Enhanced Accumulation of Pb inIndian Mustard by Soil-AppliedChelating Agents

Enhanced Accumulation of Pb in Indian Mustard by Soil-Applied Chelating Agents

M I C H A E L J .B L A Y L O C K ,*,?

D A V I D

E .S A L T ,?S L A V I K D U S H E N K O V ,?O L G A Z A K H A R O V A ,?

C H R I S T O P H E R G U S S M A N ,?Y O R A M K A P U L N I K ,?

B U R T D .E N S L E Y ,?A N D I L Y A R A S K I N ?Phytotech,Inc.,One Deer Park Drive,Suite I,

Monmouth Junction,New Jersey 08852,and AgBiotech Center,Rutgers University,Cook College,P.O.Box 231,New Brunswick,New Jersey 08903

Phytoremediation is emerging as a potential cost-effective solution for the remediation of contaminated soils.Because contaminants such as lead (Pb)have limited bioavailability in the soil,a means of solubilizing the Pb in the soil and facilitating its transport to the shoots of plants is vital to the success of phytoremediation.Indian mustard (Brassica juncea )was used to demonstrate the capability of plants to accumulate high tissue concentrations of Pb when grown in Pb-contaminated soil.Concentrations of 1.5%Pb in the shoots of B.juncea were obtained from soils containing 600mg of Pb/kg amended with synthetic chelates such as EDTA.The accumulation of Pb in the tissue corresponded to the concentration of Pb in the soil and the concentration of EDTA added to the soil.The accumulation of Cd,Cu,Ni,and Zn from contaminated soil amended with EDTA and other synthetic chelators was also demon-strated.The research indicates that the accumulation of metal in the shoots of B.juncea can be enhanced through the application of synthetic chelates to the soil,facilitating high biomass accumulation as well as metal uptake.

Introduction

The use of plants to remove toxic metals from soils (phy-toremediation)is emerging as a potential strategy for cost-effective and environmentally sound remediation of con-taminated soils (1-4).Certain plants,known as metal hyperaccumulators,have been discovered that contain unusually high concentrations of heavy metals in their tissue.Hyperaccumulators of Ni and Zn,for example,contain as much as 5%of these metals on a dry weight basis (5,1).Phytoremediation as a soil cleanup technology seeks to exploit the ability of metal-accumulating plants to extract metals from the soil with their roots and to concentrate these metals in above-ground plant parts.The metal-rich plant material can be safely harvested and removed from the site without extensive excavation,disposal costs,and loss of topsoil associated with traditional remediation practices.

The success of phytoremediation is dependent on several factors.Plants must produce sufficient biomass while ac-cumulating high concentrations of metal.The metal-ac-cumulating plants also need to be responsive to agricultural practices to allow repeated planting and harvesting of the

metal-rich tissues.In addition,these plants should prefer-entially accumulate environmentally important toxic metals (e.g.,

Pb,Cd,Cr,etc.).Known metal accumulators do not meet these criteria.The ability to cultivate a high biomass plant with a high content of toxic metals on a contaminated soil will be a determining factor in the success of phytore-mediation.Therefore,enhancing metal accumulation in existing high yielding crop plants without diminishing their yield is the most feasible strategy in the development of phytoremediation.

In addition,the availability of metal in the soil for plant uptake is another limitation for successful phytoremediation.For example,Pb,one of the most important environmental pollutants,has limited solubility in soils and availability for plant uptake due to complexation with organic matter,sorption on oxides and clays,and precipitation as carbonates,hydroxides and phosphates (6).For soils with pH between 5.5and 7.5,where Pb solubility is controlled by phosphate or carbonate precipitates,the maximum activity of Pb 2+in the soil solution is approximately 10-8.5M,or about 0.6ppb (7).The Pb 2+activity in the soil solution should remain constant regardless of the magnitude of the soil Pb concen-tration because of the equilibrium between the solution and the solid phase in the soil.Extremes in soil pH above 7.5or below 5.5will either decrease or increase the solubility accordingly.In most soils capable of supporting plant growth,however,the soluble Pb levels will remain very low and will not allow substantial uptake by the plant even if it has the genetic capacity to accumulate this metal.Vegetation growing in heavily contaminated areas often has less than 50μg/g Pb in the shoots (8).In addition,many plants retain Pb in their roots via sorption and precipitation with only minimal transport to the above-ground harvestable portions (9,4).Nevertheless,some Pb hyperaccumulators have been reported in isolated instances in extremely contaminated soils.The most frequently cited example reports Thlaspi rotundifolium (L.)Gaud.-Beaup ,with shoot Pb concentrations of 8500μg/g dry weight (10),but very few other examples are found.Cultivars of Brassica juncea (L.)Czern.(Indian mustard,a high biomass forage and oil crop)have also demonstrated the ability to accumulate as high as 1.5%Pb in shoot tissues when grown in nutrient solution with high concentrations of soluble Pb (11).At lower Pb concentrations in solution,the shoot tissue accumulations were substantially less,although root concentrations were very high.In spite of the significant capacity of B.juncea plants to concentrate Pb and translocate it to the shoots in solution culture,little uptake into the shoots was observed in B.juncea plants growing in soils where Pb bioavailability is limited (11).In this paper,we report on the enhanced uptake of Pb accumulation in soil-grown B.juncea plants with synthetic chelates.The magnitude of this enhancement may be sufficient to make phytoextraction of Pb-contaminated soils a viable environmental technology.

Materials and Methods

Growth chamber studies were conducted to evaluate the effectiveness of soil-applied chelating agents at increasing metal uptake in a known metal accumulator,Indian mustard (B.juncea ,cv.426308)(11).A Sassafras Ap silt loam soil was collected from the Rutgers University Horticultural Farm and amended with CdCO 3,CuCO 3,2NiCO 3?3Ni(OH)2?4H 2O,Pb-CO 3,and/or ZnCO 3to achieve desired concentrations.The carbonate forms were selected in order to provide the metals in a form of limited solubility dependent on the reaction of the metals with the soil.The soil was treated with lime to pH 7.3and fertilized with urea (150mg of N/kg),potassium chloride (83mg of K/kg),and gypsum (70mg of CaSO 4/kg).

*Corresponding author fax:908-438-1209;e-mail:soilrx@https://www.sodocs.net/doc/9816277013.html,.?Phytotech,Inc.?AgBiotech Center.

Environ.Sci.Technol.1997,31,860-865

860

9ENVIRONMENTAL SCIENCE &TECHNOLOGY /VOL.31,NO.3,1997S0013-936X(96)00552-4CCC:$14.00?1997American Chemical Society

The effect of pH on metal uptake was also studied by adjusting the quantity of lime added to the soil to achieve soil pH values of5.0,6.0,6.5,7.0,and7.5.The amended soils were allowed to equilibrate for a period of2weeks in the greenhouse undergoing three cycles of saturation with water and air-drying,before being remixed and planted.The soil was then placed in8.75cm diameter round pots(350g of soil/pot)and planted with B.juncea seeds.Phosphate fertilizer was added as a spot placement of triple super phosphate1cm below the seeds at planting at the rate of44mg of P/kg.After seedling emergence,the pots were thinned to two plants per pot.

The plants were grown for3weeks in a growth chamber using a16-h photoperiod and weekly fertilization treatments of16and7mg/kg N(urea)and K(KCl),respectively. Potassium salts of the synthetic chelators CDTA(trans-1,2-cyclohexylenedinitrilotetraacetic acid),DTPA(diethylene-trinitrilopentaacetic acid),EDTA(ethylenedinitrilotetraacetic acid),EGTA(ethylenebis[oxyethylenetrinitrilo]tetraacetic acid), and citric or malic acid were applied to the soil surface as solutions3weeks after seedling emergence using four replications of each treatment.The pots were placed in individual trays to prevent loss of amendments from leaching. Following the amendment applications,the soil was irrigated to field capacity on a daily basis.The plants were grown for 3weeks after seedling emergence(at the onset of flowering) before applying chelates to the soil.The plants were harvested 1week after the amendment treatment by cutting the stem 1cm above the soil surface.The plant tissue was dried at70°C and then wet-ashed using nitric and perchloric acids.The resulting solution was analyzed for metal content by induc-tively coupled plasma spectrometry(ICP)(Fisons Accuris, Fisons Instruments,Inc.,Beverly,MA).Soil samples were collected from the pots and analyzed for water-soluble metals by equilibrating2.5g of soil with25mL of0.01M KNO3for 2h.The suspensions were centrifuged,and the supernatant solution was analyzed for soluble metals by ICP.Certified National Institute of Standards and Technology plant and soil standards were carried through the digestions,extractions, and analysis as part of the QA/QC protocol.Reagent blanks and spikes were used where appropriate to ensure accuracy and precision in the analysis.

A further investigation into this observed phenomenon of chelate-enhanced uptake was conducted using a hydroponic

system in the growth chamber. B.juncea seedlings were grown for3weeks in a nutrient solution containing28.7mg/L NH4H2PO4,0.71mg/L H3BO3,164.1mg/L Ca(NO3)2,0.02mg/L CuSO4,2.66mg/L ferric tartrate,60.19mg/L MgSO4,0.45mg/L MnCl2,0.004mg/L MoO3,151.7mg/L KNO3,and0.055mg/L ZnSO4.The3-week-old plants were transferred to solutions of distilled water adjusted to either pH3.5or pH5.5using HNO3containing PbNO3and/or K2EDTA at equimolar(0.2 mM)rates.The plants were harvested1week after the imposition of the Pb and EDTA treatments.The roots and shoots were rinsed with DI water and analyzed for metal content.

A field study was also conducted at a former cable manufacturing site in Bayonne,NJ,with Pb-contaminated soil(pH8.3,1200mg of Pb/kg).The soil was fertilized with 150,44,83,and70mg/kg N,P,K,and CaSO4,respectively, before rototilling to15cm depth.The surface soil(0-15cm) was then excavated and placed in lysimeters(48-qt ice chests). Approximately65kg of soil was placed in each lysimeter and placed on the surface of the soil in the field. B.juncea seeds were planted and grown for3weeks before treatment application.Applications of EDTA and acetic acid were given as1-L solutions applied to equal5.0mmol/kg EDTA and acetic acid.A light irrigation(0.25cm)was provided following the application of the amendments.The plants were harvested1week after the amendment application.Root and shoot tissue was collected and washed with DI water to remove soil deposition before analysis.Results

Metal concentration in the soil is expected to play a large role in determining the metal uptake by the plant and the metal content of the shoot material.In the case of Pb,however,a high total Pb concentration in the soil does not necessarily result in high Pb concentrations in the shoots due to its insolubility.In soils with Pb added as lead carbonate at the rates of600,900,1200,and1800mg/kg,only very low levels of Pb(<100mg/kg)were accumulated in the shoots of B. juncea(Figure1).The accumulation was dependent on the soil concentration and increased from45to100mg/kg as the soil Pb increased from1200to1800mg/kg.

Through the addition of synthetic chelators to metal-contaminated soil,accumulation of Pb in B.juncea was enhanced.The initial study was conducted to evaluate the potential of soil-applied chelators and amendments at four different concentrations to enhance the uptake of Pb and other metals from contaminated soils.Subsequent studies were conducted using lower concentrations of EDTA to optimize chelate additions with respect to potential field applications.The application of EDTA,DTPA,CDTA,EGTA, and citric acid to the soil solubilized Pb in the soil and also increased Pb uptake and translocation to the shoots(Figure 2).The concentration of Pb in the shoots increased with the concentration of chelator applied to the soil.A dramatic increase in shoot Pb concentrations occurred between the1 and5mmol/kg EDTA treatments.Both EDTA and DTPA FIGURE1.Shoot Pb concentrations of B.juncea grown in Sassafras Ap soil(limed to pH7.3)with Pb added as lead

carbonate.

FIGURE2.Shoot Pb concentrations of B.juncea(A)and water-soluble Pb(B)in Sassafras Ap soil(limed to pH7.3)containing600 mg of Pb/kg as lead carbonate as a result of soil-applied chelating agents.

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solubilized approximately470mg of Pb/kg(2.2mmol of Pb/ kg)in the soil at the10mmol/kg rate;however,the EDTA application produced plants with1.6%Pb in the shoots as compared to only1.0%Pb in the shoots of DTPA-treated plants(Figure2A).The Pb solubilizing capacity of CDTA in the soil was similar to DTPA at the5mmol/kg rate and resulted in greater Pb uptake as compared to DTPA in the plant tissue at the5and10mmol/kg rates.EGTA was much less effective at solubilizing Pb in the soil and also at increasing Pb uptake by the plants.Citric acid produced only a small increase in Pb uptake by the plants.In the absence of an applied chelating agent,Pb uptake in the shoots was minimal.

Plant dry matter yield was also significantly affected by the application of the treatments.Plants grown in untreated or the0.1mmol/kg treated soil produced nearly twice the

biomass of the plants receiving the10mmol/kg chelate application(Table1).However,the magnitude of the metal accumulation at the10mmol/kg treatment was several thousand to over ten thousand times greater than the control treatments.The untreated plants nearly doubled their biomass during the time period between the treatments and harvest.

The uptake of Cd was also enhanced through the addition of chelating agents to the soil(Figure3).The addition of EGTA to the soil increased Cd shoot concentrations to approximately2800mg/kg in the10mmol/kg treatment as compared to only220mg/kg in the control treatments.CDTA, DTPA,and EDTA were less effective than EGTA but still produced Cd accumulations of approximately1000mg/kg at the5mmol/kg treatment and1500mg/kg at the10mmol/kg treatment of CDTA and EDTA.Increasing the DTPA rate from 5to10mmol/kg did not increase Cd uptake into the shoots. Malic acid amendments did not increase Cd uptake above the control treatment.

An additional study using a soil containing multiple metal contaminants showed that the ability of a soil-applied chelator such as EDTA to increase metal uptake is not limited to Pb and Cd.A2.5mmol/kg EDTA treatment to soil containing Cd,Cu,Ni,Pb,and Zn substantially increased the uptake of those metals to the shoots of B.juncea(Figure4).Shoot concentrations of Cu,Pb,and Zn were at or above1000mg/ kg,with Pb showing the highest concentration at almost3600 mg/kg.Converting the shoot metal concentrations to a molar basis,however,showed Cu,Pb,and Zn with similar con-centrations of15.3,17.4,and16.3μmol/g,respectively.Shoot content of Cd and Ni were significantly less with respective concentrations of4.3and3.5μmol/g.

Because Pb is conceivably the most important metallic soil contaminant,we concentrated our subsequent research efforts on the phytoremediation of Pb and on defining the role of synthetic chelates in this process.As shown in Figure 2,increasing the solubility of Pb through the addition of a chelator can produce much higher concentrations of Pb in the plant than the increase in the total Pb concentration in the soil(Figure1).This effect can be observed even at low total soil Pb concentrations.Soil containing total Pb con-centrations added as lead carbonate(150,300,450,and600 mg/kg)and amended with0.5mmol/kg EDTA produced plants with as much as5000mg of Pb/kg at the600mg/kg level,with a sharp increase in Pb content of the shoots as the soil Pb increased from150to300mg/kg(Figure5).Other mechanisms of increasing metal solubility,such as soil acidification,can also increase metal uptake somewhat,but

TABLE1.Dry Matter Yield of B.juncea Grown in Soil Treated with Chelating Agents a

concentration(mmol/kg)

chelating

agent0(g/pot DW)0.1(g/pot DW) 1.0(g/pot DW) 5.0(g/pot DW)10(g/pot DW) CDTA 2.05(0.16 2.07(0.22 1.95(0.380.99(0.09 1.13(0.04 DTPA 2.05(0.16 2.53(0.19 1.05(0.09 1.11(0.15 1.31(0.03 EDTA 2.05(0.16 2.37(0.05 1.38(0.170.93(0.08 1.02(0.13 EGTA 2.05(0.16 2.33(0.22 1.88(0.250.88(0.09 1.17(0.15 citric acid 2.05(0.16 2.08(0.13 1.54(0.120.95(0.100.89(0.18

a Plants were grown for3weeks before treatment applications.Values are means(1

SE.

FIGURE3.Effect of soil-applied chelating agents on shoot Cd concentrations in B.juncea grown in a Sassafras Ap soil amended with cadmium carbonate(100mg of Cd/kg)and limed to pH7.3.FIGURE4.Enhancement of shoot metal concentrations in B.juncea grown in Sassafras Ap soil containing added Cd,Cu,Ni,Pb,and Zn through soil-applied EDTA(2.5mmol/kg).Soil metal concentrations are given(in mg/kg)in parentheses.

FIGURE5.Accumulation of Pb in shoots of B.juncea grown in a Sassafras Ap soil(limed to pH7.3)with Pb added as lead carbonate and amended with0.5mmol/kg EDTA.

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concentrations greater than1000mg of Pb/kg in the shoot material were only observed through the addition of a chelator. Figure6shows the effect of soil pH in combination with EDTA amendments on Pb accumulation in B.juncea.Lead uptake increased slightly to445mg/kg as the pH decreased to5.0 in the absence of EDTA,but increased dramatically to almost 4000mg/kg when EDTA was added to the pH5soil.The uptake of Pb from hydroponic solutions was also strongly related to solution pH in the presence of EDTA in the solution (Figure7).At pH5.5without EDTA addition,only trace levels of Pb were found in the shoot tissue.The addition of0.2mM EDTA increased the shoot tissue Pb to approximately0.8mg/ g.At pH3.5shoot Pb uptake was substantially higher.In the absence of EDTA,the shoots accumulated6mg of Pb/g.EDTA increased the Pb uptake in this treatment to17mg/g.Root Pb concentrations decreased at the lower pH and with the addition of EDTA.Roots in the control treatment at pH5.5 contained225mg of Pb/g.Lowering the solution pH to3.5 decreased the root Pb to110mg/g,and the addition of EDTA decreased the root Pb to65and20mg/g at pH5.5and pH

3.5,respectively.

A field study was conducted to assess the effectiveness and feasibility of using a soil-applied chelator to facilitate metal removal as part of phytoremediation activities.Ad-dition of EDTA to soils in the field lysimeter studies confirmed the laboratory studies showing the effectiveness of a soil-applied chelator at increasing Pb uptake.The addition of EDTA increased Pb uptake in the shoots from28mg/kg in the control treatments to785mg/kg.Acetic acid was applied in conjunction with EDTA to determine if a reduction in soil pH would also enhance Pb uptake in the field.Application of the acetic acid with EDTA resulted in a further increase in Pb accumulation to1475mg/kg Pb in the shoots(Table2) even though the soil pH decreased only slightly from8.3to 7.8.The accumulation of Pb in the shoots was also greater than root Pb concentrations with the addition of the EDTA and acetic acid,indicating decreased binding of Pb by the root tissue as a result of the EDTA application. Discussion

Successful phytoremediation is a combination of several steps and processes.Because total metal removal is a function of the metal concentration in the shoots and the total harvestable biomass,the first step in phytoremediation is to produce high rates of plant biomass at the contaminated site.This can be accomplished through intensive cultivation,which facilitates rapid plant establishment and growth. B.juncea and other oil-seed Brassicas can produce18t/ha of biomass (12)in approximately2.5months of cultivation.Growth rates in excess of200kg ha of dry matter-1day-1are normally achieved by these plants under agricultural conditions(13). Biomass production will likely be lower for the plants cultivated on metal-contaminated sites,which are not ideal for agriculture.

The second step vital to the success of phytoremediation is the induction of Pb accumulation in the shoots.Even B. juncea,which has the genetic potential to accumulate Pb and Cd(11,14),cannot efficiently remove these metals from the soil matrix unless they are solubilized.Therefore, successful phytoremediation must involve mobilization of toxic metals into the soil solution that is in direct contact with plant roots.We have shown that this can be achieved with the use of synthetic chelates(Figure2).

In soil,the applied chelate acts first to complex the soluble metals in the soil solution.As the free metal activity decreases, dissolution of bound metal ions begins to compensate for the shift in equilibrium.This process continues until the chelate is saturated,the supply of metal from solid phases is exhausted,and/or equilibrium is achieved and the insolubility of the solid phase restricts the activity of the free metal.In the case of EDTA,the formation of Pb-EDTA is expected to be the dominant metal-EDTA complex in most soils between pH5.2and pH7.7,providing that the total Pb concentration and the solubility of the specific Pb solid phase(s)is not limiting(15).As a result,if EDTA is added in sufficient amounts,nearly all of the soluble Pb will be complexed as Pb-EDTA with only very low activities of Pb2+.For example, adding10mmol of EDTA/kg of soil(Figure2),which exceeds the quantity of Pb in the soil(approximately3mmol/kg), resulted in extraction of2.2mmol/kg of soluble Pb(73%of the total soil Pb).The remaining Pb was not available for complexation with the EDTA,presumably due to its speciation as highly stable complexes.The EDTA addition rates of5,1, and0.1mmol/kg resulted in a lower proportion of the total soil Pb being solubilized(64,22,and3%,respectively).The amount of Pb in the shoots was always directly proportional to the amount of EDTA added to the soil(Figure2).The amount of soluble Pb in the soil appears to be a key factor

FIGURE6.Shoot Pb concentrations of B.juncea grown in Sassafras Ap soil limed to different pH values with600mg of Pb/kg added as lead carbonate and amended with2.5mmol/kg EDTA.

FIGURE7.EDTA effects on Pb distribution between roots and shoots of B.juncea grown in solution culture at pH3.5and pH5.5.TABLE2.Pb Uptake in Root and Shoots of B.juncea in Field Lysimeter Studies

treatment shoot Pb(mg/kg)root Pb(mg/kg) EDTA785152

EDTA+acetic acid1471466 control28101

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to the enhancement of Pb uptake.When total soil Pb is limiting,the soluble Pb pool may also be limiting,even in the presence of a chelator such as EDTA,with respect to the uptake of Pb by the root and translocation to the shoots(Figure 5).

The greater ability of EDTA to enhance Pb uptake above that of other chelators also appears to be related to the binding capacity of EDTA for Pb relative to the other chelators.Norvell (16)ranked chelating agents for Pb2+at pH7,under slightly reduced conditions,in the order of EDTA≈DTPA>CDTA >EGTA.In our studies,EGTA was the most effective chelator for enhancing Cd accumulation in B.juncea.This observation corresponds to predictions that,above pH5.7,Cd-EGTA is expected to be the dominant metal-EGTA species(15)as compared to the less favorable Cd binding affinities of the other chelates tested.The extent of competition from other cations in the soil solution will vary between soil systems and therefore the effectiveness of the chelate,but it appears that the selection of chelators with the highest binding affinities based on existing thermodynamic data provides a good estimate of potential successful amendments for enhancing metal uptake.

Soil pH is an important parameter in determining the effectiveness of applied EDTA in enhancing metal uptake (Figure6).Corresponding results showing an increase in EDTA-extractable Pb as a function of decreasing pH have been noted in soil washing studies(17).The addition of soil-acidifying agents,such as acetic acid,had an additive effect on EDTA-mediated shoot metal uptake.Field lysimeter studies showed that amending an alkaline soil with acetic acid in the presence of EDTA almost doubled Pb accumulation in the shoots(Table2).More detailed hydroponic experi-ments(Figure7)confirmed that acidification stimulates Pb movement from roots to shoots.Lowering the pH of a hydroponic solution from5.5to3.5in the presence of EDTA dramatically increased shoot Pb accumulation and decreased Pb retention in the roots.

Lead retention in the roots is based on Pb binding to ion exchangeable sites on the cell wall and extracellular pre-cipitation,mainly in the form of lead carbonates deposited in the cell wall(9).EDTA,in combination with low pH, effectively prevents cell wall retention of Pb,thereby making it available for translocation to the shoot.This effect is observed in both the soil(Figure6)and hydroponic systems (Figure7).Adding14C-labeled EDTA-Pb to the hydroponic medium resulted in the accumulation of14C-labeled com-pounds in the shoots(data not shown).Since the HPLC retention time of the14C-labeled compound was identical to an authentic EDTA standard,we assumed that shoots of experimental plants contained[14C]EDTA.The amount of EDTA detected in the shoots was sufficient to chelate most of the Pb accumulated in the tissue.Therefore,it is likely that Pb enters the plant and is transported to the shoot as an EDTA complex.

The enhanced uptake of chelated Pb relative to ionic Pb contradicts the common understanding of chelate behavior in chelate-buffered nutrient solutions.It is generally believed that the chelated forms of metals are less available for uptake as compared to the ionic forms(18,19).Some evidence suggests that chelated metals may be absorbed by the root. Root-exuded chelating agents(phytosiderophores)complexed with Fe are readily and preferentially absorbed by plants(20). The transport of synthetic chelate-metal complexes across the membrane is not widely accepted.Bell et al.(21)reported that barley leaf concentrations of Cu,Zn,and Mn were generally greater when total Cu,Zn,and Mn concentrations were greater despite maintaining constant activities of free Cu2+,Zn2+,and Mn2+in solution through the use of chelators. The uptake of chelated metal was related to possible breaks in the root endodermis and Casparian strip and/or exchange between the chelated metal and the free metal in the rhizosphere and not uptake of the chelate metal complex (21).One important consideration is that much of the work conducted with chelates in nutrient solutions involved concentrations much lower than were applied in this study. The magnitude of the metal accumulation observed in this study(>1%shoot Pb concentrations)is also much greater than the metal accumulations observed in studies with much lower chelate concentrations,indicating that perhaps a different mechanism is involved.

Crop plants pump large amounts of water from the soils. The transpiration ratio of crop plants ranges between850 and300kg of water used/kg of dry matter produced(22).We already reported that blocking transpiration blocks Cd uptake by B.juncea plants(14).We have also observed that placing soil-grown B.juncea plants close to a fan increased EDTA-mediated Pb uptake by30%,while placing a plastic bag over a plant will reduce it by35%(data not shown).These treatments should increase and reduce transpiration,re-spectively.Thus,we propose that transpiration is a major force that drives Pb accumulation in the shoots.Mass flow of water from the soil and into the plant delivers solubilized Pb to the shoots,where water evaporates and Pb remains. We also suggest that most of the mobile Pb in the plant is chelated by EDTA,which prevents its precipitation in the roots and vascular system.In addition,EDTA may directly facilitate Pb movement through the root cell membranes and extracellular spaces.This intriguing possibility requires further study.

Our data demonstrate that accumulation of five toxic metals by B.juncea plants can be enhanced with synthetic chelates(Figure4).EDTA-enhanced accumulation of Pb was further enhanced by lowering the pH of the substrate.These amendments were applied several days before harvest when plants were already established.Applying chelates and other soil amendments in the field will certainly require further study and optimization to obtain the maximum metal uptake and removal.However,laboratory results show that plants can be treated to accumulate up to1.5%of their dry weight in Pb from soils containing600ppm Pb.Assuming6t/ha biomass,6weeks of cultivation,and three crops per season, B.juncea plants can remove180kg of Pb/ha in a growing season.Ashing of the harvested biomass would further concentrate the extracted Pb by an additional10-fold (assuming plant dry matter is10%ash)resulting in a250-fold concentration of the extracted Pb over the soil level.The duration of this extraction efficiency as the total soil Pb concentration decreases is unclear but will most probably be a function of the magnitude of the EDTA-extractable Pb pool.

We have estimated that the addition of chelators to the soil will result in an increased cost of$7.50/t of treated soil, which would not severely impact the overall cost of phy-toremediation as an alternative.Additional concerns regard-ing the potential mobility of chelated metals are being addressed through modeling of hydrological dynamics of the soil profile and monitoring of metal migration.However, the integration of intensive cropping practices and develop-ment of extensive root systems along with careful manage-ment of irrigation should result in a net water loss from the soil during the growing season(23).Phytoremediation offers an attractive alternative for remediation of metal-contami-nated soils.The results presented in this paper suggest that, with integration of this technology with the proper agro-nomical and engineering skills,phytoremediation of Pb-contaminated soils will soon become a competitive reme-diation tool.

Acknowledgments

This work was supported in part through a Small Business Innovation Research Grant(DE-FG02-95ER82053)awarded to Phytotech,Inc.by the U.S.Department of Energy.The

8649ENVIRONMENTAL SCIENCE&TECHNOLOGY/VOL.31,NO.3,1997

authors gratefully acknowledge the valuable technical as-sistance of Tracey Ledder.

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Received for review June25,1996.Revised manuscript re-ceived October22,1996.Accepted October28,1996.X

ES960552A

X Abstract published in Advance ACS Abstracts,January15,1997.

VOL.31,NO.3,1997/ENVIRONMENTAL SCIENCE&TECHNOLOGY9865

锆石SHRIMP定年原理和方法

锆石SHRIMP定年原理和方法 锆石分选采用常规重力分选和显微镜下手工挑选的方法进行，具体是将岩石样品粉碎成60目左右，通过淘洗和使用重液等物理方法分离锆石，然后在双目镜下精选、剔除杂质。然后将其与标准锆石（TEM，417 Ma）一起粘贴，制成环氧树脂样品靶，打磨抛光并使其露出中心部位，进行反射光透射光和阴极发光显微照相，阴极发光图像用以确定单颗粒锆石晶体的形态、结构特征以及标定测年点。最后，用超声波在去离子水中清洗约10分钟后，镀金膜并上机测年。在分析中，采用跳峰扫描记录Zr2O+、204Pb+、背景值、206Pb+、207Pb+、208Pb+、U+、ThO+和UO+等9个离子束峰值，每5次扫描记录一次平均值：一次离子为4.5nA，10kV的O-2，离子束直径约25~30um：质量分辨率约5400（1％峰高）：应用SL13（572Ma，U=238×10-6）标定样品的U、Th及Pb含量，用TEM（417Ma）标定样品的年龄。为了尽量降低锆石表面普通Pb和镀金过程中的污染，测定过程中先将束斑在120um 范围内扫描 5 分钟，具体测试条件及流程见Compston等（1992）、Williams（1998）、宋彪（2002）等。数据处理采用SQUID1.0和ISOPLOT 程序，普通Pb一般根据实测204Pb及Cumming等(1975)模式铅成分校正：单个测试数据误差和206Pb/238U 年龄的加权平均值误差均为95％置信度误差(1σ)，对年轻的岩浆锆石，采用206Pb/238U 年龄；对较老的继承锆石，采用207Pb/206Pb 年龄。 206Pb/238U 年龄的加权平均值,即谐和年龄，用谐和图表示，谐和图是锆石同位素地质年代学最常用的图解，它是以207Pb／235U 和206Pb/238U 为坐标，t为参 数的超越方程(207Pb/235U=t e*λ－1和206Pb/238U =t eλ－1，其中λ*和λ分别是235U 和238U的衰变常数)的轨迹――谐和线。在谐和线上的点具有一致年龄，即206Pb/238U、207Pb/235 U、207Pb/206Pb三个表面年龄相等，表明被测对象自形成以来，同位素母体子体一直处于封闭体系中。 本次研究锆石分选工作在河北地勘局廊坊实验室进行，锆石样品在北京离子探针中心完成制靶，阴极发光显微照相在中国地质科学院矿产地质研究所电子探针室完成，最后分批在北京离子探针中心和澳大利亚Curtin University of Technology 离子探针中心完成测试，测试原始数据由北京离子探针中心处理。标样为来自澳大利亚国立大学(ANU)的SL13和TEM。SL13（宝石级锆石，U含量为238μg/g，年龄为572 Ma）用于样品U含量标定。TEM（母岩为澳大利亚堪培拉附近一闪长岩体，年龄为417 Ma）用于样品年龄标定，采用公式为206Pb+/238U+=A （254UO+/238U+）。

长江三角洲晚第四纪地层锆石U-Pb年龄谱时序变化研究

长江三角洲晚第四纪地层锆石U-Pb年龄谱时序变化研 究

长江三角洲晚第四纪地层锆石 U-Pb 年龄谱时序变化研究#

10 15 20 25 30 35 40

摘要：河流系统受构造运动、气候变化与海平面升降等的影响，通过物源分析可以反演上述 过程的演化历史。选择长江三角洲现代表层沉积和钻孔（YQ03 与 HM03）中全新世与末次 盛冰期沉积等三个层位，进行重矿物组合特征、锆石 U-Pb 年龄谱系研究，发现全新世与末 次盛冰期沉积物源有较大的差别，前者的重矿物成熟度 ZTR 指数较小，来自长江支流涪江 与湘江的特征矿物榍石和锆石含量较低，见较多的古元古代锆石，但白垩纪、新元古代锆石 含量较少。可见，冰期－间冰期旋回长江沉积物源区发生了较大变化，冰期沉积物更多源自 中、下流区域，随着间冰期气候回暖，尤其是中全新受强烈的夏季风降雨影响，径流增强可 携带更多的上游的物质入海． 关键词：沉积地球化学；物源分析；锆石 U-Pb 年龄；气候变化 中图分类号：P737 Changes in zircon U-Pb Age in the late Quaternary strata of Yangtze River Delta Wang Yangyang, Fan Daidu School of Ocean and Earth Science, Tongji University, Shanghai

200092 Abstract: Fluvial systems are particularly sensivite to climatic and tectonic change, and these process could be reversed by provenance analysis. On the basis of heavy mineral and zircon U -Pb age of modern surface sediments from Yantze River delta, Holocene and late Quaternary sediments from YQ03 hole and HM03 hole, the research shows that there is a big difference between Holocene sediments and late Quaternry sediments: there are fewer titanites and zircons which are character mineral of Peijiang River and Xiangjiang River, less ZTR value, less Cretaceous and Neoproterozoic zircons but more Paleoproterozoic zircons from estuary sediments in Holocene. It suggests that the estuary sediments provenance changed during Holocene, which is related to more sediments influx from upper reaches of Yangtze River due to strong summer monsoon. Key words: Sedimentary Geochemistry; provenance analyses; Zircon U-Pb age; climate change

北大别大山坑二长花岗片麻岩的地球化学特征与锆石U_Pb年代学

第18卷第2期2003年4月 地球科学进展 ADVANCE IN EARTH SCIENCES Vol.18No.2 A pr.,2003 文章编号:1001-8166(2003)02-0192-06 北大别大山坑二长花岗片麻岩的地球化学 特征与锆石U-Pb年代学* 薛怀民1,2,董树文3,刘晓春1 (1.中国地质调查局南京地质矿产研究所,江苏南京210016; 2.中国地质调查局地质力学研究所,北京100081; 3.中国地质科学院,北京100037) 摘要:北大别东部大山坑片麻岩主要由钾长石、斜长石和石英组成,少量角闪石、黑云母和褐帘石,成分为二长花岗质。岩石以富碱(Na2O+K2O)尤其是K2O、贫Al2O3为特征,地球化学性质上表现为富集K、Rb、Th、La、Ce等大离子亲石元素而亏损Nb、Ta、U等高场强元素及Sr元素,高的Ga含量、强的负Eu异常(D Eu=0.37)和相对较强的轻重稀土分馏程度((La/Yb)N=16.75)。岩石地球化学的总体特征与南大别水吼地区的A型花岗片麻岩类似,意味着其原岩与南大别的A型花岗片麻岩一样,可能也是在拉张状态下形成的一套偏碱性的花岗岩。该二长花岗片麻岩的锆石U-Pb年龄为229.2?5.5M a,也与南大别超高压变质的年龄相似,指示北大别正片麻岩印支期可能也经历过超高压变质作用。南、北大别造山带可能具有相同的形成与演化历史,现南、北大别变质带之间的差异可能更多的是后造山历史不同,尤其是燕山期花岗岩侵入对两个带影响的不同造成的。 关键词:锆石U-Pb年龄;二长花岗质片麻岩;北大别 中图分类号:P597文献标识码:A 大别造山带因性质不同分为南大别超高压变质带和北大别变质杂岩带[1],两个带之间的巨大差异曾导致部分学者将其间的五河)水吼断裂作为华北与扬子两大板块印支期碰撞造山的缝合线[2,3]。近年来在北大别地区不仅也发现有少量的榴辉岩出露[4,5],与南大别超高压变质时代(220~245 M a[6~11])类似的年龄资料也开始见有报道[12,13],使得南、北大别之间的差异及其分隔断裂的含义有必要重新加以认识。 1区域地质与岩石学 正片麻岩是北大别变质杂岩中最基本的岩石单元,其中由奥长花岗质)英云闪长质)花岗闪长质片麻岩所构成的灰色片麻岩占到整个变质杂岩体积的80%左右[14],它们主要构成2个明显的正片麻岩穹隆[8,9],此外还有少量的二长花岗片麻岩出露。与其伴生的岩石有众多的超镁铁)镁铁质岩块(透镜体)和变质的表壳岩、混合岩及少量角闪岩、麻粒岩和榴辉岩。整个北大别地区燕山期花岗岩极为发育,占到整个基岩出露面积的50%以上,其中相当部分变质杂岩受到了混合岩化的强烈影响。 大山坑二长花岗片麻岩位于岳西东南的石竹河附近,出露面积约20km2。该片麻岩的东侧有辉长岩侵入,西侧和西南侧则被燕山期花岗岩所破坏(图1)。矿物组成包括钾长石(约占40%)、斜长石(约占30%)、石英(约占20%),少量黑云母和褐帘石。其中钾长石呈它形晶,边缘不规则,内部多具出溶条带;斜长石呈并自形板柱状,聚片双晶发育,表面泥化较强;黑云母呈不规则片状,边缘不规则,充填在钾长石和斜长石等矿物的间隙中。副矿物主要 *收稿日期:2002-05-19;修回日期:2002-09-121 基金项目:中国地质科学院地质调查项目/湖北红安地区构造和年代格架与高压)超高压变质带的演化0(编号:200013000169)资助. 作者简介:薛怀民(1962-),男,江苏泰兴人,副研究员,主要从事岩石学与地球化学研究工作.E-mail:huaiminx@https://www.sodocs.net/doc/9816277013.html,