Phytoremediation of heavy metal polluted soils and water: Progresses and perspectives

Phytoremediation of heavy metal polluted soils and water: Progresses and perspectives 1 2 first-author 1 3 corresp ‡Corresponding Author ‡Corresponding Author †E-mail:zhe@ufl.edu †E-mail:zhe@ufl.edu zhe@ufl.edu 1 3 University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, Fort Pierce, Florida 34945, USA University of Florida, Institute of Food and Agricultural Sciences, Indian River Research and Education Center, Fort Pierce, Florida 34945, USA University of Arid Agriculture, Rawalpindi, Pakistan University of Arid Agriculture, Rawalpindi, Pakistan MOE Key Laboratory of Environmental Remediation and Ecological Health, College of Natural Resources and Environmental Sciences, Zhejiang University, Hangzhou 310029, China MOE Key Laboratory of Environmental Remediation and Ecological Health, College of Natural Resources and Environmental Sciences, Zhejiang University, Hangzhou 310029, China Environmental pollution affects the quality of pedosphere, hydrosphere, atmosphere, lithosphere and biosphere. Great efforts have been made in the last two decades to reduce pollution sources and remedy the polluted soil and water resources. Phytoremediation, being more cost-effective and fewer side effects than physical and chemical approaches, has gained increasing popularity in both academic and practical circles. More than 400 plant species have been identified to have potential for soil and water remediation. Among them, Thlaspi , Brassica , Sedum alfredii H., and Arabidopsis species have been mostly studied. It is also expected that recent advances in biotechnology will play a promising role in the development of new hyperaccumulators by transferring metal hyperaccumulating genes from low biomass wild species to the higher biomass producing cultivated species in the times to come. This paper attempted to provide a brief review on recent progresses in research and practical applications of phytoremediation for soil and water resources. Environmental pollution Heavy metals Phytoremediation Soil Water INTRODUCTION 1 Land and water are precious natural resources on which rely the sustainability of agriculture and the civilization of mankind. Unfortunately, they have been subjected to maximum exploitation and severely degraded or polluted due to anthropogenic activities. The pollution includes point sources such as emission, effluents and solid discharge from industries, vehicle exhaustion and metals from smelting and mining, and nonpoint sources such as soluble salts (natural and artificial), use of insecticides/pesticides, disposal of industrial and municipal wastes in agriculture, and excessive use of fertilizers (McGrath et al., 2001 2001 ; Nriagu and Pacyna, 1988 1988 ; Schalscha and Ahumada, 1998 1998 ). Each source of contamination has its own damaging effects to plants, animals and ultimately to human health, but those that add heavy metals to soils and waters are of serious concern due to their persistence in the environment and carcinogenicity to human beings. They cannot be destroyed biologically but are only transformed from one oxidation state or organic complex to another (Garbisu and Alkorta, 2001 2001 ; Gisbert et al., 2003 2003 ). Therefore, heavy metal pollution poses a great potential threat to the environment and human health. In order to maintain good quality of soils and waters and keep them free from contamination, continuous efforts have been made to develop technologies that are easy to use, sustainable and economically feasible. Physicochemical approaches have been widely used for remedying polluted soil and water, especially at a small scale. However, they experience more difficulties for a large scale of remediation because of high costs and side effects. The use of plant species for cleaning polluted soils and waters named as phytoremediation has gained increasing attention since last decade, as an emerging cheaper technology. Many studies have been conducted in this field in the last two decades. Numerous plant species have been identified and tested for their traits in the uptake and accumulation of different heavy metals. Mechanisms of metal uptake at whole plant and cellular levels have been investigated. Progresses have been made in the mechanistic and practical application aspects of phytoremediation. They were reviewed and reported in this paper. SOURCES OF HEAVY METALS AND SOIL WATER POLLUTION 1 Land and water pollution by heavy metals is a worldwide issue. All countries have been affected, though the area and severity of pollution vary enormously. In Western Europe, 1 400 000 sites were affected by heavy metals (McGrath et al., 2001 2001 ), of which, over 300 000 were contaminated, and the estimated total number in Europe could be much larger, as pollution problems increasingly occurred in Central and Eastern European countries (Gade, 2000 2000 ). In USA, there are 600 000 brown fields which are contaminated with heavy metals and need reclamation (McKeehan, 2000 2000 ). According to government statistics, coal mine has contaminated more than 19 000 km of US streams and rivers from heavy metals, acid mine drainage and polluted sediments. More than 100 000 ha of cropland, 55 000 ha of pasture and 50 000 ha of forest have been lost (Ragnarsdottir and Hawkins, 2005 2005 ). The problem of land pollution is also a great challenge in China, where one-sixth of total arable land has been polluted by heavy metals, and more than 40% has been degraded to varying degree due to erosion and desertification (Liu, 2006 2006 ). Soil and water pollution is also severe in India, Pakistan and Bangladesh, where small industrial units are pouring their untreated effluents in the surface drains, which spread over near agricultural fields. In these countries raw sewage is often used for producing vegetables near big cities. Heavy metals that have been identified in the polluted environment include As, Cu, Cd, Pb, Cr, Ni, Hg and Zn. The sources of various heavy metals are listed in Table 1 1 . The presence of any metal may vary from site to site, depending upon the source of individual pollutant. Excessive uptake of metals by plants may produce toxicity in human nutrition, and cause acute and chronic diseases. For instance, Cd and Zn can lead to acute gastrointestinal and respiratory damages and acute heart, brain and kidney damages. High concentrations of heavy metals in soil can negatively affect crop growth, as these metals interfere with metabolic functions in plants, including physiological and biochemical processes, inhibition of photosynthesis, and respiration and degeneration of main cell organelles, even leading to death of plants (Garbisu and Alkorta, 2001 2001 ; Schmidt, 2003 2003 ; Schwartz et al., 2003 2003 ). Soil contamination with heavy metals may also cause changes in the composition of soil microbial community, adversely affecting soil characteristics (Giller et al., 1998 1998 ; Kozdrój and van Elsas, 2001 2001 ; Kurek and Bollag, 2004 2004 ). Heavy metals Sources As Semiconductors, petroleum refining, wood preservatives, animal feed additives, coal power plants, herbicides, volcanoes, mining and smelting (Nriagu, 1994 1994 ; Walsh et al., 1979 1979 ) Cu Electroplating industry, smelting and refining, mining, biosolids (Liu et al., 2005 2005 ) Cd Geogenic sources (Baize, 1997), anthropogenic activities (Nriagu and Pacyna, 1988 1988 ), metal smelting and refining, fossil fuel burning, application of phosphate fertilizers, sewage sludge (Alloway, 1995 1995 ; Kabata-Pendias, 2001 2001 ) Cr Electroplating industry, sludge, solid waste, tanneries (Knox et al., 1999 1999 ) Pb Mining and smelting of metalliferous ores, burning of leaded gasoline, municipal sewage, industrial wastes enriched in Pb, paints (Gisbert et al., 2003 2003 ; Seaward and Richardson, 1990 1990 ) Hg Volcano eruptions, forest fire, emissions from industries producing caustic soda, coal, peat and wood burning (Lindqvist, 1991 1991 ) Se Coal mining, oil refining, combustion of fossil fuels, glass manufacturing industry, chemical synthesis (e.g., varnish, pigment formulation) Ni Volcanic eruptions, land fill, forest fire, bubble bursting and gas exchange in ocean, weathering of soils and geological materials (Knox et al., 1999 1999 ) Zn Electroplating industry, smelting and refining, mining, biosolids (Liu et al., 2005 2005 ) TECHNOLOGIES FOR THE RECLAMATION OF POLLUTED SOILS 1 The cleaning of contaminated soils from heavy metals is the most difficult task, particularly on a large scale. The soil is composed of organic and inorganic solid constituents, water and mixture of different gases present in various proportions. The mineral components vary according to parent materials on which the soil had been developed under a particular set of climatic conditions. Therefore, soils vary enormously in physical, chemical and biological properties. Soil water movement is controlled by physical properties, such as soil structure and texture. The soil moisture has great bearing in controlling solute movement, salt solubility, chemical reactions and microbiological activities and ultimately the bioavailability of the metal ions. A successful phytoremediation program, therefore, must take into consideration variations in soil properties of the specific site. Different approaches have been used or developed to mitigate/reclaim the heavy metal polluted soils and waters including the landfill/damping sites. These may be broadly classified into physicochemical and biological approaches. The physicochemical approach includes excavation and burial of the soil at a hazardous waste site, fixation/inactivation (chemical processing of the soil to immobilize the metals), leaching by using acid solutions or proprietary leachants to desorb and leach the metals from soil followed by the return of clean soil residue to the site (Salt et al., 1995 1995 ), precipitation or flocculation followed by sedimentation, ion exchange, reverse osmosis and microfiltration (Raskin et al., 1996 1996 ). The physicochemical approaches are generally costly and have side effects (Raskin et al., 1997 1997 ; McGrath et al., 2001 2001 ). Biological approaches of remediation include: (1) use of microorganisms to detoxify the metals by valence transformation, extracellular chemical precipitation, or volatilization [some microorganism can enzymatically reduce a variety of metals in metabolic processes that are not related to metal assimilation] , and (2) use of special type of plants to decontaminate soil or water by inactivating metals in the rhizosphere or translocating them in the aerial parts. This approach is called phytoremediation, which is considered as a new and highly promising technology for the reclamation of polluted sites and cheaper than physicochemical approaches (Garbisu and Alkorta, 2001 2001 ; McGrath et al., 2001 2001 ; Raskin et al., 1997 1997 ). Phytoremediation, also referred as botanical bioremediation (Chaney et al., 1997 1997 ), involves the use of green plants to decontaminate soils, water and air. It is an emerging technology that can be applied to both organic and inorganic pollutants present in the soil, water or air (Salt et al., 1998 1998 ). However, the ability to accumulate heavy metals varies significantly between species and among cultivars within species, as different mechanisms of ion uptake are operative in each species, based on their genetic, morphological, physiological and anatomical characteristics. There are different categories of phytoremediation, including phytoextraction, phytofiltration, phytostabilization, phytovolatization and phytodegradation, depending on the mechanisms of remediation. Phytoextraction involves the use of plants to remove contaminants from soil. The metal ion accumulated in the aerial parts that can be removed to dispose or burnt to recover metals. Phytofiltration involves the plant roots or seedling for removal of metals from aqueous wastes. In phytostabilization, the plant roots absorb the pollutants from the soil and keep them in the rhizosphere, rendering them harmless by preventing them from leaching. Phytovolatization involves the use of plants to volatilize pollutants from their foliage such as Se and Hg. Phytodegradation means the use of plants and associated microorganisms to degrade organic pollutants (Garbisu and Alkorta, 2001 2001 ). Some plants may have one function whereas others can involve two or more functions of phytoremediation. PLANT SPECIES FOR PHYTOREMEDIATION 1 To identify plant populations with the ability to accumulate heavy metals, 300 accessions of 30 plant species were tested by Ebbs et al.( 1997 1997 ) in hydroponics for 4 weeks, having moderate levels of Cd, Cu and Zn. The results indicate that many Brasssica spp. such as B . juncea L., B . juncea L. Czern, B . napus L. and B . rapa L. exhibited moderately enhanced Zn and Cd accumulation. They were also found to be most effective in removing Zn from the contaminated soils. To date, more than 400 plant species have been identified as metal hyperaccumulators, representing less than 0.2% of all angiosperms (Brooks, 1998 1998 ; Baker et al., 2000 2000 ). The plant species that have been identified for remediation of soil include either high biomass plants such as willow (Landberg and Greger, 1996 1996 ) or those that have low biomass but high hyperaccumulating characteristics such as Thlaspi and Arabidopsis species. On worldwide basis, the number of species identified to have ability to accumulate one or more metals > 1000 mg/kg dry weight is listed in Table 2 2 (Reeves, 2003 2003 ). Metals Number of species As 04 Cd 01 Co 34 Cu 34 Pb 14 Ni >320 Se 20 The hyperaccumulators that have been most extensively studied by scientific community include Thlaspi sp., Arabidopsis sp., Sedum alfredii sp. (both genera belong to the family of Brassicaceae and Alyssum). Thlaspi sp. are known to hyperaccumulate more than one metal, i.e., T . caerulescens for Cd, Ni, Pb and Zn, T . goesingense for Ni and Zn, T . ochroleucum for Ni and Zn, and T . rotundifolium for Ni, Pb and Zn (Prasad and Freitas, 2003 2003 ). Among the genus Thlaspi , the hyperaccumulator plant Thlaspi caerulescens received much attention and has been extensively studied as potential candidates for Cd and Zn contaminated soils. Robinson et al.( 1998 1998 ) found T . caerulescens as hyperaccumulator for Cd and Zn could remove as high as 60 kg Zn/ha and 8.4 kg Cd/ha. It can accumulate as high as 2600×10 −6 Zn without showing any injury (Brown et al., 1995 1995 ) and extract up to 22% of soil exchangeable Cd from the contaminated site. It also showed remarkable Cd tolerance (Sneller et al., 2000 2000 ; Escarre et al., 2000 2000 ; Lombi et al., 2000 2000 ). T . caerulescens has higher uptake of Cd due to specific rooting strategy and a high uptake rate resulting from the existence in this population of Cd-specific transport channels or carriers in the root membrane (Schwartz et al., 2003 2003 ). METAL HYPERACCUMULATION IN VARIOUS PLANT SPECIES 1 The hyperaccumulation of metals in various plant species has been extensively investigated and to date substantial progress has been made. It becomes clear that different mechanisms of metal accumulation, exclusion and compartmentation exist in various plant species. In T . caerulescens , Zn is sequestered preferentially in vacuoles of epidermal cells in a soluble form (Frey et al., 2000 2000 ). In A . halleri leaves, Zn was found to be accumulated in the mesophyll cells (Kupper et al., 2000 2000 ; Zhao et al., 2000 2000 ; Sarret et al., 2002 2002 ). Cosio et al.( 2004 2004 ) investigated the mechanisms of Zn and Cd accumulation in three different plant species through ion compartmentation by measuring the short term 109 Cd and 65 Zn uptake in mesophyll protoplast of T . caerulescens “Ganges” and A . halleri . Their study suggests the existence of regulation mechanism on the plasma membrane of leaf mesophyll protoplast. Puschenreiter et al.( 2003 2003 ) investigated chemical changes in the rhizosphere of hyperaccumulators T . goesingense and T . caerulescens and the metal excluder T . arvense with a rhizosphere bag experiment on the contaminated and non-contaminated soils. Hyperaccumulation and depletion of labile Zn in the rhizosphere were observed for T . goesingense grown on the contaminated soil. In the non-contaminated soil, Zn was accumulated but labile Zn in the rhizosphere was not changed. Nickel present in background concentration in both soils was accumulated by T . goesingenseonly when grown on non-contaminated soil. In contrast, labile Ni in the rhizosphere increased in both soils, suggesting a general tendency of Ni mobilization by T . goesingense . Uneo et al.( 2004a 2004a ) studied the interaction between Zn and Cd in T . caerulescens in solution culture and in pot soil. Results from long term (4 weeks) and short term (1 week) solution culture experiments indicate that Cd accumulation in the shoot was not affected by the supply of a 4~10-fold excess of Zn, whereas the Cd concentration of the roots decreased with increasing Zn concentrations in the solution. The results suggest that the Ganges ecotype of T . caerulescens displayed different uptake systems for Cd and Zn and that Cd competed with Zn uptake while Zn did not compete with Cd uptake. Uneo et al.( 2004b 2004b ) investigated the uptake of Cd and Zn by T . caerulescens (the Ganges ecotype) from enriched soil with different insoluble and soluble sources of Cd and Zn. The data show that there was no significant differences in the shoot Cd concentration between the treatments with soluble or insoluble Cd compounds, even though Cd concentration in the soil solution was in the order of CdSO 4 > > CdCO 3 > CdS. Thlaspi caerulescens grown on the ZnS-enriched soil accumulated up to 6 900 mg Zn/kg in the shoots, although Zn accumulation was 1.5 times higher with the addition of more soluble compounds Zn 3 (PO 4 ) 2 or ZnSO 4 . These results indicate that the Ganges ecotype of T . caerulescens is able to utilize insoluble Cd and Zn compounds in soils. Whiting et al.( 2000 2000 ) found that the plants from T . carerulescens population that accumulated Cd also showed increased root biomass and root length after allocation into Cd-enriched soil, whereas plants from the population that did not accumulate Cd showed no such increase. T . caerulescens was grown with H . vulgare and L . heterophyllum in the field to examine the effect of rhizosphere interaction on metal uptake. The data show that the Cd concentration in H . vulgare was increased by a factor of 2.4 when it was grown along the sides of T . caerulescens without a barrier. In contrast, the uptake of Zn by H . vulgare was significantly decreased, probably through metal depletion within the zone of the Zn-hyperaccumulator rhizosphere. These results suggest that T . caerulescens may alter conditions in the shared rhizospheres and thereby affect the availability of selected metals to neighboring plants (Gove et al., 2002 2002 ). On the other hand, when S . alfredii was intercropped with a grain crop, Z . mays , heavy metals (Zn and Cu) accumulated in the grains were significantly reduced, as compared to monoculture cropping, and the intercropping improved the growth of both plant species (Liu et al., 2005 2005 ). Studies on the role of rhizosphere process in metal hyperaccumulation of Ni in T . geosingense Halacsy by Wenzel et al.( 2002 2002 ) indicate that root exudates of organic ligands may contribute to Ni hyperaccumulation in T . geosingense Halacsy. This was attributed to the ligand-induced dissolution of Ni bearing minerals in the rhizosphere of T . geosingense and appeared to be less effective in the rhizosphere of excluder Silene vulgaris and Rumex acetosella growing on the same site. Sedum alfredii Hance was identified in China as hyperaccumulator for Cd and Zn and has been intensively investigated by various researchers in their studies conducted in hydroponics and/or the uncontaminated and contaminated soils (Li H. et al., 2005 2005 ; Li T.Q. et al., 2005a 2005a ; Liu et al., 2005 2005 ; Xiong et al., 2004 2004 ; Yang et al., 2004 2004 ; 2006 2006 ). The data show that the concentrations of Cd and Zn in leaves and stems increased with increasing Cd and Zn supply levels. The distributions of the metals in different plant parts decreased in the order: stem > leaf > root for Zn and leaf > stem > root for Cd. These results indicate that S . alfredii has an extraordinary ability to tolerate Cd/Zn toxicities, and to absorb and hyperaccumulate Cd and Zn under a range of Cd/Zn combining levels. The uptake and accumulation of Cd by the mined and the non-mined ecotypes of S . alfredii indicated that the plants of the mined ecotype (ME) have higher tolerance to Cd than those of the non-mined ecotypes (NME) in terms of dry matter yield (Xiong et al., 2004 2004 ). Zinc compartmentation studies involving hyperaccumulating and non-hyperaccumulating S . alfredii plants using radioactive tracer flux technique indicate that S . alfredii H. can accumulate Zn in shoots over 2% of dry weight. Leaf and stem Zn concentrations of the hyperaccumulating ecotype (HE) were 24- and 28-fold higher, respectively, than those of the non-hyperaccumulating ecotype (NHE), whereas 1.4-fold more Zn was accumulated in the roots of the NHE. Approximately 2.7-fold more Zn was stored in the root vacuoles of the NHE, and thus became unavailable for loading into the xylem and subsequent translocation to shoots. These results also indicate that the altered Zn transport across tonoplast in the root and the stimulated Zn uptake in the leaf cells are the major mechanisms involved in the strong Zn hyperaccumulation observed in S . alfredii H. (Yang et al., 2006 2006 ). The root morphology and Zn 2+ uptake kinetics of HE and NHE of S . alfredii H. were investigated using hydroponic methods and the radiotracer flux technique. The results indicate that the root length, root surface area and root volume of NHE decreased significantly with increasing Zn 2+ concentration in growth media, whereas the root growth of HE was not adversely affected, and even promoted, by 500 µmol/L Zn 2+ . The concentrations of Zn 2+ in both ecotypes of S . alfredii H. were positively correlated with root length, root surface area and root volumes, but no such correlation was found with root diameter. The uptake kinetics for 65 Zn 2+ in the roots of both ecotypes of S . alfredii were characterized by a rapid linear phase during the first 6 h and a slower linear phase during the subsequent period of investigation. The concentration-dependent uptake kinetics of the two ecotypes of S . alfredii could be characterized by the Michaelis-Menten equation, with the V max (maximum uptake speed) for 65 Zn 2+ influx being 3-fold greater in the HE than that in the NHE, indicating that enhanced absorption into the root was one of the mechanisms involved in Zn hyperaccumulation. A significantly larger V max value suggested that there was a higher density of Zn transporters per unit membrane area in HE roots (Li H. et al., 2005 2005 ). Li T.Q. et al.( 2005b 2005b ) investigated the root morphological and physiological response of the HE of S . alfridii H. from the mined area and the NHE of S . alfridii from the agricultural area to the supplied Zn and Pb in hydroponics. The results show that Zn concentrations in the leaves and the stems of HE were 34 and 41 times higher, whereas Pb concentrations were 1.9 and 2.4 times higher, respectively than those of the NHE when grown at 1224 µmol/L Zn and/or 200 µmol/L Pb. The study also shows that the tolerance and hyperaccumulation of the HE of S . alfridii H. to Zn and Pb appear to be closely related to its high adaptation of root growth, morphology and physiology to Pb and Zn toxicity. Through its root excretion of some special substances, the plant can activate Pb and Zn in the mined soil, thus increasing their mobilization and bioavailability. The Alyssum species has been extensively studied for the hyperaccumulation of Ni. Kupper et al.( 2001 2001 ) studied the Ni uptake and cellular compartmentation in three Ni hyperaccumualtors: A . bertolonii (Desv), A . lesbiacum (Candargy), and T . goesingense (Halacsy). These three species showed similar hyperaccumulation of Ni, but T . goesingense was less tolerant to Ni than the other two species. Addition of 500 mg Ni/kg to a nutrient-rich growth medium significantly increased shoot biomass of all species. X-ray microanalysis of frozen-hydrated tissues of leaves and stems of all species showed that Ni in all species was distributed preferentially in the epidermal cells, most likely in the vacuoles of the leaves and stem. Kidd and Monterroso ( 2005 2005 ) investigated the efficiency of Alyssum serpyllifolium ssp. lusitanicum (Brassicaceae) for use in phytoextraction of polymetal-contaminated soils. The plant was grown on two mine spoil soils, one contaminated with Cr (283 mg/kg) and the other moderately contaminated with Cr (263 mg/kg), Cu (264 mg/kg), Pb (1433 mg/kg) and Zn (377 mg/kg). The results suggest that A . serpyllifolium could be suitable for phytoextraction uses in polymetal-contaminated soils, provided that Cu concentrations were not phytotoxic. Among different fern species, three accessions of P . vitta , two cultivars of P . cretica , P . longifolia and P . umbrosa were grown with 0~500 mg As/kg added to the substrate. The results show that in addition to P . vitta , P . cretica , P . longifolia and P . umbrosa also hyperaccumulate As to a similar extent. This study identified three new species of As hyperaccumulators in the Pteris genus (Zhao et al., 2002 2002 ). In another study, the speciation and distribution of As of Brake fern was investigated by Zhang et al.( 2002 2002 ), which was grown for 20 weeks in As contaminated soil. The results show that As recoveries of 85% to 100% were obtained from most parts of the plant (rhizomes, fiddle heads, young fronds and old fronds), and for roots, the corresponding value was approximately 60%. The result also demonstrates the ability of Blake fern as As hyperaccumulator, which can transfer As rapidly from soil to above ground biomass with minimal As concentration in the roots. As is found to be predominantly as inorganic species. Caille et al.( 2005 2005 ) conducted a pot experiment with 0~500 mg/kg As added as arsenate and another short term (8 h) uptake experiment with 5×10 −6 arsenate under phosphorus sufficient conditions, and grew hyperaccumulator Pteris vitta and the nonhyperaccumulator Pteris tremula . The results show that in both experiments P . vitta accumulated much more As than P . tremula without any visual toxicity symptoms. PHYTOREMEDIATION OF POLLUTED WATER 1 Rhizofiltration is the removal of pollutants from the contaminated waters by accumulation into plant biomass. Several aquatic species have been identified and tested for the phytoremediation of heavy metals from the polluted water. These include sharp dock ( Polygonum amphibium L.), duck weed ( Lemna minor L.), water hyacinth ( Eichhornia crassipes ), water lettuce ( P . stratiotes ), water dropwort [ Oenathe javanica (BL) DC], calamus ( Lepironia articulate ), pennywort ( Hydrocotyle umbellate L.) (Prasad and Freitas, 2003 2003 ). The roots of Indian mustard are found to be effective in the removal of Cd, Cr, Cu, Ni, Pb and Zn, and sunflower can remove Pb, U, Cs-137 and Sr-90 from hydroponic solutions (Zaranyika and Ndapwadza, 1995 1995 ; Wang et al., 2002 2002 ; Prasad and Freitas, 2003 2003 ). The potential of duck weed was investigated by Zayed et al.( 1998 1998 ) for the removal of Cd, Cr, Cu, Ni, Pb and Se from nutrient-added solution and the results indicate that duck weed is a good accumulator for Cd, Se and Cu, a moderate accumulator for Cr, but a poor accumulator of Ni and Pb. Dos Santos and Lenzi ( 2000 2000 ) tested aquatic macrophyte ( Eiochhornia crassipes ) in the elimination of Pb from industrial effluents in a green house study and found it useful for Pb removal. Water hyacinth possesses a well-developed fibrous root system and large biomass and has been successfully used in wastewater treatment systems to improve water quality by reducing the levels of organic and inorganic nutrients. This plant can also reduce the concentrations of heavy metals in acid mine water while exhibiting few signs of toxicity. Water hyacinth accumulates trace elements such as Ag, Pb, Cd, etc. and is efficient for phytoremediation of wastewater polluted with Cd, Cr, Cu and Se (Zhu et al., 1999 1999 ). Wang et al.( 2002 2002 ) conducted a pot experiment to test five wetland plant species, i.e., sharp dock, duckweed, water hyacinth, water dropwort and calamus for their possible use in remedying the polluted waters. The results show that sharp dock was a good accumulator of N and P. Water hyacinth and duckweed strongly accumulated Cd with a concentration of 462 and 14200 mg/kg, respectively. Water dropwort achieved the highest concentration of Hg, whereas the calamus accumulated Pb (512 m/kg) substantially in its roots. Ingole and Bhole ( 2003 2003 ) conducted hydroponic studies to investigate the uptake of As, Cr, Hg, Ni, Pb and Zn by water hyacinth from the aqueous solution at the concentrations ranging from 5 to 50 mg/L, and observed that in aqueous solutions containing 5 mg/L of As, Cr and Hg, the maximum uptake was 26, 108 and 327 mg/kg dry weight of water hyacinth, respectively. Among the ferns, Pteris vitta commonly known as Brake fern has been identified as As hyperacccumulator for As contaminated soils and waters. It can accumulate up to 7500 mg As/kg on a contaminated site (Ma et al., 2001 2001 ) without showing toxicity symptoms. One fern cultivar is available commercially for As phytoremediation and has been successfully used in field trials (Salido et al., 2003 2003 ). Li H. et al.( 2005 2005 ) conducted a laboratory study in hydroponics to test different levels of Cd on the growth and Cd uptake by three hydrophytes: Gladiolous , Isoetes taiwaneneses Dwvol and Echinodorus amazonicus . The data show that the biomass of all the plants decreased with an increase in Cd concentration from 5 to 20 mg/L. However, Cd toxic effect was greater on Isoetes taiwaneneses Dwvol and Echinodorus amazonicus than that on Gladiolous . In addition, the accumulation of Cd was higher in Gladiolous than the other two plants. Zhang et al. ( 2005 2005 ) investigated the efficiency of Cu removal from the contaminated water by Elsholtzia argyi and Elsholtzi splendens in hydroponics. The results show that Elsholtzia argyi showed better Cu phytofiltration than Elsholtzi splendens , which was associated with better ability to higher Cu concentrations and translocation to shoots. ENHANCEMENT OF PHYTOREMEDIATION BY CHEMICAL AND BIOLOGICAL APPROACHES 1 In order to cope with heavy metal contaminated soils, various phytoremediation approaches (phytostabilization, phytoimmobilization and phytoextraction) can be applied. However, the choice will depend on many factors, such as plant tolerance to pollutants, soil physicochemical properties, agronomic characteristics of the plant species, climatic conditions (rainfall, temperature), and additional technologies available for the recovery of metals from the harvested plant biomass. It appears that both chemical and biological approaches are passing through their infancy and need more efforts for their effective use in the future. The solubility of heavy metals in the polluted soils can be increased by using organic and inorganic agents, thus enhancing the phytoextraction capabilities of many plant species. Ebbs et al.( 1997 1997 ) amended the contaminated soil with Grower-Power, a commercial soil amendment that improves soil structure and fertility, and the removal of Zn by plant shoots was doubled to more than 30 000 mg Zn/pot (4.5 kg). Other applied enhancement materials include ethylene diamine tetraacetic acid (EDTA), citric acid, elemental sulfur or ammonium sulfate. Increases greater than 100 folds in Pb concentration in the biomass of crops were reported when EDTA was applied to the contaminated soils (Cunningham and Berti, 2000 2000 ). Uranium, cadmium and zinc concentrations in plant biomass were increased by the application of citric acid, elemental sulfur or ammonium sulfate, respectively (Schmidt, 2003 2003 ). In addition to the chelating material, the plant roots excrete metal-mobilizing substances called phytosiderophores. Other exudates include mugenic and deoxymugeneic acids from barley and corn, and avenic acid from oats (Welch and Norvell, 1993 1993 ). Plant roots can increase metal bioavailability by exuding protons that acidify the soil and mobilize the metals. The lowering of soil pH decreases the adsorption of heavy metals and increases their concentrations in the soil solution. Soil microbes associated with plant roots are also helpful in the phytoextraction of the heavy metals in soils through the degradation of organic pollutants. These include several strains of bacillus and psedumonos, which increase the Cd accumulation in Brassica juncea seedlings (Salt et al., 1995 1995 ). Scott Angle et al.( 2003 2003 ) determined the effect of high soil moisture content on the growth and hyperaccumulation of Ni in three different species, including Alyssum murale and Berkheya coddii and Zn hyperaccumulator T . caerulescens cultivar AB300 and AB336. The results show that hyperaccumulators grew well under high soil moisture content and the biomass of all the tested species was generally greater at higher soil moistures and inhibited at lower soil moistures. These results suggest that for successful phytoremediation of metal polluted soils, a strategy should be developed to combine a rapid screening of plant species possessing the ability to accumulate heavy metals with agronomic practices that enhance shoot biomass production and/or increase metal bioavailability in the rhizosphere. CONCLUSIONS AND PERSPECTIVES 1 The contamination of heavy metals to the environment, i.e., soil, water, plant and air is of great concern due to its potential impact on human and animal health. Cheaper and effective technologies are needed to protect the precious natural resources and biological lives. Substantial efforts have been made in identifying plant species and their mechanisms of uptake and hyperaccumulation of heavy metals in the last decade. There are genetic variations among plant species and even among the cultivar of the same species. The mechanisms of metal uptake, accumulation, exclusion, translocation, osmoregulation and copartmentation vary with each plant species and determine its specific role in phytoremediation. Variations exist for hyperaccumulation of different metals among various plant species and within populations. These variations do not correlate with either the metal concentration in the soil or the degree of metal tolerance in the plant (Pollard et al., 2002 2002 ). In order to develop new crop species/plants having capabilities of metal extraction from the polluted environment, traditional breeding techniques, hybrid generation through protoplast fusions, and production of mutagens through radiation and chemicals are all in progress. With the development of biotechnology, the capabilities of hyperaccumulators may be greatly enhanced through specific metal gene identification and its transfer in certain promising species. This can play a significant role in the extraction of heavy metals from the polluted soils. The use of cleaning technologies is site-specific due to spatial and climatic variations and is not economically feasible everywhere. Therefore, cheaper technologies are being sought for practical use. Nevertheless, the recent advances in plant biotechnology have created a new hope for the development of hyperaccumulating species. However, much research work is needed in this respect such as metal uptake studies at cellular level including efflux and influx of different metal ions by different cell organelles and membranes. Rhizosphere studies under the control and field conditions are also needed to examine the antagonistic and synergistic effects of different metal ions in soil solution and the polluted waters. In depth soil microbial studies are required to identify the micro-organisms highly associated with metal solubility or precipitations. To date the available methods for the recovery of heavy metals from plant biomass of hyperaccumulators are still limited. Traditional disposal approaches such as burning and ashing are not applicable to volatile metals; therefore, investigations are needed to develop new methods for effective recovery of metals from the hyperaccumulatior plant biomass. References BJ Alloway . Heavy Metals in Soils , 1995 . : London Blackie Academic & Professional , . Heavy Metals in Soils AJM Baker , SP McGrath , RD Reeves , 等 . N Terry , G Banuelos . Metal Hyperaccumulator Plants: A Review of the Ecology and Physiology of a Biological Resource for Phytoremediation of Metal-Polluted Soils . Phytoremediation of Contaminated Soil and Water , 2000 . : Boca Raton Lewis Publishers , 85 - 108 . . Metal Hyperaccumulator Plants: A Review of the Ecology and Physiology of a Biological Resource for Phytoremediation of Metal-Polluted Soils RR Brooks . Plants that Hyperaccumulate Heavy Metals , 1998 . : Wallington CAN International , 379 . Plants that Hyperaccumulate Heavy Metals SL Brown , RL Chaney , J Scott Angle . Zinc and cadmium uptake by hyperaccumulator thlaspi-caerulescens grown in nutrient solution . Soil Sci Soc Am J , 1995 . 59 ( 1 ): 125 - 133 . . Zinc and cadmium uptake by hyperaccumulator thlaspi-caerulescens grown in nutrient solution B Caille , C Vauleon , C Leyval . Metal transfer to plants grown on a dredged sediment: Use of radioactive isotope Hg-203 and titanium . Sci Total Environ , 2005 . 341 ( 1-3 ): 227 - 239 . 15833254 DOI: 10.1016/j.scitotenv.2004.09.020 http://doi.org/10.1016/j.scitotenv.2004.09.020 . Metal transfer to plants grown on a dredged sediment: Use of radioactive isotope Hg-203 and titanium RL Chaney , M Malik , YM Li , SL Brown , EP Brewer , J Scott Angle , AJM Baker . Phytoremediation of soil metals . Curr Opin Biotechnol , 1997 . 8 ( 3 ): 279 - 284 . 9206007 DOI: 10.1016/S0958-1669(97)80004-3 http://doi.org/10.1016/S0958-1669(97)80004-3 . Phytoremediation of soil metals C Cosio , E Martinoia , C Keller . Hyperaccumulaton of cadium and zinc in Thlaspi caerulescens and Arabidopsis hallri at leaf cellular level . Plant Physiol , 2004 . 134 ( 2 ): 716 - 725 . 14730081 DOI: 10.1104/pp.103.031948 http://doi.org/10.1104/pp.103.031948 . Hyperaccumulaton of cadium and zinc in Thlaspi caerulescens and Arabidopsis hallri at leaf cellular level SC Cunningham , WR Berti . N Terry , G Banuelos . Phytoextraction and Phytostabilization: Technical, Economic, and Regulatory Considerations of the Soil-Lead Issue . Phytoremediation of Contaminated Soil and Water , 2000 . : Boca Raton, Florida, USA Lewis Publishers , 359 - 376 . . Phytoextraction and Phytostabilization: Technical, Economic, and Regulatory Considerations of the Soil-Lead Issue MC Dos Santos , E Lenzi . The use of aquatic macrophytes ( Eichhornia crassipes ) as a biological filter in the treatment of lead contaminated effluents . Environ Technol , 2000 . 21 ( 6 ): 615 - 622 . . The use of aquatic macrophytes (Eichhornia crassipes) as a biological filter in the treatment of lead contaminated effluents SD Ebbs , MM Lasat , DJ Brady , J Cornish , R Gordon , IV Kochian . Phytoextraction of cadmium and zinc from a contaminated soil . J Environ Qual , 1997 . 26 ( 5 ): 1424 - 1430 . . Phytoextraction of cadmium and zinc from a contaminated soil J Escarre , C Lefebvre , W Gruber . Zinc and cadmium hyperaccumulation by Thlaspi caerulescens from metalliferous and nonmetalliferous sites in the Mediterranean area: Implications for phytoremediation . New Phytologist , 2000 . 145 ( 3 ): 429 - 437 . DOI: 10.1046/j.1469-8137.2000.00599.x http://doi.org/10.1046/j.1469-8137.2000.00599.x . Zinc and cadmium hyperaccumulation by Thlaspi caerulescens from metalliferous and nonmetalliferous sites in the Mediterranean area: Implications for phytoremediation B Frey , C Keller , K Zierold . Distribution of Zn in functionally different leaf epidermal cells of the hyperaccumulator Thlaspi caerulescens . Plant Cell Environ , 2000 . 23 ( 7 ): 675 - 687 . DOI: 10.1046/j.1365-3040.2000.00590.x http://doi.org/10.1046/j.1365-3040.2000.00590.x . Distribution of Zn in functionally different leaf epidermal cells of the hyperaccumulator Thlaspi caerulescens LH Gade . Highly polar metal—Metal bonds in “early-late” heterodimetallic complexes . Angewandte Chemie-International Edition , 2000 . 39 ( 15 ): 2658 - 2678 . DOI: 10.1002/1521-3773(20000804)39:15<2658::AID-ANIE2658>3.0.CO;2-C http://doi.org/10.1002/1521-3773(20000804)39:15<2658::AID-ANIE2658>3.0.CO;2-C . Highly polar metal—Metal bonds in “early-late” heterodimetallic complexes C Garbisu , I Alkorta . Phytoextraction: A cost effective plant-based technology for the removal of metals from the environment . Biores Technol , 2001 . 77 ( 3 ): 229 - 236 . DOI: 10.1016/S0960-8524(00)00108-5 http://doi.org/10.1016/S0960-8524(00)00108-5 . Phytoextraction: A cost effective plant-based technology for the removal of metals from the environment KE Giller , E Witter , SP McGrath . Toxicity of heavy metals to microorganism and microbial processes in agricultural soils: A review . Soil Biol Bichem , 1998 . 30 ( 10-11 ): 1389 - 1414 . DOI: 10.1016/S0038-0717(97)00270-8 http://doi.org/10.1016/S0038-0717(97)00270-8 . Toxicity of heavy metals to microorganism and microbial processes in agricultural soils: A review C Gisbert , R Ros , A de Haro , DJ Walker , M Pilar Bernal , R Serrano , JN Avino . A plant genetically modified that accumulates Pb is especially promising for phytoremediation . Biochem Biophys Res Commun , 2003 . 303 ( 2 ): 440 - 445 . 12659836 DOI: 10.1016/S0006-291X(03)00349-8 http://doi.org/10.1016/S0006-291X(03)00349-8 . A plant genetically modified that accumulates Pb is especially promising for phytoremediation B Gove , JJ Hutchison , SD Young , J Craigen , SP McGrath . Uptake of metals by plants sharing a rhizosphere with the hyperaccumulation Thlaspi caerulescences . Int J Phytoremediation , 2002 . 4 ( 4 ): 267 - 281 . DOI: 10.1080/15226510208500087 http://doi.org/10.1080/15226510208500087 . Uptake of metals by plants sharing a rhizosphere with the hyperaccumulation Thlaspi caerulescences NW Ingole , AG Bhole . Removal of heavy metals from aqueous solution by water hyacinth ( Eichhornia crassipes ) . J Water Supply Res Technol-AQUA , 2003 . 52 ( 2 ): 119 - 128 . . Removal of heavy metals from aqueous solution by water hyacinth (Eichhornia crassipes) A Kabata-Pendias . Trace Elements in Soils and Plants , 2001 . 3rd Ed : Boca Raton, Florida CRC Press , . Trace Elements in Soils and Plants PS Kidd , C Monterroso . Metal extraction by Alyssum serpyllifolium ssp. lusitanicum on mine-spoil soils from Spain . Sci Total Environ , 2005 . 336 ( 1-3 ): 1 - 11 . 15589245 DOI: 10.1016/j.scitotenv.2004.06.003 http://doi.org/10.1016/j.scitotenv.2004.06.003 . Metal extraction by Alyssum serpyllifolium ssp. lusitanicum on mine-spoil soils from Spain AS Knox , AP Gamerdinger , DC Adriano , 等 . DC Adriano , JM Bollag , WTJr Frankenberg , 等 . Sources and Practices Contributing to Soil Contamination . Bioremediation of the Contaminated Soils , 1999 . : Madison, Wisconson, USA ASA, CSSA, SSSA , 53 - 87 . Agronomy Series No. 37 . Sources and Practices Contributing to Soil Contamination J Kozdrój , JD van Elsas . Structural diversity of microbial communities in arable soils of a heavily industrialized area determined by PCR-DGGE finger printing and FAME profiling . Appl Soil Ecol , 2001 . 17 ( 1 ): 31 - 42 . DOI: 10.1016/S0929-1393(00)00130-X http://doi.org/10.1016/S0929-1393(00)00130-X . Structural diversity of microbial communities in arable soils of a heavily industrialized area determined by PCR-DGGE finger printing and FAME profiling H Kupper , E Lombi , FJ Zhao . Cellular compartmentation of cadmium and zinc in relation to other elements in the hyperaccumulator Arabidopsis halleri . Planta , 2000 . 212 ( 1 ): 75 - 84 . 11219586 DOI: 10.1007/s004250000366 http://doi.org/10.1007/s004250000366 . Cellular compartmentation of cadmium and zinc in relation to other elements in the hyperaccumulator Arabidopsis halleri H Kupper , E Lombi , FJ Zhao , G Wieshammer , SP McGrath . Cellular compartmentation of nickel in the hyperaccumulators Alyssum lesbiacum , Alyssum bertolonii and Tulips goesingense . J Exp Bot , 2001 . 52 ( 365 ): 2291 - 2300 . 11709579 DOI: 10.1093/jexbot/52.365.2291 http://doi.org/10.1093/jexbot/52.365.2291 . Cellular compartmentation of nickel in the hyperaccumulators Alyssum lesbiacum, Alyssum bertolonii and Tulips goesingense E Kurek , JM Bollag . Microbial immobilization of cadmium released from CdO in the soil . Biogeochemistry , 2004 . 69 ( 2 ): 227 - 239 . DOI: 10.1023/B:BIOG.0000031046.40036.df http://doi.org/10.1023/B:BIOG.0000031046.40036.df . Microbial immobilization of cadmium released from CdO in the soil T Landberg , M Greger . Differences in uptake and tolerance to heavy metals in Salix from unpolluted and polluted areas . Applied Geochem , 1996 . 11 ( 1-2 ): 175 - 180 . DOI: 10.1016/0883-2927(95)00082-8 http://doi.org/10.1016/0883-2927(95)00082-8 . Differences in uptake and tolerance to heavy metals in Salix from unpolluted and polluted areas H Li , F Cheng , A Wang , 等 . Cadmium Removal from Water by Hydrophytes and Its Toxic Effects . Proc. of the International Symposium of Phytoremediation and Ecosystem Health , 2005 . Sept. 10-13, 2005 Hangzhou, China . Cadmium Removal from Water by Hydrophytes and Its Toxic Effects TQ Li , XE Yang , ZL He , JY Yang . Root morphology and Zn 2+ uptake kinetics of the Zn hyperaccumulator of Sedum alfredii Hance . J Integr Plant Biol , 2005 . 47 ( 8 ): 927 - 934 . DOI: 10.1111/j.1744-7909.2005.00117.x http://doi.org/10.1111/j.1744-7909.2005.00117.x . Root morphology and Zn2+ uptake kinetics of the Zn hyperaccumulator of Sedum alfredii Hance TQ Li , XE Yang , XF Jin , ZL He , PJ Stoffella , QH Hu . Root response and metal accumulation in two contrasting ecotypes of Sedium alfredii Hance under lead and zinc stress . J Environ Sci Health , 2005 . 40 ( 5 ): 1081 - 1096 . DOI: 10.1081/ESE-200056163 http://doi.org/10.1081/ESE-200056163 . Root response and metal accumulation in two contrasting ecotypes of Sedium alfredii Hance under lead and zinc stress O Lindqvist . Mercury in the Swedish environment . Water Air Soil Bull , 1991 . 55 ( 1 ): 23 - 32 . . Mercury in the Swedish environment Y Liu . Shrinking Arable Lands Jeopardizing China’s Food Security , 2006 . (Available from: Http://www.worldwatch.org/node/3912 Http://www.worldwatch.org/node/3912 ) . Shrinking Arable Lands Jeopardizing China’s Food Security XM Liu , QT Wu , MK Banks . Effect of simultaneous establishment of Sedum alfridii and Zea mays on heavy metal accumulation in plants . Int J Phytoremediation , 2005 . 7 ( 1 ): 43 - 53 . 15943243 DOI: 10.1080/16226510590915800 http://doi.org/10.1080/16226510590915800 . Effect of simultaneous establishment of Sedum alfridii and Zea mays on heavy metal accumulation in plants E Lombi , FJ Zhao , SJ Dunham . Cadmium accumulation in populations of Thlaspi caerulescens and Thlaspi goesingense . New Phytologist , 2000 . 145 ( 1 ): 11 - 20 . DOI: 10.1046/j.1469-8137.2000.00560.x http://doi.org/10.1046/j.1469-8137.2000.00560.x . Cadmium accumulation in populations of Thlaspi caerulescens and Thlaspi goesingense LQ Ma , KM Komar , C Tu , W Zhang , Y Cai , ED Kennely . A fern that hyperaccumulates arsenic . Nature , 2001 . 409 ( 6820 ): 579 11214308 DOI: 10.1038/35054664 http://doi.org/10.1038/35054664 . A fern that hyperaccumulates arsenic SP McGrath , FJ Zhao , E Lombi . Plant and rhizosphere process involved in phytoremediation of metal-contaminated soils . Plant Soil , 2001 . 232 ( 1/2 ): 207 - 214 . DOI: 10.1023/A:1010358708525 http://doi.org/10.1023/A:1010358708525 . Plant and rhizosphere process involved in phytoremediation of metal-contaminated soils P McKeehan . Brownfields: The Financial, Legislative and Social Aspects of the Redevelopment of Contaminated Commercial and Industrial Properties . 2000 . (Available from: Http://md3.csa.com/discoveryguide/brown/overview.php?SID=05c43ivvp4r0detrha3d9r5g Http://md3.csa.com/discoveryguide/brown/overview.php?SID=05c43ivvp4r0detrha3d9r5g ) . Brownfields: The Financial, Legislative and Social Aspects of the Redevelopment of Contaminated Commercial and Industrial Properties JO Nriagu . Arsenic in the Environment. 1. Cycling and Characterization , 1994 . : New York John Wiley and Sons , . Arsenic in the Environment. 1. Cycling and Characterization JO Nriagu , JM Pacyna . Quantitative assessment of worldwide contamination of air water and soils by trace metals . Nature , 1988 . 333 ( 6169 ): 134 - 139 . 3285219 DOI: 10.1038/333134a0 http://doi.org/10.1038/333134a0 . Quantitative assessment of worldwide contamination of air water and soils by trace metals AJ Pollard , KD Powell , FA Harper , JAC Smith . The genetic basis of metal hyperaccumulation in plants . Crit Rev Plant Sci , 2002 . 21 ( 6 ): 539 - 566 . DOI: 10.1080/0735-260291044359 http://doi.org/10.1080/0735-260291044359 . The genetic basis of metal hyperaccumulation in plants MNV Prasad , HMD Freitas . Metal hyperaccumulation in plants—Biodiversity prospecting for phytoremediation technology . Electron J Biotechnol , 2003 . 93 ( 1 ): 285 - 321 . . Metal hyperaccumulation in plants—Biodiversity prospecting for phytoremediation technology M Puschenreiter , S Wieczorek , O Horak , WW Wenzel . Chemical Changes in the rhizospher of metal hyperaccumulator and excluder Thlaspi species . J Plant Nutri Soil Sci , 2003 . 166 ( 5 ): 579 - 584 . DOI: 10.1002/jpln.200321155 http://doi.org/10.1002/jpln.200321155 . Chemical Changes in the rhizospher of metal hyperaccumulator and excluder Thlaspi species KV Ragnarsdottir , D Hawkins . Trace metals in soils and their relationship with scrapie occurrence . Geochimica et Cosmochimica Acta , 2005 . 69 ( 10 ): A196 - A196 . . Trace metals in soils and their relationship with scrapie occurrence I Raskin , D Gleba , R Smith . Using plant seedlings to remove heavy metals from water . Plant Physiol , 1996 . 111 ( 2 ): 552 - 552 . . Using plant seedlings to remove heavy metals from water I Raskin , RD Smith , DE Salt . Phytoremediation of metals: Using plants to remove pollutants from the environment . Curr Opin Biotechnol , 1997 . 8 ( 2 ): 221 - 226 . 9079727 DOI: 10.1016/S0958-1669(97)80106-1 http://doi.org/10.1016/S0958-1669(97)80106-1 . Phytoremediation of metals: Using plants to remove pollutants from the environment RD Reeves . Tropical hyperaccumulators of metals and their potential for phytoextraction . Plant Soil , 2003 . 249 ( 1 ): 57 - 65 . DOI: 10.1023/A:1022572517197 http://doi.org/10.1023/A:1022572517197 . Tropical hyperaccumulators of metals and their potential for phytoextraction BH Robinson , M Leblanc , D Petit . The potential of Thlaspi caerulescens for phytoremediation of contaminated soils . Plant Soil , 1998 . 203 ( 1 ): 47 - 56 . DOI: 10.1023/A:1004328816645 http://doi.org/10.1023/A:1004328816645 . The potential of Thlaspi caerulescens for phytoremediation of contaminated soils AL Salido , KL Hastly , JM Lim , DJ Butcher . Phytoremediation of arsenic and lead in contaminated soils using Chinese brake ferns ( Pteris vittata ) and Indian mustard ( Brassica juncea ) . Int J Phytoremediation , 2003 . 5 ( 2 ): 89 - 103 . 12929493 DOI: 10.1080/713610173 http://doi.org/10.1080/713610173 . Phytoremediation of arsenic and lead in contaminated soils using Chinese brake ferns (Pteris vittata) and Indian mustard (Brassica juncea) DE Salt , M Blaylock , PBAN kumar , V Dushenkov , BD Ensley , L Chet , L Raskin . Phytoremediation: A novel strategy for the removal of toxic metals from the environment using plants . Biotechnology , 1995 . 13 ( 2 ): 468 - 474 . 9634787 . Phytoremediation: A novel strategy for the removal of toxic metals from the environment using plants DE Salt , RD Smith , L Raskin . Phytoremediation . Ann Rev Plant Phys Plant Mol Biol , 1998 . 49 ( 1 ): 643 - 668 . DOI: 10.1146/annurev.arplant.49.1.643 http://doi.org/10.1146/annurev.arplant.49.1.643 . Phytoremediation G Sarret , P Saumitou-Laprade , V Bert . Forms of zinc accumulated in the hyperaccumulator Arabidopsis halleri . Plant Physiol , 2002 . 130 ( 4 ): 1815 - 1826 . 12481065 DOI: 10.1104/pp.007799 http://doi.org/10.1104/pp.007799 . Forms of zinc accumulated in the hyperaccumulator Arabidopsis halleri E Schalscha , I Ahumada . Heavy metals in rivers and soils of central chile . Water Sci Technol , 1998 . 37 ( 8 ): 251 - 255 . DOI: 10.1016/S0273-1223(98)00255-8 http://doi.org/10.1016/S0273-1223(98)00255-8 . Heavy metals in rivers and soils of central chile U Schmidt . Enhancing phytoremediation: The effect of chemical soil manipulation on mobility, plant accumulation, and leaching of heavy metals . J Environ Qual , 2003 . 32 1939 - 1954 . 14674516 . Enhancing phytoremediation: The effect of chemical soil manipulation on mobility, plant accumulation, and leaching of heavy metals C Schwartz , G Echevarria , JL Morel . Phytoextraction of cadmium with Thlaspi caerulescens . Plant Soil , 2003 . 249 ( 1 ): 27 - 35 . DOI: 10.1023/A:1022584220411 http://doi.org/10.1023/A:1022584220411 . Phytoextraction of cadmium with Thlaspi caerulescens J Scott Angle , AJM Baker , SN Whiting , RL Chaney . Soil moisture effect on uptake of metals by Thlaspi , Alyssum and Berkheya . Plant Soil , 2003 . 256 ( 1 ): 325 - 332 . . Soil moisture effect on uptake of metals by Thlaspi, Alyssum and Berkheya MRD Seaward , DHS Richardson . AJ Shaw . Atmospheric Sources of Metal Pollution and Effects on Vegetation . Heavy Metal Tolerance in Plants: Evolutionary Aspects , 1990 . : Florida CRC Press , 75 - 92 . . Atmospheric Sources of Metal Pollution and Effects on Vegetation FEC Sneller , LM van Heerwaarden , H Schat . Toxicity, metal uptake, and accumulation of phytochelatins in silene vulgaris exposed to mixtures of cadmium and arsenate . Environ Toxicol Chem , 2000 . 19 ( 12 ): 2982 - 2986 . DOI: 10.1897/1551-5028(2000)019<2982:TMUAAO>2.0.CO;2 http://doi.org/10.1897/1551-5028(2000)019<2982:TMUAAO>2.0.CO;2 . Toxicity, metal uptake, and accumulation of phytochelatins in silene vulgaris exposed to mixtures of cadmium and arsenate D Uneo , FJ Zhao , JF Ma . Interaction between Cd and Zn in relation to their hyperacculation in Thlaspi caerulescens . Soil Sci Plant Nutri , 2004 . 50 ( 4 ): 591 - 597 . . Interaction between Cd and Zn in relation to their hyperacculation in Thlaspi caerulescens D Uneo , FJ Zhao , RF Shen , JF Ma . Cadmium and zinc accumulation by the hyperaccumulattor Thlaspi caerulescens from soils enriched with insoluble metal compounds . Soil Sci Plant Nutr , 2004 . 50 ( 3-4 ): 511 - 515 . . Cadmium and zinc accumulation by the hyperaccumulattor Thlaspi caerulescens from soils enriched with insoluble metal compounds PR Walsh , RA Duce , JI Finishing . Consideration of the enrichment, sources, and flux of arsenic in the troposphere . J Geophysical Res , 1979 . 84 1719 - 1726 . . Consideration of the enrichment, sources, and flux of arsenic in the troposphere Q Wang , Y Cui , Y Dong . Phytoremediation of polluted waters potential and prospects of wetland plants . Acta Biotechnol , 2002 . 22 ( 1-2 ): 199 - 208 . DOI: 10.1002/1521-3846(200205)22:1/2<199::AID-ABIO199>3.0.CO;2-T http://doi.org/10.1002/1521-3846(200205)22:1/2<199::AID-ABIO199>3.0.CO;2-T . Phytoremediation of polluted waters potential and prospects of wetland plants RM Welch , WA Norvell . Growth and nutrient-uptake by barley (hordeum-vulgare l cv herta)—studies using an n -(2-ydroxyethyl) ethylenedinitrilotriacetic acid buffered nutrient solution technique. 2. Role of zinc in the uptake and root leakage of mineral nutrients . Plant Physiol , 1993 . 101 ( 2 ): 627 - 631 . 12231718 . Growth and nutrient-uptake by barley (hordeum-vulgare l cv herta)—studies using an n-(2-ydroxyethyl) ethylenedinitrilotriacetic acid buffered nutrient solution technique. 2. Role of zinc in the uptake and root leakage of mineral nutrients WW Wenzel , M Bunkowski , M Puschenreiter , O Horak . Rhizosphere characteristics of indigenously growing nickel hyperaccumulator and excluder plants on serpentine soil . Environ Poll , 2002 . 123 ( 1 ): 131 - 138 . . Rhizosphere characteristics of indigenously growing nickel hyperaccumulator and excluder plants on serpentine soil NS Whiting , RJ Leake , PS McGrath , MJA baker . Positive response to Zn and CD by roots of the Zn and Cd hyperaccumulator Thlaspi caerulescens . New Phytol , 2000 . 145 ( 2 ): 199 - 210 . DOI: 10.1046/j.1469-8137.2000.00570.x http://doi.org/10.1046/j.1469-8137.2000.00570.x . Positive response to Zn and CD by roots of the Zn and Cd hyperaccumulator Thlaspi caerulescens YH Xiong , XE Yang , ZQ Ye , ZL He . Characteristics of cadmium uptake and accumulation by two contrasting ecotypes of sedum alfredii hance . J Environ Sci Health Part A , 2004 . 39 ( 11-12 ): 2925 - 2940 . . Characteristics of cadmium uptake and accumulation by two contrasting ecotypes of sedum alfredii hance XE Yang , HB Ye , XX Long , B He , ZL He , PJ Stoffella , DV Calvert . Uptake and accumulation of cadmium and Zinc by Sedum alfredii Hance at different Cd/Zn supply levels . J Plant Nutr , 2004 . 27 ( 11 ): 1963 - 1977 . DOI: 10.1081/PLN-200030082 http://doi.org/10.1081/PLN-200030082 . Uptake and accumulation of cadmium and Zinc by Sedum alfredii Hance at different Cd/Zn supply levels XE Yang , TQ Li , JC Yang , ZL He , LL Lu , FH Meng . Zinc compartmentation in root, transport into xylem, and absorption into leaf cells in the hyperaccumulating species of Sedum alfredii Hance . Planta , 2006 . 224 ( 1 ): 185 - 195 . 16362325 DOI: 10.1007/s00425-005-0194-8 http://doi.org/10.1007/s00425-005-0194-8 . Zinc compartmentation in root, transport into xylem, and absorption into leaf cells in the hyperaccumulating species of Sedum alfredii Hance MF Zaranyika , T Ndapwadza . Uptake of Ni, Zn, Fe, Co, Cr, Pb, Cu and Cd by water hyacinth ( Eichhornia crassipes ) in Mukuvisi and Manyame Rivers, Zimbabwe . J Environ Sci Health Part A , 1995 . 30 ( 1 ): 157 - 169 . . Uptake of Ni, Zn, Fe, Co, Cr, Pb, Cu and Cd by water hyacinth (Eichhornia crassipes) in Mukuvisi and Manyame Rivers, Zimbabwe A Zayed , S Gowthaman , N Terry . Phytoremediation of trace elements by wetland plants: 1. Duck weed . J Environ Qual , 1998 . 27 ( 3 ): 715 - 721 . . Phytoremediation of trace elements by wetland plants: 1. Duck weed WH Zhang , Y Cai , C Tu , LQ Ma . Arsenic speciation and distribution in an arsenic hyper accumulating plant . Sci Total Environ , 2002 . 300 ( 1-3 ): 167 - 177 . 12685480 DOI: 10.1016/S0048-9697(02)00165-1 http://doi.org/10.1016/S0048-9697(02)00165-1 . Arsenic speciation and distribution in an arsenic hyper accumulating plant L Zhang , S Tian , Z Ye , 等 . The Efficiency of Heavy Metal Removal from Contaminated Water by Elsholtzia argi and Elsholtzia splendens . Proc. of the International Symposium of Phytoremediation and Ecosystem Health , 2005 . Sept. 10-13, 2005 Hangzhou, China . The Efficiency of Heavy Metal Removal from Contaminated Water by Elsholtzia argi and Elsholtzia splendens FJ Zhao , E Lombi , T Breedon . Zinc hyperaccumulation and cellular distribution in Arabidopsis halleri . Plant Cell Environ , 2000 . 23 ( 5 ): 507 - 514 . DOI: 10.1046/j.1365-3040.2000.00569.x http://doi.org/10.1046/j.1365-3040.2000.00569.x . Zinc hyperaccumulation and cellular distribution in Arabidopsis halleri FJ Zhao , SJ Dunham , SP McGrath . Arsenic hyper accumulation by different fern species . New Phytologist , 2002 . 156 ( 1 ): 27 - 31 . DOI: 10.1046/j.1469-8137.2002.00493.x http://doi.org/10.1046/j.1469-8137.2002.00493.x . Arsenic hyper accumulation by different fern species YL Zhu , AM Zayed , JH Qian , M De Souza , N Terry . Phytoaccumulation of trace elements by wetland plants: II. Water hyacinth . J Environ Qual , 1999 . 28 ( 1 ): 339 - 344 . . Phytoaccumulation of trace elements by wetland plants: II. Water hyacinth Dr. M.I. Lone is obliged to the Vice Chancellor, University of Arid Agriculture, Rawalpindi, Pakistan, for sending him to USA for training in phytoremediation and Dr. Z.L. He and Dr. P.J. Stoffella for hosting this training program at the University of Florida, USA. 10.1631/jzus.B0710633 Project supported by the Higher Education Commission, Government of Pakistan for the faculty training under the R & D Project “Strengthening Department of Soil Science and Soil and Water Conservation” at the University of Florida, USA, a grant from the St. Lucie River Water Initiative (SFWMD contract No. OT060162), USA, in part, and the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT0536), China 2007-12-24 2008-01-10 2022-01-05 Copyright © 2008, Journal of Zhejiang University Science 2008 Journal of Zhejiang University-SCIENCE B (Biomedicine and Biotechnology) 03 09