Humic and Fulvic Acids - American Chemical Society

Chapter 2

Micellar Nature of Humic Colloids 1

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T. F. Guetzloff and James A. Rice

Downloaded by GEORGETOWN UNIV on August 24, 2015 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch002

Department of Chemistry and Biochemistry, South Dakota State University, Brookings, SD 57007-0896

Humic and fulvic acids are ill-defined and heterogeneous mixtures of naturally-occurring organic molecules that possess surface active properties. The molecules that comprise this mixture are also known to form aggregates of colloidal dimensions. Humic and fulvic acids are shown to be able to solubilize hydrophobic organic compounds (HOC) in a manner that is consistent with known, micelle-forming surfactants, but not at organic carbon concentrations that are environmentally relevant. In addition, it is found that some HOCs are not solubilized to the same extent as other HOCs. Some implications of the micellar nature of humic materials are briefly discussed.

Micelles are colloidal particles formed by the concentration-dependent aggregation of surfactant molecules (1). In an aqueous environment micelles form when the hydrophobic portions of the surfactant molecules begin to associate at a surfactant concentration that is referred to as the "critical micelle concentration", or CMC, as a result of hydrophobic effects. In water, a micelle has a hydrophobic core and a charged surface that is the result of the orientation of ionizable or hydrophilic functional groups out into the bulk solution. At concentrations prior to the CMC the surfactant molecules migrate to the solution-air interface which disturbs the structure of the water molecules and results in a decrease in the solution's surface tension (2). At concentrations greater than the CMC, increasing Current address: Chemistry Department, Mount Marty College, Yankton, SD 57078 Corresponding author

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0097-6156/96/0651-0018$15.00/0 © 1996 American Chemical Society

In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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amounts of surfactant result in the formation of additional micelles but the surface tension remains constant. The surface tension of a surfactant will typically undergo an abrupt transition to a constant value at the CMC. Because aqueous micelles have a hydrophobic core they can, in effect, act as a second, nonaqueous phase in a system and greatly enhance the apparent water solubility of relatively insoluble hydrophobic organic compounds (HOC). Because this solubility enhancement is only observed at, or after, the onset of micelle formation, it is a criterion for identifying the formation of a micelle (3). The coincidence of the onset of a constant surface tension and the abrupt solubilization of a HOC is a definitive test for micelle formation. It has been recognized for some time that the presence of even small amounts of humic or fulvic acid in an aqueous solution can significantly enhance the apparent water solubility of a hydrophobic organic compound (eg., 4). This observation has been extrapolated so that the ability of humic and/or fulvic acid to effect this solubilization in aqueous solutions is often attributed to the presence of micelles (4-12). Guetzloff and Rice (13) first demonstrated that humic acid will form a micelle in alkaline, aqueous solutions but not at concentrations that are likely to be encountered in natural environments. Though the solution conditions in the experiments performed to date limit the extrapolation of these results to natural systems, there are important aspects of humic substances chemistry that can be studied by understanding their process of micelle formation. For example, the hydrophobic core of an aqueous micelle provides a nonaqueous environment into which hydrophobic organic contaminants can partition. While the alkaline conditions that are necessary for micelle formation to occur in the experiments reported here make this an unlikely mechanism in a natural environment, understanding the solubilization of HOC by humic and fulvic acid may provide insight into HOC interactions with humic materials in the solid state. In an aqueous system, a micelle forms as a result of hydrophobic interactions between lyophilic portions of surfactant molecules which produces a colloidal aggregate whose surface is studded with hydrophilic functional groups. This implies a certain juxtaposition of hydrophilic and hydrophobic regions of the molecules that comprise a humic acid or fulvic acid. In order to form a micelle the molecules must be amphiphilic, that is, one portion of the molecule must be more hydrophobic while the other must be more hydrophilic. This gives indirect information on the arrangement of functional groups that might be present. Materials and Methods Materials. Humic acid (HA) and fulvic acid (FA) were isolated from a soil known as the Poinsett silt-loam using a traditional alkaline extraction procedure. Solutions of HA or FA were made by dissolving enough material in aqueous NaOH (pH=10.6) to give a range of concentrations upto -12.3 gm HA or FA per liter.



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HUMIC AND FULVIC ACIDS

Surface Tension Measurements. The surface tension of each HA or FA solution was measured using a du Nouy ring tensiometer as previously described (13). HOC Solubilization. Carbon-14 labeled DDT (2,2-bis(4-chlorophenyl)-l,l,ltrichloroethane) or pyrene were used as HOC probes to study the micelle formation phenomena. The probe dissolved in toluene was added to a culture tube, and the solvent evaporated under nitrogen. Ten milliliters of each HA or FA solution were added to the tube which was then capped with a teflon-lined lid. The tube was transferred to a water bath (25.0 °C±0.5 °C) where the solutions were equilibrated for 24 hours with periodic shaking. After equilibration the tubes where centrifuged to minimize the amount of suspended or particulate probe molecule. An aliquot of the supernatant was then withdrawn and transferred to a scintillation vial. Scintillation fluid was added to the vial and the pemission counted at a 0.5% counting error. Additional methodological details can be found elsewhere (13). Small-angle X-ray Scattering. Small-angle x-ray scattering (SAXS) measurements of the PSL humic acid were performed on the 10-meter SAXS camera at Oak Ridge National Laboratory (14). Fractal dimensions were calculated from log-log plots of the scattering intensity as function of the scattering vector (I(q)) versus the scattering vector (q). The procedure used to characterize humic acid, and determine its fractal dimension, has been described by Rice and Lin (15-16). The radius of gyration (which is defined as the root mean square of the distance from the electrons in a particle to the center of charge) of the HA sample was obtained from the slope of of a plot of In I(q) versus q (17). 2

Results and Discussion HOC Solubilization. The effect of increasing HA and FA concentrations on the solubility of DDT and pyrene are shown in Figures 1 and 2. The effect of increasing HA or FA concentration on the surface tension of the resulting solution is also shown. The surface tension of each solution decreases with increasing HA or FA concentration until at concentrations of 7 gm H A / L or 6.8 gm FA/L the surface tension becomes constant. The surface tension of the the FA solution is higher than that of the HA which indicates that FA is a weaker surfactant than humic acid under these conditions. Combined with the solubility data of the HOC probes discussed later, these values can be taken as CMC values for the formation of micelles by HA and FA. The surface tension of the FA solution (Figure 2) shows a pronounced drop just prior to becoming constant. This type of behavior is typical of mixed surfactant solutions (18). Given the heterogeneous nature of fulvic acid, the presence of this drop in the surface tension would be expected. The solution concentration of DDT is found to increase at the CMC for


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GUETZLOFF & RICE

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both HA and FA, and it increases dramatically in HA solutions with concentrations above the CMC. The coincidence of the abrupt change in slope of the surface tension plots, and the abrupt increase in the DDT concentration is direct evidence for the formation of micelles in aqueous alkaline solutions of humic materials (13). It should be reemphasized,


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• Surface Tension A DDT Concentration ° Pyrene Concentration

Humic Acid Concentration (g/U Figure 1. Effect of increasing humic acid concentration on the surface tension and apparent solubility of DDT and pyrene. The solid line indicates the position of the HA CMC. Standard deviations are less than the height of the symbols.

however, that the concentrations at which the CMC was observed in these experiments (7 gm H A / L and 6.8 gm FA/L) make it unlikely that micelle formation would take place in a natural system. The solubilization behavior of pyrene is different than that exhibited by DDT. The apparent water solubility of pyrene does increase at the CMC in


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the HA solutions, but not as abruptly as in the case of DDT. Apparently pyrene is not as readily taken up by HA micelles as is DDT. Careful examination for Figure 1 also shows that there is a gradual solubility enhancement of pyrene even before the CMC. Figure 2 shows a solubility enhancement for pyrene in the presence of even low concentrations of FA, but there is no abrupt concentration increase that can be attributed

• Surface Tension 1

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* DDT Concentration n

Pyrene Concentration

Fulvic Acid Concentration

(g/D Figure 2. Effect of increasing fulvic acid concentration on the surface tension and apparent solubility of DDT and pyrene. The solid line indicates the position of the FA CMC. When present errors bars represent ± 1 standard deviation, all others are less than the height of the symbol.

to the formation of a micelle. Thus, while FA is able to enhance the water solubility of pyrene it does not do so through micelle formation. Gauthier et ah (19-20) concluded that polyaromatic hydrocarbon binding to HA or FA was a result of van der Waals interactions between the aromatic rings of polyaromatic hydrocarbons and the aromatic components of the humic



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GUETZLOFF & RICE


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material; higher HA or FA concentrations should result in higher solution pyrene concentrations. While this mechanism can be used to explain the solubility behavior of pyrene in HA and FA solutions prior to the CMC (the HA and FA concentrations used in their studies were on the order of milligrams of humic material/L), it does not seem to explain the concentration increase at the onset of HA micelle formation which is attributed to the solubilization of pyrene in the hydrophobic core of the micelle. The difference in the ability of HA and FA to solubilize the probes could be related to the size of the molecules which comprise each humic material. Humic acid is generally believed to consist of larger molecules than does fulvic acid isolated from the same environment (21-22). The micelles formed during FA aggregation could be smaller and less able to accommodate DDT and pyrene in their interior than larger HA micelles. Estimates of the molecular dimensions of DDT and pyrene using HYPERCHEM gave lengths of 10.3 A and 9.1 A, respectively. Using SAXS measurements Thurman et al. (23) estimated the size of FA aggregates and found that every sample in the suite that they characterized had a radius of gyration of less than lOA. This gives an indication that the probe molecules being investigated are on the order of the size of the FA micelles which may interfere with the probe's solubilization. In contrast, the radius of gyration of HAs from a variety of environments have been reported in the range of 8 A to 100 A (23-24). Table I gives the radius of gyration of the HA solutions at several concentrations and the corresponding fractal dimension. The values Table I. Radius of gyration and the fractal dimension of HA at various solution concentrations. Fractal Dimension*

HA Concentration

Radius of Gyration*

2.0 gm/L

5.5 A

nd

6.0 gm/L

4.8 A

2.8

8.3 gm/L

4.8 A

2.9

11.0 gm/L

5.2 A

2.8

nd - could not be determined because scattering was very weak. The uncertainty associated with each measurement is ± 0.1. reported here are smaller than those previously reported in the literature (23-24), but given the heterogeneity of humic materials and the effect of solution parameters on their size, this is not surprising. As the solution concentration increases the particle size remains essentially constant. The fractal dimension also remains constant as the concentration increases. Neither of these measurements, however, show an abrupt change as the concentrations go from below the CMC ( 2.0 and 6.0 gm HA/L) to above


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the CMC (8.3 and 11.0 gm/L) which would be expected if all of the components were involved in the micellization process. The lack of this abrupt transition suggests that only a portion of the molecules that comprise HA are responsible for the HOC solubilization observed in Figures 1 and 2.


Summary Both HA and FA have been shown to form micelles, but not at concentrations that are environmentally relevant. The inability of FA to solubilize pyrene at concentrations above its CMC, and the lower solubility enhance of DDT in FA micelles compared to HA suggests that the smaller size of the molecules which comprise FA, and consequently the micelles that form from it, affects the solubilization phenomena. The SAXS analysis of HA does not show an abrupt change in size or fractal dimension as the solution concentration increases beyond the CMC which suggests that only a portion of the molecules which comprise HA are involved in the micellization phenomena. Acknowledgement This work was supported by the US Department of Agriculture, National Research Initiative Competitive Grants program through agreement no. 91-37102-6864. References 1. Tanford, C. The Hydrophobic Effect: Formation of Micelles and Biological Membranes; Wiley: New York, 1980. 2. Popiel, W. Introduction to Colloid Science; Exposition Press: New York, 1974. 3. Mukerjee, P.; Mysels, K. Critical Micelle Concentrations of Aqueous Surfactant Systems; Natl. Bureau Standards Data Ser., 36, National Bureau of Standards: Washington, DC, 1971, pp. 1-21. 4. Wershaw, R.; Burcar, P.; Goldberg, M. Environ. Sci. Technol. 1969, 3, 271-273. 5. Piret, E.; White, R.; H.; Walther, H.; Madden, A. Sci. Proc. R. Dublin Soc. 1960, A1, 69-79. 6. Visser, S. Nature 1964, 204, 581. 7. Boehm, P. D.; Quinn, J. G. Geochim. Cosmochim. Acta 1973, 37, 24592477. 8. Tschapek, M.; Wasowski, C. Geochim. Cosmochim. Acta 1976, 40, 1343-1345. 9. Chen Y.; Schnitzer, M. Geoderma 1978, 26, 87-104. 10. Rochus, W.; Sipos, S. Agrochim. 1978, 22, 446-454. 11. Hayano, H.; Shinozuka, N.; Hyakutake, M. Yukagaku 1982, 31, 357-362.



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12. Hayase, K.; Tsubota, H. Geochim. Cosmochim. Acta 1983, 47, 947-952. 13. Guetzloff, T.F.;Rice, J. A. Sci. Total Environ. 1994, 152, 31-35. 14. Wignall, G. D.; Lin, J. S.; Spooner, S. J. Appl. Crystallog. 1990, 23, 241246. 15. Rice, J. A.; Lin, J. S. Environ. Sci. Technol. 1993, 27, 413-414. 16. Rice, J. A.; Lin, J. S. IN Humic Substances in the Global Environment and Implications on Human Health; Senesi, N.; Miano. T. M., Eds., Elsevier B. V.: Amsterdam, 1994, pp. 115-120. 17. Chen, S.; Lin, T. IN Methods of Experimental Physics, v. 23B, Academic Press: New York, 1987, pp. 489-543. 18. Schott, H. J. Phys. Chem. 1966, 70, 2966-2973. 19. Gauthier, T. D.; Shane, E.C.;Guerin, W. F.; Seitz, W. R.; Grant, C. L. Environ. Sci. Technol. 1986, 20, 1162-1166. 20. Gauthier, T. D.; Seitz, W. R.; Grant, C. L. Environ. Sci. Technol. 1987, 21, 243-248. 21. Stevenson, F. J. Humus Chemistry; Wiley: New York, 1982, pp. 285308. 22. Wershaw, R. L.; Aiken, G. R. IN Humic Substances in Soil, Sediment, and Water: Geochemistry, Isolation, and Characterization; Aiken, G. R.; McKnight, D. M.; Wershaw, R. L.; MacCarthy, P., Eds., J. Wiley: New York, 1985, pp. 477-492. 23. Thurman, E. M.; Wershaw, R. L.; Malcolm, R. L.; Pinckney, D. J. Org. Geochem. 1982, 4, 27-35. 24. Wershaw, R. L.; Burcar, P. J.; Sutula, C. L.; Wiginton, B. J. Science 1967, 157, 1429.


Humic and Fulvic Acids - American Chemical Society

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