Phytoremediation of MetalContaminated Soils: Stabilization, Extraction, Enhancement

1
Phytoremediation of Metal-Contaminated
SoilsStabilization, Extraction, Enhancement

• Paul Schwab, Soil Physical Chemist
• Purdue University

2
Overview

• Definitions of terms and processes
• In-depth discussion of metal-phytoremediation
  topics
• Plant-based stabilization
• Extraction of metals from soils using plants
  (includes volatilization)
• Enhancement of uptake through soil amendments
• Examples of application of phytoremediation in
  the field
• Limitations and challenges to be met

3
Terms and Definitions

• Phytorestoration the use of soil amendments and
  plants to alter the chemical and physical form of
  heavy metals in the environment, thereby
  decreasing the chemical and biological
  availability to cause harm.
• Phytoextraction the removal of metal
  contaminants from the soil through planting and
  harvesting plants that accumulate metals.
• Phytovolatilization using plants to remove
  metals from soil by converting them to volatile
  forms.
• Phytostabilization plant-mediated conversion of
  soil-borne metals to unavailable forms.

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Terms and Definitions (contd)

• Soil amendment any inorganic or organic material
  added to the soil to enhance its properties.
• Chelate a soluble compound, usually organic,
  that has the capacity to complex a central metal
  species by attachment with several components.

Graphic courtesy of P. Barak, Soils Dept., Univ.
Wisc.
5
Phytostabilization

• High-cost alternatives make stabilization
  techniques attractive.
• Stabilization reduces risk to targeted biological
  organisms
• Risk is related to total soil concentrations
• However, biological exposure and pathways are
  critical as well
• Engineering alternatives include
• Capping
• Immobilization
• Vitrification

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Objectives of Phytostabilization

• Alter trace metal speciation, reducing soluble
  and exchangeable phases.
• Establish stable vegetation and minimize metal
  uptake.
• Ultimately reduce exposure of biological
  organisms to available metals.
• Enhance the biodiversity of the site (ecological
  restoration).

7
Metal cations in solution and on the soil
exchange sites comprise a large fraction of total
available metal.
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Site Stabilization Biological Approaches

• Plants perform many important functions in
  phytorestoration
• Protecting contaminated soil from wind and water
  erosion
• Reducing water movement through the soil.
• Accumulating metals in the roots.
• Precipitating metals in the roots or vicinity.
• Adsorption on the root surfaces.
• Altering the environment around the root (pH,
  redox)

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Site Stabilization Biological Approaches (contd)

• Rhizosphere microorganisms also play a role in
  phytostabilization (discussed later)
• Chosen plant species must be metal tolerant and
  adapted to site conditions
• For stabilization purposes, uptake into the
  tissue is not desirable
• Potential food chain issues
• Dispersal of senesced leaves or seeds.

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Phytostabilization Project, Bunker Hill, Idaho.
(photos courtesy of Dr. Sally Brown, University
of Washington)
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The Problem

• Located in the Coeur d'Alene River Basin, it is
  the second largest Superfund site in the nation.
• Mining and smelting of Zn, Pb, Cd, and As rich
  ores from 1916 into the 1980s resulted in heavy
  metal contamination of mountain soils.
• Phytostabilization of these soils along and metal
  contaminated mine tailings will limit the spread
  of contaminants via the Coeur d'Alene River
  system.

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Properties of Soils and Amendments(courtesy of
Dr. Sally Brown, University of Washington)
13
Aerial view of the Bunker Hill, Idaho,
phytostabilization site. (courtesy of Dr. Sally
Brown, University of Washington)
14
Erosional patterns on the hillslopes of the
Bunker Hill site. (courtesy of Dr. Sally Brown,
University of Washington)
15
Hillslope in Bunker Hill after contouring. (courte
sy of Dr. Sally Brown, University of Washington)
16
Researchers on the project Rufus Chaney and
Sally Brown (courtesy of Dr. Sally Brown,
University of Washington)
17
Chuck Henry, University of Washington (courtesy
of Dr. Sally Brown, University of Washington)
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Bunker Hill Remediation Approach

• High and low nitrogen biosolids applied at 55 and
  110 dry Mg/ha.
• Wood ash from power generation applied at 220 wet
  Mg/ha (50 Mg/ha calcium carbonate equiv.)
• Logyard waste added at a volume ratio of 15.
• Materials mixed with a front end loader.
• Applied using an Aerospread on a Rottne chassis.

19
Research plots testing the impact of various
amendments including biosolids, Fe oxides, and
limestone. (courtesy of Dr. Sally Brown,
University of Washington)
20
Plant growth in the final field of the Bunker
Hill project. Treatment on the right has low-N
biosolids. (courtesy of Dr. Sally Brown,
University of Washington)
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Bunker Hill Results

• Low-N biosolids mixtures provided rapid and lush
  growth of a variety of native seeded and
  volunteer plants.
• Ammonia volatilization from high-N biosolids
  caused initial toxicity to emerging seedlings
• Subsequent reseeding proved to be highly
  successful.
• Metals in grass harvested from these plots in
  1997-8 are comparable to plants grown on
  uncontaminated soils.

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Conclusions from Bunker Hill

• A mixture of biosolids and ash was successful in
  revegetation and erosion control.
• A combination of grasses and legumes on the plots
  indicate a self-sustaining vegetative cover.
• High initial ammonia can reduce initial
  germination. Delayed seeding can eliminate this
  problem.
• Supermulch" treatments can result in biomass far
  exceeding control soils and conventional
  hydroseeding techniques.

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Phytoextraction Metal Hyperaccumulation by Plants
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Phytoextraction Metal Hyperaccumulation by Plants

• Typical plants accumulate 100 ppm Zn, and 1 ppm
  Cd.
• Hyperaccumulators (e.g. Thlaspi spp. and Brassica
  spp) accumulate up to 30,000 ppm Zn and 1,500 ppm
  Cd in the shoots
• Hyperaccumulators exhibit few or no toxicity
  symptoms
• A normal plant can be poisoned with as little as
  1,000 ppm of zinc or 20 to 50 ppm of cadmium in
  its shoots

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Phytoextraction (continued)

• Zn, Ni and Cd can be removed from contaminated
  soil by harvesting the plant's shoots and
  extracting the metals from them
• In Thlaspi, several key sites for Zn transport
  are greatly stimulated.
• A transporter gene has been cloned and
  transferred.
• Zn transporter genes are normally regulated by
  the Zn levels in the plant
• In Thlaspi these genes are maximally active at
  all times, independent of plant zinc levels.

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Phytomining of Nickel

• Alpine pennycress, Thlaspi caerulescens, is a
  wild perennial herb found on Zn- and Ni-rich
  soils.
• Occurs in alpine areas of Central Europe as well
  as in Rocky Mountains of USA.
• Most varieties grow only 8 to 12 inches high and
  have small, white flowers.
• In 1998, ARS agronomist Rufus L. Chaney patented
  a method to use such plants to "phyto-mine"
  nickel, cobalt, and other metals.

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Rufus Chaney, one of the innovators in the field
of phytomining. (courtesy of U.S. Department of
Agriculture)
28
Thlaspi caerulescens (courtesy of U.S. Department
of Agriculture)
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Phytomining of Nickel

• Biomining is the use of plants to extract
  valuable heavy-metal minerals from contaminated
  or mineralized soils, as opposed to
  decontaminating soils.
• The crops are grown as hay.
• Plants are cut and baled after they'd taken in
  enough minerals
• The plant tissue is burned and the ash sold as
  ore.
• Ashes of alpine pennycress grown on a high-zinc
  soil in Pennsylvania yielded 30 to 40 percent
  zinc, which is as high as high-grade ore.

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Phytoextraction Case StudyOpen Burn/Open
Detonation Area at the Ensign-Bickford Company

• The OB/OD area at the Ensign-Bickford Company,
  located in Simsbury, Connecticut.
• Highly contaminated with lead due to past
  activities.
• A full-scale phytoremediation project by
  Edenspace Systems Corporation on 1.5 acres
• Successful results were obtained for 1997, which
  resulted in the increasing of the project to 2.35
  acres in 1998, combining both phytoextraction and
  phytostabilization.

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Open Burn/Open Detonation Area at the
Ensign-Bickford Company

• Pb concentrations in the soil ranged from 500 to
  5,000
• Silt loam soil with pH from 6.5 to 7.5.
• Soil amendments were applied to increase the
  mobility of the lead within the soil profile.
• Three crops were planted and harvested for the
  1998 growing season.
• Indian mustard (Brassica juncea)
• Sunflower (Helianthus annus)
• A mix of mustard and sunflower

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Brassica juncea (courtesy of U.S. Department of
Agriculture)
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Helianthus annus (courtesy of U.S. Department of
Agriculture)
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Open Burn/Open Detonation Area Results

• Phytoextraction resulted in a decrease in soil Pb
  concentrations from an initial average of 635
  mg/kg (April 1998) to 478 mg/kg (October 1998).
• After the 1998 growing season, no soil samples
  taken exceeded 4000 mg/kg.
• Prior to phytoremediation, 7 of the treatment
  area had soil lead concentrations in excess of
  2000 mg/kg
• After the treatment process only 2 still
  exceeded that amount.
• Lead uptake in Indian mustard ranged from 342
  mg/kg (first crop) to 3,252 mg/kg (third crop)
• Average Pb uptake was similar in both sunflower
  and Indian mustard -- 1000 mg/kg for sunflower
  and 1,091 mg/kg in Indian mustard.

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Enhanced/Induced PhytoextractionUse of
Synthetic Metal Chelating Agents
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Enhanced/Induced Phytoextraction

• Although phytoextraction holds promise, it has
  some limitations.
• Some plant species are too small to be effective
  at removing significant quantities of metal.
• Metals such as Pb are resistant to
  hyperaccumulation.
• Food chain issues elevated metal concentrations
  in plant tissues.
• Mobility and plant availability of metals can be
  enhanced by adding chelating agents to soils

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Mode of Action

• Typical chelates
• EDTA
• DTPA
• HEDTA
• Mechanism
• A strong, soluble metal/chelate complex.
• Metals can be removed from even some of the least
  labile pools in the soil.
• Chelate/metal complex can be assimilated intact
  by the plant roots.
• Translocation from root to shoot is enhanced.

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The influence of chelate concentration on the
total amount of Pb desorbed in four consecutive
extractions (courtesy of Edenspace)
39
Uptake of EDTA and Pb by B. juncea growing for 48
hours in a solution containing 0.5 mM Pb(NO3)2
and 1.0 mM EDTA. (courtesy of Edenspace)
40
Effects of adding a synthetic chelate (HEDTA) to
a Pb-contaminated soil (2,500 mg/kg total Pb) on
Pb accumulation in corn plants. (courtesy of
Edenspace)
41
Enhanced Phytoextraction Case StudyTrenton, New
Jersey

• Site acquired in the mid 1930's for manfacture of
  lead acid batteries.
• During the after years of battery production,
  local residents began to complain about an
  offensive smell emanating from the factory.
• Pb concentrations as high as 2,000 mg/kg in soil
• Indian mustard planted to remove lead.
• Community members participated in the planting.
• EDTA added to enhance Pb removal.

42
Initial surface soil lead concentrations and the
soil lead concentration after three
phytoremediation crops. (courtesy of Edenspace)
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Trenton, New Jersey Results

• Phytoremediation reduced the area of
  lead-contaminated soil. 
• At the initial sampling
• 40 of the area exceeded the regulatory limit of
  400 mg/kg.
• 7 exceeded 1000 mg/kg
• After three phytoremediation crops
• The area exceeding 400 mg/kg decreased from 40
  to 28
• None exceeded 1000 mg/kg.
• Phytoremediation reduced the surface area of lead
  contamination at other levels.

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Phytovolatilization
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Phytovolatilization

• Some contaminant metals are particularly amenable
  to volatilization Hg and Se.
• The plants accumulate the metal from the soil and
  convert it to a gaseous form.
• This approach to phytoremediation has received
  considerable attention lately.
• The genes associated with uptake and conversion
  have been isolated.
• Engineering plants has been taken forward.
• Extensive areas contaminated with Se have
  increased the interest in this application.

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Transpiration of Volatiles and water
Photosynthesis
Contaminant uptake
Phytovolatilization of soil contaminants.
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Phytovolatilization of Selenium

• Selenium is a true phytovolatilization success
  story.
• Much of the motivation for this research was the
  extensive Se contamination in and around
  Kesterton Reservoir, California.
• The mechanism of volatilization is complex.
• For some organics (such as TCE), the plant acts
  as a simple pipe from the soil solution to the
  air.
• For Se, uptake is an active process, and the
  plant must invoke a conversion to a volatile form.

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Selenate uptake across the root plasma membrane
is mediated by the high-affinity sulfate
transporter. (courtesy of U.S. Department of
Agriculture)
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Phytovolatilization of Selenium

• The pathway for the assimilation of inorganic
  forms of Se to SeCys in Se accumulators is
  believed to be the same as for nonaccumulators
• Se accumulators differ from nonaccumulators in
  that they metabolize the SeCys primarily into
  various nonprotein selenoamino acids.
• The synthesis of these nonprotein selenoamino
  acids probably occurs along pathways associated
  with S metabolism.
• Synthesis of SeMet and DMSe would appear to be
  rate limiting to Se volatilization.

50
Total amounts of Se removed from hydroponic media
per plant when Indian mustard plants were
supplied with different forms of Se at 20 M.
(courtesy of U.S. Department of Agriculture
51
Field Testing of Phytovolatilization

• Excellent research in this area has been
  conducted by Gary Banuelos and colleagues
• Multiple species tested
• Canola, Indian mustard, tall fescue, etc.
• Findings
• Indian mustard was found to be an excellent
  accumulator of Se
• Canola was also effective and volatilized high
  quantities
• Nearly 50 of the Se in soil volatilized in a
  single season.

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Conclusions

• The use of higher plants in remediating metal
  contaminated soils takes many forms.
• Stabilization is a suitable alternative when
  biological exposure is limited.
• Metal toxicities must be overcome.
• Amendments can be used that take advantage of our
  knowledge of the soil chemistry of metals.
• Extraction is useful for some metals and some
  soils.
• Nickel, cadmium, and zinc.
• More challenging for lead.

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Conclusions

• Enhanced extraction employs chelates to increase
  bioavailability of metals.
• Field tested for Pb and other metals.
• Added expense.
• A potential for leaching of mobilized metals
  exists.
• Volatilization is useful for a limited number of
  elements.
• Selenium, mercury.
• Complete removal from the system.
• The ultimate fate of the metal will be of
  interest.