Chapter 1
Humic and Fulvic Acids and Organic Colloidal Materials in the Environment 1
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Jeffrey S. Gaffney , Nancy A. Marley , and Sue B. Clark
Downloaded by GEORGIAN COURT UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch001
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Environmental Research Division, Argonne National Laboratory, Building 203, 9700 Cass Avenue, Argonne, IL 60439-4831 Savannah River Ecology Laboratory, University of Georgia, P.O. Drawer E, Aiken, SC 29802 2
Humic substances are ubiquitous in the environment, occurring in all soils, waters, and sediments of the ecosphere. Humic substances arise from the decomposition of plant and animal tissues yet are more stable than their precursors. Their size, molecular weight, elemental composition, structure, and the number and position of functional groups vary, depending on the origin and age of the material. Humic and fulvic substances have been studied extensively for more than 200 years; however, much remains unknown regarding their structure and properties. Humic substances are those organic compounds found in the environment that cannot be classified as any other chemical class of compounds (e.g., polysaccharides, proteins, etc.). They are traditionally defined according to their solubilities. Fulvic acids are those organic materials that are soluble in water at all pH values. Humic acids are those materials that are insoluble at acidic pH values (pH < 2) but are soluble at higher pH values. Humin is the fraction of natural organic materials that is insoluble in water at all pH values. These definitions reflect the traditional methods for separating the different fractions from the original mixture. The humic content of soils varies from 0 to almost 10%. In surface waters, the humic content, expressed as dissolved organic carbon (DOC), varies from 0.1 to 50 ppm in dark-water swamps. In ocean waters, the DOC varies from 0.5 to 1.2 ppm at the surface, and the DOC in samples from deep groundwaters varies from 0.1 to 10 ppm (1). In addition, about 10% of the DOC in surface waters is found in suspended matter, either as organic or organically coated inorganic particulates.
Structure and Composition Humic materials have a wide range of molecular weights and sizes, ranging from a few hundred to as much as several hundred thousand atomic mass units. In general, fulvic acids are of lower molecular weight than humic acids, and soil-derived materials are larger than aquatic materials (1,2). Humic materials vary in composition depending on their source, location, and method of extraction; however, their similarities are more 3
Current address: Department of Chemistry, Washington State University, P.O. Box 644630, Pullman, W A 99164-4630 0097-6156/96/0651-0002$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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1. GAFFNEY ET AL.
Organic Colloidal Materials in the Environment 3
pronounced than their differences. The range of the elemental composition of humic materials is relatively narrow, being approximately 40-60% carbon, 30-50% oxygen, 45% hydrogen, 1-4% nitrogen, 1-2% sulfur, and 0-0.3% phosphorus (3). Humic acids contain more hydrogen, carbon, nitrogen, and sulfur and less oxygen than fulvic acids. Studies on humins have shown that they are similar to humic acids except that they are strongly bound to metals and clays, rendering them insoluble (4). Substantial evidence exists that humic materials consist of a skeleton of alkyl/aromatic units cross-linked mainly by oxygen and nitrogen groups with the major functional groups being carboxylic acid, phenolic and alcoholic hydroxyls, ketone, and quinone groups (5,6). The structures of fulvic acids are somewhat more aliphatic and less aromatic than humic acids; and fulvic acids are richer in carboxylic acid, phenolic, and ketonic groups (7). This is responsible for their higher solubility in water at all pH values. Humic acids, being more highly aromatic, become insoluble when the carboxylate groups are protonated at low pH values. This structure allows the humic materials to function as surfactants, with the ability to bind both hydrophobic and hydrophilic materials. This function in combination with their colloidal properties, makes humic and fulvic materials effective agents in transporting both organic and inorganic contaminants in the environment. Colloidal Characteristics The colloidal state represents a phase intermediate between true solutions, where species are of ionic or molecular dimensions, and suspended particulates, where species are sufficiently large to settle under the influence of gravity. The colloidal range is considered to extend from 0.001 to 1.0 um or 10 to 10,000 Angstroms (Figure 1). Chemical and physical reactions are generally enhanced in colloidal systems due to the large surface areas of colloidal particles. At the same time, mobility through surface waters or groundwaters is also enhanced, approaching that for true solutions. The ranges of molecular sizes for the majority of humic and fulvic acids place them in the colloidal range when in aqueous solution. Humic colloidal materials are thought to consist of coiled, long-chain, or three-dimensional cross-linked macromolecules with electrical charges variously distributed on the particle. The presence of charged sites, arising from ionized acidic groups, results in mutual repulsion and causes maximum expansion of the molecule (2). The factors most important in controlling the molecular conformation of humic materials are concentration of the humic, pH, and ionic strength of the system (8). At high sample concentrations (>3.5 g/L), low pH (0.05 M), the humic materials are rigid uncharged colloidal particles. At low sample concentrations, high pH, and low electrolyte concentrations, humics and fulvics exist as flexible linear polyelectrolytes. In fresh waters, where both humic and ionic strength would be expected to be low, and the pH is greater than 3.0, humic materials should exist as linear polyelectrolytes. Free Radicals in Humic Materials Humic materials have a relatively high content of radicals, presumably of the semiquinone type, which are more prominent in humic acids than in fulvic acids. These radicals can exist as permanent components or as transient species, generated by pH In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
10 - 4
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100,000
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500,000
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albumin protein
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bacteria
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Figure 1. Size ranges of aqueous particles. Micrometer (urn) and Angstrom (A) units are shown using a logarithmic scale, while the nominal molecular weights are approximate and are not scaled.
Approximate Molecular 100 Weight \—
fulvic acids
aqueous salts
micron
dissolved
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Downloaded by GEORGIAN COURT UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: November 14, 1996 | doi: 10.1021/bk-1996-0651.ch001
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1. GAFFNEY ET AL.
Organic Colloidal Materials in the Environment 5
changes, chemical reduction, or solar irradiation, that have lifetimes of minutes to hours (9). These free radicals most likely play important roles in polymerization or oxidationreduction reactions. Humic materials can reduce metals with estimated reduction potentials of 0.5-0.7 eV (10). This can have a major effect on the migration of reducible cations. The absorption spectrum of an aquatic humic acid is shown in Figure 2. The absorbance values generally decrease with increasing wavelength with essentially no absorption above about 550 nm. Most of the solar energy absorbed by the humic materials is between 300 and 500 nm. Absorption of light in this region can initiate a number of photochemical processes. This light absorption can produce peroxy radicals and hydroxyl radical as well as hydrated electrons, hydrogen peroxide, singlet oxygen, and superoxide (77). These species can also promote redox reactions. In addition, humic acids can photosensitize nonpolar organics such as herbicides and possibly accelerate their decomposition and detoxification. Separation and Purification Humic and fulvic acids are traditionally extracted from soils and sediment samples as the sodium salts by using sodium hydroxide solution. The material that remains contains the insoluble humin fraction (Figure 3). The alkaline supernatant is acidified to pH 2 with HC1. The humic acid precipitates and the fulvic acid remains in solution with other small molecules such as simple sugars and amino acids. These molecules can be separated by passing the solution through a hydrophobic resin, such as the methacrylate cross-linked polymer, XAD-8. The fulvic acids will sorb to the resin while the more hydrophilic molecules pass through the column. The fulvic acid can be removed with dilute base. Aqueous samples are treated similarly beginning with the acidification step. The entire sample is then put through the hydrophobic resin, and the fulvic acids are eluted at pH 7. The humic acids are removed with 0.1 M NaOH (2). After extraction, purification of the samples can be accomplished by freeze-drying and dialysis. The use of strong acids and bases has been criticized for several reasons. They can promote degradation, decarboxylation, oxidation, and condensation reactions. Strong acids and bases can also dissolve siliceous materials and lyse cells, resulting in contamination of the sample. Other extractants have been proposed, such as sodium pyrophosphate or sodium fluoride; however, the classical procedure offers the most complete dissolution of humic material from solid samples and is still most often used (72). High-volume ultrafiltration techniques, such as hollow fiber ultrafilters, have been proposed for the separation of humic materials from aqueous samples (13-15). This technique avoids exposure to strong acids and bases and maintains the materials in aqueous concentrates closest to their natural states. The hollow fiber filters are available in nominal molecular weight cutoff ratings of 3,000, 10,000, 30,000, and 100,000 atomic mass units. In addition, flat membrane filters, which are used in stirred vortex cells, and spiral-wound cartridge filters are available at ratings of 500 and 1,000 nominal molecular weights. By using a series of filters, aqueous humic materials can be separated into size fractions, with the fulvic acids in the smaller size ranges and the humic acids in the larger.
In Humic and Fulvic Acids; Gaffney, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.
HUMIC AND FULVIC ACIDS
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