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Assessing of distribution, mobility and bioavailability of exogenous Pb in agricultural soils

Journal of Hazardous Materials 266 (2014) 182–188

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Journal of Hazardous

Materials

j o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j h a z m a

Assessing of distribution,mobility and bioavailability of exogenous Pb in agricultural soils using isotopic labeling method coupled with BCR approach

Zhi-Yong Huang a ,?,Hong Xie a ,c ,Ying-Lan Cao a ,Chao Cai b ,Zhi Zhang a

College of Bioengineering,Jimei University,Xiamen 361021,PR China

Key Laboratory of Urban Environment and Health,Institute of Urban Environment,Chinese Academy of Sciences,Xiamen 361021,PR China c

Shandong Vocational Animal Science and Veterinary College,Weifang 261061,PR China

h i g h l i g h t s

?Large amounts of exogenous Pb were found to distribute in reducible fractions.?Very few of exogenous Pb were found to distribute in acid-extractable fractions.?More than 60%of exogenous Pb in rhizosphere soils lost after planting.

Isotopic labeling method and SEP enable to explore Pb bioavailability in soil.

a r t i c l e

i n f o

Article history:

Received 5August 2013Received in revised form 14December 2013

Accepted 16December 2013

Available online 22 December 2013

Keywords:

Bioavailability Exogenous lead Agricultural soil

Isotopic labeling method

Sequential extraction procedures

a b s t r a c t

The contamination of Pb in agricultural soils is one of the most important ecological problems,which potentially results in serious health risk on human health through food chain.Hence,the fate of exoge-nous Pb contaminated in agricultural soils is needed to be deeply explored.By spiking soils with the stable enriched isotopes of 206Pb,the contamination of exogenous Pb 2+ions in three agricultural soils sampled from the estuary areas of Jiulong River,China was simulated in the present study,and the dis-tribution,mobility and bioavailability of exogenous Pb in the soils were investigated using the isotopic labeling method coupled with a four-stage BCR (European Community Bureau of Reference)sequential extraction procedure.Results showed that about 60–85%of exogenous Pb was found to distribute in reducible fractions,while the exogenous Pb in acid-extractable fractions was less than 1.0%.After plant-ing,the amounts of exogenous Pb presenting in acid-extractable,reducible and oxidizable fractions in rhizospheric soils decreased by 60–66%,in which partial exogenous Pb was assimilated by plants while most of the metal might transfer downward due to daily watering and applying fertilizer.The results show that the isotopic labeling technique coupled with sequential extraction procedures enables us to explore the distribution,mobility and bioavailability of exogenous Pb contaminated in soils,which may be useful for the further soil remediation.

1.Introduction

With the rapid development of industrialization and urban-ization,the contamination of heavy metals in agricultural soils is drawing increasing public attention in China due to the inti-mate link of heavy metals in soils to human health through food chain [1–5].As one of the toxic metals commonly found in agricultural soils,lead (Pb)has been emitted through various anthropogenic activities,such as sewage irrigation,mining and

?Corresponding author.Tel.:+865926181487;fax:+865926180470.E-mail address:zhyhuang@https://www.wendangku.net/doc/e412571754.html, (Z.-Y.Huang).

smelting operations,application of pesticides and inorganic fer-tilizers,and atmospheric deposition,and eventually accumulated on the surfaces of agricultural soils [6–12].Pb in agricultural soils at high level may result in serious reduction of crops’output and deterioration of soil quality,and inevitably enhance the risk of Pb contamination in farm products [13,14].Therefore,it is essential to gain a deep understanding of the physico-chemical and biological behaviors of exogenous Pb contaminated in soils.

It has been well recognized that the bioavailability of a metal in soils depends not only on the metal’s total concentrations,but relies on their various chemical species [15].Hence,the investigation of the mobility and bio-activity of various Pb species are very impor-tant for evaluating the metal’s potential toxicity and bioavailability

Z.-Y.Huang et al./Journal of Hazardous Materials266 (2014) 182–188183

in soils.Many methods have been applied to fractionate and assess Pb species and their bioavailability in soils[15–17].By combin-ing several speci?c extractions,sequential extraction procedures (SEPs)such as Community Bureau of Reference(BCR)and Tessier approaches are the most important methods used for soil samples [18],among which BCR and a modi?ed BCR method by de?ning the chemical species of Pb into three or four fractions have been widely applied in the assessment of Pb bioavailability in soils,despite some drawbacks,such as the lacks of speci?city and re-adsorption prob-lem during the extraction,and the dif?culty in interpreting the obtained results[19–21].

With several advantages such as non-radiation,less waste dis-posal and restriction,and convenient analysis of the isotopic tracers by inductively coupled plasma mass spectrometry(ICP-MS),sta-ble isotopic labeling technique is becoming increasingly popular in the investigation of metals’mobility and potential bioavailabil-ity in soil environment compared with the traditional radioisotope labeling technique[22,23].Previous studies have shown that lead stable isotopic labeling technique can provide helpful and effective messages on monitoring and tracing the metabolism of Pb in orga-nisms,allowing the use of very low exposure levels of Pb tracers and overcoming many of the analytical(poor precision and sensitivity) and healthy(exposure to radioactivity)limitations encountered by employing simple Pb concentration measurements or radio-Pb isotopes.For example,the enriched isotopes of206Pb were fed to rats for labeling Pb compartments in skeletal and soft tissues at very low Pb level(ng g?1)[24].Pb isotopic labeling technique can also be used to discriminate anthropogenic sources from natural source to monitor and trace the transmissions and distributions of the anthropogenic Pb in environment[25,26].For example,an enriched isotopic207Pb2+tracer was sprayed on the forest?oor to simulate rainfall in order to investigate the transmission of atmo-spheric deposit of Pb in soil pro?le[27].Additionally,many studies have been accomplished on the investigation of labile pools of Pb in soils by the method of stable isotope dilution[17,28–30].

Recently,a novel method based on the Tessier approach of SEPs combined with stable isotopic labeling technique has been proved to be a powerful tool to explore the bioactivity and toxicity of mercury in soils in our previous study[31].In this experiment, an enriched isotopic tracer of202Hg was added to simulate the contamination of exogenous Hg in soils.After incubation,the distri-bution of Hg species in soils was studied by measuring the isotope ratios of R Hg(202Hg/200Hg)in various chemical fractions partitioned with a modi?ed Tessier approach.The result demonstrated that the isotopic labeling technique coupled with the modi?ed Tessier approach was able to provide the information about the distribu-tion,mobility and potential bioavailability of exogenous Hg(II)in soils.

To our knowledge,there is little report about the bioavailabil-ity of exogenous Pb contaminated in soils investigated with the method of stable enriched isotopic technique coupled with SEPs approach.Therefore,the aim of the present study is to deter-mine the distribution,mobility and bioavailability of the species of exogenous Pb contaminated in three agricultural soils using the method of stable isotopic labeling technique combined with a four-stage BCR approach.

2.Materials and methods

2.1.Soil samples

Soil samples with different levels of Pb were collected(0–20cm) from the estuary areas of Jiulong River,locating at LongHai City, Fujian Province,China.Soil of GWDD was sampled at24 21 14.37 N,118 01 24.70E,the sampling site of SMHC soil was located at 24 24 15.98N,117 50 35.64E,and the SMNS soil was sampled at 24 25 46.65N,117 49 https://www.wendangku.net/doc/e412571754.html,nds of the three sampling sites have been used for crop planting all the year round,and the veg-etable of Chinese cabbage(Brassica chinensis L.)was planted when the soil samples were collected.At least six subsamples of topsoil were collected and blended as one soil sample at each sampling plot.After being transported to the laboratory,all soil samples were oven-dried for72h at40?C,then ground with a glass mortar and passed through an80mesh sieve,and?nally stored in polypropyl-ene bags.

2.2.Determination of soil characteristics

The basic soil characteristics,including color,and texture,etc. were measured.The soil pH,the contents of organic matters(SOM) and the cation-exchange capacity(CEC)were measured according to the approaches described in our previous report[30].Brie?y, the soil pH was measured with pH electrode in0.01mol l?1CaCl2 suspension at the soil/solution ratio of1:2.5.The contents of SOM were measured with oxidation procedure using H2SO4–K2Cr2O7 followed by a titration with ferrous sulfate.And the contents of CEC were measured using a compulsive exchange procedure by MgSO4–BaCl2.

2.3.Measurement of Pb concentrations in soils

The concentrations of Pb in soil samples before adding enriched isotopes were measured with the method of isotope dilution as described detailed in our previous report[29].After spiking the enriched isotopes of206Pb,the total concentrations of Pb in soil samples were measured with a ZEEnit700instrument of atomic absorption spectroscopy(AAS).The quality assurance of Pb measurement by AAS was achieved by measuring the standard reference material of soil(GSS-16)with88.5±1.5%of Pb recovery.

2.4.Simulation of exogenous Pb contamination in soils by spiking enriched isotopes

For simulating the contamination of exogenous Pb in soils,the enriched isotopes206Pb(>99%,Cambridge Isotope Laboratories, USA)were respectively added(in the form of Pb(NO3)2)to GWDD, SMHC and SMNS soils at the ratios of21.47mg,41.10mg and 67.36mg kg?1soil according to the levels of intrinsic Pb(i.e.,orig-inal Pb)in the three soil samples.For reducing the in?uences of the spiked isotopic reagents on the soils and the analytical errors of R Pb(isotopic ratios of208Pb/206Pb)measurement,the amounts of the enriched isotopes spiked in each of the soils were optimized based on the minimum error propagation factors of R Pb measured in the mixed soils according to the method described in our previous report[32].The mixed soils were wetted with pure water to70–80% of moisture and incubated at room temperature.The samples were blended regularly at least twice a day for full contact of the added enriched isotopes with the intrinsic Pb in soils.After incubation for30days,Pb species in soils were divided into four fractions(i.e., acid-extractable,reducible,oxidizable and residual fractions)using a four-stage BCR approach of sequential extraction procedures[33]. The extraction reagents and conditions of the BCR approach were brie?y described in Table1.The concentrations of Pb in each of fractions were measured with AAS as described above.And the iso-tope ratios of R Pb(208Pb/206Pb)in each fraction were measured with inductively coupled plasma mass spectrometry(ICP-MS).

2.5.Measurement of isotope ratios of R Pb(208Pb/206Pb)

Isotope ratios of R Pb(208Pb/206Pb)in all Pb fractions parti-tioned with BCR approach were measured with a7500a instrument

184Z.-Y.Huang et al./Journal of Hazardous Materials266 (2014) 182–188

Table1

Extraction reagent and conditions of four-stage BCR approach.

Steps Fractions Reagent and conditions

1Acid-extractable 1.5g soil was mixed with20ml0.11mol l?1

CH3COOH,then shaken16h at23?C,

centrifuged at4000rpm for20min.The

supernatant was measured for Pb

concentration and R Pb(208Pb/206Pb)value,and

the residue was used for extraction in next step 2Reducible20ml0.5mol l?1hydroxylamine hydrochloride

(pH1.5)was added to the previous residue.

The mixture was shaken16h at23?C,then

centrifuged.The supernatant was measured for

Pb concentration and R Pb(208Pb/206Pb)value,

and the residue was used for the next

extraction

3Oxidizable The residue was incubated with5ml

8.8mol l?1H2O21h at23?C,then1h at85?C.

Another5ml8.8mol l?1H2O2was added and

the mixture was shaken1h at85?C.After

adding25ml1mol l?1NH4COOCH3,the

mixture was shaken16h at23?C,then

centrifuged.The supernatant was measured for

Pb concentration and R Pb(208Pb/206Pb)value,

and the residue was used for the digestion in

the next step

4Residual0.1g of dried residue was digested and

measured for Pb concentration and R Pb

(208Pb/206Pb)value

(Agilent,USA)of ICP-MS.50ng ml?1of Pb981(National Institute of Standards and Technology,USA),a reference material consisting of natural isotopic composition,was used for the quality assur-ance of R Pb measurement.The measured isotope ratio of2.1680 (208Pb/206Pb),with an average precision of0.22%(expressed as relative standard deviation,RSD),was consistent with the cer-ti?ed value(2.1681±0.0008).In addition,50ng ml?1of Tl(GSB G62070-90,National Analytical Center of Steel Material,China)was used as an internal standard for the corrections of mass bias and the signal?uctuation during the long-term measurement of R Pb (208Pb/206Pb).

2.6.Pot experiment

Each of500g samples of GWDD,SMHC and SMNS soils spiked with the enriched isotopes of206Pb as described above was placed into a perforated plastic pot(about800ml)for planting experi-ment.Three replicates were carried out for each of the soil groups. Four seedlings(～5cm)of Chinese mustard(Brassica juncea)were planted in each pot.The plants were grown in a controlled envi-ronment with the conditions of～20?C and12dark/12h light. The moisture content was maintained at80%with pure water daily.20ml of mixed nutrient solution consisting of CO(NH2)2 (0.5g l?1),KH2PO4(0.3g l?1),MgSO4(0.05g l?1),KNO3(0.7g l?1) and Na2B4O710H2O(0.0006g l?1)was added per20days.After60 days’growth,the aboveground parts of plants were harvested.The root systems,together with adhering soils,were carefully removed from the bulk soil.The rhizosphere soils were collected by shak-ing the slightly air-dried root systems.The plants were rinsed in 50mmol l?1EDTA for5min,and then washed with deionized water and pure water.After being dried at60?C for72h,the plants were ground and digested with5ml of16mol l?1HNO3for the measure-ments of Pb concentrations by AAS and isotope ratios(208Pb/206Pb) by ICP-MS.Pb species in the rhizosphere soils were fractionated with the BCR approach as described above,and the concentrations of Pb and the R Pb values of208Pb/206Pb in each of Pb fractions were measured.2.7.Calculation method of exogenous Pb distributing in soils and plants

The isotope intensity of208Pb or206Pb in soil measured with ICP-MS is the summed values of208Pb or206Pb derived from the exogenous Pb by spiking the enriched isotopes(designated as Pb s) and the intrinsic Pb containing in natural samples(designated as Pb n),respectively.

Hence,

R m=

M208

+M208

M206

n+M206

(1)

where R m is the measured isotope ratio of R Pb(208Pb/206Pb).M208

and M206

n are the mole numbers of

208Pb and206Pb intrinsically con-

taining in soil samples,respectively.M208

s and M

206

s are the spiked mole numbers of exogenous208Pb and206Pb in the enriched iso-topes,respectively.Because the mole number of an isotope is equal to the element’s mole number multiplied with the correspond-

ing isotopic abundance,the mole numbers of M208

n,M

206

n,M

208

and M206

s can be expressed as M n·f208,n,M n·f206,n,M s·f208,s and M s·f206,s,respectively.Then,formula(1)can be presented as:

R m=

M n·f208,n+M s·f208,s

M n·f206,n+M s·f206,s(2) where M n and M s are the mole numbers of intrinsic and exogenous Pb in soils,respectively.f208,n and f206,n,and f208,s and f206,s are the isotopic abundances of208Pb and206Pb in natural soil and enriched isotopic reagent,respectively.

Formula(2)is arranged as:

M s=

M n·f208,n?R m·M n·f206,n

R m·f206,s?f208,s(3) The denominator and numerator of formula(3)are simul-taneously divided with f206,n×f206,s.In addition,the total mole number(T)of Pb should be the sum of M s and M n,(i.e.,T=M n+M s). Then,formula(3)can be arranged as:

M s[(R m·f206,s?R s·f206,s)+(R n·f206,n?R m·f206,n)]

=T(R n·f206,n?R m·f206,n)(4) where R n and R s are the isotope ratios of Pb in natural soil and the enriched isotopic reagent,respectively.T can be represented as the total concentrations of Pb in soil after spiking with the enriched isotopes,and can be measured with AAS.

Formula(4)can be also changed as:

M s=

T·(R m·f206,n?f208,n)

f208,s?R m·f206,s+R m·f206,n?f208,n(5) By formula(5),the distributions of exogenous Pb in soil,soil fractions and plants can be calculated,respectively.Through the similar steps as described above,or by M n=T?M s,the distribution of intrinsic Pb(M n)can also be calculated.

2.8.Statistical analysis

The statistical analysis was performed using the software of SPSS 11.0for Windows?.One-way ANOVA was employed as the statisti-cal program to compare the isotope ratios of Pb in different fractions of soils partitioned with BCR approach.

3.Results

3.1.Soil characteristics

All the soil samples were dark brown,and the contents of silt and sand fractions were higher than that of clay for the three soils.

Z.-Y.Huang et al./Journal of Hazardous Materials266 (2014) 182–188185

Table2

Basic characteristics and Pb concentrations of soil samples.

Soils pH values SOM(%)CEC(cmol kg?1)Concentrations of Pb(mg kg?1) GWDD 6.8 2.2 5.562.2

SMHC 5.8 2.4 5.8119

SMNS 5.1 2.4 6.2195

Data in Table2showed that the values of soil pH were less than 7.0,indicating the acidity of all the soil samples.The contents of SOM and CEC showed that the soil fertility was at medium level for all the sampled soils.The concentrations of intrinsic Pb in all the soil samples were more than50mg kg?1,a limit for Pb at pH<7.5in farmland soils set for the production of edible agricultural products in China(HJ332–2006).The levels of intrinsic Pb in SMHC and SMNS soils were about2-and4-folds higher than the limit,respectively.

3.2.Species distribution of exogenous Pb in soils

After adding the enriched isotopes of206Pb,the isotope ratios of R Pb(208Pb/206Pb)in each of fractions partitioned with BCR approach were shown in Table3.For each of the same fractions,there was no signi?cant difference of R Pb among the three soils before adding the exogenous Pb,but the average R Pb value(2.090)in acid-extractable fractions was slightly lower than that(2.098)in the other three fractions.However,signi?cant decreases(p<0.05)of R Pb were observed in most fractions after spiking the soils with the enriched 206Pb isotopes.For example,after adding the enriched isotopes,the R Pb values in the acid-extractable,reducible,oxidizable and resid-ual fractions of GWDD soil were respectively0.535,0.650,0.790 and1.817,which were signi?cantly lower(p<0.05)than those of the soils before adding the enriched isotopes except for the R Pb value in the residual fraction.After spiking the enriched isotopes, the R Pb values in acid-extractable fractions of soils decreased the most,while the R Pb values in residual fractions decreased the least.

By the measured R Pb values,the distributions of exogenous Pb in soils were estimated with formula(5)as described above.As shown in Table4,about60–85%of the exogenous Pb was found to distribute in reducible fractions,while the exogenous Pb dis-tributing in acid-extractable fractions was less than0.5%,except for GWDD soil with about1%in this fraction.The amounts of exoge-nous Pb distributing in oxidizable fractions accounted for about 8–20%of the spiked enriched isotopes.The amounts of exoge-nous Pb in the three soils ranged in the same distribution order: reducible>oxidizable≥residual>acid-extractable fractions.

3.3.Concentrations of Pb in rhizosphere soils

After spiking the soils with the enriched isotopes,pot exper-iments were carried out.The isotope ratios of R Pb(208Pb/206Pb) in various fractions of rhizosphere soils partitioned with the BCR approach were measured when plants were harvested.As shown in Fig.1,except for the residual fractions,the R Pb values in all Pb fractions signi?cantly increased(p<0.05)after planting,espe-cially for those in the acid-extractable fractions.For example,R Pb

Pb fractio ns

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Pb fr action s

https://www.wendangku.net/doc/e412571754.html,parison of isotope ratios of R Pb(208Pb/206Pb)in various Pb fractions of soils before and after planting.Different letters for the same fraction indicate the signi?cant differences(p<0.05)

Table4

Distributions of exogenous Pb in various Pb fractions of soils.

Fractions GWDD(mg kg?1)SMHC(mg kg?1)SMNS(mg kg?1) Acid-extractable0.2327(1.08)a0.06904(0.168)0.2886(0.428) Reducible18.40(85.7)27.66(67.3)39.92(59.2) Oxidizable 1.710(7.96)7.398(18.0)13.48(20.0) Residual 1.132(5.27) 5.974(14.5)13.68(20.3)

a Values in parenthesis were the percentages of the exogenous P

b various frac-tions in soils.

values in the acid-extractable fractions of GWDD,SMHC and SMNS soils after planting increased from0.535,0.450and0.611to0.783, 0.672and0.812,respectively.Obvious increases of R Pb values were also observed in reducible and oxidizable fractions for all the three

Table3

Isotope ratios of R Pb(208Pb/206Pb)in various Pb fractions of soils before and after spiking the enriched isotopes of206Pb.

Fractions GWDD SMHC SMNS

Uncontaminated soil Contaminated soil Uncontaminated soil Contaminated soil Uncontaminated soil Contaminated soil

Acid-extractable 2.091±0.005a0.535±0.003b 2.090±0.004a0.450±0.014b 2.090±0.009a0.611±0.004b Reducible 2.098±0.013a0.650±0.005b 2.099±0.009a0.587±0.007b 2.096±0.017a0.750±0.006b Oxidizable 2.101±0.010a0.790±0.017b 2.099±0.015a0.674±0.014b 2.095±0.008a0.821±0.007b Residual 2.100±0.005a 1.817±0.005a 2.097±0.007a 1.457±0.012b 2.099±0.007a 1.409±0.011b

Different letters indicated the signi?cant differences(p<0.05)of R Pb in soils before and after spiking enriched isotopes.

186Z.-Y.Huang et al./Journal of Hazardous Materials266 (2014) 182–188

Table5

Concentrations of exogenous Pb in various fractions of soils after planting.

Fractions GWDD(mg kg?1)SMHC(mg kg?1)SMNS(mg kg?1)

After planting Decrease percentages(%)a After planting Decrease percentages(%)After planting Decrease percentages(%)

Acid-extractable0.0608473.80.0259262.40.0682376.4

Reducible 6.94262.38.87567.915.1862.0

Oxidizable 1.12534.2 2.88261.0 2.78279.4

Residual 6.238?451b 5.14613.912.607.89

Sum13.4037.616.9358.830.6354.5

a Decrease percentages were calculated by comparing the amounts of exogenous P

b in each of fractions before and after planting.

b Negative value is due to the increase of the amount of exogenous Pb in residual fraction after planting compared with that of exogenous Pb before planting.

rhizosphere soils after planting.However,only slight increase of R Pb was observed in residual fractions for all the three soils compared with those of the soils before planting.

After planting the amounts of exogenous Pb in various fractions of rhizosphere soils,calculated with formula(5),were shown in Table5.Result showed that the total amounts of exogenous Pb in the three rhizosphere soils were signi?cantly lower(p<0.05) than those in the soils before planting.In addition,the amounts of exogenous Pb in all the four fractions decreased after planting, except for the residual fraction of GWDD soil with an increase from 1.132mg kg?1to6.238mg kg?1.Except for GWDD soil with37.6% of decrease,more than50%of total exogenous Pb decreased in each of the rhizosphere soils of SMHC and SMNS samples after planting.

3.4.Uptake of exogenous Pb in plants

The isotope ratios of R Pb(208Pb/206Pb)and the uptake of exoge-nous Pb in the harvested plants were shown in Table6.R Pb in the roots,stems and leaves of plants were signi?cantly lower(p<0.05) than those(R Pb=2.112)of the plants grown in the soils without adding the enriched isotopes.The distributions of exogenous Pb in stems and leaves were similar in each of the soil groups,but the uptakes of exogenous Pb in roots were much more than those in stems and leaves.Considering the biomasses of harvest plants, the total amounts of exogenous Pb accumulated in plants grown in GWDD,SMNC and SMNS soils were31.0,55.6and88.5?g,respec-tively.

4.Discussion

Because of various anthropogenic activities,the emission of contaminant Pb has considerably increased the metal’s levels in agricultural soils,which potentially increases the health risk on human body through food chain.Because of the potential differ-ences of physico-chemical and biological activities of exogenous Pb compared with those of intrinsic Pb in soils,the distribution,mobil-ity and bioavailability of exogenous Pb in soil matrices are needed to be deeply explored for further controlling and reducing the haz-ard of the contaminant in soils.Many methods have been found suitable to evaluate the distribution,mobility and bioavailability of Pb in soils,such as various single chemical extraction[34–38], sequential extraction procedures[39,40],diffusive gradients in thin ?lms(DGT)[41,42],and E value which is by tracking the depletion of isotopically exchangeable Pb in soil suspension and estimating the bio-available Pb with the method of isotope dilution[29,30,43,44], etc.However,it is dif?cult to?nd an appropriate method to clearly differentiate the chemical and biological behaviors between exoge-nous Pb2+and intrinsic Pb in the matrices of soils up to now[45]. In the present study,a novel method using a stable isotopic label-ing technique combined with a four-stage BCR approach of SEPs is proposed to reveal the distribution,mobility and bioavailability of exogenous Pb in agricultural soils.

By spiking the stable enriched isotopes of206Pb,the contamina-tion of exogenous Pb2+ions in soils was simulated.Data in Table3showed that the R Pb values in acid-extractable fractions decreased the most among the four Pb fractions after adding the enriched iso-topes,indicating that the species of intrinsic Pb in acid-extractable fractions were labile and adequately exchanged with the spiked exogenous Pb in soils.It is well known that the species of Pb in acid-extractable fractions are considered to be capable of immediately presenting Pb2+ion activities in neutral and acidic soil solutions, and therefore easily assimilated by plants[46].Therefore,the pool of Pb in acid-extractable fractions re?ects the metal’s potential phytotoxicity and bioavailability in soils[47].But data in Table4 showed that the amounts of exogenous Pb distributing in the acid-extractable fractions only accounted for less than0.5%of the added exogenous Pb except for1%in GWDD soil,indicating that most of the exogenous Pb2+ions were not in the forms of acid-extractable fractions in soil matrices.

While the least decrease of R Pb values was observed in the resid-ual fractions after adding the enriched isotopes,indicating that the species of intrinsic Pb in this fraction were chemical stable. Pb in residual fraction presents in the crystal lattices of lithogenic and pedogenic silicate-binding mine,and is dif?cult to be available for plants in soils under natural conditions[48].Result in Table4 showed that only small amount of the exogenous Pb2+ions dis-tributed in the residual fractions except for those in the SMHC and SMNS soils.The signi?cant decreases of R Pb observed in the residual fractions of SMHC and SMNS soils might be due to the precipitation of exogenous Pb2+ions by ligands in the soils,but the real reason remains unknown and is worth studying further.

As shown in Table4,about60–85%of exogenous Pb was found to distribute in reducible fractions,indicating that the exoge-nous Pb2+ions in soils mainly distributed in this fraction.Pb in reducible fractions is associated with iron and manganese oxide/hydroxide,and may be adsorbed to reducible Fe/Mn oxides in soils[49,50].Although the chemical activity and bioavailability of Pb in reducible fractions are less than those of the acid-extractable fractions in soils,Pb species in this fraction can be released and subsequently assimilated by plants when the oxidation-reduction potential of soil solutions decreases[51].The results in Table4 indicated that large amounts of exogenous Pb2+in soils distribut-ing in the reducible fractions could prevent the immediate toxic impacts of the exogenous Pb2+on plants,but Pb species in this fraction still posed potential phyto-toxicity and bioavailability to plants.

The amounts of exogenous Pb2+distributing in oxidizable frac-tions,associated with SOM and sul?des in soils[49],accounted for about8–20%of the spiked enriched isotopes.The relative large proportions of exogenous Pb2+in SMHC and SMNS soils might be due to the higher contents of SOM in the two soils compared with that in GWDD soil as shown in Table2.Pb in oxidizable fractions of soils may also present potential biological impacts on plants in soils because Pb species in this fraction can be released at strong oxidizing conditions in soil environment.

For further investigating the mobility and bioavailability of exogenous Pb in soils,pot experiments were carried out in the stimulated soils.Except for R Pb in residual fractions,results in

Z.-Y.Huang et al./Journal of Hazardous Materials 266 (2014) 182–188

187

Table 6

Isotope ratios (R Pb )of 208Pb/206Pb and uptake amounts of exogenous Pb in plants.

Soil groups

Roots

Stems

Leaves

Total amounts of exogenous Pb in plants (?g)

R Pb

Amounts (?g)

R Pb

Amounts (?g)

R Pb

Amounts (?g)

GWDD 1.123±0.01017.1 1.197±0.0077.03 1.350±0.025 6.8031.0SMNC 0.902±0.01229.8 1.060±0.00812.8 1.206±0.00713.055.6SMNS

0.838±0.006

47.3

0.976±0.012

20.3

1.138±0.008

20.9

88.5

Fig.1showed that the values of R Pb in all Pb fractions signi?-cantly increased (p <0.05)after planting,which indicated that the rhizospheric Pb in acid-extractable,reducible and oxidizable frac-tions might be readily mobilizable and potentially bio-available.The greatest increases of R Pb in acid-extractable fractions indi-cated that Pb species in this fraction were very active and might be readily bio-available in soils.Besides,the species of exogenous Pb in reducible and oxidizable fractions might also be mobilizable and bio-available in soils,because the concentrations of exogenous Pb in rhizospheric soils after planting as shown in Table 5decreased by 64%in reducible fractions and 58%in oxidizable https://www.wendangku.net/doc/e412571754.html,-pared with the exogenous Pb in acid-extractable,reducible and oxidizable fractions,exogenous Pb in residual fractions was inert in soils because only slight increases of R Pb values in this fraction were observed after planting.Because large amounts of exoge-nous Pb in acid-extractable,reducible and oxidizable fractions lost after planting,the amounts of exogenous Pb in the three fractions were summed up (i.e., BCR )for the bio-available evaluation of the exogenous Pb in soils.The results in Table 5showed that after planting the values of BCR amazingly decreased by 60–66%com-pared with those of the three soils before planting (as shown in Table 4).

But what reasons might account for the loss of exogenous Pb from the rhizospheric soils?Was the exogenous Pb lost in rhizo-spheric soils totally attributed to the uptake of the grown plants?To answer these questions,the mass balances of exogenous Pb in soil-plant system were carried out to evaluate the bioavailability of the spiked 206Pb in soils.Data in Table 6showed that the uptake of exogenous Pb by plants only accounted for a few fractions of the spiked Pb in the three soils.For example,the loss of exogenous Pb in GWDD soil after planting was 4036?g,calculated based on the residual concentration of exogenous Pb in rhizospheric soil (as shown in Table 4)and the total exogenous Pb spiked in 500g soil for pot experiment.But the total amount of exogenous Pb accumulated in the harvested plants (including roots,stems and leaves)grown in GWDD soil was only 31.0?g.Obviously,large amounts of exoge-nous Pb lost in rhizospheric soils could not be simply attributed to the uptakes of the grown plants.Except for the small amount of exogenous Pb assimilated by the plants,most of the metal lost in rhizophsere soils during the planting might derive from the metal downward migration or even loss from the pots because of the daily watering and applying fertilizer.In addition,the effect of aging dur-ing planting could also affect the species distribution of Pb in soils.It has been demonstrated that most weakly bound fraction of metals tends to decrease during aging [52].After adding the enriched iso-topes,the soils have been incubated for 30days before planting,but a slow continuous retention of exogenous Pb in soil solid phases occurred causing the changes of the proportional distribution of Pb in rhizophsere soils.Therefore,the massive loss of exogenous Pb observed in rhizosphere soils should mainly result from the analyt-ical soil samples,because only the rhizospheric soils were collected and measured in the present experiment.A non-planted soil spiked with the enriched isotopes should also be simultaneously set as the control group in our further study.

Many researches have been accomplished to investigate the bio-chemical behaviors of Pb in rhizospheric and bulk soils.It has been

reported that Pb in micro-rhizosphere could be activated due to the root exudates in root system and the microorganisms on root sur-faces [53,54].Because of the activation,the biochemical activities of Pb in rhizosphere soils might be stronger than those in bulk soils,which indicate that rhizospheric Pb might be readily assimilated by plants or other microorganisms.Therefore,it is con?rmed that the exogenous Pb in soils is active and potentially bio-available for plants.

5.Conclusions

By spiking the stable enriched isotopes of 206Pb,the contam-ination of exogenous Pb 2+ions in soils was simulated.Through the isotopic labeling technique coupled with the BCR approach of sequential extraction procedures,the distribution,mobility and bioavailability of exogenous Pb contaminated in soils was https://www.wendangku.net/doc/e412571754.html,rge amounts of exogenous Pb were found to distribute in the reducible fractions,while the amounts of exogenous Pb distribut-ing in the acid-extractable fractions only accounted for less than 0.5%of the added enriched isotopes except for GWDD soil with about 1%in this fraction.More than 60%of exogenous Pb in acid-extractable,reducible and oxidizable fractions of rhizosphere soils lost after planting,but only a few of the lost metal were attributed to the uptake of plants.

Acknowledgements

The work was supported by grants from the National Natu-ral Science Foundation of China (40771185),the Natural Science Foundation of Fujian Province of China (2012J01046),the Sci-ence and Technology Planning Project of Fujian Province,China (2012Y0052),the Science and Technology Planning Project of Xia-men,China (3502Z20113024),the Foundation of the Key Project Laboratory of Urban Environment and Health,Institute of Urban Environment,Chinese Academy of Sciences (KLUEH201303),and the Foundation for Innovative Research Team of Jimei University (2010A007).

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