[Advances in Chemistry] Aquatic Humic Substances Volume 219 (Influence on Fate and Treatment of Pollutants) || Interactions of Hazardous-Waste Chemicals with Humic Substances

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  • 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 de-solubilization of metal compounds and organic compounds by humic substances, precipitation reactions, and oxidation-reduction phe-nomena. All three major types of humic substancesfulvic acid, humic acid, and huminmay interact with hazardous-waste chem-icals. 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; po-lycyclic 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 haz-ardous-waste chemical species and the possible uses of humic sub-stances 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 5-2393/89/0219-(X)83$(W.(X)/0 1989 American Chemical Society

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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 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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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 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 substancesfulvic 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 humtes 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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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 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 humtes 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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    In Aquatic Humic Substances; Suffet, I., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1988.

  • 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 humtes.

    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 othe...

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