration data is that no membrane has pores small enough to separate completely dissolved metal complexes from metal ions adsorbed on colloidal particulate matter. Metal species between 0.4 and 0.2 pm probably include both metal complexes and adsorbed metal ions. Equilibrium dialysis (ED). The ED experiment is set up with two solutions: a solution with metal ions and ligands outside bags made of dialysis membranes and a solution with neither constituent inside the bags ( 3 1 ) . Ideally the membranes will allow only hydrated metal ions to pass through to the inside. Thus, one can measure free metal ion inside the bag and total metal ion (hydrated and complexed) outside it. A plot of free metal ion vs. total metal ion will show a sharp endpoint (CL) if the conditional stability constant K is sufficiently large. This CL value indicates the quantity of ligands or complexing groups in the solution. A recent paper ( 2 2 )describes tests of
this method with titrations of 6.25 pM eth lenediaminetetraacetic acid by Cut* and Cd2+;the experimental CL values were within 2.4% of the theoretical 6.25-pM value. Initial experiments in our laboratory with soil-derived fulvic acid (SFA) demonstrated that Cu2+ and Cd2+ penetrate the dialysis membrane, whereas SFA does not ( 2 2 ) . Experiments were performed with 0.001 M KNO,, an ionic strength similar to that of fresh water. Titrations of IO mg/L SFA with Cu*+ and Cd” showed t h a t thc titration endpoint CL depended on the pH and on which metal ion was used: CLincreased from pH 5 to 8 and is higher for Cu2’ than for Cd2* (Table 3). Earlier ion-selective electrode studies (18, 3 2 ) indicated these trends. After the completion of ED experiments with SFA, this approach was extended to seven freshwater samples from southeastern New Hampshire ( 3 3 ) . Equilibrium dialysis does not require sample modification such as addition ofa supporting electrol)te. so the samples had only to he filtered through a 0.4-pm polycarbonate membrane before titration nith Cu2+ or Cd2’. Base was added to maintain the original pH. The measured CL values (Table 3) are generally larger for Cu2’ (1-15 p M ) t h a n for Cd2+ (0-10 p M ) as we expected from experiments with isolated SFA (18.22, 32). Nonseparation techniques. Unfortunately, adsorption occurs with all separation techniques in which a membrane, chromatographic material. solid electrode, or mercury electrode touches a solution containing humic matter. This is a problem that ultrafiltration, equilibrium dialysis, and chromatography (separation techniques) share with two nonseparation techniques: ion-selective clectrode potentiometry and voltammetry. I n contrast, the problem of shifting equilibrium is more common with separation techniques (particularly chromatography) than with nonseparation ones, although the equilibrium can be shifted during voltammetry experiments. Nonseparation approaches cover relatively few metal ions. For example. voltammetry is commonly used for Cu2+,Cd2+,Pb2*, and Zn2+. The applicability of other nonseparation techniques is shown in Table 2 . Emiron. Sci. Technol.. VoI. 16, No. 9. 1982
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TABLE 3
Total ligand concentration (C,) for 10 mglL soil fulvic acid (SFA) solutions and some New Hampshire fresh waters as determined by equilibrium dialysis CL,CM
-mP*
pn
/a
P
Electron paramagnetic resonance (EPR). EPR can provide many types of information for complexes that contain paramagnetic species. In one study, EPR showed that SFA could reduce vanadium(V) to vanadium(1V) (34). In another vanadium study, EPR allowed measurement of the distance between complexing sites, the conditional stability constants for those sites, and the aggregation of vanadyl-SFA complexes (35). Senesi and cuworkers found both strong and weak Fe3+ sites on humic and fulvic acids (36). The EPR results yielded information on site symmetries and on the resistance to reduction of Fe3+ at the two types of sites. Lakatos and co-workers ( 3 7 ) observed EPR signals in systems containing humic acid and Mn2+, Cuz+, VOz+, Mo(VI), Mo(V), and Cr3+. Ion-Selectiue Electrode (ISE). The detection limits of ion-selective electrodes are too high to measure the concentration of free metal ion in mast unmodified samples of natural water. Moreover, ISE experiments cannot be done at very low ionic strength. However, ion-selective electrodes are widely used to measure the complexation of metal ions to isolated humic matter in solutions with metal-ion concentrations higher than those usually found in the environment (4,38-41). An early project carried out by this research group employed an ISE to study Cu2+ binding to water-derived fulvic acid (WFA) and SFA in 0.1 M KNO3 (32). Because the total concentrations of ligand and metal ion and the concentration of free metal ion are known, one could calculate conditional stability constants (K values) based on the Scatchard binding model (42.43). SWA
Envimn Sci Tedmol.. Voi. 16.No. 9, 1082
CY*+
16.2 24.1 28.7 8.6 10.7 1.1 2 Id 5.0 11.9 15.1
ca2+
8.0 20.3 24.3 0
It was found that increased pH resulted in higher K values, and more Cu2+ bound to SFA or WFA sites. Also, the average K value increased as more ligand was added during the titration. This result was expected, because fulvic acid is a mixture of ligands that have different affinities for metal ions such as Cu2+.Typical K values arebetweenlX106andIOX106.The extent of Cu2+binding can be related to the total acidity (carboxyl plus phenol content) of SFA ( I 3.4 meq/g) and WFA (10.5 meq/g). For complexation of Cd2+ by SFA and WFA, average K values decrease during titration with metal ion titrant, and increase with increasing pH (18). Also, the total binding of Cd2+ by fulvic acid increased with increasing pH. These results are similar to those for the Cu2+ work (32). The major new finding of the Cd2+study was that K also depends on total fulvic acid concentration. For example, at pH 6, K for complexes of SFA and Cd2+ decreases from 2.9 X 104 to 1.2 X lo4 as the SFA concentration increases from 19 to 360 mg/L. This dependence necessitates titration with a concentrated Cd2+ solution and maintenance of a fairly constant SFA concentration. In contrast, the Cu*+SFA system gives the same results for either Cu2+ or SFA titrant. Pb2+ binding to SFA and WFA by ISE ( 4 4 ) was also studied. An important discovery was that even small amounts of PbZ+caused aggregation (formation of solids) in fulvic acid solutions, as indicated by light scattering measurements. For example, aggregation occurred in a 32 mg/L solution of SFA at pH 4 and 5 when the Pb2+
concentration was 50 p M . This aggregation occurs well before the approximately 400 pmol of carboxyfic and phenolic complexation groups are saturated with Pb2+, which indicates that the solid-phase, lead-fulvate aggregates should be able to do two things: physically adsorb and chemically complex additional Pb2+ ions. However, because K values should reflect only solution-phase complexation, data for solutions without aggregates were used. As in the Cu2+ ( 3 2 ) and Cd2+ (18) studies, K increased with pH. For example, K values for Pb2+-SFA complexes are 1 X IO4 at pH 4.0, and 2 X lo6 at pH 6. These ISE studies demonstrate that under the same conditions, Cu2+- and Pb2+-fulvate complexes have similar K values, whereas those of Cd2+-fulvate are about 100 times lower. Cu2+ and Pb2+ differ in that Pb2+-fulvates aggregate at much lower M2+/fulvic acid mole ratios; Cd2+ is even less effective than Cu2+ in aggregating fulvic acid (45). Differential pulse anodic stripping uoltammetry (DPASV). Researchers commonly use DPASV (ES&T, Vol. 16, No. 2, p. 104A) experiments to determine metal-ion speciation in natural water samples, because the technique can measure very low concentrations of metal ions-down to about 1 X M ( 1 1 ) . Despite its popularity, DPASV has several drawbacks. DPASV experiments, like those with ISE, need a supporting electrolyte, and work only for certain metal ions, including Pb”, Cu2+, Cd2+,and Zn2f. DPASV has two additional major drawbacks that limit its effectiveness in natural water matrices. First, DPASV disturbs the equilibrium between free and complexed metal ions: Dissociation of metal complexes often occurs during the plating step. Therefore, the measured stripping current is composed of two parts: the diffusion current caused by dissolved, hydrated metal ions, those that were not part of complexes; and the kinetic current, which arises from metal ions that have just dissociated from complexes. A measure of only the concentration of hydrated metal ions requires that the kinetic current be subtracted from the stripping current. This differentiation is theoretically and experimentally difficult. Second, humic matter adsorbs on the mercury electrode, mak-
ing it difficult to interpret results (46, 47). Our recent paper (48) described a DPASV method that overcame the problems of kinetic currents and fulvic acid adsorption. We titrated 10, 20, and 40 mg/L SFA solutions in 0.1 M K N 0 3 with Cu2+at pH 5 and 6 , and measured the total stripping current, which was recorded along with the total amount of metal ion added. The data were computer fitted to an equation developed by Shuman and Cromer ( 4 9 ) .One computer program calculates, among other things, the total ligand concentration in the solution (CL), and the conditional stability constant (K); a second program calculates stripping currents with the kinetic current removed, as well as corrected CL and K values. The contribution from kinetic cup rent proved to be substantial for titrations of SFA by Cu2+. For example, in the titration of 20 mg/L SFA at pH 6 , the CL values increased by 22% as a result of the kinetic-current correction. That is, the uncorrected data underestimated the total ligand concentration by 22%. The problem of fulvic acid adsorption onto the mercury electrode is overcome by in situ calibration curves. Calibration curves done in .the absence of fulvic acid lead to erroneous values for hydrated (free) Cu2+ concentrations. The correct concentrations for free Cu2+ allow calculation of K by any desired means of data treatment. Fluorescence spectrometry ( F S ) . Humic matter and natural water samples fluoresce (22,50,51),but the fluorescence of organic ligands is quenched by complexation to paramagnetic metal ions, that is, those with unpaired electrons ( 5 2 , 53). Therefore, the intensity of humic matter fluorescence decreases during titration by such metal ions. FS experiments have several advantages over other methods. First, fluorescence is measurable in solutions with concentrations of dissolved organic carbon even lower than those found in many samples of natural water (detection limit is less than 1 mg/L). Second, FS experiments, unlike DPASV and ISE, need no supporting electrolyte. Third, unlike all other methods discussed here, FS measures the concentration of free ligands, rather than that of free or total metal ions. This third advantage re-
sults in a direct measurement of CL. The major disadvantage of FS is that it is very effective only with strongly binding, paramagnetic metal ions, such as Cu2+. SFA and WFA fluorescence spectra exhibit a broad, featureless emission peak at 445-450 nm upon excitation at 350 nm (50).Two requirefnents must be met before FS is used for binding studies with fulvic acid and metal ions: Uncomplexed metal ions must not quench fluorescence, and metal-ion quenching of fluorescence must be proportional to metal-ion complexation. In initial studies (50) we demonstrated that the first requirement was met by observing that at pH 1.4, at which divalent metal ions are not complexed to fulvic acid, its fluorescence is unquenched, even at very high concentrations of metal ions. As for the second requirement, we showed with the model ligand salicyclic acid (ohydroxybenzoic acid) that the percentage of Cu2+ bound (calculated from the known stability constant) and the percentage of fluorescence quenched are equal during titration by Cu2+ at pH 6. These two preliminary experiments encouraged us to use fluorescence as a probe to study metal ion complexation by fulvic acid. Accordingly, we titrated 32 mg/L solutions of SFA and WFA with Cu2+, Pb2+, Co2+, and Ni2+ at various pH values (50). For any specified metal ion/fulvic acid mole ratio, the percentage of fluorescence quenched for each metal ion increased as pH increased. This trend agrees with complexation studies done by ISE (18,32, 44, 4 3 , which demonstrated increased K values (and hence, increased complexation) at higher pH values. The effectiveness of metal ions in quenching FA fluorescence is: Cu2+ > Pb2+ > Co2+ = Ni2+ > Cd2+. C U ~ + and Pb2+ form strong fulvic acid complexes with similar K values, but paramagnetic Cu2+ is much more effective than diamagnetic Pb2+ in quenching fluorescence. The weakly bound paramagnetic ions Co2+ and Ni2+ quench some fluorescence, and the weakly bound and diamagnetic Cd2+ ion has no effect on fluorescence intensity. Finally, we demonstrated with FS and ISE titrations that Cu2+ and Pb2+ binding to fulvic acid is proportional to fluorescence quenching.
Our recent results ( 5 4 ) extend the earlier work by describing a quantitative method for determining micromolar CL values of fluorescing ligands for metal ions. We initially titrated 3 6 - p M solutions of the model compound L-tyrosine with Cu2+to test an equation and a curve-fitting program that we developed. The observed and calculated CLvalue of 33 yM is close to the 3 6 - y M known concentration. The average K value of 5.8 X lo4 at pH 6 agrees well with the first conditional stability constant (K) of 5.9 X lo4 (55). We also found that the residual fluorescence, after complexes have been formed, was only 2.6 f 1.7% of the original fluorescence exhibited by the free ligand. Experiments with 10 mg/L (1 6 p M ) solutions of SFA at pH 5 , 6 , and 7 yielded CL values of approximately 20 p M . Earlier ISE titrations ( 3 2 ) also showed that, on average, there was more than one complexing site per average SFA molecule (42). The conditional stability constant K increased with increasing pH, a result seen with the other techniques. In contrast to the L-tyrosine trials, the residual fluorescence for SFA was substantial, about 20%. This relatively high residual may occur because the fluorescence efficiency of the complex is about 20% that of the free ligand. Alternatively, it is possible that the residual fluorescence is attributable to nonbinding fluorescent molecules in the SFA mixture. In this case, the binding material, as in the L-tyrosine experiments, would be quenched almost completely. Fluorescence spectrometry has several advantages, including the required detection limit, for determining micromolar complexing capacities of natural organic matter. It gives results that are comparable to those from DPASV, ISE, and dialysis/atomic absorption experiments. Fluorescence, which differentiates free and bound ligand, is an excellent complement to the other techniques for measuring complexing capacity, which distinguish between free and bound metal ions. Furthermore, FS is fairly rapid and requires no supporting electrolyte. Measurement of solution scattering with the FS instrumentation gives valuable information on aggregation and precipitate formation; this is important, because the usual goal in
Environ. Sci. Technol., Vol. 16, No. 9, 1982
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fulvic acid and
4.0
Strong sites: 5.6 w sn-: 4.0 Strong snes- 6 o Week sites: 4 1 Swng sites: 6.3 Weak sites: 3 8
measuring complexing capacity and stability constants is an understanding of solution-phase equilibria. By carefully considering both fluorescence intensities and scattering, one can distinguish between metal-ion complexation, which is a solution-phase process, and adsorption, which occurs only when a solid phase is present. Work is continuing in our laboratory on the application of fluorescence to natural water samples. Modeling Various models have been used to explain complexation of metal ions by fulvic acid. The Scatchard method (42, 43) applies to a model in which fulvic acid has distinct classes of sites, each with a common ability tocomplex metal ions. The calculation method described by Buffle and co-workers (56) applies to a system with two types of complexes: one average fulvic acid molecule and one metal ion (1:l complex) and two average fulvic acid molecules and one metal ion (2:l complex). A simpler model system than either of these is the one in which only 1:l complexes are postulated. More complex models are also available (4.57). Certain experimental conditions and different metal ions or samples of fulvic acid may favor one model over the others. A complete discussion of various models and their merits would be tedious and, possibly, not helpful. Table 4 shows results of stabilityconstant calculations for fulvic acid complexes with strongly bound Cuz+ and weakly bound Cd2+ ions (58). For each metal ion, the stability constants have been determined by applying two different calculation schemes to the same data set. The results for the two calculation methods are quite similar, 510A
Envimn. Sci. T e d l ~ I . VoI. . 16. Na. 9. 1982
3.0
3.2
35
3.8
indicating, perhaps, that the two models in each case are about equally appropriate (or inappropriate). Tbe two sets of results for either metal ion would work about equally well if incorporated into more complex models that include both inorganic and organic factors affecting metal-ion speciation. Avoiding pitfalls Complexes between metal ions and naturally occurring organic matter can be studied, but the commonly used methods have pitfalls. Data must be interpreted with attention to possible shifts in equilibrium during measurement, adsorption of organic matter on electrodes and membranes, and the appearance of a solid phase if aggregation occurs. In addition, the researcher needs to consider such factors as isolation procedures, as well as the need to modify natural water samples when trying to draw conclusions about environmental processes from the results of laboratory experiments. Acknowledgment This work was supported in part by Office of Water Resources Technology Grant BOOCNH through the University of New Hampshire Water Resources Research Center, and National Science Foundation Grants OCE 77-08390 and OCE 7910571.
Prior to publication, this article was read and commented on for suitability as an ES&T feature by Russell F. Christman, Chairman, Department of Environmental Science and Engineering, University of North Carolina, Chapel Hill, N.C. 27514, and John Ertel, Department of Oceanography, WV-IO, University of Washington, Seattle, Wash. 98105. References (I)Reuter. J . H.; Perdue, E. M. Geochim. Cosmochim. Acta 1977.41, 325-34.
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Robert A. Sanr (leji) is a senior seienrisr with fhe groundwater consultingfirm of Ceraghty & Miller, Inc.. where he prepares monitoring programs /or groundwater contomination studies pnd interprets the resulting chemical data. He srudiedar the Uniuersity of New Hampshire during 197540. and was a member ofthe Manhattan College Chemistry Deparrment faculry during 1980-81.
J a m H.Weber (righf) is a professor of chemisfry at the Uniuersiry of New Hampshire. He lecfures on general and inorganic chemistry. His research group does organometallic chemistry as well as environmental coordinarion chemistry.
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CIRCLE 8 ON REAOER SERVICE CARD Enuirm. Sci. Techmi.. Vol. 16. No.9.1982
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