Phytoremediation: Principles and Techniques
PRINCIPLE AND THEORY
What is phytoremediation? • Phytoremediation is the use of trees and plants to help clean up toxic waste sites. Or • Phytoremediation is the name given to a set of technologies that use plants to clean contaminated sites, soil or groundwater.
• Phytoremediation is not only a growing science, it's also a growth industry. • One report estimates that the phytoremediation market in the United States will expand from $16.5-$29.5 million in 1998 to $55-$103 million by 2000 and to $214-$370 million by 2005.
•The term phytoremediation (phyto = plant and remediation = correct evil) is relatively new, coined in 1991.
Diagram showing phytoremediation to clean-up groundwater.
However, the phytoremediation technique is not new. • Basic information for what is now called phytoremediation comes from a variety of research areas including constructed wetlands, oil spills, and agricultural plant accumulation of heavy metals.
Principles • This technique can be used for the remediation of organic contaminants (phytodegradation) as well as inorganic contaminants (phytoextraction).
Phytoremediation of inorganic contaminants • Because inorganic contaminants e.g. metals cannot be biodegraded. • Remediation of metal contaminated soil can then be done by phytoextraction.
• Phytoextraction is the removal of inorganic contaminants by aboveground portions of the plant. When the shoots and leaves are harvested, the inorganic contaminants are reclaimed or concentrated from the plant biomass or can be disposed.
• Perhaps not surprisingly, phytoremediation as an environmental cleanup technology was initially proposed for the remediation of metal contaminated soil.
Phytoextraction is the use of higher plants to remove metals from polluted soil by translocate metal ions to the stems and leaves.
Organic contaminants can be removed by phytovolatilization and phytodegradation.
Phytoextraction • Plants have a natural property to take up metals. Some ions such as Cu, Co, Fe, Mo, Mn, Ni, and Zn, are essential mineral nutrients. • Others, however, such as Cd and Pb, have no known physiological activity in plants.
Phytoextraction (cont.) • Suitable plant species not only tolerate the presence of heavy metals, they have a strong appetite for them, and take them up so they accumulate in the plant. • More than 400 different kinds of plants are known to accumulate toxins.
Numbers of metal hyperaccumulator plants
Metal % in dry leaf matter
Cd Co Cu Pb Mn Ni Zn
>0.01 >0.1 >0.1 >0.1 >1.0 >0.1 >1.0
No. of species No. of families
1 28 37 14 9 317 11
1 11 15 6 5 37 5
Examples of Hyperaccumulator Plants And Plants used in phytoremediation
• Vetiver grass combined with applications of zeolite is being used to remove boron, cadmium and lead from Indonesian soils contaminated by sludge from the textile industry.
In Thailand, the silverback fern Pityrogramma calomelanos is being used to remove arsenic from the soil.
Thlaspi caerulescens, one of the many species of stone cress, is a hyperaccumulator of copper, lead, cadmium and zinc.
Brassica juncea, has been tested worldwide and also in Taiwan, and has shown to be a hyperaccumulator of lead and selenium.
• Metal tolerance can prevent plants from phytotoxicity by metal exclusion and metal detoxification. • The excluders prevent metal uptake into roots avoiding translocation and accumulation in shoots. • The hyperaccumulators absorb high levels of metals in cells.
Phytotoxicity • A primarily effect of metal toxicity is the
inhibition of a number of cytoplasmic enzymes. • Therefore the homeostasis of metal ions in the cytoplasm is essential in avoiding metal toxicity. • Metal tolerance is a major characteristic of hyperaccumulator species.
• Hyperaccumulators are species capable of accumulating metals at levels 100-fold greater than those typically measured in shoot of the common nonaccumulator plants. • A hyperaccumulator will concentrate: > 10 ppm Hg > 100 ppm Cd > 1000 ppm Co, Cr, Cu, Pb > 10,000 ppm Zn, Ni
Detoxification & Sequestration
Xylem loading & translocation
Thlaspi caerulescens (Alpine penny cress) T. caerulescens was shown to accumulate up to 26,000 ppm without showing any injury. While most plants exhibit toxicity symptoms at Zn concentrations of about 100 ppm. This species extracted up to 22% of Cd contaminated site and show remarkable Cd tolerance .
Differences between hyperaccumulator and non hyperaccumulator The expression of a gene, ZNT1, encoding a Zn transporter was stimulated in T. caerulescens compared with a non accumulator relative, T. arvense. High expression of ZNT1 in T. caerulescens is related to a mechanism that allows detoxification of ionic zinc to less active compounds and stored in the leaf cells.
Zn concentration in root ก Zn ก
Zn concentration in shoot
T. caerulescens T. arvense
Detoxification and Sequestration After transport across the tonoplast, Zn is precipitated in the vacuole of leaf cells. Precipitation in the vacuole as Znphytate has been proposed to account for Zn inactivation in the vacuole.
Table 2 Tolerance, Expressed as the External EC50 for Root Growth ( the Concentration in the Test Solution which Inhibits Root Elongation by 50% ) in a 4 Day Test Species ( population ) Thalspi caerulescens ( Luxemburg) T. caerulescens( Belgium ) T. caerulescens ( Italy ) T. arvense ( The Netherlands) Silene vulgaris ( The Netherlands) S. vulgaris ( Imsbach ) S. vulgaris ( Germany )
Cu 0.5 1 1 0.5 3 170 3
Zn 160 2300 1200 20 140 1800 1800
EC50 ( uM ) Cd 8 230 60 1 20 170 110
Ni 150 600 800 40 15 50 50
Co 250 200 300 15 110 240 220
There are differences in uptake capacity due to ecotype.
Soil microorganisms can enhance phytoremediation
Soil microorganisms • Soil microorganisms can alter chemical properties of the soil with subsequent effects on the mobility of metal contaminants and the potential for root uptake. • Bacteria can enhance mobility of metal contaminants via soil acidification, or, in contrast, to decrease their solubility due to precipitation as sulfides.
Fungal symbiotic associations (mycorrhizae ) • Enhance root absorption area, and stimulate the acquisition of plant nutrients including metal ions. • Inhibition of Zn translocation from root to shoot in grass and maize. • This effect appeared to be speciesspecific (host preference is an important characteristic of the symbiotic association).
•Microbes have been shown to catalyze redox transformations leading to change in soil bioavailability and potential for phytoextraction. •Mycorrhizal fungi may play an important role in hyperaccumulation in some metals.
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Phytoremediation initiation in Thailand Kleety Mine
Potential Hyperaccumulators (cont.)
Where are we at? • Results is positive and we are still researching for the best hyperaccumulators.