Environ. Sci. Techno/. 1995, 29, 622-628
Potentiometric Titrations of Humic Substances: Do Ionic Strength Effects Depend on the Molecular Weight? J A M E S H . EPHRAIM,* C A T H A R I N A PETTERSSON, MARIA NORDEN, AND BERT ALLARD Department of Water and Environmental Studies, Linkoping University, S-581 83 Linkoping, Sweden
That the solution chemistry of humic substances is perturbed by ionic strength effects and functional group heterogeneity is now widely accepted. Application of the two-phase approach employing concepts from the Donnan equilibrium and the electrochemical unit cell had earlier postulated the ionic strength effects to be due to the presence of higher molecular weight moieties in the fulvic acid “assemblage”. Potentiometric titrations of a number of aquatic fulvic and humic acids (isolated by an initial preconcentration on diethylaminoethyl (DEAE) cellulose prior to their adsorption on XAD-8 resins) are presented as a function of ionic strength and their molecular weight distributions. Even though an apparent correlation between molecular weight distribution and ionic strength effects was observed,for humic substances of similar molecular weight, ionic strength effects seem to increase with an increase in the polydispersity of the molecule.
Introduction Until recently, the need to consider separately the ionic strength effects and functional group heterogeniety effects was not incorporated into modeling of proton and metal ion binding by humic substances (1). A number of researchers have since incorporated the concept of separating ionic strength effects from function group heterogeneity into models that either belong to the discrete multiligand school (2-6) or the continuous distribution approach (7-13). In efforts to explain the electrostatics in the equilibria of humic substances, some recent studies have envisaged the humic substance molecule to be constituted of rigid impermeable spheres (2-4) or cylinders (7-9) forming an ensemble of identical heterogeneous polyelectrolytes for which an average potential holds near all groups. For the smaller molecules, the ionic strength effects on the potentiometric behavior attributed to the radial distribution of the operating electrostatic field have been shown to be significantly smaller than that of the large particles. Application of the Debye-Huckel theory to describe the dissociation properties of hypothetical spherical and rigid “humic molecules” indicates that ionic strength effects are more pronounced the higher the molecular weight (3). Adaptation of the spherical and cylindrical double-layer model has also indicated that ionic strength effects increase with an increase in the molecular weight of the molecules (8, 9). In this paper, it has been our objective to identify possible correlations between ionic strength effects and the molecular weights for the various fulvic acids extracted from different origins. An additional objective has been the presentation of potentiometric titration results of a number of fulvic acids, which have been interpreted using the approach developed earlier ( 1 ) .
Essential Concepts of the Approach The unified physicochemical approach developed earlier has employed knowledge from the application of the Gibbs-Donnan concept to the potentiometric titrations of synthetic polymers (14). The humic substance is first and foremost conceptualized as an assemblage of relatively small amphiphilic moieties that are slightly different but composed of four to five predominant separate acidic sites with each site characterized by a distribution of average acid strengths. In such conceptualization, the humic molecule could either be arigid or flexible sphere, rod-like, or a flat plate depending on a number of factors including pH, ionic strength, temperature, and concentration. The perturbing factors to the dissociation properties of these humic molecules have been identified as ionic strength effects arising from charge buildup on the assemblage and the functional group heterogeniety (1). Most researchers estimate the ionic strength effect, Le., the electrostatic effects, by invoking the rigid sphere or cylinder (2-4, 7-9) and applying a combination of the Debye-Huckel theory and variations of the Poisson-Boltzman distribution functions. In the unified physicochemical approach, the ionic strength effects, Le., the electrostatic correction term, are obtained experimentally (1,15). The methodology for such ~
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* Corresponding author: e-mail address:
[email protected].
022 IENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 3, 1995
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C 1995 American Chemical Society
experimental determination employs the Gibbs-Donnan equilibrium concept as its basis (14, 16). The logic behind the unified physicochemical approach emphasizes the need to accuratelyestimate the water uptake by the solvent sheath surrounding a charged polymeric molecule (14). Such an objective is achieved by expressing the equilibrium condition with the following equation:
I I
I
(2)
for the “critical”bulk electrolyteconcentration. The critical bulk electrolyte concentration is the concentration after which no ionic strength effects are observed on the pKa versus degree of neutralization, a, curve (determined to be I = 1.00 M) (1). To achieve data fitting at any other ionic strength, the counterion concentration correction term, ApK ( I ) , must be included in eq 2 as follows:
(d) Steps a and b above are changed until the following relationship is achieved, i.e.
(4) and the residual lacomputed - aexperimentd12 reaches aminimum for the set of data points. In eq 4, ai is the degree of ionization for each acid site; fi is the fraction of the total acid capacity determined in aqueous titrations due to the
Dworptlon wlth 0.3 M NaOH
i t
ai = (1 + 10‘pK~-pH’)-’
Adsorptlon on DEAE~sllulose
i
I
I
where pKaappand pKLntrin are the respective apparent and intrinsic pKof the monomeric acid repeated in the polymer. The terms pM,’ and pMp- are the respective activities of the counterions in the solution and polymer domain. A detailed derivation of eq 1is presented elsewhere (14).From eq 1 above, it can be seen that the difference between the apparent and the intrinsic dissociaton constants may be estimated by the ratio of the counterion activities in and out of the polymer domain. Thus, changes in the solvent sheath contents of apolymeric molecular may be estimated from potentiometric titrations as a function of varying counterion concentrations, pM,’, as long as the volume of the polymer and activity coefficients in the polymer domain can be estimated (14,17). Based on the conceptualization that humic substances are composed of four to five separate acidic moieties, the potentiometric titrations are interpreted for the system at the ionic strength where separate phase effects are minimal, i.e., I > 1.00 M. The steps involved in assigning pKa values to the envisaged acidic moieties are presented below: (a) Four to five “average” sites are projected, and their respective abundances are estimated from nonaqueous titrations and titrations in the presence of increasing amounts of Cu(I1)and Eu(II1)or La(II1) (1,16,18,19).From nonaqueous titrations, abundances of moieties of -COOH type and -OH type are estimated ( I , 18, 19). From the titrations in the presence of increasing amounts of Cu(1I) and Eu(II1) or La(III), abundances of possible chelating moieties are estimated (1, 15, 16, 18, 19). (b) Initial guesses of their pKa values are made. (c) The degree of neutralization of each of the envisaged “average”titratable site, ai, is computed using the following equation:
NATURAL WATER
I I
I I I I
t
I I
t
Addlflcatlon wlth concentrated HCI (to pH 2) Separatlon by Centrlfugatlon
/
I Soluble fractlon
Adsorptlon on XAD-8
\ Insoluble fradlon HUMIC ACID
Desalting wlth water
Desorption wlth 0.1 M NaOH
I I
t I I
t
catton exchange, I+ Lyophlllzatlon
I
I
t
FULVIC ACtD
FIGURE 1. Scheme showing the isolation and purification of aquatic humic substances (adapted from ref 24).
ith site; acomputed and aexperimental are the respective overall computed and experimental degree of neutralization ( 1 ) .
Experimental Section Isolation and Purification of Humic Substances. The fulvic and humic acids employed in this study have been isolated and purified following the scheme shown in Figure 1 (20-24). The DEAE method was well suited to our particular situation because it eliminated the addition of strong acid to the water samples, especially for groundwater systems. That there may be differences in the characteristics of the final product as a result of the method of extraction/ isolation cannot be overemphasized, but the humic substances compared in this paper have been isolated and purified by the same procedure. As a result, it is hypothesized that perturbations to the final product are similar, thus allowing comprehensive comparison. Determination of Physicochemical Parameters. Elemental analyses were performed at Mikro Kemi AB at Uppsala, Sweden. The molecular weight distributions of the various humic substances were determined using gel filtration. A HPLC (Waters) coupled with a 7.5 x 300 mm column filled with a silica-based material that has been modified by vinylic alcohol (TSK2000SW) was employed for such determinations. Polystyrene sulfonates of known molecular weights were used as reference substances. It is again considered that a similar method (gel filtration) employed to determine the molecular weight distributions would cancel out perturbations arising intrinsically from the method, thus justifjmgthe comparison of the molecular weight distributions. PotentiometricTitrationsin Aqueous and Nonaqueous Media. Potentiometric titrations were performed in aqueous medium as a function of ionic strength, Le., from 0.001 to 0.10 M NaC104 ( I ) . Details of the method including necessary precautions are given elsewhere (1, 15). Application of an internal reference compound, p-hydroxyVOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
623
0
8.00 7,OO P
TABLE 1
Properties of Humic Substances: Correlations between Ionic Strength Effects and Molecular Weight
6.00
B 5,oo
Y
P
4,OO
sample
acidity (mequivlg) M.4
3,OO
Fulvic Acidse Bersbo RBdsla Finnsjan Fjallveden
2,oo 0.20
0.40
0,60
0.80
1,oo
degree of neutralization
Stripa Tiveden Vadstena Aspo Nordic ref Fanay-Auget'es
4.65 4.60 4.98 5.14 5.42 3.72 4.78 5.46 9.86 7.53 5.07
Bersbo soil Bersbo aqua Gorleben Nordic ref
1.48 4.44 3.03 4.82
GideB
8'oo 7,OO
T
b
2650 2520 2650 1700 1600 1090 1850 1050 1920 3360 900
1.51 1.59 1.51 1.36 1.39 1.17 1.48 1.24 1.26 1.54 1.20
0.30 0.12
0.30 0.17 0.35 0.00 0.20 0.00 0.00 0.00 0.14 0.23 0.12
0.00 0.00 0.00 0.00 0.50 0.00 0.16 0.45 0.01
Humic Acidsa
2,oo 0,20
17650 4.71 6100 1.97 11800 4.75 11190 2.54
NAb 0.25 0.43 0.13
0.22 0.31 0.30
0.06
I
0,40
0,60
0,80
aThe origin of the humic substances is described elsewhere (24). NA = not available.
LOO
degree of neutralization
8.00
C
8,OO
7,OO
! I 6poo
7,OO
? 5,OO
Y
a 4.00 3,OO 6.00
2,oo 0,20
0,40
0,60
0,80
1,oo
degree of neutralization
FIGUREZ. Potentiometrictitrations of threefulvic acids as a function of ionic strength(NaC104): (0)0.001 M, (A) 0.010 M, (e)0.100 M. (a) Gideii FA, M,,, = 1600. (b) Tiveden FA, A& = 1850. (c) FjHllveden FA, M,,, = 1700.
benzoic acid, in nonqueous titrations facilitated the determination of carboxylic (-COOHI and alcoholic (-OH) moieties in the various humic substances (18). Details of the nonaqueous titrations and the criterion for the selection of the internal reference compound are given elsewhere (18, 25).
Results PotentiometricTitrations as a Function of Ionic Strength. To facilitate the comparison of results obtained from potentiometric titrations, plots of apparent dissociaton constants (pKaapp)versus the degree of neutralization of all titratable acids (a)were made at different ionic strengths and for all the humic substances employed (1, 15). That the calculated pKaappvalues increased with an increase in a for all the fulvic acids demonstrates the heterogeneic and polymeric nature of the materials. The dependence of pKaapPversus a curve on the ionic strength is attributable to the polymeric nature of the molecule in solution. In the development of the unified approach, the effect of ionic strength on the pKaappversus a curve has been demonstrated to level off when the ionic strength is equal to or greater than 1.00 M salt concentration, Le., I z 1.00 M (I, 15, 19). The results of pKaaPPversus a curve show that ionic strength effects were different for different humic sub624
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 3. 1995
P 5,oo
Y
4.00
3.00
2,oo 0,20
0.40
0.60
0,80
1,oo
depree o( dissociation
FIGURE 3. Composite titration curves (p&ppvenus a)for 12different fulvic acid samples at different ionic strengths of 0.001.0.010, and 0.100 M NaCIOI. This figure is envisaged to give the upper and lower boundaries for the pKaappversus a curves for the 12 aquatic fulvic acid samples.
stances (Figure2). Tiveden FA exhibited properties similar to typical linear polyelectrolytes whereas for Fjdlveden FA no effect of ionic strength was observed. The ionic strength effects on the potentiometric behavior are compared for the various humic substances in relation to their molecular weight distributions (Table 1). In the first column of Table
Gorleben
Nordic ref.
Fanay-Augeres 0
1
2
3
4
5
6
7
acid capacity, meq/g HA
Bersbo Finnsj6n R6dsla Vadstena Gide& Tiveden Fanay - Augeres Fjallveden
0,OO
1,00 2,OO
3,OO 4,OO
5,OO 6,OO 7,OO
Acid Capacity, meq/g FA FIGURE 4. Comparison of aqueous and nonaqueous acid capacities for selected fulvic and humic acids: (a) humic acids; (b) fulvic acids. Note the closeness of the aqueous acid capacity to the nonaqueous -COOH capacity.
1 is the name of the area from which the humic material was isolated and extracted. In the second column, the titratable acidic capacity determined in aqueous medium is presented. The third and fourth columns contain the weight-average molecular weight (as determined by gel filtration chromatography) and ratio of weight-average molecular weight to number-average molecular weight (a measure of polydispersity). In the fifth column, the difference in PKa values at a = 0.5 between ionic strength of 0.001 and 0.01 M is presented for selected humic substances, while in the sixth column, the corresponding difference between ionic strength of 0.01 and 0.10 M is presented. It must be noted that the a = 0.5 value criterion was chosen onlyfor convenienceand that the ionic strength effect are the same even if charge balance (A-) is plotted against pH instead of pKaappversus a. From the table, it is difficult to identify a clear correlation between molecular weight (M,) and ionic strength effects. Even though higher molecular weight humic substances
appeared to exhibit considerable ionic strength effects, it cannot be stated that because a particular humic substance has a higher molecular weight the ionic strength effects on its potentiometric behavior will be large. However, for humic substances of similar molecular weight (M,) highly polydispersed molecules (withhigh M,/M, ratio) showed an apparent enhanced ionic strength effect. A plot of pKaaPPversus a curve for a number of fulvic acid samples (12 samples) is presented in Figure 3. This figure, which is a composite for the 12 samples at three different ionic strengths (0.001,0.010, and 0.100 M NaClOd, may be construed as an effort to determine the upper and lower boundaries of pKaappversus a curves for the humic substances under typical environmental conditions. From this figure, it is observed that the maximum displacement of pKa as a result of change in ionic strength and origin of sample is -2.0 pKunits for the a range of 0.30-1.0. It is observed that, at the lower pH values (lower a)where humic substances are more hydrophobic, considerable perturbaVOL. 29, NO. 3, 1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY 1625
a
tion to the pK, versus a curves is ohservedwhile at the high pH such perturbation is minimal. An overall pKaapp for aquatic fulvic acids may he crudely estimated at a = 0.5 in the midpoint ofthe handas4.0 f 1.0to be usedinchemical speciation programs and in performance assessment modeling where the effect of humic substances on the distribution and transport of metal ions is an objective. Such value will he justifiable for, at least, the 12 aquatic fulvic acid samples. If the philosophy that humic substances have more in common than differences (26)is invoked, then hy default such an average value will he appropriate for aquatic fulvic acids in general. Nonaqueous Titrations. Nonaqueous titration results for a number of fulvic and humic acids are presented in Figure 4. The carboxylic acid capacity in the nonaqueous titrations ranges from 4.2 to 6.4 mequivlg while the acid capacity in the aqueous titrations ranges from 4.6 to 5.4 mequivlg for the fulvic acids. The -OH capacity, which is acombination of strong alcohols and phenols, ranges from 0.8 to 2.5 mequivlg for the fulvic acid. The hypothesis is 626. ENVIRONMENTAL SCIENCE &TECHNOLOGY I VOL. 29, NO. 3. 1995
that extra weak acid moieties, which are nontitratahle in aqueous medium, may he detected in the nonaqueous titrations. The total acid capacity in the nonaqueous titrations is thus often larger than the aqueous capacity (Figure 4). An exception, however, is observed for the Finnsjon fulvic acid where the sum of the nonaqueous capacity is equal to the aqueous acid capacity (Figure 4). Thisohservationis rationalized hypostulatingthat the -OH groups determined in the nonaqueous titrations were "strong" en.ough to facilitate their detection in the aqueous titrations. The nonaqueous titrations of the humic acids yielded interesting values. The humic acids isolated using t h e m - 8 method, Bersho aquatic HA (Table 1)and Nordic reference HA, showed significant dissolution in aqueous medium at a pH as low as 3.0 and yielded an apparently highacidcapacity, i.e.,4.44and4.82mequiv/g,respectively. For the humic acids, the aqueous capacities ranged from 1.5 to 4.8 mequivlg while the nonaqueous carboxylic capacityrangedfrom 1.2 to 4.1 mequivlg. The -OH capacity ranged from 1.0 to 1.5 mequivlg for the humic acids.
Despite the fact that the resolution of the two inflection points in the nonaqueous titrations are dependent on the collective acid strengths of the acids, no pK, values can be assigned to moieties in aqueous medium owing to differences in their respective dielectric constants. The information obtained in the nonaqueous titrations are thus restricted to elucidation of the acidic spectrum existent in the humic substance molecule and as a means for comparing humic substances from different sources (16). Acid Site Heterogeneity Resolutions. Following the steps outlined above (16, 2 3 , the predominant acidic moieties for some select humic substances have been resolved. Results for samples for which all experimental data had been attained (including titrations at the critical ionic strength, Z = 1.00 M NaC104) are presented as pie charts in Figure 5 (1, 15, 19). In the figure, the percentage of each acidic site and its corresponding pKa are shown for the various fulvic acids. The site heterogeneity resolutions made for titrations in aqueous medium show that a large fraction of most fulvic acids have pK, values under 4.0, suggesting probable -COOH functionality (27). This hypothesis is corroborated by the closeness of aqueous acid capacity and the -COOH capacity in nonaqueous titrations (Figure 4).
Discussions and Conclusions Recently, the effect of ionic strength on the potentiometric titrations of humic substances have been attributed to the buildup of charge on the macromolecules of the humic substances as follows: the larger the molecule, the more pronounced the charge buildup and thus the more pronounced the ionic strength effects (3, 8). To allow such conclusions, both the monomeric molecular unit and the resultant polymer were assumed to be rigid spheres (2, 3) or either rigid spheres or cylinders (79). In this paper, potentiometric titrations of fulvic and humic acids from different origins and with different molecular weights have shown that ionic strength effects are not solely dependent on the molecular weight of the humic substance. Whereas humic substances with higher molecular weight had the tendency to show larger ionic strength effects, for samples with similar molecular weight, a larger ionic strength effect was observed for those molecules that are more polydispersed (as determined by the ratio of M,IM,). This observation suggests that the electrostatics in humic substance equilibria may be considerably affected by the polydispersity and conformation of the humic substance, contrary to the hypothesis of De Wit et al. (8). The effect of ionic strength changes on the pKa versus a curve for aquatic fulvic acids is small as compared to that for typical synthetic polymers (14). This small effect is of considerable relevance in exercises concerning the incorporation of natural organic acids into chemical speciation programs for the environment. The fact that at times ionic strength effects are observed and the fact that at times they are not observed seem to corroborate the contention that the behavior of humic substances in solution lies in between that of the polyelectrolyte and that of simple electrolytes, i.e., humic substances probably behave as “oligoelectrolytes”in solution (4, 28). In effortsto determine the effect of organic acids on the mobility of trace metals in the environment, it is necessary to determine the concentrations of these natural organic acids and to characterize them. Steps to affect such
characterization have been developed earlier (11,and some results have been presented in this paper. A number of researchers have developed concepts describing ion binding to humic substances following our initial presentation that functional group heterogeneityand “separate phase” effects brought about by ionic strength changes are the two complicating factors influencing the solution chemistry of humic substances (I). Model V developed by Tipping (2) and the approach of De Wit et al. (7-9) are examples of such new approaches. Whereas model Vand the approach by De Wit et al. emphasize data fitting, our approach places a greater emphasis on the chemical characterization of the fulvic acid. For example, De Wit et al. using their master curve approach describethe Bersbo fulvic acid as composed of eight sites per molecule (8)with an affinity distribution characterized by a large peak with a maximum in the log Krange 3-4. The description is equivalentto the statement that Bersbo FA has eight (identical?) sites with an average pKa in the range 3-4. It is our contention that such a description is less valuable for experimental efforts aimed at identifymg the functional group heterogeneity in the fulvic acid. Observations from nonaqueous titrations (15, 18) and derivatization methods (19) do not support such a description of the Bersbo fulvic acid. Our approach of describing the fulvic acid in terms of a discrete number of sites with corresponding pK, values and probable functionalities offers ideas in functional group characterization (29) and is amenable to verification by the utilization of literature values of stability constants describing the complex formation between a particular functional site on the fulvic acid molecule and a given metal ion, e.g., Cu2’ (30).
Acknowledgments Research grants from the Swedish Natural Science Research Council and the Swedish Nuclear Fuel and Waste Management Company are gratefully acknowledged. Valuable comments from an unknown referee are gratefully acknowledged.
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(11) Thakur, A. K.; Munson, P. J.; Hunston, D. L.; Rodbard, D. Anal. Biochem. 1980, 103, 240-254. (12) Shuman, M. S.; Collins, B. J.; Fitzgerald, P. J.; Olsen, D. L. In Aquatic and Terrestrial Humic Materials; Christman, R. F., Gjessing, E. T., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; p p 349-370. (13) Gamble, D. S.; Underdown, A. W.; Langford, C. H. Anal. Chem. 1980, 52, 1901-1908. (14) Marinsky, J. A. InlonExchangeand SoZventExtraction;Marinsky, J. A,, Marcus, Y., Eds.; Marcel Dekker Inc.: New York, 1993; Vol. 11, pp 237-334.
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(25) Ephraim, J. H.; Allard, B. Inlon Exchangeand SoluentExtraction; Marinsky, J. A., Marcus, Y., Eds.; Marcel Dekker Inc.: New York, 1993; VO~.11, pp 335-367. 126) Pettersson. C.: Euhraim. J.: Allard. B. Finn. Humus News 1991, 3, 133-138. (27) Perdue, E. M. In Humic substances in soil, sediment and water. Gemhemisty, isolation and characterisation; Aiken, G. R., McKnight, D. M., Wershaw, R. L., MacCarthy, P., Eds.: Wiley-Interscience: New York, 1985; pp 493-526. (28) Aiken, G. R.; McKnight, D. M.; Wershaw, R. L.; MacCarthy, P., Eds.: Humic Substances in Soil, Sediment and Water: Wiley Interscience: New York, 1985. (29) Arsenie, I.; Borkn, H.; Allard, B. Sci. Total Enuiron. 1992, 116, 2 13-220. (30) Ephraim, J. H.; Allard, B. Enuiron. Int. 1994, 20, 89-95.
Received f o r review May 9, 1994. Revised manuscript received December 5, 1994. Accepted December 6, 1994.@
ES9402819 %Abstractpublished in Advance ACS Abstracts, January 15, 1995.
