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

210Lone et al.1 J Zhejiang Univ Sci B 2008 9(3):210-220Joumal of Zhejiang Univerity SCIENCE BISSN 1673-1581 (Print); ISSN 1862 1783 (Online)www. ziu.edu.cn/jzus; ww.sprngerini. comE-mail: jzus@zu edu.cnJZsReview:Phytoremediation of heavy metal polluted soils and water:Progresses and perspectivesMohammad lqbal LONE'2, Zhen-i HE1I3, Peter J. STOFFELLA', Xiao-e YANG'(Universiy ofFlorida, Institnte ofFood and Agriculwral Sciences, Indian River Research and Education Center. Fort Pierce, Florida 34945, USA)(University ofArid Agriculure, Rawalpindi, Pakistn)(MOE Key Laboratory ofEmironmental Remediation and Ecological Heath, Cllege of Narural Resources and Environmental Sciences,Zhejiang University, Hangzhou 310029, China)'E-mail: zhe@ufl.eduReceived Dec. 24, 2007; revision accepted Jan.10, 2008Abstract: Environmental pllution afets the quality of pedosphere, hydrosphere, atmosphere, lithosphere and biosphere. Greateforts have been made in the last two decades to reduce pollution sources and remedy the pllued soil and water resources.Phytoremediation, being more cost-fctive and fewer side effects than plhysical and chemical approaches, has gained increasingpopularity in both academic and practical circles. More than 400 plant species have been inified to have potential for soil andwater remeiation. Among them, Thlaspi, Brassica, Sedum afredi H., and Arabidopsis spcies have been mostly studied. Itis alsoexpected that recent advances in biotechnology will play a promising role in the development of new hyperaccumulators bytransferring metal hyperaccumulating genes from low biomass wild species to the higher biomass producing cultivated species inthe times to come. This paper atempted to provide a brief review on recent progresses in research and practical applications ofphytoremediation for soil and water resources.Key words: Environmental pollution, Heavy metals, Phytoremediation, Soil, Waterdoi: 10.163 l/jzus.B0710633Document code: ACLC oumber; X5INTRODUCTIONindustrial and municipal wastes in agriculture, andexcessive use of fertilizers (McGrath et al, 2001;Land and water are precious natural resources onNriagu and Pacyna, 1988; Schalscha and Ahumada,which rely the sustainability of agriculture and the1998). Each source of contamination has its owncivilization of mankind. Unfortunately, they havedamaging effects to plants, animals and ultimately tobeen subjected to maximum exploitation and severelyhuman health, but those that add heavy metals to soilsdegraded or polluted due to anthropogenic activities.and waters are of serious concern due to their persis-The pollution includes point sources such as emission, tence in the environment and carcinogenicity to bhu-effluents and solid discharge from industries, vehicleman beings. They cannot be destroyed biologicallyexhaustion and metals from smelting and mining, and but are only transformed from one oxidation state ornonpoint sources such as soluble salts (natural andorganic complex to another (Garbisu and Alkorta,arificial), use of insecticides/pesticides, disposal of 2001; Gisbert et al, 2003). Therefore, heavy metalpollution poses a great potential threat to the envi-↓Corresponding authorronment and human health.'Project supported by the Higher Education Commission, Govern-lity of soils andment of Pakistan for the faculty training under the R & D Project"Strengthening Department of Soil Science and Soil and Water Con- water中国煤化工tamination, conm-servation" at the University ofFlorida, USA, a grant from the SL. Lucie tinuouTYHC N M H Glevelop technolo-River Water Initiative (SFWMD contract No. OT060162), USA, ingies that are easy to use, sustainable and economicallypart, and the Program for Changjiang Scholars and Innovative Re-search Team in University (No.IRT0536), Chinafeasible. Physicochemical approaches have beenLone et al.1J Zhejiang Univ Sci B 2008 9(3):210-220211widely used for remedying polluted soil and water,vegetables near big cities.especially at a small scale. However, they experienceHeavy metals that have been identified in themore difficulties for a large scale of remediation be-polluted environment include As, Cu, Cd, Pb, Cr, Ni,cause of high costs and side effects. The use of plant Hg and Zn. The sources of various heavy metals arespecies for cleaning polluted soils and waters namedlisted in Table 1. The presence of any metal may varyas phytoremediation has gained increasing atetion from site to site, depending upon the source of indi-since last decade, as an emerging cheaper technology.vidual pollutant. Excessive uptake of metals by plantsMany studies have been conducted in this field in themay produce toxicity in human nutrition, and causelast two decades. Numerous plant species have beenacute and chronic diseases. For instance, Cd and Znidentified and tested for their traits in the uptake and can lead to acute gastrointestinal and respiratoryaccumulation of different heavy metals. Mechanismsdamages and acute heart, brain and kidney damages.of metal uptake at whole plant and cellular levels have High concentrations of heavy metals in soil canbeen investigated. Progresses have been made in thenegatively affect crOP growth, as these metals inter-mechanistic and practical application aspects of fere with metabolic functions in plants, includingphytoremediation. They were reviewed and reportedphysiological and biochemical processes, inhibitionin this paper.of photosynthesis, and respiration and degenerationof main cell organelles, even leading to death ofplants (Garbisu and Allkorta, 2001; Schmidt, 2003;SOURCES OF HEAVY METALS AND SOIL~ Schwartz et al, 2003). Soil contamination with heavyWATER POLLUTIONmetals may also cause changes in the composition ofLand and water pollution by heavy metals is aTable 1 Different sources of heavy metalsworldwide isue. All countries have been affected,HeavySourcesthough the area and severity of pollution vary enor-metalsmously. In Westerm Europe, 1 400000 sites were af-As Semiconductors, petroleum refining, wood pre-servatives, animal feed additives, coal powerfected by heavy metals (McGrath et al, 2001), ofplants, herbicides, volcanoes, mining andwhich, over 300000 were contaminated, and the es-smeling (Nriagu, 1994; Walsh et al, 1979)timated total number in Europe could be much larger,Cu Electroplating industry, smeling and refining,mining, biosolids (Liu et al, 2005)as pollution problems increasingly occurred in Cen-d Geogenic sources (Baize, 1997), anthropogenictral and Eastem European countries (Gade, 2000). Inactivities Nriagu and Pacyna, 1988), metalUSA, there are 600000 brown fields which are con-smelting and refining, fossil fuel burming, ap-taminated with heavy metals and need reclamationplication of phosphate fertilizers, sewagesludge (Alloway, 1995; Kabata-Pendias,(McKeehan, 2000). According to government statis-2001)tics, coal mine has contaminated more than 19000 kmCr Electroplaing industry, sludge, solid waste,of US streams and rivers from heavy metals, acidtanneries (Knox et al, 1999)mine drainage and polluted sediments. More thanPb Mining and smelting of maealliferous ores,burming of leaded gasoline, municipal sewage,100000 ha of cropland, 550000 ha of pasture andindustrial wastes enriched in Pb, paints (Gis-50000 ha of forest have been lost (Ragnarsdottir andbert et al, 2003; Seaward and Richardson,Hawkins, 2005). The problem of land pollution is also1990)Ig Volcano eruptions, forest fire, emissions froma great challenge in China, where one sixth of totalindustries producing caustic soda, coal, peatarable land has been polluted by heavy metals, andand wood burning (Lindqvist, 1991 )more than 40% has been degraded to varying degreeSe Coal mining, oil refining, combustion of fossildue to erosion and desertification (Liu, 2006). Soilfuels, glass manufacturing industry, chemicalsynthesis (e.g, vamish, pigment formulation)and water pollution is also severe in India, PakistanNi Volcanic eruptions, land fil, forest fire, bubbleand Bangladesh, where small industrial units arebursting and gas exchanee in ocean, weath-pouring their untreated effluents in the surface drains,中国煤化Inaterials (Knoxwhich spread over near agricultural fields. In thesetn.MHC N M H Gg and refining,countries raw sewage is often used for producing. mining, blosollds (Lu er al, 2005)212Lone et al. 1J Zhejang Univ SciB 2008 9(3):210-220soil microbial community, adversely affecting soilprocesses that are not related to metal assimilation], .Characteristics (Giller et al, 1998; Kozdroj and van and (2) use of special type of plants to decontaminateElsas, 2001; Kurek and Bollag, 2004).soil or water by inactivating metals in the thizosphereor translocating them in the aerial parts. This ap-proach is called phytoremediation, which is consid-TECHNOLOGIES FOR THE RECLAMATION OFered as a new and highly promising technology for thePOLLUTED SOILS .reclamation of polluted sites and cheaper than phys-icochemical approaches (Garbisu and Alkorta, 2001;The cleaning of contaminated soils from heavyMcGrath et al, 2001; Raskin et al, 1997).metals is the most difficult task, particularly on a largePhytoremediation, also referred as botanicalscale. The soil is composed of organic and inorganicbioremediation (Chaney et al, 1997), involves the usesolid constituents, water and mixture of differentof green plants to decontaminate soils, water and air.gases present in various proportions. The mineralIt is an emerging technology that can be applied tocomponents vary according to parent materials onboth organic and inorganic pollutants present in thewhich the soil had been developed under a particular soil, water or air (Salt et al, 1998). However, theset of climatic conditions. Therefore, soils varyability to accumulate heavy metals varies signif-enormously in physical, chemical and biologicalcantly between species and among cultivars withinproperties. Soil water movement is controlled byspecies, as different mechanisms of ion uptake arephysical properties, such as soil structure and texture.operative in each species, based on their genetic,The soil moisture has great bearing in controllingmorphological, physiological and anatomical char-solute movement, salt solubility, chemical reactionsacteristics. There are different categories of phy-and microbiological activities and ultimately thetoremediation, including phytoextraction, phytofil-bioavailability of the metal ions. A successful phy-tration, phytostabilization, phytovolatization andtoremediation program, therefore, must take intophytodegradation, depending on the mechanisms ofconsideration variations in soil properties of the spe-remediation. Phytoextraction involves the use ofcific site.plants to remove contaminants from soil. The metalDifferent approaches have been used or devel-ion accumulated in the aerial parts that can be re-oped to mitigate/reclaim the heavy metal pollutedmoved to dispose or bumt to recover metals. Phyto-soils and waters including the landill/damping sites.fitration involves the plant roots or seedling for e-These may be broadly classified into physicochemicalmoval of metals from aqueous wastes. In phytostabi-and biological approaches.lization, the plant roots absorb the pollutants from theThe physicochemical approach includes exca-soil and keep them in the rhizosphere, rendering themvation and burial of the soil at a hazardous waste site,harmless by preventing them from leaching. Phyto-fixation/inactivation (chemical processing of the soilvolatization involves the use of plants to volatilizeo immobilize the metals), leaching by using acidpollutants from their foliage such as Se and Hg.solutions or proprietary leachants to desorb and leachPhytodegradation means the use of plants and ass0-the metals from soil followed by the retumn of cleanciated microorganisms to degrade organic pollutantssoil residue to the site (Salt et al, 1995), precipitation(Garbisu and Alkorta, 2001). Some plants may haveor flocculation followed by sedimentation, ion ex-one function whereas others can involve two or morechange, reverse osmosis and microfiltration (Raskinfunctions of phytoremediation.et al, 1996). The physicochemical approaches aregenerally costly and have side effects (Raskin et al,1997; McGrath et al, 2001).PLANT SPECIES FOR PHYTOREMEDIATIONBiological approaches of remediation include: (1)use of microorganisms to detoxify the metals by va-中国煤化工_with the ability tolence transformation, extracellular chemical precipi-accussions of 30 planttation, or volatilization [some microorganism canspeci:fYHC N M H G(1997) in hydro-enzymatically reduce a variety of metals in metabolic ponics for 4 weeks, having moderate levels ofCd, CuLone et al. 1J Zhejiang Univ SiB 2008 93)210220213and Zn. The results indicate that many Brassica Spp.due to specific rooting strategy and a high uptake ratesuch as B. juncea L., B. juncea L. Czem, B. napus L.resulting from the existence in this population ofand B. rapa L. exhibited moderately enhanced Zn andCd-specific transport channels or carriers in the rootCd accumulation. They were also found to be mostmembrane (Schwartz et al, 2003).effective in removing Zn from the contaminated soils.To date, more than 400 plant species have been iden-tified as metal hyperaccumulators, representing lessMETAL HYPERACCUMULATION IN VARIOUSthan 0.2% of all angiosperms (Brooks, 1998; Baker etPLANT SPECIESal, 2000). The plant species that have been identifiedfor remediation of soil include either high biomassThe hyperaccumulation of metals in variousplants such as willow (1 andberg and Greger, 1996) orplant species has been extensively investigated and tothose that have low biomass but high hyperaccumu-date substantial progress has been made. It becomeslating characteristics such as Thlaspi and Arabidopsis. clear that different mechanisms of metal accumula-species. On worldwide basis, the number of speciestion, exclusion and compartmentation exist in variousidentified to have ability to accumulate one or moreplant species. In T. caerulescens, Zn is sequesteredmetals >1000 mg/kg dry weight is listed in Table 2preferentially in vacuoles of epidermnal cells in a(Reeves, 2003).soluble form (Frey et al, 2000). In A. halleri leaves,Zn was found to be accumulated in the mesophyllTable 2 The number of plant species that are reportedcells (Kupper et al, 2000; Zhao et al, 2000; Sarret etto have hyperaccumulation traits (metal concentrational, 2002). Cosio et al.(2004) investigated the>1000 mg/kg dry weight) (Reeves, 2003)mechanisms of Zn and Cd accumulation in three dif-Metals Number of species| Metals Number of speciesferent plant species through ion compartmentation byAs)4measuring the short term 10Cd and 6Zn uptake inCd)1Ni>320mesophyll protoplast of T. caerulescens “Ganges"Co34se2and A. halleri. Their study suggests the existence ofCuregulation mechanism on the plasma membrane ofleaf mesophyll protoplast.The byperaccumulators that have been mostPuschenreiter et al.(2003) investigated chemicalextensively studied by scientifc community include changes in the thizosphere of hyperaccumulators T.Thlaspi sp., Arabidopsis sp, Sedum alfredi sp. (both goesingense and T. caerulescens and the metal ex-genera belong to the family of Brassicaceae ancluder T. arvense with a rhizosphere bag experimentAlyssum). Thlaspi sp. are known to byperaccumulate on the contaminated and non-contaminated soils.more than one metal, i.e., T. caerulescens for Cd, Ni, Hyperaccumulation and depletion of labile Zn in thePb and Zn, T. goesingense for Ni and Zn, T. ochro- thizosphere were observed for T. goesingense grownleucum for Ni and Zn, and T. rotundifolium forNi, Pb on the contaminated soil. In the non-contaminatedand Zn (Prasad and Freitas, 2003). Among the genus soil, Zn was accumulated but labile Zn in theThlaspi, the hyperaccumulator plant Thlaspi rhizosphere was not changed. Nickel present incarilescens reeived much atention and has been background concentration in both soils was accumu-extensively studied as potential candidates for Cd andlated by T. goesingenseonly when grown onZn contaminated soils. Robinson et al.(1998) foundT. non-contaminated soil. In contrast, labile Ni in thecaerulescens as byperaccumulator for Cd and Znrhizosphere increased in both soils, suggesting acould remove as high as 60 kg Zn/ha and 8.4 kg Cdha. general tendency of Ni mobilization by T. goesin-It can accumulate as high as 2600x10“Zn without gense. Uneo et al.(2004a) studied the interactionshowing any injury (Brown et al, 1995) and extract between Zn and Cd in T. caerulescens in solutionup to 22% of soil exchangeable Cd from the con-culture and in nt soil._ Results from long term (4taminated site. It also showed remarkable Cd toler- weeks中国煤化Iution culture ex-ance (Sneller et al., 2000; Escarre et al, 2000; Lombi perimY片CNMHGtionintheshootet al, 2000). T. caerulescens has higher uptake ofCd was not aTTectea y Ine suppiy oI a 4~10-fold excess214Lone et al. 1J Zhejang Univ Sci B 2008 9(3):210-220of Zn, whereas the Cd concentration of the roots de- Halacsy by Wenzel et al.(2002) indicate that rootcreased with increasing Zn concentrations in the so-exudates of organic ligands may contribute to Nilution. The resuts suggest that the Ganges ecotype of hyperaccumulation in T. geosingense Halacsy. ThisT. caerulescens displayed different uptake systemswas attributed to the ligand-induced dissolution ofNifor Cd and Zn and that Cd compeied with Zn uptake bearing minerals in the rhizosphere of T. geosingensewhile Zn did not compete with Cd uptake. Uneo etand appeared to be less effective in the rhizosphere ofal.(2004b) investigated the uptake of Cd and Zn by T.excluder Silene vulgaris and Rumex acetosellacaerulescens (the Ganges ecotype) from enriched soilgrowing on the same site.with different insoluble and soluble sources of Cd andSedum alfredi Hance was identified in China asZn. The data show that there was no significant dif-hyperaccumulator for Cd and Zn and has been inten-ferences in the shoot Cd concentration between thesively investigated by various researchers in theirtreatments with soluble or insoluble Cd compounds,studies conducted in hydroponics and/or the uncon-even though Cd concentration in the soil solution wastaminated and contaminated soils (Li H. et al, 2005;the order of CdSO,>>CdCO;>CdS. ThlaspiLi T.Q. et al, 2005a; Liu et al, 2005; Xiong et al,caerulescens grown on the ZnS-enriched soil accu-2004; Yang et al, 2004; 2006). The data show that themulated up to 6900 mg Zn/kg in the shoots, althoughconcentrations of Cd and Zn in leaves and stems in-Zn accumulation was 1.5 times higher with the addi-creased with increasing Cd and Zn supply levels. Thetion of more soluble compounds Zn3(PO4)h or ZnSO4.distributions of the metals in different plant partsThese results indicate that the Ganges ecotype of T. decreased in the order: stem>leaf>root for Zn andcaerulescens is able to utilize insoluble Cd and Znleaf>stem>root for Cd. These results indicate that S.compounds in soils.alfredi has an extraordinary ability to tolerate Cd/ZnWhiting et a.(2000) found that the plants from T.toxicities, and to absorb and hyperaccumulate Cd andcarerulescens population that accumulated Cd also Zn under a range of Cd/Zn combining levels. Theshowed increased root biomass and root length afteruptake and accumulation of Cd by the mined and theallocation into Cd-enriched soil, whereas plants from non mined ecotypes of s. alfredi indicated that thethe population that did not accumulate Cd showed noplants of the mined ecotype (ME) have higher toler-such increase.ance to Cd than those of the non-mined ecotypesT. caerulescens was grown with H. vulgare and(NME) in terms of dry matter yield (Xiong et al,L. heterophylum in the field to examine the effect of 2004).rhizosphere interaction on metal uptake. The dataZinc compartmentation studies involving hy-show that the Cd concentration in H. vulgare was peraccumulating and non-hyeraccumulating S. al-increased by a factor of2.4 when it was grown along fredi plants using radioactive tracer fux techniquethe sides of T. caerulescens without a barrier. In indicate that S. alfredi H. can accumulate Zn incontrast, the uptake of Zn by H. vulgare was signif- shoots over 2% of dry weight. Leaf and stem Zncantlty decreased, probably through metal depletion concentrations of the hyperaccumulating ecotype (HE)within the zone of the Zn-byperaccumulator were 24- and 28 fold higher, respectively, than thoserhizosphere. These results suggest that T. caerules- of the non-hyperaccumulating ecotype (NHE),cens may alter conditions in the shared thizospheres whereas 1.4-fold more Zn was acumulated in theand thereby afet the availability of selected metals roots of the NHE. Approximately 2.7-fold more Znto neighboring plants (Gove et al, 2002) On the other was stored in the root vacuoles of the NHE, and thushand, when S. alfredii was intercropped with a grain became unavailable for loading into the xylem andcrop, z. mays, heavy metals (Zn and Cu) accumulated subsequent translocation to shoots. These results alsoin the grains were significantly reduced, as compared indicate that the altered Zn transport across tonoplastto monoculture cropping, and the intercropping im- in the root and the stimulated Zn uptake in the leafproved the growth of both plant species (Liu et al, cells a中国煤化工_Ived in the strong2005).Zn hy上n s. alfredi H.Studies on the role of thizosphere process in (YangYHCNMHGmetal byperaccumulation of Ni in T. geosingenseThe root morphology and Zn^* uptake kinetics ofLone et al. IJ Zhejang Univ Scl B 2008 9(3):210-220215HE and NHE of S. alfredi H. were investigated using similar hyperaccumulation of Ni, but T. goesingensehydroponic methods and the radiotracer fux tech- was less tolerant to Ni than the other two species.nique. The results indicate that the root length, root Addition of 500 mg Ni/kg to a nutrient-rich growthsurface area and root volume of NHE decreased sig-medium significantly increased shoot biomass of allnificantly with increasing Zn2* concentration in. species. X-ray microanalysis of frozen-hydrated tis-growth media, whereas the root growth ofHE was notsues of leaves and stems of all species showed that Niadversely affected, and even promoted, by 500 in all species was distributed preferentially in theμmol/L Zn2*. The concentrations of Zn2+ in bothepidermal cells, most likely in the vacuoles of theecotypes of S. alfredi H. were positively correlated leaves and stem. Kidd and Monterroso (2005) inves-with root length, root surface area and root volumes,tigated the efficiency of Alyssum serpllifolium ssp.but no such correlation was found with root diameter. lusitanicum (Brassicaceae) for use in phytoextractionThe uptake kinetics for 6Zn2+ in the roots of bothof polymetal-contaminated soils. The plant wasecotypes of S. alfredi were characterized by a rapid grown on two mine spoil soils, one contaminated withlinear phase during the first 6 h and a slower linearCr (283 mg/kg) and the other moderately contami-phase during the subsequent period of investigation. nated with Cr (263 mg/kg), Cu (264 mg/kg), Pb(1433The concentration-dependent uptake kinetics of themg/kg) and Zn (377 mgkg). The results suggest thattwo ecotypes of S. alfredi could be characterized by A. serpyllifolium could be suitable for phytoextractionthe Michaelis-Menten equation, with thVmauses in polymetal-contaminated soils, provided that(maximum uptake speed) for° 'Zn^+ influx being Cu concentrations were not phytotoxic.3-fold greater in the HE than that in the NHE, indi-Among different fem species, three accessionscating that enhanced absorption into the root was one of P. vitta, two cultivars of P. cretica, P. longifoliaof the mechanisms involved in Zn hyperaccumulation. and P. umbrosa were grown with 0~500 mg As/kgA significantly larger Vmax value suggested that there added to the substrate. The results show that in addi-was a higher density of Zn transporters per unit tion to P. vitta, P. cretica, P. longifolia and P. um-membrane area in HE roots (Li H. et al., 2005). .brosa also hyperaccumulate As to a similar extent.Li T.Q. et al.(2005b) investigated the rootThis study identified three new species of As hyper-morphological and physiological response of the HE accumulators in the Pteris genus (Zhao et al, 2002).of S. alfridi H. from the mined area and the NHE ofS.In another study, the speciation and distribution of Asalfrdi from the agricultural area to the supplied Zn of Brake fem was investigated by Zhang et al.(2002),and Pb in hydroponics. The results show that Zn which was grown for 20 weeks in As contaminatedconcentrations in the leaves and the stems ofHE were soil. The results show that As recoveries of 85% to34 and 41 times higher, whereas Pb concentrations100% were obtained from most parts of the plantwere 1.9 and 2.4 times higher, respectively than those (rhizomes, fiddle heads, young fronds and old fronds),of the NHE when grown at 1224 pumol/L Zn and/or and for roots, the corresponding value was approxi-200 μmol/L Pb. The study also shows that the toler- mately 60%. The result also demonstrates the abilityance and hyperaccumulation of the HE ofS. alfidiH. of Blake fem as As hyperaccumulator, which canto Zn and Pb appear to be closely related to its high transfer As rapidly from soil to above ground biomassadaptation of root growth, morphology and physiol- with minimal As concentration in the roots. As isogy to Pb and Zn toxicity. Through its root excretion found to be predominantly as inorganic species.of some special substances, the plant can activate Pb Caille et al.(2005) conducted a pot experiment withand Zn in the mined soil, thus increasing their mobi-0~500 mg/kg As added as arsenate and another shortlization and bioavailability.term (8 h) uptake experiment with 5x10~ arsenateThe Alyssum species has been extensively stud-under phosphorus sufficient conditions, and grewied for the hyperaccumulation of Ni. Kupper et hyperaccumulator Pteris vitta and the nonhyperac-al.(2001) studied the Ni uptake and cellular com-cumul:ts show that inpartmentation in three Ni byperaccumualtors: A. ber- both e:中国煤化工d much more Astolonii (Desv), A. lesbiacum (Candargy), and T.thanYHCN M H G toxicity symp-goesingense (Halacsy). These three species showed toms.216Lone et al. 1J Zhejiang Univ Sci B 2008 9():210-220PHYTOREMEDIATION OF POLLUTED WATERsubstantially in its roots. Ingole and Bhole (2003)conducted hydroponic studies to investigate the up-Rhizofiltration is the removal of pollutants fromtake of As, Cr, Hg, Ni, Pb and Zn by water byacinththe contaminated waters by accumulation into plantfrom the aqueous solution at the concentrationsbiomass. Several aquatic species have been identifiedranging from 5 to 50 mg/L, and observed that inand tested for the phytoremediation of heavy metalsaqueous solutions containing 5 mg/L of As, Cr andfrom the polluted water. These include sharp dock Hg, the maximum uptake was 26, 108 and 327 mgkg(Polygomum amphibium L.), duck weed (Lemna mi-dry weight of water hyacinth, respectively.nor L.), water hyacinth (Eichhormia crassipes), waterAmong the fems, Pteris vitta commonly knownlettuce (P. stratiotes)， water dropwort [Oenatheas Brake fem has been identified as As hyperaccu-javanica (BL) DC], calamus (Lepironia articulate),mulator for As contaminated soils and waters. It canpennywort (Hydrocotyle umbellate L.) (Prasad andaccumulate up to 7500 mg As/kg on a contaminatedFreitas, 2003). The roots of Indian mustard are found site (Ma et al, 2001) without showing toxicityto be efctive in the removal of Cd, Cr, Cu, Ni, Pbsymptoms. One fem cultivar is available commer-and Zn, and sunflower can remove Pb, U, Cs-137 and cially for As phytoremediation and has been sucSr-90 from hydroponic solutions (Zaranyika andcessfully used in field trials (Salido et al., 2003).Ndapwadza, 1995; Wang et al, 2002; Prasad andLi H. et al.(2005) conducted a laboratory studyFreitas, 2003).in hydroponics to test different levels of Cd on theThe potential of duck weed was investigated by growth and Cd uptake by three bhydrophytes:Zayed et al.(1998) for the removal ofCd, Cr, Cu, Ni,Gladiolous, Isoetes taiwaneneses Dwvol and Echi-Pb and Se from nutrient-added solution and the results nodorus amazonicus. The data show that the biomassindicate that duck weed is a good accumulator for Cd,of all the plants decreased with an increase in CdSe and Cu, a moderate accumulator for Ct, buta poor concentration from 5 to 20 mg/L. However, Cd toxicaccumulator of Ni and Pb. Dos Santos and Lenzieffect was greater on Isoetes taiwaneneses Dwvol and(2000) tested aquatic macrophyte (Eiochhornia cras- Echinodorus amazonicus than that on Gladiolous. Insipes) in the elimination of Pb from industrial efflu- addition, the accumulation of Cd was higher inents in a green house study and found it useful for Pb Gladiolous than the other two plants. Zhang et al.removal. Water hyacinth possesses a well-developed (2005) investigated the eficiency of Cu removal fromfibrous root system and large biomass and has been the contaminated water by Elsholzia argyi and EI-successfully used in wastewater treatment systems to sholtzi splendens in hydroponics. The results showimprove water quality by reducing the levels of or- that Elsholtia argyvi showed better Cu phytofilrationganic and inorganic nutrients. This plant can also than Elsholzi splendens, which was associated withreduce the concentrations of heavy metals in acid better ability to higher Cu concentrations and trans-mine water while exhibiting few signs of toxicity. location to shoots.Water hyacinth accumulates trace elements such asAg, Pb, Cd, etc. and is eficient for phytoremediationof wastewater polluted with Cd, Cr, Cu and Se (Zhuet ENHANCEMENT OF PHYTOREMEDIATION BYal, 1999).CHEMICAL AND BIOLOGICAL APPROACHESWang et al.(2002) conducted a pot experiment totest five wetland plant species, i.e, sharp dock,In order to cope with heavy metal contaminatedduckweed, water hyacinth, water dropwort and sois, various phytoremediation approaches (phy-calamus for their possible use in remedying the pol- tostabilization, phytoimmobilization and phytoex-luted waters. The results show that sharp dock was a traction) can be applied. However, the choice willgood accumulator of N and P. Water hyacinth anddepend on many factors, such as plant tolerance toduckweed strongly accumulated Cd with a concen-polluta中国煤化工.roperties, agro-tration of 462 and 14200 mg/kg, respectively. Water nomic:pecies, climaticdropwort achieved the highest concentration of Hg, conditMHC N M H G and additionalwhereas the calamus accumulated Pb (512 m/kg) technologies available for the recovery ofmetals fromLone et al./J Zhejang Univ Sci B 2008 9(3):210-220217the harvested plant biomass. It appears that bothshould be developed to combine a rapid screening ofchemical and biological approaches are passingplant species possessing the ability to accumulatethrough their infancy and need more efforts for theirheavy metals with agronomic practices that enbanceeffective use in the future.shoot biomass production and/or increase metalThe solubility of heavy metals in the pollutedbioavailability in the rhizosphere.soils can be increased by using organic and inorganicagents, thus enhancing the phytoextraction capabili-ties of many plant species. Ebbs et al.(1997) amendedCONCLUSIONS AND PERSPECTIVESthe contaminated soil with Grower-Power, a com-mercial soil amendment that improves soil structureThe contamination of heavy metals to theand fertility, and the removal of Zn by plant shootsenvironment, i.e, soil, water, plant and air is of greatwas doubled to more than 30000 mg Zn/pot (4.5 kg).concern due to its potential impact on human andOther applied enhancement materials include ethyl- animal health. Cheaper and efective technologies areene diamine tetraacetic acid (EDTA), citric acid,needed to protect the precious natural resources andelemental sulfur or ammonium sulfate. Increases biological lives. Substantial efforts have been made ingreater than 100 folds in Pb concentration in theidentifying plant species and their mechanisms ofbiomass of crOPS were reported when EDTA was uptake and hyperaccumulation of heavy metals in theapplied to the contaminated soils (Cunningham andlast decade. There are genetic variations among plantBerti, 2000). Uranium, cadmium and zinc concentra- species and even among the cultivar of the sametions in plant biomass were increased by the applica-species. The mechanisms of metal uptake, accumula-tion of citric acid, elemental sulfur or ammonium tion, exclusion, translocation, osmoregulation andsulfate, respectively (Schmidt, 2003). In addition tocopartmentation vary with each plant species andthe chelating material, the plant roots excrete determine its specific role in phytoremediation.metal-mobilizing substances called phytosideropho- Variations exist for hyperaccumulation of differentres. Other exudates include mugenic and deoxy-metals among various plant species and withinmugeneic acids from barley and com, and avenic acidpopulations. These variations do not correlate withfrom oats (Welch and Norvell, 1993). Plant roots caneither the metal concentration in the soil or the degreeincrease metal bioavailability by exuding protons thatof metal tolerance in the plant (Pollard et al, 2002). Inacidify the soil and mobilize the metals. The loweringorder to develop new crop species/plants having ca-of soil pH decreases the adsorption of heavy metals pabilties of metal extraction from the polluted envi-and increases their concentrations in the soil solution.ronment, traditional breeding techniques, hybridSoil microbes associated with plant roots are also generation through protoplast fusions, and productionhelpful in the phytoextraction of the heavy metals inof mutagens through radiation and chemicals are all insoils through the degradation of organic pollutants. progress. With the development of bitechnology, theThese include several strains of bacillus and pse-capabilties of hyperaccumulators may be greatlydumonos, which increase the Cd accumulation inenhanced through specific metal gene identificationBrassica juncea seedlings (Salt et al, 1995).and its transfer in certain promising species. This canScott Angle et al(2003) determined the efectof play a significant role in the extraction of heavyhigh soil moisture content on the growth and hyper- metals from the polluted soils. The use of cleaningaccumulation of Ni in three different species, in- technologies is site-specific due to spatial and cli-cluding Alyssum murale and Berkheya coddi and Znmatic variations and is not economically feasiblehyperaccumulator T. caerulescens cultivar AB300 everywhere. Therefore, cheaper technologies areand AB336. The results show that hyperaccumulators being sought for practical use. Nevertheless, the re-grew well under high soil moisture content and thecent advances in plant biotechnology have created abiomass of all the tested species was generally greaternew hope for the development of hvDeraccumulatingat higher soil moistures and inhibited at lower soilspecies中国煤化Irk is needed inmoistures. These results suggest that for successfulthis res;Y片C N M H Gadies at celularphytoremediation of metal polluted soils, a strategylevel inuiunng cunun aiu uiiun Ul different metal218Lone et el.1J Zhejiang Univ SciB 2008 93)210-2202ions by different cell organelles and membranes.716-725. (doi:10.1 104/p.103.031948]Rhizosphere studies under the control and field con-Cunningham, S.C, Berti, W.R., 2000. Phytoextraction andPhytostabilization; Technical, Economic, and Regulatoryditions are also needed to examine the antagonisticConsiderations of the Soil-Lead Issue. In: Terry, N, Ba-and synergistic effects of different metal ions in soilnuelos, G (Eds.), Phytoremediation of Contaminated Soilsolution and the polluted waters. In depth soil micro-and Water. Lewis Publishers, Boca Raton, Florida, USA,bial studies are required to identify the mi-p.359-376.cro-organisms highly associated with metal solubility Dos Santos, M.C, Lenzi, E, 2000. The use of aquatic macro-or precipitations. To date the available methods forphytes (Eichhornia crassipes) as a biological filter in thetreatment of lead contaminated effluents. Emviron.the recovery of heavy metals from plant biomass ofTechnol, 216)615-622.hyperaccumulators are still limited. Traditional dis-Ebbs, S.D, Lasat, M.M, Brady, D.J, Cormish, J, Gordon, R.,posal approaches such as burming and ashing are notKochian, L.V, 1997. Phytoextraction of cadmium anapplicable to volatile metals; therefore, investigationszinc from a contaminated soil. J. Environ. 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