Grantee Research Project Results
Final Report: The Secondary Structure of Humic Acid and its Environmental Implications
EPA Grant Number: R822832Title: The Secondary Structure of Humic Acid and its Environmental Implications
Investigators: Von Wandruszka, Ray
Institution: University of Idaho
EPA Project Officer: Packard, Benjamin H
Project Period: August 1, 1995 through September 1, 1998
Project Amount: $323,438
RFA: Exploratory Research - Chemistry and Physics of Water (1995) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Safer Chemicals
Objective:
The study described here deals with the fundamental and applied chemistry of dissolved humic acids (HAs). This group of substances is a major part of the humus in soil and water, i.e. the material that results from the decay of organic material and gives the soil its brown color. Derived from both plant and animal matter, it is widely distributed in natural matrices and has a major influence on their properties. These include the retention of man-made pollutants by soils, and the ability of surface and ground water to transport them. This latter aspect is the focus of the work summarized here.Many pollutants, including petroleum products, are hydrophobic - they are incompatible with water, and have low solubility in it. One of the most important properties of HA is its detergent character, i.e. its ability to solubilize such hydrophobic materials. This is a major cause of their dispersal through soil and water, as illustrated by the familiar spread of pollutant plumes from leaking underground storage tanks and other point sources. Plumes consisting of e.g. gasoline or jet fuel would have little tendency to be carried through the dry upper layers of the soil (the vadose zone) by percolating water, were it not for the HA dissolved in it. The behavior of HA in aqueous solution is therefore of considerable interest, especially in view of the fact that it exists in many varieties, depending on age and origin, and that its detergent character is strongly influenced by other substances present in the environment.
Summary/Accomplishments (Outputs/Outcomes):
In the fundamental portion of the funded work we have shown that aqueous HA displays distinct detergent behavior at concentrations as low as 5-10 ppm. We have found that this can be ascribed to the formation of humic "pseudomicelles" in water. These are submicroscopic aggregates of HA molecules that are analogous to the micelles formed by soaps and other surface active compounds. As such, they have nonpolar cores, comparable to miniature oil drops, and polar surfaces that make them water compatible. Their structure in HAs is less well defined than it is in synthetic detergents, due to variations in molecular size and composition of HA. The effects, however, are similar. We have shown that HA pseudomicelles can form by both intra- and intermolecular processes. In the intramolecular case, HA polymers coil and fold to create molecular domains that may be likened to knots in a string. The insides of these structures provide hospitable sequestration sites for hydrophobic pollutants that are thereby solubilized and transported. The intermolecular process, on the other hand, involves the association of discrete HA molecules, both large and small. The effect is again similar, but it requires greater HA concentrations to bring it about.We have shown that both mechanisms of HA aggregation are promoted by the presence of positive ions in the solution, and by increased temperature. The former means that HA is a better detergent in the presence of dissolved salts. This is especially true in cases where the salts contain multivalent metal ions such as Mg2+ and Sm3+. Monovalent ions such as Na+ and H+ also have an enhancing effect, but to a somewhat lesser degree. In the case of H+, this implies that lowering the pH boosts the detergent character of HA. In all instances, however, a point is reached where the presence of the ions causes macroscopic HA aggregation, leading to its precipitation from solution.
The nature of HA itself (and there are many different types) also has a profound influence on its detergent qualities. We found that molecular size and flexibility are important variables in this regard. HAs containing larger and/or more flexible polymers form pseudomicelles more effectively and are better detergents. In keeping with this, we have shown that fulvic acid (HA's smaller "cousin") has little tendency to aggregate and is a relatively poor detergent.
Temperature plays a remarkable and unexpected role in HA detergency. As the temperature of a solution is increased, HAs tend to aggregate more and become more effective detergents. We have found that this even leads to a phenomenon referred to as "clouding", which is well known in surfactant chemistry. It involves the "squeezing out" of water by aggregating HA polymers, which then collapse onto themselves and form larger assemblies.
We have shown that all forms of HA aggregation are diminished when the humic polymers are reduced in size. In the laboratory, this was brought about by UV irradiation (photolysis), which rendered the HA molecules smaller and unable to form pseudomicelles. The detergent character decreased commensurately. We also demonstrated that certain HA purification procedures that are traditionally used to isolate the material from soil, in fact cause polymer size reduction and other chemical changes. Consequently, HAs obtained in this way are not always representative of the unadulterated material.
In the applied portion of the funded research, which was a direct consequence of the fundamental studies, we have shown that crude HA can be used effectively for water purification. Potentially large-scale application is made possible by the availability of a crude humic blend (~80% HA), which, as a mined material, is plentiful and inexpensive. We have conducted both continuous flow and batch decontamination studies, and have found that both metallic and neutral organic contaminants can be effectively removed from polluted natural water (e.g. mine run-off). In the case of the flow studies, solid HA was mixed with sand and packed into a column. Polluted water was pumped through this column, and the contaminants were retained by the HA medium. In batch decontamination, dissolved HA and lime were added to the water, forming a sludge that contained 90+% of the contaminants. The column process is especially attractive, because it is suitable for acidic waters (see e.g. the Berkeley Pit superfund site in Montana), in which the solid HA does not dissolve. After use, the spent HA packing can be landfilled or incinerated. In the case of metal removal, contaminant reclamation is even possible.
Journal Articles on this Report : 15 Displayed | Download in RIS Format
Other project views: | All 36 publications | 16 publications in selected types | All 15 journal articles |
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Engebretson RR, Von Wandruszka R. The effect of molecular size on humic acid associations. Organic Geochemistry 1997;26(11-12):759-767. |
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Engebretson RR, von Wandruszka R. Kinetic aspects of cation-enhanced aggregation in aqueous humic acids. Environmental Science & Technology 1998;32(4):488-493. |
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Engebretson RR, von Wandruszka R. Effects of humic acid purification on interactions with hydrophobic organic matter: evidence from fluorescence behavior. Environmental Science & Technology 1999;33(23):4299-4303. |
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Evdokimov E, von Wandruszka R. Decontamination of DDT-polluted soil by soil washing/cloud point extraction. Analytical Letters 1998;31(13):2289-2298. |
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McCarroll M, Toerne K, von Wandruszka R. Micellar fluidity and preclouding in mixed surfactant solutions. Langmuir 1998;14(11):2965-2969. |
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McCarroll ME, Toerne K, von Wandruszka R. Preclouding in mixed micellar solutions. Langmuir 1998;14(21):6096-6100. |
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Ragle CS, Engebretson RR, von Wandruszka R. The sequestration of hydrophobic micropollutants by dissolved humic acids. Soil Science 1997;162(2):106-114. |
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von Wandruszka R, Ragle C, Engebretson R. The role of selected cations in the formation of pseudomicelles in aqueous humic acid. Talanta 1997;44(5):805-809. |
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von Wandruszka R. The micellar model of humic acid: evidence from pyrene fluorescence measurements. Soil Science 1998;163(12):921-930. |
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von Wandruszka R, Schimpf M, Hill M, Engebretson R. Characterization of humic acid size fractions by SEC and MALS. Organic Geochemistry 1999;30(4):229-235. |
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Yates III LM, Engebretson RR, Haakenson TJ, von Wandruszka R. Immobilization of aqueous pyrene by dissolved humic acid. Analytica Chimica Acta 1997;356(2-3):295-300. |
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Yates III LM, von Wandruszka R. Functional group analysis of Suwannee River fulvic acid with reactive fluorescent probes. Fresenius' Journal of Analytical Chemistry 1999;364(8):746-748. |
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Yates III LM, von Wandruszka R. Decontamination of polluted water by treatment with a crude humic acid blend. Environmental Science & Technology 1999;33(12):2076-2080. |
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Yates III LM, von Wandruszka R. Effects of pH and metals on the surface tension of aqueous humic materials. Soil Science Society of America Journal 1999;63(6):1645-1649. |
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Young C, von Wandruszka R. A comparison of aggregation behavior in aqueous humic acids. Geochemical Transactions 2001;2:16-20. |
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Supplemental Keywords:
Scientific Discipline, Water, Hydrology, Physics, Chemistry, Engineering, Chemistry, & Physics, transport model, subsurface, nonpolar micropollutants, hydrophobic chemicals, humic acid, sediment comparison data, colloidRelevant Websites:
http://www.chem.uidaho.edu/~rvw/index.htmlProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.