IntroductionBeside thenatural activities, almost all human activities also have potentialcontribution to Arsenic contamination in the environment as side effects occurs in many parts of theworld and is a global problem. In many areas As level has crossed the safethreshold level. Large-scale groundwater pollution by geogenic arsenic (As) inWest-Bengal and Bangladesh has recently promoted this element into anenvironmental pollutant of prime concern. Epidemiological studies have documentedvarious adverse effects on the human population. Arsenic contaminated soils,sediments, and sludge are the major sources of arsenic contamination of thefood chain, surface water, groundwater, and drinking water (WT Frankenberger & Arshad, 2002). Other potentialsources of arsenic contamination are the chemicals used extensively inagriculture as pesticides, insecticides, defoliants, wood preservatives, andsoil sterilants (AZCUE & NRIAGU, 1994). Currently availabletechniques for the remediation of As contaminated soil are very expensive andtime-consuming, often hazardous to workers, and capable of producing secondarywastes (LOMBI, ZHAO, DUNHAM, & MCGRATH, 2000).
Phytoextraction, the use of green plants to clean up contaminated soil, hasattracted attention as an environmentally- friendly, low-input remediationtechnique. It uses plants that extract heavy metals from the soil andaccumulate it in the harvestable, above ground biomass.Severalcomprehensive reviews on phytoremediation and phytoextraction have beenrecently published. In the present review, phytoextraction of arsenic fromcontaminated soils by the recently discovered arsenic hyperaccumulator ferns,with emphasis on the recent studies developed in order to understand andenhance the arsenic removal process. ARSENIC In theenvironment As originates from both geochemical and anthropic sources. Concentrationof As normally varies from below 10 mg kg-1 in non-contaminated soils to ashigh as 30,000 mg kg-1 in contaminated soils (ADRIANO, 1986). Through theproduction or the use of arsenical pesticides (fungicides, herbicides, andinsecticides) human activities have caused an accumulation of arsenic in soils.Because of manufacture of As-based compounds, smelting of arsenic-containing ores,and combustion of fossil fuels, As contamination is affecting soils, water, andatmosphere and has been identified as a major toxic contaminant in manycountries (AZCUE & NRIAGU, 1994).
The form andspeciation of arsenic has great influence on bioavailability, toxicity, andchemical behavior of arsenic compounds. Normally As can exit essentially infour oxidation states: (-3), (0), (+3), and (+5). Arsenate (As(+5)) andarsenite (As(+3)) are the main forms present in soils (HARPER & HASWELL, 1988). Remediation of arsenic contaminated soilsThere are many remediationtechniques are available to address contamination problems but high capital expenditure, unsuitability for largeareas, and environmental disruption are some of the disadvantages of thosetechniques, which differ from cost intensity and timeframe. No singlesoil remediation technique is unique for all situations. Contaminated sitecharacteristics should be investigated carefully, contaminant problem,treatment options, and treatment timeframe must be considered. Some selected current remediation technologies forarsenic-contaminated soil, adapted from USEPA (2002), follow:a)Excavation – Commonly used ex-situ remediationmethod that involves the physical removal and disposal of contaminated soil indesignated landfill. Even though it produces rapid remediation results, excavationis often expensive because of the operation, transport, and special landfillrequirements (USEPA, 2002).
b)Capping – In-situ method.A hard cover is placed on the surface of the contaminated soil. Capping is alsoa rather simple method that reduces the contaminant exposure. However, it doesnot remove contaminants from the soil (USEPA, 2002).c)Solidification and stabilization – In-situ methodwhere the contaminated soil is mixed with stabilizers reducing the mobility ofarsenic in soil.
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The drawbacks to these remediation techniques are that theycan be relatively costly (USEPA, 2002).d) Vitrification – In-situ method where arsenic ischemically bonded inside a glass matrix forming silicoarsenates.e) Soil washing/Acid extraction – Ex-situ treatment based on the suspensionor dissolution of arsenic in a water-based wash solution to concentrate thecontaminant.f) Soilflushing: In-situ method that uses water, chemicals ororganics to mobilize arsenic and flush it from the soil.
g) Phytoremediation/phytoextraction: In-situ methodusing plants to up-take arsenic from soil.PHYTOREMEDIATION Phytoremediation includes any remediation process which utilizes plantsto either remove pollutants or render them harmless in soil and water systems,it can be applied for both organic and inorganic pollutants present in soil,water, and air (SALT, SMITH, & RASKIN, 1998). The term phytoremediation includes several strategies:Phytoextraction: Phytoextraction is a sub-process of phytoremediation in which plantsremove dangerous elements or compounds from soil or water, most usually heavymetals, metals that have a high density and may be toxic to organisms even atrelatively low concentrations. Phytostabilization: Phytostabilization involves the reduction of the mobility of heavy metalsin soil. Immobilization of metals can be accomplished by decreasing wind-blowndust, minimizing soil erosion, and reducing contaminant solubility orbioavailability to the food chain. Fig: heavy metals uptake byplant through phytoremediationPhytoimmoblization: Phytoimmoblization is the use of plantsto reduce the bioavailability and mobility of pollutants by altering soilfactors that lower pollutant mobility by formation of precipitates and insolublecompounds, as well as by sorption on roots.Phytovolatilization: A form of phytoremediation in which substances from thesoil are released into the air, sometimes after being broken down into volatilecomponents.Rhizofiltration:plants get rid of contaminants present in solution surrounding the root zone byadsorption or precipitation onto their roots or absorption of contaminants intotheir roots from the solution.
This technique is used to clean contaminated watersuch as groundwater or a waste stream. Hypothesis: Hyper-accumulatorsplants can take up and concentrate in excess of 0.1% of a given element intheir tissue (Brooks, 1998). In higher plantsmetal hyper-accumulation is a very rare phenomenon.
Till now, only 400 plantspecies have been identified as metal hyper-accumulators, representing <0.2%of all angiosperms (Brooks, 1998). Very recentlyhyper-accumulation of As was discovered and most of the plants are fern speciesand first of them was Pteris vittata L. (MA, et al.
, 2001). PvACR3, is a keyarsenite As(III) antiporter in the As hyperaccumulator fern Pteris vittata. If PvACR3 gene isexpressed to other plants it can create the ability to take up As and enhancethe tolerance level. Objectives: Creating hyperaccumulation plants for As take up by using PvACR3 gene.
Flowering plants aremissing PvACR3 gene. We can choose a flowering plant which is preferable forour country environment and make it transgenic plant to take up As. Tagetes erectais a very common flower in Bangladesh. PlantSelection: Tagetes erecta is a very common flower in Bangladesh. We can use this plant as ourtarget organism. Thereare several reasons to choose this plant as target plant….
.Ø Availablein Bangladesh.Ø Grows well in almost any sort of soil (Shores,ponds, springs, quiet waters in streams, ditches, wetlands, wet meadows,waterside swamps and meadows which are prone to flooding, damp hollows inbroad-leaved forests, sometimes underwater). So, by usingthis plant as transgenic plant we can remove the As contamination from bothsoil, ground water. Methodology: cDNAsynthesis of Pteris vittata :By usingfollowing primers 5?-ATGGAG AAC TCA AGC GCG GAG CGG A-3?and5?-CTA AAC AGA AGG CCCCTT CCT CTG A-3?,PvACR3Coding sequence (CDS) can be cloned from a cDNA library of the arsenic hyper-accumulatingplant fern Pteris vittata. Fig: cDNA synthesis Generationand Selection of Transgenic Tagetes: We canuse binaryvector process of Ti plasmid vector for this process.
Adapters can be added to PvACR3CDS by use of the following primers: 5?-acg ggg gac tct aga gga tcc ATG GAG AACTCA AGC GCG GAG CGG A-3?and5?-ggg aaa ttc gag ctcggt acc CTA AAC AGA AGG CCC CTT CCT CTG A-3?. CloneEZ PCR cloning kit (Genscript) isused in PCR. The PCR product needs to be cloned into the 35S promoter cassetteof pSN1301 (derived from pCAMBIA1301, CAMBIA) between BamHI and KpnIrestriction sites by recombination. In this way the constructed binary vectorwill be pSN1301-PvACR3. Agrobacterium strain C58 can be transformed with the binaryvector pSN1301-PvACR3 by electroporation.
The Agrobacterium culture can be usedto transform Tagetes erecta by Agrobacterium-mediated dip floraltransformation.
