6

Interactions of Hazardous-Waste Chemicals with Humic Substances

Stanley E. Manahan

Department of Chemistry, University of Missouri, Columbia, MO 65211

Hazardous-waste chemicals may interact in a variety of ways with humic substances. These interactions include solubilization and de­solubilization of metal compounds and organic compounds by humic substances, precipitation reactions, and oxidation-reduction phe­nomena. All three major types of humic substances—fulvic acid, humic acid, and humin—may interact with hazardous-waste chem­icals. Among the more important classes of hazardous waste chemicals that may be influenced by the presence of humic substances are sparingly soluble organic compounds, particularly organohalides; po­lycyclic aromatic hydrocarbons; heavy metal ions; soluble oxidants; iron compounds; aluminum compounds; and strong acids and bases. This chapter discusses the interaction of humic substances with haz­ardous-waste chemical species and the possible uses of humic sub­stances for the treatment of such chemicals.

IN ASSESSING THE FATES AND EFFECTS of hazardous-waste chemicals, it is important to consider the potential environmental chemical interactions of these wastes with humic substances from water, soil, codisposed municipal refuse, or the wastes themselves. Among the types of interactions possible between humic substances and hazardous wastes, and the effects that occur from such interactions, are the following: (1) precipitation of humic acid by reaction with waste acid, such as steel pickling liquor acid waste; (2) solu­bilization of humic acid by reaction with waste alkali; (3) complexation of heavy metal ions by humic substances; (4) reduction of metal species by humic substances [e.g., reduction of iron(III) to iron(II) or chromium(VI) to

(X 5̂-2393/89/0219-(X)83$(W.(X)/0 © 1989 American Chemical Society

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84 AQUATIC HUMIC SUBSTANCES chromium(III)]; (5) effects of humic substances on the solubilities of organic wastes; and (6) effects of humic substances on the sorption of hazardous-waste species (e.g., complexation of metal cations).

Humic substances sometimes can be used in the treatment and detox­ification of hazardous-waste chemicals. Examples include the application of peat as a matrix for the growth of white rot fungus used to degrade organic wastes and the reduction-sorption of chromate wastes by surface-oxidized low-rank coal.

Hazardous Wastes A chemical waste may be deemed hazardous because of toxicity, ignitability, reactivity, corrosivity, radioactivity, or a combination of these characteristics. Relatively large numbers of people may be exposed to low levels of toxicants through drinking-water sources contaminated by hazardous-waste constit­uents leached from disposal sites, a process that can be affected by interaction with humic substances.

Numerous methods are used to reduce the hazards of waste chemicals. These methods can be broadly categorized as destruction, deactivation, and stabilization.

The most common means of destruction is the incineration of organic chemicals to C 0 2 , H 2 0 , and other inorganic species (such as HC1 in the case of chlorinated hydrocarbons). Wet oxidation of chemicals in water can be accomplished with air at high temperatures (175-345 °C) and high pressures (20.4-204 atm) (J).

The objective of deactivation is to convert the waste to a nonhazardous material disposable in a conventional landfill or, for aqueous wastes, in a publicly owned treatment works. Dilute hazardous-waste solutes may be removed from water by processes such as activated carbon adsorption of organic compounds, sulfide precipitation ofheavy metals, or reverse osmosis.

Some wastes, such as radionuclides, cannot be destroyed or deactivated at a reasonable cost with available technology, and stabilization is required prior to disposal (2). Stabilization prevents contamination of air or water by hazardous substances and allows safer handling and transport. In favorable cases, stabilized wastes can be incorporated with landfill at construction sites or even added to building materials (3). Stabilization technologies include mixing wastes with cement, conversion of inorganic wastes to pozzolanic concrete materials produced by mixing lime with fly ash and other siliceous materials, and vitrification to produce a glassy solid.

Hazardous-waste chemicals include a wide variety of species. Insofar as interaction with humic substances is concerned, the most important of these species include sparingly soluble organic compounds, particularly or­ganohalides; polycyclic aromatic hydrocarbons; heavy metal ions; soluble oxidants; iron compounds; aluminum compounds; and strong acids and bases.

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6. MANAHAN Hazardous-Waste Chemicals and Humic Substances 85

Humic Substances

Humic substances are colored organic substances found in natural waters, soil, peat, and other sources where plant material has undergone partial degradation. The exact nature of humic substances defies a concise definition because of their variable compositions and properties. They are normally defined in terms of three somewhat overlapping kinds of substances—fulvic acid, humic acid, and humin (4). Humic acid and fulvic acid are soluble in strong basic solution, from which humic acid precipitates upon acidification, leaving fulvic acid in solution. Humin is not extractable in aqueous solution of even strong base; it can be isolated as a suspension in methyl isobutyl ketone (5).

Despite the widespread occurrence of humic material in natural waters and terrestrial systems and detailed study by a host of investigators, knowl­edge of the exact nature and formation mechanism of humic material is still incomplete. Its formation probably involves a combination of degradation and condensation of plant residual material. Soil scientists long ago recog­nized the importance of "humus" in maintaining soil texture, moisture con­tent, and fertility (6). More recently, environmental scientists have become interested in humic substances because of their interactions with soil and water pollutants and their influence on treatment processes. Prominent among the latter are problems encountered in trying to remove humic-bound metals (e.g., iron) in water treatment and the role of humic substances in forming trihalomethane contaminants in water chlorination (7).

Beliefs about the properties of humic substances have undergone some modification in recent years in light of more sophisticated studies. For ex­ample, although they are somewhat aromatic, aquatic humic substances may have less aromaticity than was once widely believed (8). The literature abounds with diverse estimates of the molecular weights of humic sub­stances, ranging from a few hundred (fulvic acid) to several hundred thousand (humin materials). More recent estimates have tended to be lower (9, 10), for example, about 800 for fulvic acid and 1500-3000 for humic acid (4). With the use of newly developed flow field-flow fractionation, molecular weight distributions have been determined for fulvates and humâtes from various sources (4). The molecular weight values obtained ranged from 860 for a Suwannee stream fulvate to 4050 for a Leonardite coal humate. Inter­mediate values obtained were 1010 for a Mattole soil fulvate, 1490 for a Suwannee stream humate, and 2430 for a Washington peat humate. Each of these numbers represents a distribution of values for each source of humic substance.

Regardless of uncertainties such as molecular weight, humic substances have some well established functionalities and properties. They are polye-lectrolytes with various functional groups attached to the hydrocarbon skel­eton. P rominen t among these groups are carboxyl and phenol ic functionalities responsible for the acid-base, complexation, and salt for-

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86 AQUA™ HUMIC SUBSTANCES mation capabilities of humic substances. Methoxyl groups are also present. Humic molecules are subject to strong intermolecular association (II).

Acid-Base Solubility Behavior of Humic Substances with Hazardous Wastes Interaction between humic substances and hazardous-waste chemicals is most likely to occur when such chemicals have been disposed of under­ground. In older sites containing chemical wastes mixed with municipal refuse, the decay of the refuse can generate humic materials. Humic sub­stances are present in soil lining and covering underground disposal sites and are mixed with waste chemicals placed in lagoons, trenches, or pits. Infiltration of surface water or groundwater containing humic substances can result in their contacting disposed chemical wastes. It is even plausible that humic materials such as peat or weathered, surface-oxidized coal residues may be codisposed with waste chemicals. There are several potential sources of humic substances, a condition that should be considered in assessing environmental chemical processes involving landfilled chemicals.

A major factor in considering interaction of humic substances with waste chemicals is the solubility of the humic materials, including the influence of chemical species on the solubility, which depends predominantly on the acid-base precipitation behavior of humic substances. The most likely route for the mobilization of large quantities of humic material is through contact with strong base from waste caustics, such as that discarded after use for removal of sulfur compounds from petroleum products. Humic acid solu-bilized by this route could move some distance from a waste disposal site and precipitate upon neutralization of the base or dilution. However, lower molecular weight fractions could stay in solution, acting as water pollutants per se or in the form of chelates with metal ions.

Humic substances in solution can be precipitated by contact with waste chemicals in two major ways. Acidification, such as by waste steel pickling liquor, causes humic acid to precipitate. This phenomenon is unlikely to cause any problems; humic-chelated heavy metal ions could conceivably be released, perhaps in a more mobile form.

Soluble humic substances can be precipitated and colloidal humic ma­terials aggregated by contact with multiply charged metal ions. The most likely way for this to happen is through contact with C a 2 + ion, which abounds in water at many waste-disposal sites because of the widespread use of lime in waste treatment. Fly ash, sometimes used to cover disposal lagoons for closure, is another possible source of calcium. The precipitation of calcium humâtes should be beneficial in removing humic organic matter from waste-site leachate. Coprecipitation of heavy metal ions with calcium humate would be beneficial in removing these pollutant species. Even some organic species might be removed from aqueous solution or suspension by the humate

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6. MANAHAN Hazardous-Waste Chemicals and Humic Substances 87

precipitate. It is difficult to imagine any harmful effects that might arise from formation of insoluble humâtes.

Interaction of Humic Substances with Waste Heavy Metal Ions The ability of humic substances to act as chelating agents for metal ions, including trace metals in water (12), is well known. The knowledge base in this area continues to expand steadily, and extensive coverage of this topic is found in the scientific literature. The particular effects that humic sub­stances have on chelatable metals in hazardous wastes depend upon the types of metal species, the nature of the humic substances, and the chemical environment with respect to acidity-alkalinity, oxidation-reduction, and the presence of competing species. (Examples of competing species are calcium in competition with other metals for humic ligands or complexing species such as cyanide that compete with humic ligands for metal ions). Under some conditions, fulvate species act to keep metal ions in solution as pre­dominantly anionic chelates that are relatively hard to remove from water (hazardous-waste leachate) by natural or treatment processes. However, in­teractions of fulvates with metal ions are involved in the removal of fulvic acid from water by coagulation with alum (13) or iron(III) salts. Spent steel pickling liquor is a common source of iron(III) salts at waste chemical disposal sites. In some cases the chelation of metal ions as soluble humic species may prevent precipitation of the metals by precipitate-forming anions such as C 0 3

2 ~ , O H " , and S 2 _ . Precipitation by S 2 " could be particularly significant under anoxic conditions, in which the presence of sulfide ion is often assumed to keep heavy metal ions in an insoluble form.

Most research on humic substances, including their metal-binding char­acteristics, has concentrated on the fulvic acid and humic acid fractions, and comparatively little attention has been given to the humin fraction (5). The lower priority given humin is understandable in light of its lack of solubility and widely held reputation for intractability. A recently published method for the extraction of at least some fractions of humin material (5) may enable more complete investigations of this class ofhumic substance. Clearly, humin deserves careful consideration in assessing the fates of heavy metal ions from hazardous-waste disposal sites. Because humin is a water-insoluble material with ion-exchange and metal-binding capabilities, it can act as an ion ex­changer and sink for heavy metals from waste chemical leachate. This effect is generally beneficial because soil humin acts to immobilize heavy metal ions in leachate passing through it. More research is needed in assessing the influence ofhumic substances on the fates of heavy metal wastes because of binding between the two types of materials.

Humic substances influence speciation of metals by chelation. The other major influence of humic substances on metal speciation is through oxida-

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88 AQUATIC HUMIC SUBSTANCES tion-reduction reactions, which can occur in two major ways. Humic material is an active oxidation-reduction system with an E° value for humic acid estimated to be 0.70 V (see Table I) (14); it normally functions as a reducing agent. The second major influence of humic substances on oxida­tion-reduction of metal species is stabilization of the reduced cationic form by chelation. For example, as shown in Table I, the unchelatable oxoanion C r 2 0 7

2 " is reduced by humic acid to chelatable cationic C r 3 + , which tends to shift the half-reaction for the reduction of C r 2 0 7

2 " to the right in the presence of humic substance.

In addition to the reductions of metal species shown in Table I, humic acid has been reported to reduce ionic mercury to elemental mercury(O) (15). The accumulation of vanadium and molybdenum in peats (which are humic materials) and coals (which undergo a humic stage as they develop) has been attributed to the reduction by humic substance of soluble V 0 3 " and M o 0 4

2 " to chelatable cationic species (16). The reduction of acidic iron(III) to iron(II) and subsequent retention of

the iron(II) product has been demonstrated for surface-oxidized bituminous coal (17). This granular solid material produced by contacting the coal with 6 M nitric acid and bubbling air through the suspension, followed by washing with base to remove base-soluble organic products, can be regarded as a humin material.

Interaction of Humic Substances with Waste Organic Compounds

A large fraction of waste chemicals consigned to landfill disposal consists of water-insoluble, hydrophobic organic compounds. Typical of these com­pounds and prominent among them are numerous chlorinated hydrocarbons. Another example is bis(2-ethylhexyl) phthalate, widely used as a plasticizer and found ubiquitously in environmental samples. Most of the hydrophobic organic waste compounds are poorly biodegradable (i.e., they are biore-fractory). Those compounds more dense than water (predominantly halo-genated hydrocarbons) belong to the class of insoluble sinking pollutants,

Table I. Oxidation-Reduction Reactions Involving Humic Acid

E ° ( V ) 1.69 1.33 1.23 1.00 0.77 0.70

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6. MANAHAN Hazardous-Waste Chemicals and Humic Substances 89

which tend to settle to the bottom of lagoons or other disposal sites and to be transported as "globs" of liquid along the bottoms of natural water systems, such as aquifers (18).

The role that humic substances may play in the transport, reactivity, and fate of hydrophobic organic compounds has been succinctly summarized by Caron and Suffet (J9). The transport, reactivity, and fate of nonpolar organic compounds in the aquatic and terrestrial environments are influ­enced by their association with dissolved, colloidal, and undissolved humic substances. The degree of association is affected by the nature of the com­pound and humic material, the concentrations of both, p H , calcium ion concentration, and the presence of other organic and ionic solutes. The octanol-water partition coefficient (KQJ is useful for predicting the degree of association, which is usually significant when Κ exceeds 105. Attempts to correlate the degree of association with measurable characteristics of humic substances have not been notably successful, and Caron and Suffet point out the shortcomings of the widespread practice of using base-extracted humic substances and commercial humic acids for studies of association. The de­velopment of reliable methods for predicting the association between hy­drophobic organic waste compounds and the humic substances that they are likely to contact in waste-disposal sites would be very useful in predicting the fates and effects of the wastes.

The enhancement of the solubility of nonpolar organic compounds by dissolved humic substances appears to increase with organic compound mo­lecular weight and with lower polarity of dissolved humic material (20) when tested with the compounds ρ,ρ'-ΌΌΎ, {1, r-(2,2,2-trichloroethylidene)bis[4-chlorobenzene]}, 2,4,5,2' ,5'-PCB, 2,4,4'-PCB, 1,2,3-trichlorobenzene, and lindane. Findings such as these suggest that humin, the highest-molecular-weight, least-polar fraction of humic substance, should have an especially high affinity for nonpolar organic compounds. However, because humin is insoluble, it should have the effect of decreasing the solubilities of nonpolar organic compounds and immobilizing them. This phenomenon could be a factor in restricting the movement of nonpolar organic compounds at haz­ardous-waste sites.

Effects of Humic Substances on Hazardous Waste Leachate Treatment One of the most common remedial actions taken at a hazardous-waste dis­posal site is the treatment of leachate water from the site to remove heavy metals, toxic inorganic species (e.g., cyanide), and organic contaminants. The presence of humic substances can have significant effects upon the processes for removing leachate solutes. One of the more obvious examples is the solubilization of metal ions as humic complexes. Solubilization may

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90 AQUATIC HUMIC SUBSTANCES prevent the removal from leachates of dissolved heavy metals by standard techniques, such as precipitation.

In some cases the presence ofhumic substances can lead to undesirable byproducts as the result of side reactions from processes used to remove other contaminants from leachate. For example, the destruction of dissolved cyanide by chlorination would lead to the formation of trihalomethane by­products in the presence ofhumic substances.

Biological treatment of biodegradable organic contaminants, usually by an activated sludge process, is an operation commonly applied to hazardous-waste leachates. The presence ofhumic substances may well affect biological waste treatment. In some cases the effects could be beneficial; for example, toxic heavy metal ions may be bound to humic materials, and the bound metals are less toxic to the microorganisms carrying out the biodégradation processes. In general, association ofhumic substances with nonionic organic compounds reduces the toxicity and bioaccumulation of the organic com­pounds, apparently by reducing their ability to traverse biological mem­branes (21). This phenomenon could reduce the biodégradation of organic compounds.

Activated carbon sorption is often employed to remove refractory or­ganic compounds and other solutes from hazardous chemical waste leachate. By competing with other organic matter, humic materials in the leachate should generally reduce the capacity and effectiveness of activated carbon to remove organic contaminants from leachate.

Humic material in hazardous-waste leachate can affect membrane pro­cesses (reverse osmosis) and resin processes (ion exchange) used to treat the leachate. Humic substances are known to foul reverse osmosis membranes, a phenomenon that can be prevented by precipitation and coagulation of the humic material with iron(III) or aluminum(III) (22). Membrane fouling by humic substances normally leads to a lower flux and reduces separation efficiency. In some cases the presence of a layer of organophilic humic substance on a membrane can have the beneficial effect of making it a more effective filter for the retention of organic solutes, but the overall effect is generally too unpredictable for deliberate use in leachate treatment.

Uses of Humic Substances for Hazardous Waste Chemical Treatment The preceding discussion has pointed out some of the possible, largely det­rimental, effects of the interactions ofhumic substances with hazardous waste chemicals, particularly heavy metals and nonpolar organic compounds. How­ever, humic substances have several important properties that suggest their beneficial uses for the treatment and detoxification of hazardous chemicals. Prominent among these properties are the ability to chelate heavy metals, reduce oxidized metal species, and bind to nonpolar organic compounds.

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6. MANAHAN Hazardous-Waste Chemicals and Humic Substances 91

For the treatment and immobilization of hazardous chemical species, the insoluble humin fraction is the most useful. It is readily available in inex­pensive raw materials, such as peat treated to remove soluble constituents or surface-oxidized low-rank coal. Some waste chemical byproducts can be used to prepare and pretreat humic materials to be employed for waste treatment. Examples include waste caustic for the removal of soluble humic fractions in the preparation of humin; waste nitric acid for the surface oxi­dation of low-rank coal to give it humic properties; and lime, aluminum salts, and iron salts for the coagulation and precipitation of humic materials to be used in waste treatment.

As an example of humic materials applied to wastewater treatment, the use ofhumic acid-fly ash mixtures has been described (23). Applications of humic acids or humâtes for the purification of highly polluted water, such as hazardous-waste leachate, include the following:

1. Neutralization of acids by exchange of calcium, magnesium, or sodium on humâtes for H + ion in acidic water, accompanied by the formation and settling of insoluble humic acid.

2. Removal of heavy metals by chelation.

3. Removal of anions, such as phosphates, cyanide, and organic anions by mixed ligand complexation.

4. Sorption of organics from water.

5. Clarification of suspended matter by precipitation and floc-culation of humic acid and insoluble metal humâtes.

As an example of the use ofhumic substances for the removal of potential air pollutants, the sorption of sulfur dioxide by humic acid-fly ash mixtures (24) and by sodium humate (25) has been described. The major mechanism for sorption of S 0 2 was found to be formation of hydrogen sulfite (HS0 3~) in the presence of basic humâtes. However, under conditions of low p H and high temperature, evidence was found for the formation of a humate complex of S0 2(aq).

The concentration and immobilization of hazardous-waste chemicals, such as heavy metals and refractory organic compounds, on relatively small masses of humic material provide several possibilities for hazardous-waste chemical treatment. These include incineration of sorbed organics and humic material under conditions that retain heavy metals with the ash and biological treatment and degradation of organic compounds on the humic substance matrix.

References 1. Kiang, Y.-H.; Metry, A. A. Hazardous Waste Treatment Technology; Ann Arbor

Science: Ann Arbor, 1982.

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92 AQUATIC HUMIC SUBSTANCES 2. Tucker, S. P.; Carson, G. A. Environ. Sci. Technol. 1985, 19, 215-220. 3. Malone, P.; Jones, L. Guide to the Disposal of Chemically Stabilized and So­

lidified Waste; U.S. Environmental Protection Agency. U.S. Government Print­ing Office: Washington, DC, 1980; SW-872.

4. Beckett, R.; Jue, Z.; Giddings, J. C. Environ. Sci. Technol. 1987, 21, 289-295. 5. Rice, J. Α.; MacCarthy, P. Abstracts of Papers, 193rd National Meeting of the

American Chemical Society, Denver, CO; American Chemical Society: Wash­ington, DC, 1987; ENVR 199.

6. Bohn, H. L.; McNeal, B. L.; O'Connor, G. A. In Soil Chemistry, 2nd ed.; Wiley-Interscience: New York, 1985; Chapter 5, pp 135-152.

7. Christman, R. F.; Norwood, D. L.; Millington, D. S.; Johnson, J. D.; Stevens, A. A. Environ. Sci. Technol. 1983, 17, 625-628.

8. Thurman, Ε. M.; Malcolm, R. L. In Aquatic and Terrestrial Humic Materials; Christman, R. F.; Gjessing, E. T., Eds.; Ann Arbor Science: Ann Arbor, 1983; pp 1-23.

9. Malcolm, R. L. In Humic Substances in Soil Sediment and Water; Aiken, C. R.; McKnight, D. M.; Wershaw, R. L.; MacCarthy, P., Eds.; Wiley-Interscience: New York, 1985; Chapter 7, pp 181-209.

10. Thurman, Ε. M. Organic Geochemistry of Natural Waters; Martinus Nijhoff/ Junk: The Hague, 1985; pp 304-312.

11. Wershaw, R. L.; Pinckney, D. J. J. Res. U.S. Geol. Surv. 1973, 1, 701-707. 12. Hart, Β. T. Environ. Technol. Lett. 1981, 2, 95-110. 13. Dempsey, Β. Α.; Ganho, R. M.; O'Melia, C. R. J. Am. Water Works Assoc.

1984, 76, 141-150. 14. Szilagyi, M. Fuel 1974, 53, 26. 15. Alberts, J. J. Science (Washington, D.C.) 1974, 184, 895. 16. Szalsy, Α.; Szilagyi, M. Advances in Organic Geochemistry 1965, Proceedings

of the 4th International Meeting on Organic Geochemistry; Pergamon Press: New York, 1968.

17. Harlan, Sandra J. M.A. Thesis, University of Missouri, 1974. 18. Meyer, R. Α.; Kirsch, M.; Marx, L. F. Detection and Mapping of Insoluble

Sinking Pollutants; U.S. Environmental Protection Agency, Municipal Environ­mental Research Laboratory: Cincinnati, OH, 1981; EPA-600/S2-81-198.

19. Caron, G.; Suffet, I. H. Abstracts of Papers, 193rd National Meeting of the American Chemical Society, Denver, CO; American Chemical Society: Wash­ington, DC, 1987; ENVR 4.

20. Kile, D. E.; Brinton, T. I .; Chiou, C. T. Abstracts of Papers, 193rd National Meeting of the American Chemical Society, Denver, CO; American Chemical Society: Washington, DC, 1987; ENVR 5.

21. McCarthy, J. F. Preprint of an extended abstract, Division of Environmental Chemistry, 193rd National Meeting of the American Chemical Society, Denver, CO; American Chemical Society: Washington, DC, 1987, Vol. 27, No. 1, pp 286-288.

22. Schippers, J. C.; Verdous, J.; Hofman, J. M. Desalination 1980, 32, 103-112. 23. Green, J. B.; Manahan, S. E. In Chemistry of Wastewater Technology; Rubin,

A. J., Ed.; Ann Arbor Science: Ann Arbor, 1978; pp 373-401. 24. Green, J. B.; Manahan, S. E. Fuel 1981, 60, 330-334. 25. Green, J. B.; Manahan, S. E. Fuel 1981, 60, 488-494.

RECEIVED for review August 4, 1987. ACCEPTED for publication December 1, 1987.

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