Novel Strategies for the Removal of Toxic Metals from Soils and

Research: Science and Education

Novel Strategies for the Removal of Toxic Metals from Soils and Waters D. Max Roundhill Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409; [email protected]

Environmental chemistry is an area of increasing interest both to chemists and to the general public. Much of this awareness is a result of the bad publicity that chemistry has received because of some of its activities. A challenge to chemists is to show that they are the ones who can redress these past mistakes, and, through green chemistry, avoid repeating them. One aspect of chemical pollution is metals. Many educators recognize the environmental problems that are caused by toxic metals, and are aware of the public interest in learning more about this aspect of chemistry. A problem with introducing this subject into the curriculum, however, is that much of the available information is widely dispersed in the scientific literature, and many of these sources are not easily accessible to many educators. The purpose of this article is to collect this information into a single source, thereby encouraging science teachers to incorporate some or all of the material into their curricula. In addition, this article also provides readers with recent general references to the topic for those educators who wish to introduce the material to students in a more extensive form or in a greater depth than is offered here. This article addresses the important factors that are involved in the removal of toxic metals. The metals that are the most problematic are mercury, cadmium, lead, copper, chromium, and the radioactive actinides. As a consequence these metals need to be removed or stabilized in situ before they become incorporated into the food chain. These metals can be present as free or complexed cations, or as anionic complexes. Each presents its own challenges for both its detection and removal. This article focuses on metals that are toxic because of either their chemical or radioactive properties. The individual toxicities of these metals are discussed, as are other factors that are important when choosing a remediation strategy (1–3). The focus of the article is the role played by chemistry in making the extraction methods efficient and environmentally acceptable. Occurrence, Toxicities, and Remediation Strategies People have used metals for industrial, agricultural, and military purposes for several centuries. As a result metals are widely dispersed in different chemical forms. In addition to uses, there are environmental problems resulting from their mining, extraction, and purification. In many cases these metals are present as mixtures rather than as a metal residue in pure form. As a result, if these metals are to be recovered in pure form, it will be necessary for selective extraction processes to be developed. Because these metals are toxic they cannot just be ignored and left unrecovered. For actinides and radioactive elements the situation is somewhat different. These materials are stored in a limited number of sites, and are stored as mixtures in solutions that have high salt concentrations. These materials have a high www.JCE.DivCHED.org

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priority for recovery or disposal because they are often the result of weapons production, and their present storage facilities are unsuitable for long-term use. Heavy metals are present in both soils and waters. Each site presents its own challenges for their removal. In soils metals are tightly bound because cationic metal ions strongly pair with the anionic zeolite structure of the soil. In waters metals may be present under conditions of either high acidity or basicity. Special conditions also prevail when the metal is radioactive because the chosen extractant must resist decomposition by the nuclear decay of the metal. In choosing a remediation approach several factors must be considered. One requirement in choosing a method is that it must not leave toxic residues that must themselves be subsequently removed. For chemical extraction methods this can be avoided either by eliminating the use of toxic compounds or by ensuring that they subsequently undergo complete biodegradation to nontoxic compounds. Environmentally acceptable methods with soils use either electrokinetic or phytoremediation methods where no new chemical species are added during the removal process. Different options suitable for removing toxic metals are described below.

Toxicities of Mercury, Cadmium, and Lead Although recognition of the toxicological effects of mercury have resulted in its use being curtailed, there are still various means by which it can become spread throughout the environment. Mercury has been used in thermometers, electronics, fertilizers, and pharmaceuticals. In the U.S., waste incinerators and coal-fired utilities now represent some twothirds of the mercury released into the environment. A particularly significant problem with mercury is that it can undergo methylation. Methylation results in the mercury being converted into a more lipophilic form that enters the food chain in products such as seafood (4). Warnings for fish consumption begin at levels of 0.16 ppm mercury. The combination of the airborne movement of elemental mercury and its solubilization through methylation means that the problem of mercury in seafood will be a long term one. The neurological and toxic effects of mercury ingestion are well known (5, 6). Indeed, the English phrase “mad as a hatter” results from the toxicological properties of the mercury compounds used in hat making. In the mid-1800s, hat manufacturers began to use an aqueous mercury nitrate solution to soften, compress, and shape animal furs in a process known as felting. The outcome was skin absorption of mercury solution and inhalation of mercury vapor. The personality changes and tremors induced by mercury poisoning were also known as the “Danbury shakes”. The madness in hatters also received publicity in the novel Alice in Wonderland where it is noted that both the Hatter and the March Hare are mad. Nevertheless, this diagnosis is rather suspect since it was a casual remark by the Cheshire Cat.

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Lead has been used in paints, fuels, batteries, ammunition, and water pipes. The toxicity of lead is particularly problematic for children. Because of the use of lead paint in housing, many young children have enough lead in their blood to place them at risk, and children with high lead levels are six times more likely to have reading disabilities (7). The threshold for lead levels is 10 µg per deciliter of blood. Since lead poisoning is mainly silent, most poisoned children have no symptoms. As a result, the majority of cases go undiagnosed and untreated. Because lead has been widely used, large amounts are dispersed worldwide (8, 9). Chelation therapy is used in the treatment of lead poisoning (10). Cadmium is used in batteries and electronics, and is a metal of toxicological concern. Although cadmium is only poorly absorbed by humans from the gastrointestinal tract, it is absorbed from the respiratory tract. The half-life of cadmium in the body is 10–30 years. The metal binds to metallothionein, a low molecular weight protein with a high affinity for cadmium. This very long biological half-life results in cadmium’s being a poison that accumulates. Acute cadmium poisoning usually results from inhalation of cadmium dusts or the intake of cadmium salts. When the concentration of cadmium in the kidney reaches 200 µg per g there is renal injury. Other health problems that have been attributed to cadmium are hypertension and itai-itai disease (11). Cadmium is emitted from volcanic eruptions and released from metal smelting and processing operations. Tobacco smoke is also a source of cadmium ingestion because each cigarette contains 1 to 2 µg of cadmium. Since the blood cadmium levels for smokers can be up to four times higher than those of non-smokers, smoking can lead to a significant buildup of cadmium over a lifetime (12).

Toxicities of Copper and Chromium Copper has wide industrial applications. Among its uses are electrical wiring and circuitry, plumbing materials, and heat exchangers. The toxicological effects of copper have been the subject of some debate. The metal is one of the essential elements for life, a fact that has been recognized since around 1920. Since the metal is also beneficial to plant growth, the normal diet of humans contains sufficient copper to meet daily needs. The two main diseases that are associated with copper are Wilson’s disease and Menkes’ disease. Both are disorders of copper metabolism. If untreated, the former condition can lead to a fatal accumulation of copper in the liver, brain, and kidney. Chromium is used industrially in plating baths and in the production of steels. Chromium(VI) is used in oil drilling fluids and in cleaning fluids. Chromium(VI) is a carcinogen in humans and animals in addition to being both mutagenic and genotoxic. Chromium(VI) requires intracellular reduction for activation, and this in vivo reduction can produce several reactive intermediates such as chromium(V) and chromium(IV) that can target and damage DNA (13). The principal chemical form of chromium(VI) in solution at physiological pH is chromate. Chromium(VI) enters cells through the general anion channel, resulting in a rapid accumulation of high concentrations of intracellular chromium (14). By contrast, water soluble chromium(III) compounds are not carcinogenic, possibly because they do not cross plasma membranes (15, 16). Nevertheless, the final intracel276


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lular reduction product of chromium(VI) is chromium(III), which forms amino acid nucleotide complexes. Mussels, which have been widely used as biological monitors of coastal contamination, assimilate chromium (17). Only chromium(VI) from the dissolved phase and chromium(III) from ingested food contribute to chromium accumulation in marine mussels. The uptake of the toxic chromium(VI) is especially important from solution at lower salinities. The toxicity of chromium (VI) has recently received widespread attention because of the movie Erin Brokovich, starring Julia Roberts. This movie is based on chromium(VI) contamination of the groundwater in Hinkley, California, and the involvement of the Pacific Gas and Electric Company.

Toxicities of Actinides Actinides have become an environmental problem over the past 50 years because of their use in both nuclear weapons and reactors. Disposal of the products of nuclear fission is now a major problem. These nuclear materials present a long-term environmental problem because of their extremely long half-lives as α-particle emitters. These metals, if ingested, become localized into the bone structure where both chemical toxicity and radiation damage can be harmful. One of the strategies for remediating the sites where actinides are stored is to selectively separate the radioactive metals from the other waste materials that have been combined with them. There is some urgency to remediate these sites because both chemical- and nuclear-induced reactions are problematic, and because the storage tanks are leaking into the ground. Although the high cation charge on the actinide ions will limit their migration in the surrounding soil, it is vital that they do not contaminate groundwaters. One common perception of the toxicity of actinides is that the chemical toxicity of plutonium is extremely high, and that the ingestion of even minuscule amounts of the element is fatal. Because plutonium has now been manufactured for over 50 years, and because a few accidents with it have occurred, the chemical toxicity of the element is now known to be less than was originally believed. Strategies for Removing Metals The most commonly used treatment methods for heavy metal-containing waste include precipitation, solvent extraction, activated carbon adsorption, treatment with ion exchange resins, and bioremediation. Other methods that are less widely used include reverse osmosis, electrolysis, cementation, irradiation, zeolite adsorption, evaporation, membrane processes, and ion flotation (18). Solvent extraction occurs when a metal ion associates with an organic complexant to form a coordination compound that is transferred from the aqueous to the organic phase in a two-phase system. While much laboratory work has been carried out with chloroform as the organic phase, low-volatility aliphatic hydrocarbons are preferable for industrial applications because of their lower toxicity. Carbon dioxide is also being used for the solid–liquid extraction of heavy metals from contaminated soils and waters. This fluid under supercritical conditions has the potential to become technologically important once its properties are better understood, and more complexants are available that are compatible with it (19, 20). Two other methods that are receiving attention for the removal of metals from soils are


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electrokinetic extraction and phytoremediation. Both are discussed later. Actinides are usually recovered by liquid– liquid extraction methods with tributyl phosphate in the plutonium–uranium extraction (PUREX) process (21). O O P O

Complexants Used for Metal Extraction When choosing extractants, several factors must be considered. One is how strongly the extractant binds to the metal in an in situ environment that contains other complexants such as chloride, nitrate, or sulfate. As a result, extractants are usually not simple monodentate complexants. Multidentate complexants such as chelates have higher metal ion stability constants than do monodentates. Chelates have at least two donor atoms coordinated to the metal (22). These atoms are usually oxygen, nitrogen, or sulfur, but selenium, tellurium, phosphorus, arsenic, and others may also be donors. In designing extractants for positively charged metal cations an advantage is gained if the chelate has acidic hydrogens that are substituted by metal cations during complexation. This leads to formation of an uncharged complex that will likely have a higher compatibility with a hydrophobic organic solvent. The complex to be extracted, even if uncharged, should be as hydrophobic as possible, especially if it is desired to employ an organic solvent for phase transfer extraction. An early report of chelate extraction by Cazeneuve (23) showed how chromium could be extracted into benzene as its 1,5-diphenylcarbohydrazide complex. Macrocycles are also used as extractants, especially for alkali and alkaline earth metal ions. These complexants have the same donor atoms as chelates, but have two advantages. One is the macrocyclic effect that gives such complexes higher stabilities than their open chain analogs, and the second is the selectivity achieved by matching the size of the macrocycle cavity with that of the metal ion. The complexation of both multidentates and macrocycles have been studied by molecular mechanics calculations, and patterns observed that can aid in choosing selective complexants (24). Calixarenes, cyclic condensation products derived from a phenol and formaldehyde, are also used as extractants. These are conformationally mobile oligomers that can be chemically modified to function as preorganized chelate-type complexants (25). The current state of the art in extractant design is the incorporation of a macrocycle into the calixarene framework. This strategy has allowed for the development of new calixcrown extractants that have extremely high selectivities for metal ions such as cesium(I) (26). Other strategies to occlude metal ions involve the use of concave hydrocarbons as host molecules. These compounds that represent three-dimensionally clamped analogs of πprismands show exceptionally high selectivity for metal ions (27). Another approach uses molecular replication to generate molecules having selective metal ion recognition. These systems are often assembled using hydrogen bonding and molecular self-assembly to generate the compounds (28, 29). Metallomacrocycles are yet another class of compound that may become important in the development of new extractants, especially for situations where very high selectivity is required (30). Radionuclides present additional problems because the extractant must be stable to radiation. Commonly used acwww.JCE.DivCHED.org

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P

O

1

O

2

Figure 1. Structures of tributyl phosphate (1) and trioctylphosphine oxide (2).

tinide extractants are phosphates and phosphine oxides such as tributyl phosphate (TBP, 1) and trioctylphosphine oxide (TOPO, 2) (Figure 1). Extractions using 1 may involve the presence or absence of a solvent. The complexation of 1 and 2 with uranium(VI) gives adducts of stoichiometry UO 2(NO3)2(TBP)2 and UO 2(NO3) 2(TOPO)2 (31, 32). Among the solvents used as the organic phase are dodecane (31, 33) and liquid carbon dioxide (34–36). The solubility of uranium(VI) in carbon dioxide correlates with the fluid density, which can be changed by modifying the operating pressure of the extraction. The selectivity between uranium(VI), plutonium(IV) and hydrogen ion also depends on this operating pressure. Thus one can optimize pressure or temperature to increase selectivity in supercritical fluid extractions and metal separations (35). The stoichiometry of the uranium(VI)–TBP complexes formed and the kinetics of extraction into supercritical carbon dioxide are similar to those found for conventional solvents. Supercritical carbon dioxide can also be used to extract uranium(VI) and thorium(IV) by using a binary mixture of TBP and a fluorinated β-diketone (37). Oxoanions present a different challenge because their periphery of oxygen atoms results in the metal center being unable to directly bind to the donor atoms of the extractant. Examples of oxoanions are chromate and pertechnetate. Conventional methods for chromate involve reduction of the chromium(VI) to chromium(III) followed by Cr(OH)3 precipitation at a pH in the 8–10 range (38–40). Heterocyclic amines such as 4-(5-nonyl) pyridine have been used as liquid– liquid extractants for oxoanions from nitric acid solutions. This compound has been used for the separation of pertechnetate from uranium (41), and for the extraction of chromate from a 0.25 M nitric acid solution. At high chromium concentration the metal is extracted as the pyridinium salt of HCr2O7. The chromium can be subsequently transferred back from the organic phase either by lowering the acid concentration or by reducing the extract from its chromium(VI) state to chromium(III) with ascorbic acid, hydrazine, or sodium thiosulfate (42). Extraction from Aqueous Solutions

Surfactants Surfactants are molecules that have both a hydrophobic and a hydrophilic end. In aqueous solution they aggregate


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to form micelles with the hydrophilic groups at the outer region (Figure 2). These properties of surfactants allow them to function as extractants for the transfer of metal ions from an aqueous to an organic phase. Surfactants become highly aggregated in the organic phase to form reverse micelles. The metal ions become encapsulated in the hydrophilic inner region of the micelle. At the interface between the aqueous and organic layers the surfactant molecules align along the boundary. Since the aggregation of surfactants into micelles is a dynamic process, the cavity size can change to accommodate metal ions of different sizes. As a consequence, selectivity can be difficult to achieve. Since the outer part of the reverse micelle has bound protons or cations, ionization of these cations after metal ion complexation into the inner part of the micelle can lead to charge neutralization and subsequent phase transfer of the metal ion micellar host into the organic phase (43–45).

Liquid–Liquid Extraction Since the extraction of a metal cation from an aqueous phase to an organic phase results in a disruption of the hydration spheres about the aqueous phase metal ion, ion hydration is an important aspect of extraction from an aqueous into an organic phase. The three-dimensional hydrogenbonded structure of water results in dissolved ions ordering the water structure over a large distance. As a consequence, ions have distinct first- and second-hydration spheres, and frequently ion-induced ordering of the water structure at even greater distances into the water lattice. Water-miscible liquids such as alcohols frequently reduce the hydration energies of the dissolved ions because of their involvement with the water hydrogen bonding. Cations having a high charge density induce order in the solution, while ones of low charge density have the opposite effect. For anions, fluoride induces order in aqueous solution. Halides that are lower in the periodic table, along with pseudohalide ions, have the opposite effect. Ions such as nitrate, phosphate, and sulfate show an intermediate effect. The perchlorate ion reduces order in solution, thereby resulting in a net favorable entropy effect and an increased extraction coefficient. The hydration energies (given as free energies ∆Ghyd) of a group of selected cations and anions (Table 1) show a large increase in the hydration energy as the charge on the ion increases. Partial substitution of non-aqueous solvents for water leads to a decrease in the hydration of the cation, which subsequently reduces the energy required for its desolvation. As a consequence, in a liquid–liquid separation procedure it is advantageous to have an organic phase that has some solubility in water. This effect can be transferred to ion exchange separations where a solution of 20% ethanol in water is used rather than pure water itself (46). Aqueous Biphasic Systems Aqueous biphasic systems are used for the extraction and separation of metal ions. Biphasic systems form when certain water-soluble polymers are combined, either together or with inorganic salts, over particular concentration ranges. A widely used system consists of water and polyethylene glycol (CH2CH2O)n . Polyethylene glycol (PEG), an inexpensive material obtained from ethylene oxide, forms two phases in the presence of salts. In some cases the metal ion can be di278


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Aqueous Phase

Organic Phase

Figure 2. Schematic diagram of a micelle.

Table 1. Gibbs Free Energies (kcal mol 1) of Hydration of Selected Ions Cation

∆Ghyd

Anion

∆Ghyd

Cs

66.5

I

59.0

NH4

68.1

Br

67.9

K

79.3

CN

70.5

Na

97.0

NO3

71.7 74.8

Li

120.8

Cl

H

259.2

NO2

Pb2

340.6

OH

102.8

Cd2

419.5

C1O4

102.8

2

420.6

F

103.1

Fe2

450.5

H2PO4

111.1

Fe3

1002.0

CrO42

227.1

Cr3

1006.0

SO42

258.1

Pu4

1462.7

CO32

314.3

4

1567.9

Hg

U

3

PO4

78.9

660.9

NOTE: Values are taken from Extraction of Metals from Soils and Waters; Plenum: New York, 2001.

rectly extracted into the PEG-rich phase in the absence of a complexant, but better selectivity can often be achieved by adding a water-soluble complexant (47). Advantages of the aqueous biphasic PEG system is that it is inexpensive, nonflammable, and non-toxic. With this system two immiscible aqueous phases can be obtained by the addition of inorganic salts. A wide range of salts have been used. The more negative the Gibbs free energy of hydration of the ions in the salt, the greater is its effect in causing phase separation. The effect is additive for the cation and anion in the salt. A disadvantage of this system is the difficulty of recovery of the organic-rich phase. In systems that use only an aqueous and an organic phase the metal ions that are extracted into the organic phase can be recovered by extraction of the metal ion back into an aqueous phase. For aqueous PEG, however, recovery is more difficult because the phase separation step has been achieved by the addition of ammonium salts, and


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PEG/H2O/XⴚMⴙ

Upper Layer

PEG/H2O

Lower Layer

drops stirred in an aqueous source phase. Separation is rapid because of the high surface area of the organic layer. The source phase is the bulk liquid, and the receiving phase is the water trapped within the oil drop. By contrast, a supported liquid membrane comprises an organic carrier phase immobilized in a porous polymer support. The support can be in the form of a flat sheet or a hollow fiber, with the latter being preferable because of its high surface area. Examples of metals that have been extracted using liquid membranes are lead and cadmium (52), uranium (53), and cesium (54). Extraction from Soils

Figure 3. Schematic diagram for biphasic PEG system liquid membranes.

ⴙ

MA

ⴙ

MAⴙ

MB ⴙ

MB

ⴙ

MB

MAⴙ MAⴙ

source phase

ⴙ

MA

ⴙ

MA

MAⴙ liquid membrane

receiving phase

Figure 4. Schematic diagram for a liquid membrane. Reprinted with permission from Extraction of Metals from Soils and Waters ; Plenum: New York, 2001.

PEG recovery requires removal of these salts. The metal ion M is complexed by the anion X (usually thiocyanate, iodide, or bromide), which is then extracted into the upper layer (Figure 3). Examples of metals that have been extracted using the biphasic PEG system are copper (47), sodium and cesium (48), technetium (49), and chromium (50). The latter system involves transferring chromium between the two phases by converting it from its chromium(VI) to its chromium(III) oxidation state. Liquid membranes are useful for the separation of metal ions (51). Liquids that are immiscible with both the source phase and the receiving phase can be used as liquid membranes. High transport rates are obtained with such systems because diffusion is rapid between liquid phases. By adding complexants, high selectivity for the ions MA over MB can be achieved (Figure 4). Furthermore, since the liquid membrane zone occupies a small volume between the source and receiving phases, only small amounts of complexant are required to achieve selective transport. These systems can be considered to be analogs of the biological membranes that provide a barrier between an intracellular and an extracellular aqueous environment. Different configurations can be employed to achieve a liquid membrane system. Bulk liquid membranes comprise an aqueous source phase and receiving phase separated by a waterimmiscible organic liquid in a U-tube. An emulsion liquid membrane comprises a dispersion of water-containing oil www.JCE.DivCHED.org

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Many metals can be removed from soil by washing with detergents such as sodium dodecyl sulfonate (SDS). Several problems exist with this method. One of these is that the method is non-selective, and a second is that it is usually necessary to excavate the soil to another site for cleansing. Other more selective methods are therefore usually preferable.

In Situ Stabilization One approach to dealing with metal contamination in soils is in situ stabilization. The in situ stabilization can involve converting the metal to an insoluble form, such as an insoluble sulfide, thereby converting the metal into the same chemical composition as the mineral from which it was originally extracted. Examples are the conversion of lead and mercury to their sulfides, or chromium to its oxide. Another alternative involves solidification and stabilization into Portland cement prior to burial and disposal. An agriculturalbased alternative is also viable. This method uses green plants and soil amendments to achieve in situ stabilization (55). Electrokinetic Extraction An alternative to soil washing is the use of in situ electric fields to dissolve and then transport heavy metal ions in soils and clays (56). This electrokinetic extraction method is particularly useful where soil removal is difficult or prohibitively expensive, or where detergent washing will destroy the soil integrity. The efficiency of the method depends on the soil type and water content. For dry and sandy soils water must be added so that there is a mobile phase through which the metal ions can migrate. This electrochemical (electrophoretic) processing of soils by the use of an in situ direct current results in the development of electrical, hydraulic, and chemical gradients within the soil matrix. These gradients produce a net movement in both water (electroosmosis) and ionic species (electromigrations). Because the technique involves the application of an electrical potential there is also the electrolysis of water into hydrogen and oxygen. This electrolysis is a significant cost deterrent to the use of electrokinetic extraction since the major cost is the electrical power, and a portion of this is wasted in the water electrolysis. The primary electrochemical reactions at the anode and cathode produce protons (H) and hydroxyl anions (OH), which are transported through the soil along with the metal ions M: Anode: 2H2O 4e → O2 4H Cathode: 4H2O 4e → 2H2 4OH The generation of an acid front within the soil that moves from anode to cathode results in both the solubiliza-


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tion of basic metal hydroxides, carbonates, or adsorbed species, and the protonation of any basic organic functional groups in the soil humus, giving these components more cationic character. An ion exchange process occurs within the soil, with the solubilized ionic species being carried by both the electric field and the electroosmotic movement of water to the cathode. At the cathode, reduction of the metal ions to the metal can occur as in an electrolytic purification process. More commonly, however, the electrokinetic extraction process is not carried out to this degree of completeness, but is instead used to achieve concentration of the metal ions in the vicinity of the cathode. This concentration of heavy metals or radioactive metals into a chosen region of the soil site is particularly valuable if this concentrates the metals into a location where a relatively small quantity of soil can be easily excavated and cleansed off-site by soil washing or other techniques. However, a problem with the method is selectivity, since the applied voltage migrates all cations in the direction of the cathode. The electrokinetic method involves inserting electrodes in the soil and applying a potential across them. By moving the location of the electrodes periodically, the remediated metals can be migrated across the site in a manner that resembles zone refining. In this technique the application of an in situ direct current produces an electroosmotic water flow, and H and OH fronts that move through the soil in opposite directions (57–61). The acid front that moves from the anode to the cathode dissolves metals that are absorbed in the soil (Figure 5). The highly alkaline interstitial water near the cathode then causes heavy metals to precipitate, limiting the extent of metal ion migration and the effectiveness of the reclamation process. This precipitation of metal hydroxides can be avoided, however, if the metal is strongly bound to a complexant. Electrokinetic extraction has been used for copper, lead, cadmium, and mercury (59, 62), chromium (63–65), and for radionuclides that have become bound to soils or building materials (66).

Phytoremediation and Bioremediation Toxic metals are problematic in soils because not only are they occluded into the zeolite soil structure, but they are also absorbed into the humus and biomass present in soils.

anode +

cathode – ⴙ

H H2O

OHⴚ

OHⴚ Hⴙ

Mⴙ Hⴙ

H2O

OHⴚ

H2O OHⴚ

Figure 5. Electroosmotic flow. Reprinted with permission from Extraction of Metals from Soils and Waters; Plenum: New York, 2001.

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These humic and fulvic substances in soils contain compounds that act as chelating agents to these metals, thereby contributing to the difficulty of their removal. In addition, these substances often contain redox-active agents that can convert the adsorbed metals into ones that have different oxidation states, or reduce metal ions down to the metallic state. Although the removal of metals from soils poses several challenges, the presence of good chelating agents in humic and fulvic substances is one of the obstacles that must be overcome if phytoremediation or bioremediation is to be the method of choice for metal removal. Phytoremediation is the use of green plants to remove pollutants from the environment. Two recent reviews have been written on this subject (67, 68). Among the types of phytoremediation currently in use are phytoextraction and rhizofiltration. Phytoextraction is defined as the use of metalaccumulating plants that concentrate them into the harvestable parts. Rhizofiltration is the use of plant roots to absorb metals from aqueous waste streams. Phytoextraction can be carried out either with or without added chelate complexant to assist in removing the metals. In certain cases the addition of chelating agents enhances the accumulation of metals by plants, especially if the chelate has a strong affinity for the targeted metal. Nevertheless, a consideration when using this method is the requirement that the chosen chelate must be biodegradable or readily removed from the contaminated site. Alternatively, phytoremediation can rely only on the physiological processes that allow plants themselves to accumulate metals. A disadvantage of this approach is that growth rates are slow, and the selectivity for particular metals is likely to be low. In the future, however, genetic engineering could be useful in producing plants that have both higher growth rates and metal selectivities. Bioremediation involves the use of biological remedies for pollution reduction (69). For metals this detoxification process must involve processes such as the oxidation or reduction of the metal center either to make it less water soluble, so that it precipitates and can be removed in solid form, or to convert it to a more volatile form that can be removed in the gas phase. In choosing a bioremediation strategy for metals, the biological system must be able to tolerate the concentration of metal that is present at the site. Among the toxic metals that are common soil contaminants and need to be removed are cadmium, lead, copper, chromium, and actinides. When considering phytoremediation or bioremediation strategies for these metals, a high selectivity against others such as sodium and calcium is necessary, since they are common components of soils. Good tolerance is required toward high concentrations of metals that are toxic to plants as well as to mammals. Finally, for transuranic elements, tolerance to radioactivity is another requirement. For cadmium phytoremediation, both water hyacinth and various grasses are effective (70, 71); for lead and copper, alfalfa is used (72); and for chromium, both Indian mustard and sunflowers are effective (73), as is buckwheat (74). For the transuranic elements, trees are effective (75). Bioremediation has been used for mercury, cadmium, copper, lead, and chromium. Mercuric ion reductase catalyzes the reduction of mercury(II) to elemental mercury (76), biomass absorbs cadmium, copper, and lead, with subsequent elution of the metals being achieved by pH gradient elution


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(77). Since numerous microorganisms reduce chromate to chromium(III), several options are available for this detoxification of chromium (78). Complementary Effect of Complexants Complexants can also be used in conjunction with phytoremediation and bioremediation methods. In some cases the metals are absorbed into the roots of the plants in free form in the absence of complexants, but for other systems the method is more efficient with the metal in complexed form. Another application of complexants in phytoremediation is that in some cases they assist the migration of the metals from the roots to the shoots. Comparison of Methods Of these methods, surfactants and liquid–liquid extraction are the most widely used after simple adsorption onto a solid phase. Each is inexpensive if the liquids can be recycled in pure form. For liquid–liquid extraction this recycling is important, since the goal of metal removal projects may be the purification and re-use of water. As a result the organic phase must be non-toxic. Since PEG is non-toxic, this aqueous biphasic system eliminates the need for toxic organics. Electrokinetics is a specialized method that is useful where soils cannot be excavated, such as around trees or under buildings. As mentioned, however, it is costly in electrical power. Phytoremediation is starting to becoming the method of choice because it is non-polluting, does not require soil excavation, and is more acceptable to the public than chemical methods. Acknowledgment We thank the U.S. Army Research Office, the U.S. Department of Energy through the Pacific Northwest National Laboratory, the National Science Foundation, and the Robert A. Welch Foundation for support of our research in this area. Literature Cited 1. Roundhill, D. M. Extraction of Metals from Soils and Waters; Plenum: New York, 2001. 2. Ochiai, E.-I. J. Chem. Educ. 1995, 72, 479. 3. Carter, D.E., Fernanado, Q. J. Chem. Educ. 1979, 56, 490. 4. Craig, P. J. Organometallic Compounds in the Environment; Craig, P. J., Ed.; Wiley: New York, NY, 1986; Chapter 2. 5. The Merck Index, Centennial (11th) ed.; Merck & Co. Inc.: Rahway, NJ, 1989; 5801: Mercury. 6. The Pharmacological Basis of Therapeutics, 7th ed.; Gilman, A. G., Goodman, L. S., Rall, T. W., Murad, F., Eds.; Macmillan: New York, 1985; pp 1611–1614. 7. Waldman, S. Lead and Your Kids. Newsweek, July 15, 1991, 42–48. 8. Jandreski, M. A. Clinical Chem. News January 1994. 9. Morton, A. P.; Partridge, S.; Blair, A. Chem. In Britain 1985, October, 923. 10. Fang, X.; Fernando, Q.; Chemistry Res. Toxicol. 1995, 8, 525. 11. Klaasen, C. D. in The Pharmacological Basis of Therapeutics, 7th ed.; Gilman, A. G., Goodman, L. S., Rall, T. W., Murad,

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