Phytoremediation A Potential Tool of Bioremediation

Phytoremediation has been used effectively to remediate inorganic and organic contaminants in soil and groundwater. The idea of using plants to extract metals from contaminated soil was reintroduced and developed by Utsunamyia [100] and Chaney [101. The first field trial on Zn and Cd phytoextraction was conducted by Baker et al. [102]. Phytoremediation is currently divided into the following areas: (a) Phytofiltration: the use of plants and plant root associated with microorganisms to absorb and degrade the pollutants, mainly metals and organic pollutants, from water and aqueous waste streams; (b) phytostabilization: the use of plants to reduce the bioavailability of pollutants in the environment; (c) phytoextraction: the use of pollutant-accumulating plants to remove metals or organics from soil by concentrating them in the harvestable parts; (d) phytovolatilization: the use of plants to volatilize pollutants; and the use of plants to remove pollutants from air. The different types of phytoremediation is summarized in Table 3.

Table 3 Different mechanisms of phytoremediation [1]

Process

Mechanisms

Contaminants

Phytofiltration

Rhizosphere accumulation

Organics, inorganic

Phytostabilization

Complexation

Inrganic

Phytoextraction

Hyperaccumulation

Inorganic

Phytovolatilization

Volatilization

Organics, inorganic

Table 4 summarizes the cost of different remediation technologies. Among the listed remediation technologies, phytoextraction is one of the lowest cost techniques for contaminated soil remediation. There is a need to develop suitable cost-effective biological soil remediation techniques to remove contaminants without affecting soil fertility.

Before application of any kind of phytoremediation strategy in a contaminated site, ecological evaluation has to be analyzed. Specially, scientist should consider the local ecological relationship of the studied plant/s.

Table 4. Cost of different remediation technology [103]

Process

Cost (US$/ton)

Other factors

Vitrification

75-425

Long-term monitoring

Land Filling

100-500

Transport/excavation/ monitoring

Chemical treatment

100-500

Recycling of contaminant

Electrokinetics

20-200

Monitoring

Phytoextraction

5-40

Phyomass disposal

5.1. Phytofiltration

Phytofiltration is the use of plant roots (rhizofiltration) or seedlings (blastofiltration) to absorb or adsorb pollutants, mainly metals, from water and aqueous waste streams [104]. Mechanisms involved in the biosorption process include chemisorption, complexation, surface and pore adsorption-complexation, ion exchange, microprecipitation, hydroxide condensation onto the biosurface, and surface adsorption [105].

Plant roots or seedlings grown in aerated water mixed with polluted effluents absorb, precipitate and concentrate toxic metals from it [106, 107]. Rhizofiltration uses terrestrial plants instead of aquatic plants because the former feature much larger fibrous root systems covered with root hairs with extremely large surface areas.

Table 5. Summary of recent works on Phytofiltration

Plant Species

Metals

Treatments

References

B. juncea,

Cu, Cd,

Roots of hydroponically grown

[108]

H. annuus

Cr, Ni, Pb, and Zn

terrestrial plants used to remove toxic elements from aqueous solutions

Sunflower plants

U

Rhizofiltration of U in water by roots of sunflower plants

[117]

Water Hyacinth

As, Cd Cr, Cu, Ni, and Se

The abilities of water hyacinth to take up and translocate six trace elements - As, Cd, Cr, Cu, Ni, and Se were studied under controlled conditions

[110]

Duckweed

Hg

Effects of pH, copper and humic acid

[113]

Duckweed (Lemna

Fe and Cu

Solutions enriched with 10, 2 0,

[120]

minor L.) and water

4 0, and 8 0 ppm of these 2 metal

velvet (Azolla

ions, renewed every 2 days over a

pinnata)

14-day test period.

Fern (Pteris Sp.)

As

Solar-powered hydroponic Technique, small scale clean-up.

[107, 118]

Eichhornia crassipes, Azolla

As ,

Hydroponic technique, industrial

[121]

filiculoides, Lemna minor,

Metals

wastewater and aqueous solution

Lemna gibba, Ceratophyllum

demersum, Nymphaea

spontanea, Nymphaea alba L., V. Spiralis, Nelumbo nucifera,

Myriophyllum spicatum,

Potamogeton lucens, Salvinia

herzogoi, Myriophyllum brasillensis, Cabomba sp.,

Myriophylhum aquaticum,

Ludwigina palustris and

Mentha aquatic, Scapania undulata and floating

macrophytes Pistia stratiotes

E. splendensand E. argyi

Cu

Hydroponic technique, contaminated water

[122]

Limnocharis flava (L.) .

Cd

Cd Low level Cd contaminated water

[123]

The process involves raising plants hydroponically and transplanting them into metal-polluted waters where plants absorb and concentrate the metals in their roots and shoots [4, 108-110]. Plant roots can solubilize heavy metals by acidifying their soil environment with protons extruded from the roots. A lower pH releases soil bounded metal ions into the soil solution. Solubilized metal ions may enter the root either via the extracellular or intracellular pathways. As they become saturated with the metal contaminants, roots or whole plants are harvested for disposal [109, 110].

Dushenkov et al. [108] elucidated that the translocation of metals to shoots would decrease the efficiency of rhizofiltration by increasing the amount of contaminated plant residue required for disposal. However, Zhu et al. [110] suggest that the efficiency of the process can be increased by using plants with a heightened ability to absorb and translocate metals. Many aquatic plants have the ability to remove heavy metals from water, including water hyacinth (Eichhornia crassipes, [110, 111], pennywort (Hydrocotyle umbellata L., [112], and duckweed (Lemna minor L., [113]. However, these plants have limited potential for rhizofiltration because they are not efficient in removing metals as a result of their small, slow growing roots [108]. The high water content of aquatic plants complicates their drying, composting, or incineration. P. australis has high capability to remove metals from wetlands [114-116]. Sunflower (Helianthus annus L.) and Indian mustard (Brassica juncea Czern.) are the most potential terrestrial plants for removing metals from water solution. The roots of Indian mustard are effective in capturing Cd, Cr, Cu, Ni, Pb, and Zn [108], whereas sunflower removes Pb, U [117] and 137Cs from hydroponic solutions A novel phytofiltration technology has been proposed by Sekhar et al. [119] for removal and recovery of lead (Pb) from wastewaters. Plants used for phytofiltration should be able to accumulate and tolerate significant amounts of the target metals, in conjunction with easy handling, low maintenance costs, and a minimum of secondary waste requiring disposal. It is also desirable for plants to produce significant amounts of root biomass or root surface area. Table 5 is showing the examples of phytofiltration.

5.2. Phytostabilization

Use of certain plant species to immobilize contaminants in soil, through absorption and accumulation by roots, adsorption onto roots or precipitation within the root zone and physical stabilization of soils is called phytostabilization. Phytostabilization reduces the mobility of contaminants and prevents migration to groundwater or atmosphere. The process can re-establish a vegetative cover at sites where natural vegetation is lacking due to high metal concentrations [124-125].

The main approaches to re-vegetation are : a. Metal-tolerant species may be used to restore vegetation to such sites, thereby decreasing the potential migration of contaminants through wind, transport of exposed surface soils, leaching of soil and contamination of groundwater [126] b. Soil amendments can reduce the metal contaminaton to groundwater [127] which optimize the Soil agronomic condition. The soil may require lime addition, fertilization (nitrogen, phosphorous, potassium, and other mineral nutrients), carbon addition, and soil conditioners, such as aged manure, sewage sludge, compost, straw or mulch [128129]. Unlike other phytoremediative techniques, phytostabilization is not anticipated to remove metal contaminants from a site, but somewhat to stabilize them by accumulation in roots or precipitation within root zones, reducing the risk to human health and the environment The interference to site activities may be less than with more intrusive soil remediation technologies. The heavy metal uptake potential largely varies with plant species, metal availability in the system and other environmental conditions.

The metal accumulation by various wetland plants were studied in many parts of the world, such as, Salvania natanas [130], Lemna polyrrhiza [131, Ceratophylum demersum L., Spirodela polyrrhiza (L.) Schleid, Bacopa monnieri, Hygrorrhiza aristata [132], 1995),

Eichornia crassipes [133-135], Typha latifolia and Phragmites australis [136-139]. Phytostabilization is most effective for treating a wide range of sites where large areas are subject to surface contamination [140-141].

Table 6. Summery of the recent papers of phytostabilization

Plant Species

Metals

Treatments

References

B. juncea

Cd

Soil amendments -liming materials, phosphate compounds and biosolids

146

B. juncea

Zn, Cu, Mn, Fe, Pb and Cd

organic amendments (cow manure and compost) and lime

[149, 150]

Lolium italicum and Festuca arundinaceae

Pb and Zn

Compost at two rates (10%, and 30% v/v)

[151]

H. hirta and Z. fabago

Pb, Zn and Cu

Characterization of soil and plant samples from a mine tailing located in South-East Spain for further phytostabilisation

[152]

Atriplex lentiformis(Torr.) S. Wats

As, Cu, Mn, Pb, Zn

Greenhouse study using compost

[127]

Pistacia terebinthus Bieberstein

Cu

Field study using chicken fertilizer and 1:1 soil and mine waste

(Pinus taeda) and Virginia (Pinus virginiana) pine trees

Fe, Mn, Al

A plot design consisting of three subsurface treatments (1) ripping and compost amended, (2) ripping only, and (3) control

[154]

Lolium perenne L

Zn, Pb, and Cd

Mine waste combined with synthetic (Calcinit + urea + PK14% + calcium carbonate) or organic (cow slurry) amendment,

[12S]

Wheat plant Pea Plants

Zn, Cu, Mn, Ni, Cd, Cr, Pb

A greenhouse experiment: Soil amended with industrial sludge (10 %, 20%, 30%), as well as lime treatments (0.5% and 1%)

[9S-99]

Canna indica L.

Cr, Fe, Cd, Ni,Cu, Zn, Mn, Pb

A pot experiment: Soil amended with industrial sludge (10 %, 20%, 30%)

[129]

However, some highly contaminated sites are not suitable for phytostabilization, because plant growth and survival is impossible. Phytostabilization has advantages over other soil-remediation practices as it is less expensive, easier to implement, and preferable aesthetically [141-142]. Yoon et al. [143] evaluated the potential of 36 plants (17 species) growing on a contaminated site and found that plants with a high bio-concentration factor (BCF, metal concentration ratio of plant roots to soil) and low translocation factor (TF, metal concentration ratio of plant shoots to roots) have the potential for phytostabilization. The lack of appreciable metals in shoot tissue also eliminates the necessity to treat harvested shoot residue as a hazardous waste [109]. Smith and Bradshaw [144] led to the development of two cultivars of Agrostis tenuis Sibth and one of Festuca rubra L which are now commercially available for phytostabilizing Pb-, Zn-, and Cu-contaminated soils. Stabilization also involves soil amendments to promote the formation of insoluble metal complexes that reduce biological availability and plant uptake, thus preventing metals from entering the food chain [145-146, 141]. One way to facilitate such immobilisation is by altering the physicochemical properties of the metal-soil complex by introducing a multipurpose anion, such as phosphate, that enhances metal adsorption via. anion-induced negative charge and metal precipitation [146]. Addition of sludge, or compost together with lime to raise soil pH [99,129], is a common practice for immobilizing heavy metals and improving soil conditions, to facilitate re-vegetation of contaminated soils [147].

Soil acidification, due to the oxidation of metallic sulphides in the soil, increases heavy metal bioavailability; but liming can control soil acidification; also, organic materials generally promoted fixation of heavy metals in non-available soil fractions. Revegetation of mine tailings usually requires amendments of phosphorus, even though phosphate addition can mobilize arsenic (As) from the tailings. Leachates and uptakes of As were found to be higher with an organic fertilizer amendment than superphosphate, particularly in combination with barley [148].

Active phytoremediation followed by natural attenuation, was effective for remediation of the pyrite-polluted soil [149, 150]. Root-to-shoot translocation factors were smaller in amended versus control plants, indicating a reduction in the risk of metals entering the food chain through phytostabilization [98-99, 128-129]. Recent research results on phytostabilization are summarized in Table 6.

5.3. Phytoextraction

Phytoextraction or phytoaccumulation, involves the uptake and translocation of heavy metal or inorganics from the soil by plant roots into easily harvestable shoot which must be disposed of properly after they are harvested. The term phytoremediation is a concept, whereas phytoextraction is a specific clean-up technology. Plant-based environmental remediation technology, or phytoremediation, has been widely pursued in recent years as an in situ, cost-effective potential strategy for the cleanup of trace-metals from contaminated sites [4]. The development of a commercially feasible technology (phytoextraction) depends on several factors including: identifying or creating an ideal phytoextraction plant, optimizing soil and crop management practices, and developing methods for biomass processing and metal extraction [155]. There are three sets of ratios of concentrations, which should ideally be considered in plant uptake studies. (a) Root/soil: this ratio gives information concerning the uptake of an element by the root from the soil. It suggests the bioavailability of that element and its uptake by the root from the soil gives some information as to whether it is accumulated or excluded by the root system. (b) Leaf/root: this ratio shows if there is free movement between root and aerial parts, or if the element is accumulated in either roots or leaves. (c) Leaf/soil: this ratio depends on ratios (1) and (2). In plants without leaves the root/shoot concentration ratio is measured [156]. Depending on the following three arbitrary ratios of elemental concentrations in plants and soils, plants can be categorized as: (a) Accumulator: plant/soil ratio>1.5 (b) Concentration indicator: plant/soil ratio between 1.5 and 0.5 (c) Excluder- concentration indicator: plant/soil ratio between 0.5 and 0.1. [157-158]. The three categories of plants in response to metal accumulations in their body has shown in figure I [159].

Metal Coiic. in soil i

Figure 1. Conceptual response strategies of metal concentrations in plant tops in relation to increasing total metal concentrations in the soil[159].

Some plants which grow on metalliferous soils have developed the ability to accumulate massive amounts of indigenous metals in their tissues without symptoms of toxicity are calles hyperaccumulator [160-162]. Examples of commonly reported hyperaccumulators are given in Tables 7. Plants that grow on soils with metal concentrations that are normally toxic are "metal tolerant", or "metallophytes" Some of these plants exclude the toxic metals from their tissue, other assimilate the metals present to such a degree that they are termed "accumulator". Accumulators are defined as plants with metal concentration in their tissues greater than that of soil [163]. Some plants have a natural ability to absorb and accumulate trace elements in their tissues. They have adaptations that enable them to survive and to reproduce in soils heavily contaminated with Zn, Cu, Pb, Cd, Ni, and As. Such plant species are divided into two main groups: the so-called pseudometallophytes that grow on both contaminated and non-contaminated soils, and absolute metallophytes that grow only on metal contaminated and naturally metal rich soils [164]. Over 400 hyperaccumulator plants have been reported, including members of the Asteraceae, Brassicaceae, Caryophyllaceae, Cyperaceae, Cunouniaceae, Fabaceae, Flacourtiaceae, Lamiaceae, Poaceae, Violaceae, and Euphobiaceae [165] Table 7 is showing the accumulation potentials of heavy metals in few important plants. These plants are selected and planted at a site based on the metals present and site conditions. After they have grown for several weeks or months, the plants are harvested. Planting and harvesting may be repeated to reduce contaminant levels to permissible limits. The time required for remediation depends on the type and extent of metal contamination, the duration of the growing season, and the efficiency of metal removal by plants, but it normally ranges from 1 to 20 years. This technique is suitable for remediating large areas of land contaminated at shallow depths with low to moderate levels of metal-contaminants [166, 167].

Sebertia Acuminate
Table 7. Examples of Phytoaccumulators and their accumulation potential [173]

Plants

Metal

Content (mg kg-1)

Reference

T. caerulescens

Zn

39,600 (shoots)

[174]

T. caerulescens

Cd

1,800

[175]

Ipomea alpine

Cu

12,300

[175]

Sebertia acuminate

Ni

25% by wt. dried sap

[176]

Haumaniastrum robertii

Co

10,200

[177]

A. racemosus

Se

14,900

[178]

P. vittata

As

27,000

[179]

Berkheya coddii

Ni

5,500

[180]

Iberis intermedia

Ti

3,070

[181]

Table 8. Sumary of recent work on Phytoextraction

Plant species

Metal

Results

Reference

Pistia stratiotes

Ag, Cd, Cr, Cu, Hg, Ni, Pb and Zn

All elements accumulated mainly in the root system.

[196]

Spartina plants

Hg

Organic Hg was absorbed and transformed into an inorganic form (Hg+, Hg2+) and accumulated in roots

[197]

H. annuus

Pb

Pb concentrated in the leaf and stem indicating the prerequisites of a hyperaccumulator plant

[198]

H. indicus

Pb

Heavy metal mainly accumulated in roots and shoots

[199]

Sesbania drummondii

Pb

Pb accumulated as lead acetate in roots and leaves, although lead sulfate and sulfide were also detected in leaves, whereas lead sulfide was detected in root samples.

Lead nitrate in the nutrient solution biotransformed to lead acetate and sulfate in its tissues.

Complexation with acetate and sulfate may be a lead detoxification strategy in this plant species

[200]

Lemna gibba

As

A preliminary bioindicator for As transfer from substrate to plants. Used for As phytoremediation of mine tailing waters because of its high accumulation capacity

[201]

P. longifolia and P. umbrosa

As

Suitable for phytoremediation in the moderately contaminated soils

[202]

Plant species

Metal

Results

Reference

Alyssum

Ni

Majority of Ni is stored either in the leaf epidermal cell vacuoles, or in the basal portions of the numerous stellate trichomes. The metal concentration in the trichome basal compartment was the highest ever reported for healthy vascular plant tissue, approximately 15-20% dry weight

[203]

Solanum nigrum and C. Canadensis

Cd

High concentration of Cd accumulated. Tolerant to combined action of Cd, Pb, Cu and Zn

[204]

T. caerulescens

Cd

High Cd-accumulating capability, acquiring Cd from the same soil pools as non-accumulating species.

[205]

Arabis gemmifera

Cd and Zn

Hyperaccumulator of Cd and Zn, with phytoextraction capacities almost equal to T. caerulescens

[206]

Sedum alfredii

Cd

Mined ecotype had a greater ability to tolerate, transport, and accumulate Cd, compared to non-mined ecotype

[207]

Stanleya pinnata

Se

Adapted to semi-arid western U. S. soils and environments.

Uptake, metabolism and volatilization of Se

[208]

Austromyrtus bidwilli.

P. acinosa Roxb

Mn

Australian native hyperaccumulator of Mn, grows rapidly, has substantial biomass, wide distribution and a broad ecological amplitude

[209-210]

Brassica juncea

Pb, Cd, Cu, Ni, Zn

simultaneous accumulation of several metals after applying metal chelates

(EDTA, EGTA etc.)

[155]

Thlaspi caerulescens Silene vulgaris, respectively

Cd Zn

Continous phytoextraction

[182]

V. baoshanensis S. alfredii, R. crispus

V. baoshanensis, S. alfredii, Rumex K-1, and R. crispus

Zn, Cd Pb

Higher bioaccumulation factors were found for Cd in V. baoshanensis and for Zn in S. alfredii and these resulted in greater extractions of Cd and Zn, respectively. The extraction ability of R. crispus to remove Cd and Zn was considerable, due to its higher biomass. Addition of EDTA enhanced the accumulation of Pb in shoots of V. baoshanensis, S. alfredii, Rumex K-1, and R. Crispus

[211]

The basic strategies of phytoextraction are (a) development of chelate-assisted phytoextraction, which can be called induced phytoextraction; and (b) long-term continuous phytoextraction. If metal availability is not adequate for sufficient plant uptake, chelates or acidifying agents may be added to the soil to release them [65, 168-169]. Several chelating agents, such as EDTA (ethylene diamine tetra acetic acid), EGTA (ethylene glycol-O,O'-bis-[2-amino-ethyl]-N,N, N',N',-tetra acetic acid), EDDHA (ethylenediamine di o-hyroxyphenylacetic acid), EDDS (ethylene diamine disuccinate) and citric acid, have been used to enhance phytoextraction by mobilizing metals and increasing metal accumulation in different studies [170-171]. However, there is a potential risk of leaching of metals to groundwater, and a lack of reported detailed studies regarding the persistence of metal-chelating agent complexes in contaminated soils [172]. Some of the recent reports on phytoextraction are summarized in Table 8.

The best known metal hyperaccumulator may be Thlaspi caerulescens (alpine pennycress). While most plants show toxicity symptoms at Zn accumulation of about 100 ppm, T. caerulescens was shown to accumulate up to 26,000 ppm without showing any damage [182]. Many hyperaccumulators, including T. caerulescens, have been shown to colonize metal-rich soils such as calamine soil (soil enriched in Pb, Zn, and Cd). Ebbs et al. [183] reported that B. juncea, while having one-third the concentration of Zn in its tissue, is more effective at removing Zn from soil than Thlaspi caerulescens, a known hyperaccumulator of Zn. The advantage is due primarily to the fact that B. juncea produces ten-times more biomass than T. caerulescens.

Metal hyperaccumulator species, capable of taking up metals in the thousands of ppm, possess additional detoxification mechanisms. Understanding the mechanisms of rhizosphere interaction, uptake, transport and sequestration of metals in hyperaccumulator plants will lead to designing novel transgenic plants with improved remediation traits ([184]. For example, research has shown that in T. goesingense, a Ni hyperacccumulator, high tolerance was due to Ni complexation by histidine, which rendered the metal inactive [185-186]. Sequestration in the vacuole has been suggested to be responsible for Zn tolerance in the shoots of the Zn-hyperaccumulator T. caerulescens [169, 187]. Cadmium, a potentially toxic metal, has been shown to accumulate in plants where it is detoxified by binding to phytochelatins [188-190].

Liu et al. [191] performed a survey of Mn mine tailing soils with high concentrations of Mn, Pb, and Cd and eight plants growing on mine. It was found that Poa pratensis, Gnaphalium affine, Pteris vittata, Conyza Canadensis and Phytolacca acinosa possessed specially good metal-enrichment and metal-tolerant traits.

The effectiveness of phytoextraction of heavy metals in soils also depends on the availability of metals for plant uptake [192]. The rates of redistribution of metals and their binding intensity are affected by the metal species, loading levels, aging and soil properties [193].

Generally, the solubility of metal fractions is in the order: exchangeable > carbonate specifically adsorbed > Fe-Mn oxide > organic sulfide > residual.

Kufka and Kuras [194] reported that the process of metal uptake and accumulation by different plants depend on the concentration of available metals in soils, solubility sequences, the plant species growing on these soils and soil pH, EC, CEC, OC, etc.

The use of hyperaccumulators in phytoextraction is limited by some factors (a) Hyperaccumulators often take up a specific metal. (b) Most hyperaccumulators grow slowly and have small biomass. (c) The plants often grow in remote areas and are rare; in certain cases, their habitat is threatened by mining and other development activities. (d) Little is known about their breeding potential, pest management, agronomic characteristics and physiology (Cunningham, 1995). Therefore, using wild plants as a seed source is also unreliable. Moreover, the selection and testing of multiple hyperaccumulator plants could enhance the rate of phytoremediation, giving this process a promise one for bioremediation of environmental contamination [195].

Table 9. Major factors limiting the success and applicability of phytoextraction [214]

Plant-based biological imitation

Regulatory limitations

Other limitations

1) Low plant tolerance

1) Lack of cost and performance data

1) Contaminant beneath root zone

2) Lack of contaminant translocation from root shoot

2) Regulators unfamiliarity with the technology

2) Lengthy process

3) Small size of remediating plants

3) Disposal of contaminated plant waste

3) Contaminant in biologically unavailable form

4) Risk of food chain contamination

4) Lack of remediating plant Species

The limitations of phytoextraction have shown in table 9. Phytoextraction involves repeated cropping of plants in contaminated soil until the metal concentration drops to an acceptable level. The waste volume can be reduced by thermal, microbial, physical or chemical means. If phytoextraction could be combined with biomass generation and its commercial utilization as an energy source, then it could be turned into a profitable operation, with the residual ash available to be used as an ore [65, 212-213]. Phyto-mining includes the generation of revenue by extracting soluble metals produced by the plant biomass ash, also known as bio-ore.

5.4. Phytovolatilizaton

Phytovolatilization involves the use of plants to take up contaminants from the soil, transforming them into volatile form and transpiring them into the atmosphere. Phytovolatilization occurs as growing trees and other plants take up water and the organic and inorganic contaminants. Some of these contaminants can pass through the plants to the leaves and volatilise into the atmosphere at comparatively low concentrations [215]. In recent researches have focused on naturally-occurring or genetically-modified plants capable of absorbing elemental forms of these metals from the soil, biologically converting them to gaseous species within the plant, and releasing them into the atmosphere. Selenium phytovolatilization has received the most attention in these days [215-218] because this element is a serious problem in many parts of the world where there are Se-rich soil [177]. The release of volatile Se compounds from higher plants was first reported by Lewis et al. [219]. Terry et al. [216] report that members of the Brassicaceae are capable of releasing up to 40 g Se ha-1 day -1 as various gaseous compounds. Some aquatic plants, such as cattail

(Typha latifolia L.), have potential for Se phytoremediation [220]. The volatilization of Se and Hg is also a enduring site solution, because the inorganic forms of these elements are removed, and gaseous species are not likely to redeposit at or near the site [221-222].In addition, sites that utilize this technique may not need much management after the original planting. This remediation method has the additional benefits of negligible site disturbance, less erosion, and no requirement for dispose of contaminated plant matter [222]. The transfer of Hg (O) to the atmosphere would not contribute significantly to the atmospheric pool. This technique appears to be a promising tool for remediating Se- and Hg- contaminated soils. Volatilization of arsenic as dimethylarsenite has also been hypothesized as a resistance mechanism in marine algae. Ma et al. [223] recently discovered the first known and extremely efficient arsenic hyperaccumulating plant, Pteris vittata which is also effective at volatilizing As; it removed about 90% of the total uptake of As from As-contaminated soils in the greenhouse, where the environment was similar to the subtropics [224]. Studies on arsenic uptake and distribution in higher plants indicate that arsenic predominantly accumulates in roots and that only small quantities are transported to shoots. However, plants may enhance the biotransformation of arsenic by rhizospheric bacteria, thus increasing the rates of volatilization [4].

However, phytovolatilization should be avoided for sites near population centres and at places with unique meteorological conditions that promote the rapid deposition of volatile compounds. Hence the consequences of releasing the metals to the atmosphere need to be considered carefully before adopting this method as a remediation tool.

Continue reading here: Conclusion

Was this article helpful?

0 0