Unlike the hazardous organic constituents, metals cannot be degraded or readily detoxified. The presence of metals among wastes can pose a long-term environmental hazard. The fate of the metal depends on its physical and chemical properties, the associated waste matrix, and the soil. Significant downward transportation of metals from the soil surface occurs when the metal retention capacity of the soil is overloaded, or when metals are solubilized (e.g., by low pH). As the concentration of metals exceeds the ability of the soil to retain them, the metals will travel downward with the leaching waters. Surface transport through dust and erosion of soils are common transport mechanisms. The extent of vertical contamination intimately relates to the soil solution and surface chemistry.
Properties and behavior of specific metals are discussed below:
Arsenic: Arsenic (As) exists in the soil environment as arsenate, As(V), or as arsenite, As(III). Both are toxic; however, arsenite is the more toxic form and arsenate is the most common form. (Note: Arsenic is not a true metal; however, it is included here as it is one of the eight RCRA metals.)
The behavior of arsenate in soil seems analogous to that of phosphate because of their chemical similarity. Like phosphate, arsenate is fixed to soil, and thus is relatively immobile. Iron (Fe), aluminum (Al), and calcium (Ca) influence this fixation by forming insoluble complexes with arsenate. The presence of iron in soil is most effective in controlling arsenate's mobility. Arsenite compounds are 4 to 10 times more soluble than arsenate compounds. Under anaerobic conditions, arsenate may be reduced to arsenite. Arsenite is more subject to leaching because of its higher solubility.
The adsorption of arsenite is also strongly pH-dependent. One study found increased adsorption of As(III) by two clays over the pH range of 3 to 9 while another study found the maximum adsorption of As(III) by iron oxide occurred at pH 7.
Barium: Barium (Ba) metal does not occur in nature. The most common ores are the sulfate (barite) and the carbonate (witherite). The largest end use of barium metal is as a 'getter' to remove the last traces of gases from vacuum and television picture tubes. The most important compounds are the peroxide, chloride, sulfate, carbonate, nitrate, and chlorate. Barium peroxide is used as a bleach, in dyes, fireworks and tracer-bullets, and in igniter and welding materials. Barium sulfate is used as permanent white in paint, in X-ray diagnostic work, in glassmaking, and as a pigment in Lithopone (with zinc sulfide). Barite is extensively used as a wetting agent in oil-well drilling fluids, and also in making rubber. The carbonate is used as a rat poison, while the nitrate and chlorate give colors in pyrotechny. All barium compounds that are water or acid soluble are poisonous.
Barium is released to water and soil in the discharge and disposal of drilling wastes, from the smelting of copper, and the manufacture of motor vehicle parts and accessories. In water, the more toxic barium salts are likely to precipitate out as the less toxic insoluble sulfate or carbonate. Barium is not very mobile in most soil systems. Adsorption of barium was measured in a sandy soil and a sandy loam soil at levels closely corresponding to those expected for field conditions. In general, sludge solutions appeared to increase the mobility of elements in a soil. This is due to a combination of complexation by dissolved organic compounds, high background concentration and high ionic strengths of the soil solution.
Cadmium: Cadmium (Cd) most often occurs in small quantities associated with zinc ores, but also with copper and lead ores. It is used primarily for metal plating and coating operations, including transportation equipment, machinery and baking enamels, photography, and television phosphors. It is also used in nickel-cadmium and solar batteries, in pigments, as a stabilizer in plastics and synthetic products, as alloys and the other purposes. It also is used in many types of solder (e.g., silver solder).
Cadmium oxide and sulfide are relatively insoluble while the chloride and sulfate salts are soluble. The adsorption of cadmium onto soils and silicon or aluminum oxides is strongly pH-dependent, increasing as conditions become more alkaline. When the pH is below 6-7, cadmium is desorbed from these materials. Cadmium has considerably less affinity for the absorbents tested than do copper, zinc, and lead and might be expected to be more mobile in the environment than these materials. Studies have indicated that cadmium concentrations in bed sediments are generally at least an order of magnitude higher than in the overlying water.
Addition of anions, such as humate or tartrate, to dissolved cadmium cause an increase in adsorption. The mode by which cadmium is sorbed to the sediments is important in determining its disposition towards remobilization. Cadmium found in association with carbonate minerals, precipitated as stale solid compounds, or co-precipitated with hydrous iron oxides would be less likely to be mobilized by resuspension of sediments or biological activity. Cadmium absorbed to mineral surfaces (e.g., clay) or organic materials would be more easily bioaccumulated or released in the dissolved state when sediments are disturbed, such as during flooding.
Chromium: Chromium (Cr) can exist in soil in three forms: the trivalent Cr(III) form, Cr+3, and the hexavalent Cr(VI) forms, (Cr2O7)-2 and (CrO4)-2. Hexavalent chromium is the major chromium species used in industry; wood preservatives commonly contain chromic acid, a Cr(VI) oxide. The two forms of hexavalent chromium are pH dependent; hexavalent chromium as a chromate ion (CrO4)-2 predominates above a pH of 6; dichromate ion (Cr2O7)-2 predominates below a pH of 6. The dichromate ions present a greater health hazard than chromate ions, and both Cr(VI) ions are more toxic than Cr(III) ions.
Because of its anionic nature, Cr(VI) associates only with soil surfaces at positively charged exchange sites. This association decreases with increasing soil pH. Iron and aluminum oxide surfaces adsorb the chromate ion at an acidic or neutral pH. Industrial applications, except leather tanning, use Cr(VI) but the reaction rate limits the conversion from Cr(VI) to Cr(III) under typical environmental conditions.
Chromium (III) is the stable form of chromium in soil. Cr(III) hydroxy compounds precipitate at pH 4.5 and complete precipitation of the hydroxy species occurs at pH 5.5. In contrast to Cr(VI), Cr(III) is relatively immobile in soil. Chromium (III) does, however, form complexes with soluble organic ligands, which may increase its mobility.
Regardless of pH and redox potential, most Cr(VI) in soil is reduced to Cr(III). Soil organic matter and Fe(II) minerals donate the electrons in this reaction. The reduction reaction in the presence of organic matter proceeds at a slow rate under normal environmental pH and temperatures, but the rate of reaction increases with decreasing soil pH.
Copper: Soil retains copper (Cu) through exchange and specific adsorption. Copper adsorbs to most soil constituents more strongly than any other toxic metal, except lead (Pb). Copper, however, has a high affinity to soluble organic ligands; the formation of these complexes may greatly increase its mobility in soil. Copper has high toxicity to aquatic organisms.
Lead: Lead is a heavy metal that exists in three oxidation states: O, +2(II), and +4(IV). Lead is generally the most widespread and concentrated contaminant present at a lead battery recycling site (i.e., battery breaker or secondary lead smelter).
Lead tends to accumulate in the soil surface, usually within 3 to 5 centimeters of the surface. Concentrations decrease with depth. Insoluble lead sulfide is typically immobile in soil as long as reducing conditions are maintained. Lead can also be biomethylated, forming tetramethyl and tetraethyl lead. These compounds may enter the atmosphere by volatilization.
The capacity of soil to adsorb lead increases with pH, cation exchange capacity, organic carbon content, soil/water Eh (redox potential), and phosphate levels. Lead exhibits a high degree of adsorption on clay-rich soil. Only a small percent of the total lead is leachable; the major portion is usually solid or adsorbed onto soil particles. Surface runoff, which can transport soil particles containing adsorbed lead, facilitates migration and subsequent desorption from contaminated soils. On the other hand, ground water (typically low in suspended soils and leachable lead salts) does not normally create a major pathway for lead migration. Lead compounds are soluble at low pH and at high pH, such as those induced by solidification/stabilization treatment. Several other metals are also amphoteric, which strongly affects leaching. If battery breaking activities have occurred on-site, and the battery acid was disposed of on-site, elevated concentrations of lead and other metals may have migrated to ground water.
Mercury: Mercury is extremely toxic and very mobile in the environment. In soils and surface waters, volatile forms (e.g., metallic mercury and dimethylmercury) evaporate to the atmosphere, whereas solid forms partition to particulates. Mercury exists primarily in the mercuric and mercurous forms as a number of complexes with varying water solubilities. In soils and sediments, sorption is one of the most important controlling pathways for removal of mercury from solution; sorption usually increases with increasing pH. Other removal mechanisms include flocculation, co-precipitation with sulfides, and organic complexation. Mercury is strongly sorbed to humic materials. Inorganic mercury sorbed to soils is not readily desorbed; therefore, freshwater and marine sediments are important repositories for inorganic mercury.
Selenium: Selenium (Se). Selenium occurs in nature usually in the sulfide ores of the heavy metals and constitutes about 0.09 ppm of the earth's crust. It is the most strongly enriched element in coal, being present as an organoselenium compound, a chelated species, or as an adsorbed element. Selenium is used extensively in the manufacture and production of glass, pigments, rubber, metal alloys, textiles, petroleum, medical therapeutic agents, and photographic emulsions. Selenium dioxide is the most widely used selenium compound in industry. It is used as an oxidizing agent in drug and other chemical manufacture; a catalyst in organic syntheses; and an antioxidant in lubricating oils. (Note: Selenium is not a true metal; however, it is included here as it is one of the eight RCRA metals.)
The toxicity of selenium depends on whether it is in the biologically active oxidized form. In alkaline soils and oxidizing conditions, selenium may be oxidized sufficiently to maintain the availability of its biologically active form, and cause plant uptake of the metal to be increased. In acidic or neutral soils, it tends to remain relatively insoluble and the amount of biologically available selenium should steadily decrease. Selenium volatilizes from soils when converted to volatile selenium compounds (e.g., dimethyl selenide) by microorganisms.
Silver: Silver (Ag) occurs naturally and in ores such as argentite (Ag2S) and horn silver (AgCl). Lead, lead-zinc, copper, gold, and copper-nickel ores are the principal source. It is used for jewelry, silverware, etc., and in photography, dental alloys, solder and brazing alloys, electrical contacts, high capacity silver-zinc and silver-cadmium batteries, mirrors, and coinage. Silver nitrate, the most important silver compound, is used extensively in photography. Silver iodide is used in seeding clouds to produce rain. Silver chloride has interesting optical properties as it can be made transparent; it also is a cement for glass.
While silver itself is not considered to be toxic, most of its salts are poisonous due to the anions present. Silver compounds can be absorbed in the circulatory system and reduced silver deposited in the various tissues of the body. Silver is regulated under RCRA.
Zinc: Clay carbonates, or hydrous oxides, readily adsorb zinc (Zn). The greatest percentage of total zinc in polluted soil and sediment is associated with iron (Fe) and manganese (Mn) oxides. Rainfall removes zinc from soil because the zinc compounds are highly soluble. As with all cationic metals, zinc adsorption increases with pH. Zinc hydrolyzes at a pH >7.7. These hydrolyzed species strongly adsorb to soil surfaces. Zinc forms complexes with inorganic and organic ligands, which will affect its adsorption reactions with the soil surface.