remediation of metal-contaminated sites using plants

18
Remediation of Metal-Contaminated Sites Using Plants Azadeh Azadpour John E. Matthews Azadeh Axadpour, Ph.D., currently a research microbiologist at Dynamac Corporation, assists US. EPA a t Robert S. Kerr Environ- mental Research Labora- tory {RSKERL) with site characterization and bioremediation. Her research interests include bioremediation of soil and groundwater contaminated with hazardous wastes using indigenous microorgan- isms.John E. Matthews, environmental scientist, is the president of JMCO, Inc., Environmental Consulting. He worked at EPA-RSKERL for 26 years in subsurface investigations and land treatment. Heavy metal contamination of soil resulting from anthropogenic sourcesposes a significant challenge in many industrialized societies. TBe current technologies employed for removal of heavy metals often involve expensive ex-situ processes requiring sophisticated equipment and re- moval, transportation, and purification of the soil. Generally, in-situ remedial technologies are favored to ex-situ methods for detoxification, neutralization, degradation, or immobilization of contaminants. In-situ bioremediation is increasingly favored because of its effectivenessand low cost. A new type of bioremediation, known as vegetative remediation or ‘fphytoremediation, uses metal-tolerant hyperaccumulatorplants to take up metal ions from soils and store them in their abovegroundparts To select the appropriate phytoremediation technology, one must understand the technical feasibility, cost effectiveness, and availability of the suitableplant species. Equally important is determining whether the site’s soil conditions are optimal to enhance or restore the soil biological activity. Befbre phytoremediation can be exploited on a contaminated site, greenhouse- scale confirmatoy testing is necessay to measure plant uptake and correlateshootmetal concentrations to availablesoil metals. These tests also validate that the harvesting and subsequent disposal of metal-containing plant tissues are environmentally safe and manageable. By definition, a heavy metal has an atomic number greater than or equal to 20, with a specific weight of five grams or more per cubic centimeter. Although all heavy metals differ significantly in their chemical properties and biological effects, they are referred to collectively as “trace elements.” As a whole, trace metals are classified as environmental pollutants due to their toxic effects on the biosphere (Lepp, 1981). Sources of metal contamination abound in the global ecosystem. Smelting, mining operations, fertilizer factories, urban runoff, and automo- bile emissions account for most of the metal-related environmental pollution. Sewage disposal practices and pesticides containing metals also are significant sources for toxic metal contamination. Nevertheless, natural geological formations contribute the largest percentage of trace metals to the soil (Jackson et al., 1990). CCC 1051-5658/96/060301-18 0 1996 John Wiley & Sons, Inc. 1

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Page 1: Remediation of metal-contaminated sites using plants

Remediation of Metal-Contaminated Sites Using Plants

Azadeh Azadpour John E. Matthews

Azadeh Axadpour, Ph.D., currently a research microbiologist at Dynamac Corporation, assists US. EPA at Robert S. Kerr Environ- mental Research Labora- tory {RSKERL) with site characterization and bioremediation. Her research interests include bioremediation of soil and groundwater contaminated with hazardous wastes using indigenous microorgan- isms. John E. Matthews, environmental scientist, i s the president of JMCO, Inc., Environmental Consulting. He worked at EPA-RSKERL for 26 years in subsurface investigations and land treatment.

Heavy metal contamination of soil resulting from anthropogenic sourcesposes a significant challenge in many industrialized societies. TBe current technologies employed for removal of heavy metals often involve expensive ex-situ processes requiring sophisticated equipment and re- moval, transportation, and purification of the soil. Generally, in-situ remedial technologies are favored to ex-situ methods for detoxification, neutralization, degradation, or immobilization of contaminants. In-situ bioremediation is increasingly favored because of its effectiveness and low cost. A new type of bioremediation, known as vegetative remediation or ‘fphytoremediation, ” uses metal-tolerant hyperaccumulator plants to take up metal ions from soils and store them in their abovegroundparts To select the appropriate phytoremediation technology, one must understand the technical feasibility, cost effectiveness, and availability of the suitableplant species. Equally important is determining whether the site’s soil conditions are optimal to enhance or restore the soil biological activity. Befbre phytoremediation can be exploited on a contaminated site, greenhouse- scale confirmatoy testing is necessay to measure plant uptake and correlateshoot metal concentrations to availablesoil metals. These tests also validate that the harvesting and subsequent disposal of metal-containing plant tissues are environmentally safe and manageable.

By definition, a heavy metal has an atomic number greater than or equal to 20, with a specific weight of five grams or more per cubic centimeter. Although all heavy metals differ significantly in their chemical properties and biological effects, they are referred to collectively as “trace elements.” As a whole, trace metals are classified as environmental pollutants due to their toxic effects on the biosphere (Lepp, 1981).

Sources of metal contamination abound in the global ecosystem. Smelting, mining operations, fertilizer factories, urban runoff, and automo- bile emissions account for most of the metal-related environmental pollution. Sewage disposal practices and pesticides containing metals also are significant sources for toxic metal contamination. Nevertheless, natural geological formations contribute the largest percentage of trace metals to the soil (Jackson et al., 1990).

CCC 1051 -5658/96/060301-18 0 1996 John Wiley & Sons, Inc.

1

Page 2: Remediation of metal-contaminated sites using plants

AZADEH AZADPOUR JOHN E. M A ~ H E W S

I 1 I I I I 1

I + 6121.5Carrot Roots 1647.4 Broccoli Roots

I 1628.2 Lettuce Roots

I 1356.7 Cauliflower Roots

r 490.5 Spinach Roots

L I 57 1.2 PCI ROOIS

667.7 Lettuce Leaves 663.2 Oat Roou

398.0 Radish Tops 294.4 Carrot Tops - 268.5 Broccoli Leaves

239.3 Spinach Leaves 198.6 Cauliflower Leaves

Some heavy metals pose more concerns than others because of their extensive environmental distribution. The literature provides a substantial body of evidence (Hammons et al., 1978; Lagerwerff, 1971; Miller et al., 1976; and John et al., 1972a) suggesting that common vegetables take up cadmium to concentrations (Exhibit 1) considered dangerous to human health (Friberg et al., 1979). It has been shown that cadmium exhibits significant phytotoxicity effects resulting in: root damage (Lagerwerff and Biersdorf, 1972); chlorosis and necrosis (Haghiri, 1973; Page et al., 1972; and Turner, 1973); disruption of chlorophyll synthesis (Barcelo et al., 1988); alteration of water balance (Poschenrieder et al., 1989); and reduction in growth (Root et al., 1975). The presence of cadmium also may affect soil zinc levels (Hawf and Schmid, 1976).

To decontaminate soils associated with heavy metals, environmental and economic constraints require development of innovative in-situ technologies as alternatives to those currently in practice. Currently available remedial technologies employ soil immobilization or metal extraction techniques (U.S. Army, 1987). Extraction techniques include soil

m 29.8 Carrot Tubers I 29.2 Pea Pods

19.7 Pea Seeds

Exhibit 1. Cadmium Content of Plants Grown in 200 mg Cadmium per 100 g Soil

Cadmium Content (ppm)

2 REMEDIATION/SUMMER 1996

Page 3: Remediation of metal-contaminated sites using plants

REMEDIATION OF LMETAL-CONTAMINATED SITES USING PLANTS

Significant phytoaccumulation of heavy metals from contaminated soils can succeed only i f normal phytotoxic effects can be overcome.

excavation and washing (U.S. EPA, 1991). Using plants as multipurpose soil remediators has gained considerable interest since studies have shown that metal-tolerant hyperaccumulator plants can extract and accumulate high concentrations of toxic metals from soils (Chaney, 1983; Baker and Brooks, 1989; and Brown et al., 1994).

Two principal factors influence whether or not plants can be used to remediate metal-contamination soils: (1) phytotoxic effects and (2) bioaccumulation potential. Therefore, this article primarily focuses on:

effects of toxic metals in the soil-plant systems and feasibility of using plant species at metal-contaminated sites to uptake considerable amounts of phytotoxic elements.

PHYTOTOXICITY Significant phytoaccumulation of heavy metals from contaminated

soils can succeed only if normal phytotoxic effects can be overcome- either with proper management or by selecting appropriate plant species tolerant to high metals concentrations. Many researchers have examined the phytotoxic impacts of heavy metals in soils (Allinson and Dzialo, 1981; Brown et al., 1972; Brown and Jones, 1975; Chaney and Giordano, 1977; Foy, 1973; Foy, 1974a; and Foy, 1974b). They found that plants may develop ion stress in soils containing heavy metals (metalliferous), even at low concentrations. In the early stage of metal uptake, biochemical changes induce physical damage to plant membranes, alteration of enzymatic activities, and prevention of root growth (Cumming and Taylor, 1990; Kennedy and Gonsalves, 1987; and Van Assche and Clijsters, 1988). Physiological responses to these disturbances lead to another series of events causing plant hormone imbalance, lack of essential nutrients, cessation of photosynthesis, and the uneven distribution of water through- out plants (Barcelo and Poschenrieder, 1990). Collectively, these factors prevent plant growth by inhibiting cellular expansion.

The physiology of heavy metal toxicities in plants varies extensively. Unfortunately, characteristic symptoms of phytotoxicity often do not manifest themselves early enough to detect and prevent substantial ecological loss. Foy et al. (1978) found stunting and chlorosis to be the two most general symptoms of the phytotoxicity. Chlorosis was linked to interaction of excess metals (copper, nickel, and zinc) with foliar iron (Fe). These researchers also noted that “the stunting commonly observed with metal toxicity can be due to a specific toxicity of the metal to the crop, antagonism with other nutrients in the crop, or inhibition of root penetration in the soil.” Furthermore, they noted that most of the early problems affected root systems, including restricted development and concomitant nutrient uptake. It was also observed that seasonal fluctua- tions in soil temperature and moisture content influence these effects. For example, aluminum toxicity, a major growth-limiting factor for highly acidic soils, reduces plant rooting depth, increasing drought susceptibility, which, in turn, decreases nutrient uptake (Foy, 1974a; and Konzak et al., 1977).

REMEDIATION/SUMMER 1996 3

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AZADEH AZADPOUR JOHN E. MATTHEWS

At the cellular level, the presence of aluminum in soil significantly reduces cell wall elasticity due to a crosslink of the pectin molecules. On the other hand, cadmium, zinc, and lead directly bind to the pectin molecules to reduce elasticity. Binding of the latter ions occurs regardless of pectin location. Cross-linking or binding of heavy metals to the pectin molecules significantly increases the rigidity of the cell wall and ultimately induces a large barrier against plant growth. This was demonstrated using a population of peas (Foy, 1774a). Metal toxicity also directly reduces cell wall synthesis; however, little is known about the effect of heavy metals on these processes (Fry, 1986).

Cell membranes generally are considered the prime target of heavy metal injury (Wainwright and Woolhouse, 1977). Thus, as the result of membrane destruction, plasmalemma becomes distorted and leaks K-ions, resulting in a loss of concentration in the root cells. This potassium leakage does not result from a reduction in adenosine triphosphate (ATP) due to the sodium and potassium ions pump or from an absence of membrane integrity, but from the direct binding of metal ions to the membrane (Barcelo and Poschenrieder, 1790). Cadmium toxicity in thylakoid membranes targets the structure, function, and composition of the photosystem I1 (Becceril et al., 1988; and Maksymiec and Baszynski, 1988). In contrast, during the early stages of growth, cadmium toxicity directly influenced Calvin cycle reactions, inhibiting energy consumption in bean plants (Phaseolus uulgaris L. cv. Scarlett), rather than affecting properties of the photosystem I1 (Krupa et al., 1793).

Although phytotoxic effects of metals uptake have been shown to differ significantly for each metal, other physical and chemical factors may play central roles in plants’ toxicity:

Cell membranes generally are considered the prime target of heavy metal injury.

1. Plant genotypes. Significant variation in metal tissue concentra- tions of plants grown on the same soil is determined by their genotypes (Haghiri, 1973; Cottenie and Kiekens, 1774; Page, 1974; Bingham et al., 1776; Giordano and Mays, 1977; Doyle et al., 1978; Mitchell et al., 1978; Davis and Carlton-Smith, 1780; and Smilde, 1981). For example, Barman and La1 (1774) showed that Solanum tuberosum, Allium satiuum, and Brassica oleracea var. capitata were zinc accumulators, while Spinacea oleracea and Croton bonplandianum were lead accumulators. Cadmium was accumu- lated in Solanum tuberosum, Brassica oleracea var. capitata, and Spinacea oleracea. Copper was accumulated in most of the above species (Exhibit 2). Soil pH. Increases in soil pH generally reduce the solubility of metals and, therefore, their uptake in plants (Wilkins, 1978; Dowdy, 1975; and Page, 1974). In this respect, ultrabasic serpen- tine soils were found to reduce nickel bioavailability, and despite high concentrations of this element in the soil, symptoms of nickel toxicity were not observed in oats (Slingsby and Brown, 1777). There are exceptions for molybdenum and selenium which become more available for plant uptake in neutral and alkaline soils (Page, 1974).

2 .

4 REMEDIATION/~UMMER 1996

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REMEDIATION OF METAL-CONTAMINATED SITES USING PLANTS

84

2 Q .. a fi

o m

g

$

- .- - -

Exhibit 2. Concentration of Pb, Zn, Cd, and Cu in the Leaf Dry Matter of a Range of Species Growing on a Naturally Metalliferous Soil

1 - Tlduspi ulpe.rtrc - - Minuurtiu vcrnu - Arnreriu nturiritnu - Silenc vulguris 1 Llrtus cortlicululus - Festucu ctrinu - Thymus scrpylluni - RU~IICXUCC~OSLI I Euphrmiu strictu - Achillcu millc/olium = Violu cukuminuriu

Cumpunuh rolundifiik~

5 8 ‘ vi N

- .- o m - g -

1

I ThIu.cpi ulpestrc - - Minuurtiu vcrtiu - I Arnrrriu nruritimu

Silenc vulguris Lotus corrriculurus Festucu osiriu

w - Cunipiinuh ri~tunduiih - Tlrynrus serpyllum I - Rurnex i~ccIosu

Euplrrusiu srrirru - Achilleu millcfiolium I Violu culunririuriu

$: 0

vl 2 . a - u , - v 1 - 0

3 1 g -

.-

Anncrio nruritirnu Silcnc vulguris

Lnrus cunriculutus Festucu ovinu

Cumpunulu rutundifoliu Thymus serpyllurn

Rumex i~cc~osu Euphrusiu stricfu

Achillcu millefoliutn p Violu culurninuriu

Adapted from Ernst, 1975.

- 5 , 0

vl 4 b”

B - - c d r

.- 0 v1 -

3. Cation exchange capacity (CEC). Increase in the CEC of soil reduces plant metal uptake (Giordano and Mays, 1977).

4. Organic matter and phosphate. Generally, availability of a toxic metal to plants is more enhanced in its inorganic forms than in its organic forms (King and Dunlop, 1982).

5. Natural and synthetic complexing agents. The presence of these agents increases the solubility of metals, making them more available to plant roots. Because alkaline soils enhance this effect, synthetic chelators such as ethylene-diaminetetra-acetate (EDTA)

Armeriu muritirnu Silenc vulguris - hitus corniculurus - Festuco ovirru

Cumpunulu rri!undijoliu Tlrynrus serpyllum

Runiex ucetosa F - Euphrusiu srrictu

I Achillcu millcfolium i Violu culuminariu

REMEDIATION/SUMMER 1996 5

Page 6: Remediation of metal-contaminated sites using plants

AZADEH AZADPOUR JOHN E. MAITHEWS

Plants belonging to the metalliferous soils (metallophytes) are among tolerant species.

(Davis and Carlton-Smith, 19801, and diethylene-triaminepenta- acetic acid (DTPAXBrown et al., 1994) are used for the separation of heavy metals from sewage sludge and/or decontamination of nuclear facilities.

Plants do not generally take up considerable amounts of metals because of lack of bioavailability of the metals (Jackson and Alloway, 1992). Although the total heavy metals content in soil reflects the extent of contamination, it does not truly indicate metal bioavailability or mobility. Environmental factors and transformations in the soil may decrease or increase the bioavailability of metals to plants (Giordano et al., 1975; and Giordano and Mays, 1977). The ultimate goal in using vegetative remediation is to enhance bioavailability without inducing phytotoxicity. To do so, one should consider not only soil concentration and solubility of heavy metals, but also the plant’s range of tolerance as described by Davis and Beckett (1978). Availability of metals beyond the upper limit of the tolerable range, toxicity threshold (T), will adversely affect plant growth. Factors influenc- ing availability of metals are depicted in Exhibit 3.

PLANT TOLERANCE TO HEAVY METALS The movement of metals into the plant shoot is confined by the plant’s

roots (Jarvis, 1976). It is well-established that the root cell walls are the primary sites of complexing metal ions in plants (Ashida et al., 1963; Ernst, 1972; and Paton and Budd, 1972). Two important mechanisms enable plants to escape metal toxicity: avoidance and tolerance (Cumming and Taylor, 1990; and Baker et al., 1991). In avoidance, plants prevent the uptake of metals. Since tolerant species cannot avoid uptake, metals accumulate in plant tissues at various degrees. Perhaps large production in plants of certain organic molecules containing ligands plays a significant role in metal detoxification (Jackson et al., 1990). For example, cadmium and copper form complexes that are more stable with ligands containing sulfur and nitrogen centers than the oxygen-containing ligands.

Plants belonging to the metalliferous soils (metallophytes) are among tolerant species. Tolerance in natural populations of plants may evolve as follows:

1. translocation and localization of metals in nonsensitive plant parts; 2. development of metal resistant enzymes; and 3. alterations in metabolic pathways, so that the presence of metals

does not intervene or affect metabolic by-products.

Among the three adjustments, metal-resistant enzymes play the most important role in the development of tolerance (Mansfield, 1976).

Grasses are frequently used as models for uptake studies because they are abundant in contaminated soils, have simple structures, and grow quickly to maturity. Metal tolerance usually is determined by measuring root growth o r plant yield in culture solutions containing various concentrations of toxic metal(s1, as compared to a control solution containing no metal species

6 REMEDIATION/SUMMER 1996

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REMEDIATION OF METAL-CONTAMINATED SITES USING PLANTS

Exhibit 3. Factors Affecting Availability in Soil to Plant Roots

Primary Mineral

weathering

uptake complex with insoluble organic byplants\, J MR/ matter

- - - - - - - - -_ - -_ - -

incorporation into microbial

tissue

i M++&MCh : I f=lon complex

surface adsorpdon. oxidation

reduction I1

occlusion in developing precipitates

-1 solid state

diffusion into soil materials

precipitation of oxides or phosphates

for Fe and Mn

Adapted from Hodgson, 1963.

(Wilkins, 1978). Within various plant species, individuals tolerant to high concentrations of metals have been identified (Bingham et al., 1976; and Coughtrey and Martin, 1978). In practice, the most fundamental problem to overcome is relating the concentration of toxic ions in the soil to those in soil solutions (Wilkins, 1978). Most studies have been performed under controlled conditions using excess metal concentrations in culture solutions in which the entire ion content is bioavailable. Thus, information regarding standing plants receiving sewage sludge or industrial effluent is incomplete since the total soil metal content does not reflect the amount bioavailable for plant uptake (Slingsby and Brown, 1977).

According to Wilkins (1978), tolerance to toxic metals is genetic. However, this adaptation has not been detected in plants growing in noncontaminated soils (Simons, 1977). Tolerant plants could avoid stress and maintain normal functions without prior exposure to heavy metal(s). In fact, transfer of tolerance at the genetic level caused the differences in susceptibility among individuals from the same species toward a particular metal(s) substrate (Wilkins, 1978). In contrast, Baker (1981) showed that a wide variety of plants (grasses, forbs, and trees) surviving in soils containing excess amounts of zinc, lead, cadmium, and copper have adapted to tolerate metals. The adaptive nature of tolerance was claimed

REMEDIATION/~UMMER 1996 7

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AZADEH AZADPOUR JOHN E. MATIXEWS

since plants were unable to avoid the uptake of toxic ions. The same author classified plants growing in metalliferous soils as either accumulators or excluders. Accumulators concentrated metals in the plant parts above ground, whereas the excluders had low concentrations of metals in the shoot regardless of the soil metal content. Although the actual mechanism of tolerance was mainly an “internal” process, accumulators and excluders differed in their detoxification site, which was within the shoot for accumulators and within the root for excluders. However, without exception, concentrations of toxic ions were higher for tolerant than for nontolerant individuals (Baker, 1978 and 1981).

Another possible mechanism that plants use to avoid toxicity is storing toxic metals in plant vacuoles by chemical chelation process or excreting them from leaves or fruits (Barman and Lal, 1994; and Leita et al., 1989). This pattern is not consistent for all the metals and is a characteristic of the plants grown on the metalliferous sites.

PHYTOREMEDIATION OF METALS

Soil Stabilization Using plants to cover metal-contaminated areas limits the movement

of metals within and/or from the soils despite wind or water erosion (McLean et al., 1974; Smith and Bradshaw, 1979; and Thornton, 1974). The effectiveness of vegetative remediation in soil stabilization and groundwa- ter protection was described by workers at the Great Plains-Rocky Mountain Hazardous Substance Research Center (Schwab et al., 1793). This project involved remediating Superfund sites containing elevated levels of toxic metals (arsenic, zinc, lead, cadmium, and chromium) resulting from mining and smelting activities. In this view, plants may provide an excellent measure of the hydrosphere and atmosphere pollution control. Antonovics et al. (1971) suggested that, in addition to soil stabilization, plants also contribute to organic matter and humic content of the soil surface by: Plants may provide an

excellent measure of

the hydrosphere and 1. enhancing soil nutrients; atmosphere pollution control.

2. complexing metal ions with organic matter, thus reducing

3. improving soil texture. bioavailability and retarding mobility; and

Plant species that have been used as “indicators” in prospecting for heavy metals provide valuable resources for colonization on metalliferous soils. A comprehensive list and a thorough classification of such plants was provided by Antonovics et al. (1971). According to Lambinon and Auquier (1964), metallophytes are divided into: (1) absolute metallophytes (e.g., Viola cakaminuria; 7;6alspzalpestmsp. cakaminure;Minuartiavemassp. hercyaica) and (2) local metallophytes (e.g., Armeria maritima). Isolation of many metal- tolerant species such as Festucu ovina on metal-contaminated mine wastes was reported by Bradshow et al. (1978); and Baker (1981) indicated that i%hspi a@estre, Minuartia verna, and Amzeria maritima are among species

8 REMEDIATION/SUMMER 1996

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REMEDIATION OF METAL-CONTAMINATED SITES USING PLANTS

The industry seeks more promising in- situ ecotechnological methods that meet regulatory requirements, are cost-effective, and provide human health protection.

capable of accumulating high concentrations of heavy metals. Comprehen- sive review of the literature by Simons (1977) indicated that Agrostistenuis and festuca ovina are among the most widely used plants in the metal uptake studies. Furthermore, many metallopyhtes were isolated from temperate areas, and it was apparent that xeromorphism (drought resistance) is a common characteristic of species adapted to grow on metalliferous soils (Barcelo and Poschenrieder, 1990).

Metal-tolerant species provide more effective cover for soils contami- nated with heavy metals (mine spoils, sewage sludge, industrial effluent, etc.) when coupled with other physicochemical processes to inhibit movement. Since a metal must move from soil to plant roots to induce phytotoxicity, it must be in a solution form to be taken up by plants. Therefore, reducing metal solubilization (Kiekens et al., 1980) effectively decreases bioavailability. One or more of the following processes may contribute to the immobilization of heavy metals:

1. precipitation or coprecipitation; 2. biological uptake or complexation; and 3. adsorption.

To prevent plant toxicity, workers often have combined several factors to enhance immobilization of toxic ions. For example, increases in pH and CEC were possible with the addition of lime, clay, cation exchange resins, and peat. In fact, it was suggested that phytotoxicity of arsenic was more influenced by its chemical form (water-soluble versus non-water-soluble) than by the total amount present in contaminated soils (Woolson et al., 1971).

Several helpful reports on the use of metallophytes for revegetation of metal toxic mine wastes and areas polluted by human activities appear in the literature (Humphreys and Bradshow, 1977; and John et al., 1972b). According to these reports, combining metal-tolerant species with proper application of fertilizers and pH adjustments resulted in the successful and rapid revegetation of contaminated soils. Characteristically, these plants are among hardiness species that hold soil in place by having deep roots to penetrate soil, and high water requirements to take up large amounts of water and dry the soil.

Phytoremediation Because conventional remedial technologies, such as soil excavation

and soil washing, are expensive, the industry seeks more promising in-situ ecotechnological methods that meet regulatory requirements, are cost- effective, and provide human health protection (Mench et al., 1994). Since numerous plants are tolerant to metal ions, plants may act not only to stabilize metal-contaminated soils, but also to reduce metal concentrations. According to Cunningham et al. (1995), a wide variety of plant species have the potential use for metal remediation. Certain plant and tree sap may contain by dry weight up to 3-percent zinc and 25-percent nickel, respectively, without apparent harm. Exploiting these metal accumulators opens new frontiers in remediation technology.

REMEDIATION/SUMMER 1996 9

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AZADEH AZADPOUR JOHN E. MATTHEWS

High levels of metals in the soil inhibit vital activities of soil microflora required for mineralization and biochemical transformation of nitrogen, carbon, sulphur, and phosphorus compounds.

To test remedial potential of hyperaccumulators in metal removal, ten plant species were grown on a test site in the United Kingdom where metal- contaminated sludge from London was applied for 20 years (Parry, 1775). Results indicated that the most effective species in phytoremediation of temperate regions were Kblaspi caerulescem, T. ochmleucum, Alyssum murale, A . teniurn, and A . lesbiacum. With the use of high water-demanding vegetation and trees with extensive root systems to remediate soil and aquifers less than 30 feet deep, Tree MediationTM provided a remedial alternative to the pump-and-treat system (Gatliff, 1774). Vegetation may act as a natural pump in extracting water from an aquifer, thereby controlling the migration of a contaminated plume. This system has been effective in mitigating the migration of heavy metals to an aquifer in South Carolina. Plants also can absorb and metabolize organic compounds (Bell and Sferra, 1972). The role of plants in the remediation of soil and groundwater contaminated with metals and organic materials was discussed at the Third International Symposium of In-Situ and On-Site Bioreclamation in a session devoted to “phytoremediation” as an emerging technology.

As indicated by Brown et al. (1774), successive cropping by hyperaccumulator species potentially can phytoremediate and reclaim soils contaminated with heavy metals. In fact, colonizers of contaminated sites have evolved the natural ability to uptake and translocate the metal ions from roots to their shoots in concentrations many times higher than in the soils on which they were grown (Cannon, 1760). Therefore, the consequences of entering these contaminated plants into the food chain must be considered, particularly for lead and cadmium because of their threat to humans Uorgensen, 1973; and Mench et al., 1774). The concern for cadmium contamination is more serious due to its high mobility and bioavailability (‘John, 1773). The body of scientific evidence provides numerous data for bioaccumulations of heavy metals in edible plants. Leaves of vegetables such as lettuce and spinach accumulate zinc, cadmium, and copper; roots of radish and turnip accumulate zinc and cadmium (Giordano and Mays, 1777).

High levels of metals in the soil inhibit vital activities of soil microflora required for mineralization and biochemical transformation of nitrogen, carbon, sulphur, and phosphorus compounds. This nutrient cycling is essential for the continuity of plant nutrients. Despite many attempts to establish vegetation at Blue Mountain, the lack of success was linked to the elimination of microbial activities in decomposition and regeneration of organic substrates (Oyler, 1988). Adverse effects of soil heavy metals on soil microorganisms and their activities are reported also by McGrath et al. (1788), Chaudri et al. (1773), and Brookes et al. (1786). When metal content of the soil does not inhibit microorganisms, symbiotic association of the microbial community Rhizobium sp. with the plant roots (legumes) has been shown to benefit plant growth due to its ability to fix nitrogen. In fact, metal-tolerant legumes are required for long-term success for revegetation on the polluted soils (Taylor et al., 1792). Mycorrhizal infection of metal- tolerant species also was advantageous to increase the host plant minerals and nutrients uptake, thereby facilitating colonization on the metal-

10 REMEDIATTON/SUWR 1996

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REMEDIATION OF METAL-CONTAMINATED SITES USING PLANTS

Phytoaccumulation from any soil is influenced by all the cations and sesquioxides present.

REMEDIATION/SUMMER 1996

polluted soils (Bradly et al., 1982; and Killham and Firestone, 1983). As described by Bradly et al. (1982), the mechanism of tolerance in mycor- rhizal plants included exclusion of metals in the shoots by complexing them in the root. During the complexing process, hyphal bodies provided extended surface areas within the root cells for the adsorption of higher metal ions. Since metal accumulation by the fungi may protect the plant roots from metal injury, these plants are significant sources of vegetation for covering metal-polluted areas; however, they are not desirable for the extraction of toxic metals from the soil (Bradly et al., 1982).

Nodulation by noncompatible Rbizobiurn sp. caused the nitrogen level in a legume species, flatpea, to resemble that in grasses. The nitrogen level was unlike that in other legumes in a metal-contaminated soil amended with sludge (Oyler, 1988). Whereas, the lack of nitrogen-fixing ability of mycor- rhizal plants under metal stress was linked to the abundance of nitrogen from sludge amendments. Soil-indigenous arbuscular mycorrhizal fungi (enhancer of phosphorus, zinc, and copper plant root uptake) were more sensitive to high phosphate concentrations than to elevated metals present in soil amended with sewage sludge (Weissenhorn et al., 1995).

As indicated by Seeger (1982), mushrooms (Mucrolepiotuprecera) are not dependable bioindicators of cadmium and mercury pollution. This is because mushrooms have extensive enrichment capacity for cadmium accumulation even if isolated from unpolluted areas (Dolischka, 1981). Mushrooms hyperaccumulate lead and cadmium pollution, as shown in fruiting bodies of mushrooms (Baszdiomycetes) growing in the vicinity of roadsides. Also, although they accumulate higher concentrations of copper than the higher plants, their uptake of mercury is relatively low (Kalac et al., 1991).

Even though little information is available to fully understand the mechanism of tolerance, clearly tolerance is metal-specific (Dorrington and Pyatt, 1983). Adaptation, by selection, is developed toward a specific metal ion present in the soil at a higher concentration than those present at background levels. With a few exceptions (Foy, 19781, co- or multiple- tolerances, as reported by Cox and Hutchinson (19801, are not described. This may pose a problem in using a single plant species as the sole accumulators of toxic metals, since most contaminated sites contain a mixture of metals. In reality, phytoaccumulation from any soil is influenced by all the cations and sesquioxides present (Woolson et al., 1971).

When selecting standing plant species to accumulate metals from contaminated sites, note that many metallophytes are excluders, and relatively few metallophytes are capable of storing toxic ions in their shoots. For example, Agrostis sp. tolerated metals by sequestering them in their roots (Ernst, 1976). Furthermore, this selection becomes more complicated by differential bioaccumulation of heavy metals in various vegetables and weeds. It has been shown that heterogeneous accumula- tion in various plant parts ranged from negligible amounts to beyond “critical concentrations” (i.e., the least tissue metal concentration exerting toxic effects) (MacNicol and Beckett, 1985). Maximum concentrations localized in the edible plant portion followed by the nonedible leaves and shoots. According to Al-Attar et al. (1988), the order of toxicity for growth

11

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AZADEH &ADPOUR JOHN E. MATIIIEWS

of perennial ryegrass, Loliumperenne, based on the critical concentrations were as follows:

Sludges are amended to soils with p H values of six or higher.

Shoots Cd>Hg>Ti>Se Roots Hg>Cd>Ti>Se

Concentrations of cadmium which may affect the metabolism of field crops are highly variable and even unknown for many crops. Most frequently, data reported in the literature come from experiments conducted under controlled environments, which may not reflect the extent of the toxic event taking place in nature.

Land application of sewage sludge presents another potential use of plants for in-situ removal of heavy metal. This technology has not been actively pursued in this country because of the potential health hazard for humans. Nevertheless, it has gained some interest among scientists because of its cost effectiveness as a viable solution to waste disposal problems (Baker et al., 1991; and Jorgensen, 1993). Depending on the source, sludges vary in their chemical composition and metal content. Thus, the sludge type determines exposure hazards and risks. In general, cadmium, selenium, and molybdenum pose concerns typically related to animal safety (by direct exposure or consumption of contaminated plants). According to Page (17741, selenium and molybdenum generally occur in small concentrations, and only cadmium, which readily accumulates in high concentrations in the plant shoots, poses a serious problem.

The quantity of sewage sludge that can be applied to soil is usually determined by the amount of heavy metals in the sludge related to the pH and CEC of the soil (Jorgensen, 1993). To avoid leaching problems, sludges are amended to soils with pH values of six or higher (except for boron, molybdenum, and selenium). It has been suggested that the addition of lime and organic matter with the sludge would increase soil pH and would complex heavy metals, thus reducing toxicity. In doing so, metals would accumulate in the top soils (20 cm) and become more bioavailable for plant uptake. To reduce plant bioavailability, King and Dunlop (1782) have suggested using soils rich in organic matter. Soil additives were also found beneficial in site restoration, since immobilizing cadmium and lead from sludge-contaminated soils reduced plant uptake in the presence of lime or hydrous manganese oxide (Mench et al., 1994).

Although efforts conventionally are directed toward reducing plant uptake on sludge-amended soils, Jorgensen (1973) conducted experi- ments using EDTAsolution for watering soil from a shooting range and was able to remove 11.5 percent of the soil lead during a single harvesting of common maize plants grown in various pots. The same author speculated that the combination of EDTA and maize plants will be adequate for the significant removal of the soil lead after six to ten harvestings.

SUMMARY OF PRACTICAL APPLICATIONS OF METAL-TOLERANT PLANTS

Success of any phytoremediation process depends on plant growth and concentration of metals accumulated in each harvesting (Baker et al., 1991).

~~~

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REMEDIATION OF ~~ETAL-CONTAMINATED SITES USING PLANTS

An increasing number of laboratories actively pursue using plants to remove, contain, or render harmless contaminants.

In general, however, metal-tolerant hyperaccumulators do not have high rates of growth (Chaney, 1983). Ernst (1988) indicated that when normal cropping was employed with hyperaccumulators, the cleanup process was extremely slow. In fact, extraction of metals from mine and smelter wastes with metal- tolerant plants by normal cropping was considered impractical, since negligible amounts of contaminants have been removed in each century (Ernst, 1988). Therefore, the major goals of surface mine revegetation are soil stabilization and erosion prevention prior to any post-mining land use (Roberts et al., 1988). While depth of soil has been recognized as the prime reason for plants’ inability to extract sufficient amounts of metals from the mine wastes, the scenario differs for soil amendment materials, such as municipal sewage sludge or other industrial metalliferous wastes. Where the plant root systems can be penetrated throughout the depth of materials, phytoremediation may provide a practical, effective, low-cost, and low-maintenance technology for the removal process (Baker et al., 1991).

The available data suggest that an increasing number of laboratories actively pursue using plants to remove, contain, or render harmless contami- nants (Cunningham et al., 1995). This approach plays a significant role in protecting the global environment by remediating hundreds of contaminated sites (Schwab et al., 1993). Still, this technology faces many practical problems. Even if successful in cleaning up contaminated soil, many environmental questions remain, such as biocycling of metals in the ecosystem through plant uptake and the disposal of plant tissues containing heavy metals. Toward this end, phytoremediation of metal-contaminated soils and sludge may provide both partial decontamination and site restoration for the polluted sites in situations where rapid and costly treatment is not appropriate. This technology is still in its infancy. Most of the predictions for its use are based on the results from small-scale projects. Certainly, further investigations and more field data are needed to evaluate generalized use, health effects, and economic feasibility Uorgensen, 1993; Baker et al., 1991; and Brown et al., 1994).

ACKNOWLEDGMENTS We are grateful to J. Choate and C. House for technical assistance and to Dr. J. Keeley for helpful discussions. H

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