| Recently, there is considerable interest in developing cost—effective,environmentally friendly and plant-based technologies for the remediation of soils andwaters contaminated with heavy metals. This technology is termed phytoremediation.The technique of phytomining involves the use of hyperaccumulator plants to growand concentrate a metal. Subsequently, the crop is harvested and the metal exteracted.Hyperaccumulator plants were originally defined by Brooks et al. as taxa containing＞1000 mg kg-1(ppm) Ni in their dry biomass. The extreme level of metal tolerance invascular plants is hyperaccumulation. Hyperaccumulators are defined as higher plantspecies whose shoots contain＞100 mg kg-1 Cd,＞1000 mg kg-1Ni, Pb and Cu, or＞10 000 mg kg-1Zn and Mn when grown in metal-rich soils. Tolerance and metalaccumulation are two essential characteristics required for phytoremediation andphytomining. Hyperaccumulator is the most ideal plant used for phytoremediation andphytomining. So the present investigation aims to identify plant species that have anability to successfully grow and reproduce in adversely impacted mining environment,and to asses their metal accumulation capacities, and then to study the mechanisms ofmetal accumulation in plants. This information would be helpful in determining ifthose plant species are suitable for ecosystem recovery on mining areas andphytoremediation of metal-contaninated soils and water.To investigate the plant species composition and their ability to accumulateheavy metals at mining area in Xiangxi area, Southern China, we sampled plants andsoil samples twice at four contaminated sites in the Xiangxi area respectively in 2003autumn and winter. The concentrations of Cd, Pb, Zn and Cu in more than 363samples of 125 plant species and 20 soil samples were analyzed in the present study.The average concentrations of Cd, Pb, Zn and Cu in the plants were 19, 81,637 and 8mg kg-1, respectively. The highest concentration of Cd in above-ground plant tissueswas found to be 287 mg kg-1in the leaves of Lobelia chinensis Lour. at the Datianwansite, followed by Solamim nigrum L., Hydddrocotyle sibthorpioides and Phytolaccaamericana with 99, 75 and 71 mg kg-1 Cd in the leaves. They might be potential Cdhyperaccumulators. At the three contaminated sites, some dominant and relative dominant species with high accumulation potential of metals, such as Kalimeris indice(L.) Sch.-Bip. and Solanum nigrum L., might be suitable for use in thephytoextraction of contaminated soils. The dominant and relative dominant specieswith low accumulation of metals and dense fibrous root systems, such as lmperatacylindrical (L.) Beauv. var. major C. E. and Miscanthus floridulus (Labill.) Warb.,might be suitable for stabilizing such metal contaminated sites.To validate Cd accumulation capacities of the three plants, Lobelia chinensisLour., Solanum nigrum L. and Phytolacca americana L., in the present study, theconcentrations of Cd in the three plant species, and the corresponding soil samplesfrom the mining and smelting contamination sites in the Xiangxi area of HunanProvince, China were further studied. The results showed that the averageconcentrations of Cd were 55.2, 36.9 and 141 mg kg-1in P. americana leaf,, S. nigrumleaf and L. chinensis shoot, respectively. The highest Cd concentration of 402 mg kg-1was found in the leaves of P. americana. Significant relationships were observedbetween the concentrations of Cd in the aerial tissues of the three plants and those ofcorresponding soils on a logarithmic scale. The average bioaccumulation factors ofCd in P americana (31 samples), S. nigrum (22 samples) and L. chinensis (10samples) were 1.19, 2.96 and 1.63, respectively. Hydroponic experiments were alsoconducted to investigate the Cd uptake ability and mechanism of the three plants insolutions. The maximum Cd concentration in the aerial tissues of P. americana, S.nigrum and L. chinensis were 1150, 1110 and 414 mg kg-1 under the laboratoryhydroponic conditions, respectively. The results suggested that P. americana, L.chinensis and S. nigrum may be considered to be Cd hyperaccumulators. Among thethree plant species under study, P. americana might be a suitable candidate for use inthe phytoremediation of metal contaminated soils due to its fast growth rate, highaboveground biomass, and elevated adaptability to a wide range of soil Cdconcentrations.The concentrations of Mn in P. americana and the corresponding soil samplesfrom the mining and smelting contamination sites in the same sites were furtherstudied. The results showed that the average concentration of Mn was 2020 mg kg-1inP. americana leaf. The highest Mn concentration of 13400 mg kg-1was found in theDatianwanⅡsite. Significant relationships were observed between the concentrationsof Mn in the aerial tissues of the plant species and those of corresponding soils on alogarithmic scale. The bioaccumulation factors of Mn in P. americana averaged 0.58, and ranged from 0.60-3.90. Hydroponic experiments were also conducted toinvestigate the Mn uptake ability and mechanism in solutions. The maximum Mnconcentration in the leaf of P americana was 11600 mg kg-1in the 10000μM Mntreatment for 15 days. The results of regeneration seedling experiment showed thatMn concentration in the leaf P. americana was also up to 11600 mg kg-1in the 5000μM Mn treatment for 90 days, besides the regeneration seedling of P americana alsocontained high concentration Mn in the leaf up to 9700 mg kg-1in the 5000μM Mntreatment for 24 days after first harvested. These results suggested that P americanamay be considered to be a manangese hyperaccumulator. As a manangese andcadmium hyperaccumulator, P. americana with high biomass may have a greatpotential for future use in phytoremediation of heavy metals compound contaminatedsoil and studies on the mechanisms of rhizosphere interaction, uptake, transport andsequestration of Cd and Mn.To understand Cd adsorption manner by P americana, hydroponics experimentwas conducted. The growth and Cd bioaccumulation were compared between two Pamericana populations respectively from Huaihua and Jishou. The growth wasremarkably inhibited and Cd contents in leaf and root were significantly increasedwith the Cd concentration increasing in the nutrition solution. No significantdifference in the growth between the Huaihua and Jishou P americana populations,but Cd content in the root of Huaihua population was markedly higher than thepopulation of Jishou. Extrinsic ABA and metabolic inhibitor NaN3 drastically reducedthe extent of Cd bioaccumulation in the leaf of two P. americana populations. ABAcan induce stomata closure and reduce the stomata transpirations. Besides, NaN3 isthe inhibitor of the H+-ATPase in the plasma membrane and vacuole membrane.Therefore, these results suggested that Cd adsorption manner by P americanainvolved passive and active adsorption two processes.We examined the relationships between the concentrations of Cd and Mn andother transition metal in leaves of forty P americana samples from the field surveys.The concentrations of Cd significantly positively related with the concentrations of Znand Cu, but remarkably negatively correlated with the concentrations of Mg. On theother hand, marked positive relationships were found between the contents of Mn andthe contents of Mg and Ni, but significantly negative relationship between thecontents of Mn and Ca. Under hydroponics condition, the effect of the interactionbetween Cd and Mn on the growth and the concentrations of Cd and Mn were studied. The results showed that Cd toxicity to P. americana was weakened and Cdconcentrations in leaf, stem and root all decreased due to addition of Mn. Theconcentrations of Mn in leaf, root and stem decreased slightly due to the competitionof between Cd and Mn. These results suggested that Cd2+, Mn2+ and Mg2+may beabsorbed and transported via common pathway.We examined the relationships between the concentrations of Cd and Mn andsulfate in leaves of forty P. americana samples from the field surveys. Theconcentrations of Cd and Mn significantly positively correlated with theconcentrations of sulfate. The result indicated that Cd and Mn detoxification involvedin chelation of Cd and Mn in the cytosol by some thiol compounds. We furtherinvestigated the effect of Cd on the amino acid, glutathione and protein andnon-protein thiol contents of leaf and root in P americana under hydroponicscondition. The data showed that no significant deference was found after Cd and Mntreatment in the contents of amino acid in leaves and roots. But the contents of proteinand non-protein thiol markedly increased and the contents of GSH and GSSG in leafand root observably decreased with the Cd concentration increasing in the nutritionsolution. GSH is the precursor for the enzymatic synthesis of phytochelatins (PC).Besides, the protein thiol in leaf of P. americna slightly decreased, non-protein andglutathione increased with the Mn concentrations in the solutions. All the resultssuggested that the phytochelatins was the potential ligands with Cd in leaf of P.americana, and the detoxification mechanisms were different between Cd and Mn.In addition, we also investigated in Donghe River in Jishou City located innorthwest Hunan Province, China and found the submerged plant community with thedominant species Potamogeton pectinatus and Potamogeton malaianus. In the study,heavy metal concentrations in the leaves of aquatic plants P. pectinatus and P.malaianus and the corresponding water and sediment samples from the Donghe Riverwere studied. Results showed that the elevated levels of metals were found in thesediments, especially Cd, Mn and Pb in comparison with the ecotox threshold valuesof soil and water developed by the Chian. Environmental Protection Agency. Thehighest concentrations of Cd, Pb, Cu, Zn and Mn in the leaves of P. pectinatus were596, 318, 62.4, 6590 and 16000 mg kg-1(DW), respectively. Significant relationshipswere observed between the concentrations of Zn, Cu and Mn in the leaves of bothaquatic plants and those in water. Laboratory experiments were also conducted toinvestigate the abilities of P. pectinatus to remove heavy metals from contaminated water. The average removal efficiencies were 84％, 90％, 91％, 77％and 84％for Cd,Cu, Pb, Zn and Mn, respectively, within the 5 h treatment. The results showed that P.pectinatus had high capability of removing heavy metals directly from water.To further estimate the application worth of P pectinatus in the water pollutioncontrol and treatment, biosorption of metals by P. pectinatus in the laboratoryexperiment was carried out to investigate Cd and Pb adsorption properties and theeffects of water temperature, pH value and salinity on Cd and Pb adsorption of Ppectinatus. Results were compared with other materials. Data obtained from the intialadsorption studies indicated that P. pectinatus was capable of removing Cd and Pbfrom solution. The metal biosorption was fast and equilibrium was attained within 20min. Overall adsorption processes of Cd and Pb with the increasing time were all bestdescribed by pseudo second-order kinetics. Data obtained from further batchadsorption with the metal concentration increasing studies conformed well to theLangmuir Model. Maximum adsorption capacities of Cd and Pb onto P. pectinatuswere 32368, 24776 mg kg-1. The metal concentrations in the leaf of P. pectinatus werehighest at the 20C compared to 5, 10 and 30℃. Other condition parameter notchanging, the concentration of Cd in leaf of P. pectinatus decreased but theconcentration of Pb increased with the pH value from 3 to 7 and salinity from 0.5 g/Lto 10 g/L increasing. P pectinatus more often has a high biomass and tolerates verywide ranges of temperature, pH value and salinity. Therefore, P. pectinatus can beused as a biological indicator while determining aquatic environmental pressures andused for phytoremediation of multiple metals contaminated water.
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