Evaluating the bioavailability of heavy metals in natural-zeoliteamended aquatic sediments using thin-fi lm diffusive gradients

Juanying Li ,Chen Zhang ,Jianwei Lin ,Jie Yin ,Jiayan Xu ,Yiqin Chen ,*

a Key Laboratory of Exploration and Utilization of Aquatic Genetic Resources,Shanghai Ocean University,Ministry of Education,Shanghai 201306,China

b College of Marine Ecology and Environment,Shanghai Ocean University,Shanghai 201306,China

1.Introduction

China is a major producer and consumer of aquatic products,and the expansion of the related industry has given it an important statusin the national economy(Liet al.,2009).Aquatic products are an important source of high quality protein,and are beneficial for consumer health(Zhang&Jiang,2010),but the accumulation of heavy metals in aquatic products is a cause for concern(Wu et al.,1999;Yang,Yao,&Xu,2012).Previous research showed that heavy metals can be readily enriched in aquatic organisms(Tao et al.,2012;Turkmen,Turkmen,&Tepe,2009).The heavy metals ingested by human consumption of aquatic products represent a health risk as they accumulate in the body(J¨arup,2003).Kwok,Liang,and Wang(2014)found that the level of Cd and Pb in the muscle of the fish from the Pearl River estuary exceeded the permissible limits proposed by the Food and Agriculture Organization(FAO).The same was true for the contents of Cd(as high as 0.27-0.29 mg/kg)in the shell fish from Yangkou Port,China(Liu,Wang,&Yu,2010).Relatively higher levels of Zn and Cu in the fish from the Luoyang area was also reported by Quan,Gong,and Cui(2010).Wang,Zhao,and Chen(2007)also reported that the amount of Hg accumulated in the fish was 1000 times higher than that of the surrounding water in Baiguishan Reservoir,China.Futhermore,numerous studies have indicated that the source of heavy metals in aquatic products is closely related to the sediment type(Yi,Yang,&Zhang,2011;De et al.,2004;Kalantzi,Shimmield,&Pergantis,2013).Therefore,it is important to remediate the contaminated sediments by reducing the bulk burden and the bioavailability of heavy metals.

There are several techniques available for remediation of heavymetals in sediments,including the utilization of the specific cell morphology and physiological metabolism of actinomycetes(Alvarez,Saez,&Costa,2017),the absorption and chelation of heavy metals by plant roots(Sarwar,Imran,&Shaheen,2016),the high surface activity of biosurfactants(Gnanamani et al.,2010)and the high porosity of large-surface bio-carbons(Li,Wu,&Zhang,2015).Natural zeolite is one kind of alkali-metal-or alkalineearth-metal-framed aluminosilicate mineral,which has selective adsorptive properties,a high ion-exchange performance and a large adsorption capacity and so has been deployed in environmental remediation.Natural zeolite can also be used to control the release of nitrogen and phosphorus from the sediments(Montalvo et al.,2011;Nakhla,Zhu,&Cui,2007).

Recent studies have focused on the feasibility of natural zeolite for heavy-metal remediation in sediments.Motsi,Rowson,and Simmons(2009)reported a removal rate of 99%for Hg using a natural zeolite-molecular seive.Hui,Chao,and Kot(2005)explored the ability of synthetic natural zeolite to remove heavy metals in water and found that the removal rate of heavy metals was above 99%when the dosage was 1g/L.However,most studies have failed to consider strategies to reduce the bioavailiability of heavy metals,and have concentrated on the reduction of the bulk content of heavy metals in sediments.In this context,diffusive gradient in thin-film technology(DGT)was introduced to evaluate how it changes the bioavailiability of heavy metals in the sediments treated with natural zeolites in the present study.

DGT consists of a diffusion phase and a bonding phase and is a new type of in-situ passive sampling tool.The diffusion phase is a hydrogel or semipermeable membrane of a certain pore size.The binding phase is a polymer which is capable of providing the functional groups for coordination electron pairs.The role of the binding phase is to coordinately bind the diffusing metals,so that the partitioning of the metals between the diffusive phase and the bonding phase is reduced to a minimum(Gimpel,Zhang,&Hutchinson,2001).Empirically,the concentrations of heavy metals in the pore water of the sediments after immersion of a DGT device for 24 h is used for calculating the metal concentrations(Ding,Xu,Sun,Yin,&Zhang,2010).Therefore,DGThas been widely used for determining the reactive(free-dissolved)heavy metal content both from pore water and sediment(Davison&Zhang,1994;Xu et al.,2013).Traditional procedure used to determine the free-dissolved heavy metals includes centrifugation,and flocculation(Hawthorne,Grabanski,Miller,&Kreitinger,2005),which is far more laborious and less reliable than the DGT method.Moreover,DGTcan be employed in the field without disturbing the formation of the sediment,providing in-situ monitoring data.

Furthermore,DGT has been used to predict the uptake of heavy metals by freshwater snails,and has the potential to predict the content of heavy metals in shell fish(Yin,Cai,&Duan,2014).It was also reported that the enrichment effect of mussels and DGT on heavy metals was significantly relevant(Webb&Keough,2002).To our knowledge,DGT has not been used as a passive sampling tool for evaluating the bioavailability of heavy metals in aquatic ponds during sediment remediation,or as a biomimetic tool to infer the accumulation of heavy metals in bethic organisms.

Therefore,in the present study,DGT devices were used as passive samplers,Venerupis philippinaram was used for heavy metal bioaccumulation experiments,and natural zeolites were used as the remediative material in sediments.In this study we explore;(1)the effect of natural zeolite on reducing the bioavailability of heavy metals in sediments and their bioaccumulation in V.philippinaram and,(2)the feasibility and reliability of using DGT to predict the bioaccumulation of heavy metals in benthic organisms.

2.Materials and methods

2.1.Sediment and natural zeolite

Surface sediment samples from Dafeng of Jiangsu Province(N33°16′34.45′′,E120°50′47.64′′)and Yangshan Port of Shanghai(N30°38′35.59′′,E122°03′22.72′′)in China were collected using a Peterson sampler(PSC-1/16).The samples were put in foil bags and placed on ice,then delivered to the lab and stored at-20°Cuntil analysis.Natural zeolite was purchased from Jinyun,Zhejiang Province,China.The cation exchange capacity of this natural zeolite is 1300-1800 mmol/kg,the molar ratio of Si/Al is 4.25-5.25,and the chemical composition is:SiO2(70%),Al2O3(12%),Fe2O3(0.87%),K2O(1.1%),CaO(2.6%),MgO(0.13%)and Na2O(2.6%).X ray diffraction analysis revealed that the natural zeolite contained 66%clinoptilolite,19%mordenite and 15%silica.The natural zeolite was grounded and seived to obtain particles of 0.15-0.18 mm diameter,and then washed 5 times with deionized water prior to the experiments.

The samples collected from Dafeng coastland were used for the natural zeolite adsorption experiment and bioaccumulation experiment,while the samples from Yangshan Port were used as the blank controls.The bioaccumulation experiments were divided into two groups:a natural zeolite-amended group and a natural zeolite-free group.Each group had two replicates.

2.2.Subject organism and DGT device

V.philippinaram were purchased from Shanghai Luchao aquatic market and were acclimated to laboratory conditions for more than three weeks before using them for experiments.During V.philippinaram acclimation,the salinity and temperature of the seawater was 25±2 and 20±1°Crespectively,and the water was aerated continuously.The photoperiod was 16 h light followed by 8 h dark,and V.philippinaram was fed daily with a regular amount of Phaeodactylum(2 g per tank)(Ngo,Pinch,&Bennett,2016).

The DGT device were purchased from Nanjing Intelligent Environment Technology Co.,Ltd..The three components of the DGT,i.e.a Zr O-Chelex fixed film,an agarose diffusion film and a PVDF filter,were superimposed on a new flat plastic case.The thickness of the DGT fixed film and diffusion layer was 0.40 and 0.90 mm,respectively,and the device window area was 150 mm×20 mm(length×width).The DGT device was cut into six pieces and one piece(0.0065 g)represented one DGT in the present study.

2.3.Use of natural zeolite for adsorbing the free-dissolved heavy metals in sediment

Approximately 2 kg(wet weight with 30%water content)of the surface sedimentscollected in the Dafeng coastland wasplaced in a 5 L glass tank,and the natural zeolite was then added at a dry weight ratio of 10%and homogenized(Aggeliki,Anthimos,&Ioannis,2001).We previously examined the influence of the amount of the zeolite on the survival of benthic organisms and found that at zeolite concentrations of less than 10%,the survival rate was above 95%,but as the dosage was increased the survival rate dropped sharply to less than 80%(data not shown).Thus,the dosage of zeolite used was 10%since it yields the best adsorption with a high survival rate of organisms.The temperature of the sediments was recorded and two DGT devices were inserted into the natural zeolite-amended sediments every 12 h,and then taken out without breaking or pressing the exposure window.The surface of the DGT device was then rinsed with deionized water and the fixed films were removed from the cases and the membrane was discarded.Excess water was wiped from the fixed film,and then the fixed film was placed into a 30 m Lglass tube and immersed in 10 m LNaOH(1 mol/L)for 24 h and then in 10 m LHNO3for a further 24 h.The extracts were then filtered using a 0.22μm glass filter before analysis.The chemical reagents used in the experiments were all analytical grade.

2.4.Calculation of the free-dissolved concentration of heavy metals in the sediment using DGT

The accumulation of heavy metals in the fixed film M(ng)of the DGT was calculated after introducing an extraction factor fe,i.e.the extraction rate of heavy metals(Wang,Ding,&Gong,2016).The equation is shown below.

Where Ceis the measured concentration of heavy metals(μg/L)in the fixed film,Veis the volume of the extraction liquid(15 m L),Vgis the volume of the fixed film (1 m L).The values of feis 1.03,0.96,0.94,0.94 and 0.74 for Cu,Pb,Cd,Cr and As,respectively(Wang et al.,2016).The concentration of heavy metals in the interface between the DGT device and the pore water within a given period of time,i.e.CDGT,was then calculated as shown below(Ding et al.,2010).

Where M is the accumulated mass of Cd over the deployment time(ng),Δg is the thickness of the diffusive film(0.09 cm),and D is the diffusion coefficient of heavy metals in the diffusion layer(cm2/s),which was obtained from a DGT website(www.dgtresearch.com).A is the area of the film(5 cm2),and t(s)is the time the DGT was in the sediments(Ding et al.,2011).

2.5.Bioaccumulation experiment

The bioaccumulation experiment was carried out according to EPA Method 600/R-99/064.Approximately 2 kg of the surface sediments(wet weight with 30%water content)collected from the Dafeng coastl and was placed in a 5 Lglass tank,and natural zeolite was added at a dry weight ratio of 10%and homogenized.In parallel,a natural zeolite-free tank with the sediments from Dafeng coastland and a blank control tank with the sediments from Yangshan Port were also prepared.The surface of the sediments were then covered with 4 Lartificial seawater(Method ISO-10253).25 V.philippinarams of similar size(length 32±2 mm and hight 12±2 mm)and 14 DGT devices were added into each tank.The bioaccumulation experiment lasted for 28 days in line with the EPA recommended method.Every four days,two V.philippinaram were sampled from all tanks,freeze-dried and analyzed as described previously(Liu,Feng,&Qiu,2012;Webb&Keough,2002);two DGT devices were taken out of the sediments and extracted as mentioned above;～2 g of sediment was sampled and analyzed according to Method GB17378.5-2007.

2.6.Instrument method

The samples were measured using an Agilent 7700x ICP-MS.The procedure was modified from the method reported by Sánchez López,Gil Garcia,Sánchez Morito,and Martınez Vidal(2003).The operating conditions are listed below:Radio Frequency Power,1500W;Carrier Gas Flow,0.9 L/min;Auxiliary Gas Flow,0.8 L/min;Nebulizer Gas Flow,0.5 L/min;Plasma Gas Flow,15 L/min;Lens Voltage,7.25 V.The calibration range was 0.001-50 pg/m L for all the heavy metals analysed.

2.7.Quality control

For every five samples analysed,a blank sample was included to check for laboratory contamination.Mili Q water was used as the blank samples.The average recoveries of spiked heavy metals were from 99%to 105%(in sediments),100%-105%(in organisms)and 96%-104%(in DGTs).Replication of recoveries showed good reproducibility with a coefficient of variation(CV)of less than 20%for all the samples(Supporting Information Table S1).The limit of quantification for compounds was de fined as the instrument detection limit(Supporting Information Tables S2-4).

2.8.Statistics

The absorption rate constant was calculated using Origin Pro8(Origin Lab Corp.USA).A t-test and a linear-regression analyses were used to determine the relationship between the concentrations of heavy metals in DGT and in V.philippinaram and was performed using Graph Pad Prism 6(Graph Pad Software,Inc.,USA).

3.Results and discussion

3.1.The role of natural zeolite in reducing the free-dissolved concentration of heavy metals in sediment

In order to determine the adsorptive effect of natural zeolite on heavy metals in sediments,the free-dissolved concentrations of heavy metals in sediment were measured using the DGT devices after the addition of natural zeolite into the sediments.The measured concentrations of different kinds of heavy metal(CDGT,calculated according to Eq.(2))versus time is shown in Fig.1.

As can be seen from Fig.1,after the natural zeolite was added to the sediments,the free-dissolved concentration of heavy metals in the sediment,expressed as CDGTdecreased as the duration of zeolite exposure increased,until it reached an equilibrium after～24 h.The CDGTwas reduced from 2.7 mg/L to 0.88 mg/L for Cu,from 0.014 mg/Lto 0.003 mg/Lfor Pb,from 0.012 mg/Lto 0.003 mg/Lfor Cd,from 0.035 mg/L to 0.013 mg/L for Cr,and from 0.02 mg/L down to 0.006 mg/Lfor As,with a percent change of 67%,81%,72%,62%and 71%,respectively.Thus,adding natural zeolite into sediments can reduce in a relatively short period of time the available concentrations of heavy metals in sediment pore water.As the heavy metals in the sediment pore water is the most easily absorbed and is the bioavailable fraction for organisms,natural zeolite is therefore an effective control of heavy metal pollution in sediments.

The rate of the reduction of CDGTfor the heavy metals tested was different,which might be related to the variable levels of heavy metals in the sediment,since initial concentration is an important factor affecting adsorption kinetics(Chen et al.,2017).The amount of Cu in the collected sediment was 2.7 mg/L,while the contents of the other heavy metals was much lower(i.e.,＜0.04 mg/L).Moreover,heavy metal interactions during the competitive adsorption may also influence the adsorption rates.Different speciations and proportions of the heavy metals associated with the exchangeable fraction,carbonate bound fraction,Fe-Mn oxides bound fraction,organic matter bound fraction,sulfide bound fraction,etc.(Tessier,P Campbell,Bisson,&.,1979)may also affect adsorption and the natural zeolite adsorption is mainly limited to the exchangeable fraction of heavy metals.

Fig.1.Bioavailable concentrations of heavy metals(C DGT)in sediment amended with natural zeolite(error bars represent mean±SD,n=2).

3.2.The role of natural zeolite in reducing bioaccumulation of heavy metals in V.philippinaram

Presumably,the part of the heavy metals adsorbed by natural zeolite represents the most available part for the organisms in sediments.The effect of natural zeolite on the bioaccumulation of heavy metals in V.philippinaram was investigated in an in vivo experiment.

As can be seen from Fig.2,the amount of the heavy metals in V.philippinaram(CVP,μg/g dw)in the blank control group did not change significantly over time,indicating that the process of heavy metal enrichment in V.philippinaram was not influenced by the experimental process or the lab environment.

For the natural zeolite-free group,the CVPvalues increased gradually from the first day to the 28thday of the experiment,which is consistent with previous studies(Liu et al.,2010).The CVPincreased from 8.1 up to 10μg/g(dw)for Cu,from 3.2 up to 4.7μg/g(dw)for Pb,from 2.4μg/g up to 3.9μg/g(dw)for Cd,from 0.82 up to 0.97μg/g(dw)for Cr,and from 3.2μg/g up to 3.4μg/g(dw)for As.Biota sediment accumulation factor(BSAF=heavy metals in V.philippinaram/heavy metals in sediments)(Fu,Meng,&Wang,2013)was used to express the accumulation effect of heavy metal in V.philippinaram(Table S2 and S4).The measured BSAFwas 1.2,0.14,2.0,0.05,and 0.13 for Cu,Pb,Cd,Cr and As,respectively.The values were similar to those reported by Fu et al.(2013)who analyzed Paphia undulata from the Beibu Gulf,Guangxi Province in China,and the BSAFvalues of Cu,Pb,Cd,Cr and As were 1.3,0.03,10,0.23 and 0.42.

Similarly,the CVPvalues in the natural zeolite-amended group increased with of experiment,but not as steeply as in the natural zeolite-free group.The CVPincreased from 8.1 to 9.2μg/g(dw)for Cu,from 3.2 up to 4.2μg/g(dw)for Pb,from 2.4 up to 3μg/g(dw)for Cd,from 0.82 up to 0.91μg/g(dw)for Cr,and from 3.2 up to 3.3μg/g(dw)for As.The BSAFvalues of Cu,Pb,Cd,Cr and As was calculated to be 0.91,0.13,1.5,0.02 and 0.10 respectively in this group.The CVPand BSAF values both decreased noticeably in comparison with the natural zeolite-free group,indicating that natural zeolite can effectively reduce the bioavailability of heavy metals in sediments.

The bioaccumulation of heavy metals in V.philippinaram followed the first-order kinetic equation(CA=C0+CW(K1/K2)(1-e(-K2t)))(Yang et al.,2012).The constant K1is the absorption rate of heavy metals by the organism,constant K2is the excretion rate of heavy metals by the organism,CWis the chemical concentration in the sediments and CAis the chemical concentration in the organism.The calculated K1values of heavy metals in V.philippinaram are indicated in Fig.2 and Table 1.

It is evident from Table 1 that the K1of the natural zeolite-free group was higher than the natural zeolite-amended group,which suggested that natural zeolite inhibited the absorption rate of heavy metals by V.philippinaram,and confirmed the efficiency of natural zeolite in reducing the bioavailable fraction of heavy metals in sediment.

3.3.The role of natural zeolite on the accumulation of heavy metals into DGT

After adding natural zeolite into the sediments,the enrichment of heavy metals in V.philippinaram decreased significantly(t-test,P＜0.01),which might be due to the decreased bioavailability of heavy metals,as it is the most important driving force for bioaccumulation.Thus,the influence of natural zeolite amendment on the bioavailability of heavy metals in sediment using DGT devices(the accumulation of heavy metals in the fixed film M(ng)of the DGT was calculated according to Eq.(1))was further analyzed,using the DGTas a proxy for organisms living in sediment.The dry weight of the DGT was 0.0065 g and the results of the experiment are shown in Fig.3 and Table S3.

The concentration of the heavy metals(μg/g gel)in the sediments increased gradually with time in the zeolite-free group(Fig.3).The concentration of Cu,Pb,Cd,Cr and As in the DGT increased from 0 to 5.6,0.054,0.034,0.12 and 0.061μg/g gel,respectively.Since the accumulation of heavy metals in the DGT increased linearly with time,the slope of the regressed line wasused to characterize the rate of heavy metal enrichment.The enrichment rate for Cu,Pb,Cd,Cr and As was 0.19,1.9×10-3,1.2×10-3,4.4×10-3and 2.2×10-3,respectively.The concentration in the DGT of Cu,Pb,Cd,Cr and As in the zeolite-amended group increased with time from 0 to 2.9,0.036,0.02,0.1 and 0.04μg/g gel,respectively.The enrichment rate for Cu,Pb,Cd,Cr and As was 0.1,1.2×10-3,7×10-4,3.6×10-3and 1.4×10-3,respectively.Natural zeolite amendment led to both less adsorption capacity and adsorption rate of heavy metals by the DGT device.

Table 1 Absorption rate constant K1 of different heavy metals.

Fig.2.The heavy metal concentrations in V.philippinaram in the bioaccumulation experiment(error bars represent mean±SD,n=3).

Fig.3.The heavy metal concentrations in DGT in the bioaccumulation experiment(error bars represent mean±SD,n=2).

In comparison,the decrease in heavy metals in the DGT device after natural zeolite amendment was 45%,33%,41%,19%and 34%respectively for Cu,Pb,Cd,Cr and As,while the decrease in V.philippinaram was 44%,37%,54%,30%and 59%(Figs.1 and 2 and Table S3).The disparity might be mainly due to the different characteristics of the DGT device and V.philippinaram.The enriching process of heavy metals in V.philippinaram is complicated,and bioturbation might change the physical and chemical composition of sediments,thus affecting the occurrence,migration and transformation characteristics of heavy metals in sediments and the accumulation in organisms.In comparison,the DGTdevice is relatively stationary and biologically inert,and heavy metal accumulation is totally dependent on the available fraction in sediments.It is a purely physical adsorption process and does not involve biological activities.Therefore,for simulation of biological absorption of heavy metal in sediments by the DGT device,the deviations in prediction and simulation caused by extracellular perturbations and in vivo transformations need to be taken into account.

3.4.Biomimetic prediction of heavy metals in V.philippinaram using DGT

Enrichment of heavy metals in V.philippinaram and the DGT device in the zeolite-amended sediments was observed,despite the different enrichment efficiencies.The relationship between the two kinds of enrichment was further analyzed in the current study.Comparison of the heavy metal enrichment in V.philippinaram and in the DGT device revealed a significant positive correlation(P＜0.001,Fig.4),which indicated that the DGT device is a promising proxy for the prediction of heavy metal bioavailability and accumulation in organism living in sediments.

3.5.Comparison with previous studies

Roulier,Tusseau-Vuillemin,Coquery,Geffard,and Garric(2008)compared the relationship between the heavy metal content of the genus,mosquitoes,and a DGTover 7 days.The results showed that both Cu and Pb had a good linear relationship,but there was no obvious relationship for Cd.A similar result was also found in another study(Dabrin et al.,2012).Our study extended the heavy metal exposure time to 28 days and analyzed the relationship between the heavy metal content of clams and DGTand revealed good correlations for Cu,Pb and Cd.The different results obtained between our and previous studies may be due to the different duration of the experiment,the test organisms and the different compositions of the sediments analyzed.Simpson et al.(2012)also used double-shelled organisms for experiments that lasted for 30 days,but only Cu was studied.Both studies showed a good linear relationship between the accumulation of Cu by DGTs and the organisms.

Taking the aquatic habits of aquatic products into consideration,Amato,Simpson,Jarolimek,and Jolley(2014)placed the DGT within 5 mm of the sediment-water interface for 10 days of experimentation with amphipoda,whereas we inserted the DGT device into the sediment during the experiment for clams.Zn in the sediments of shallow estuary bays has previously been studied with a mixture of sand and zeolite(Simpson,Pryor,Mew burn,Batley,&Jolley,2002),however,the sand piles were ineffective,the simulated tidal process of the study was quite different from the actual situation and thus did not provide insight into real live processes in the real field.In our study,the zeolite was mixed with the bottom sludge at an early stage.After aging and addition of water,the experiment was then carried out to ensure that the experimental environment and the field water environment were not excessively different from each other.

4.Conclusions and prospects

Fig.4.Correlation between the heavy metal content in V.philippinaram and DGT device.

Adding natural zeolite to sediments(10%dry weight)can effectively decrease the free-dissolved concentration of heavy metals,and the equilibrium partition of heavy metals into natural zeolite can be reached in a relatively short period of time.In addition,natural zeolite amendment can also significantly reduce the accumulation of heavy metals in V.philippinaram and the first order absorption rate K1.Therefore,natural zeolite can be used as an ideal remediation material to control heavy-metal-polluted sediments.The reduction in the bioavailable heavy metals after natural zeolite amendment was generally between 60%and 70%,which could be further improved by exploiting other artificial/modified zeolites that have a different composition or characteristic.

The accumulation of heavy metals in V.philippinaram was significantly related to the adsorption of heavy metals into the DGT device,which indicated that the DGT device can not only used as a passive sampler reflecting the free-dissolved concentration of heavy metals in sediments,but also has potential as a proxy to predict the likely bioaccumulation of heavy metals in benthic organisms.The results of our study provide insight into the utilization of DGT devices in costal sediments and V.philippinaram in laboratory.The accumulation of heavy metals was affected by the experimental design,including the DGT weight and the duration of the experiment.Further studies focused on the effect of sediment characteristics and organism taxa,the in-situ environment on DGT application and the comparison between DGT method and the traditional method is warranted.

Acknowledgements

The present research was supported by Shanghai Science and Technology Commission Key Support Fund (18050502100),Shanghai Ocean University Technology Development Fund(A2-0203-00-100223)and Shanghai Ocean University Doctoral Foundation(A2-0203-00-100352).

Appendix A.Supplementary data

Supplementary data related to this article can be found at https://doi.org/10.1016/j.aaf.2018.05.003.

Aggeliki,M.,Anthimos,X.,&Ioannis,P.(2001).Stabilization Pb,Zn,and Cd-Contaminated soil by means of natural natural zeolite.Soil&Sediment Contamination An International Journal,10(3),251-267.

Alvarez,A.,Saez,J.M.,&Costa,J.S.D.(2017).Actinobacteria:Current research and perspectives for bioremediation of pesticides and heavy metals.Chemosphere,166,41-62.

Amato,E.D.,Simpson,S.L.,Jarolimek,C.,&Jolley,D.F.(2014).Diffusive gradients in thin films technique provide robust prediction of metal bioavailability and toxicity in estuarine sediments.Environmental Science and Technology,48(8),4485-4494.

Chen,M.,Ding,S.,Zhang,L.,Li,Y.,Sun,Q.,&Zhang,C.(2017).An investigation of the effects of elevated phosphorus in water on the release of heavy metals in sediments at a high resolution.The Science of the Total Environment,575,330-337.

Dabrin,A.,Durand,C.L.,Garric,J.,Geffard,O.,Ferrari,B.J.D.,&Coquery,M.(2012).Coupling geochemical and biological approaches to assess the availability of cadmium in freshwater sediment.The Science of the Total Environment,424,308-315.

Davison,W.,&Zhang,H.(1994).In situ speciation measurements of trace components in natural waters using thin-film gels.Nature,367(6364),546-548.

De,M.S.,Fowler,S.W.,Wyse,E.,et al.(2004).Distribution of heavy metals in marine bivalves,fish and coastal sediments in the Gulf and Gulf of Oman.Marine Pollution Bulletin,49(5-6),410.

Ding,S.M.,Jia,F.,Xu,D.,Sun,Q.,Zhang,L.,Fan,C.,et al.(2011).High-Resolution,two-dimensional measurement of dissolved reactive phosphorus in sediments using the diffusive gradients in thin films technique in combination with a routine procedure.Environmental Science&Technology,45(22),9680-9686.

Ding,S.M.,Xu,D.,Sun,Q.,Yin,H.B.,&Zhang,C.S.(2010).Measurement of dissolved reactive phosphorus using the diffusive gradients in thin films technique with a high-capacity binding phase.Environmental Science&Technology,44(21),8169-8174.

Fu,W.C.,Meng,F.P.,&Wang,Z.F.(2013).Heavy metals in the intertidal sediments and two marine bivalves along the Beibu Bay:Contamination status and bioaccumulation.Acta Scientiae Circumstantiae,33(5),1401-1409(in Chinese).

Gimpel,J.,Zhang,H.,&Hutchinson,W.(2001).Effect of solution composition,flow and deployment time on the measurement of trace metals by the diffusive gradient in thin films technique.Analytica Chimica Acta,448(1-2),93-103.

Gnanamani,A.,Kavitha,V.,Radhakrishnan,N.,et al.(2010).Microbial products(biosurfactant and extracellular chromate reductase)of marine microorganism are the potential agents reduce the oxidative stress induced by toxic heavy metals.Colloids and Surfaces B:Biointerfaces,79(2),334-339.

Hawthorne,S.B.,Grabanski,C.B.,Miller,D.J.,&Kreitinger,J.P.(2005).Solid-phase microextraction measurement of parent and alkyl polycyclic aromatic hydrocarbons in milliliter sediment pore water samples and determination of K-DOC values.Environmental Science&Technology,39(8),2795-2803.

Hui,K.S.,Chao,C.Y.H.,&Kot,S.C.(2005).Removal of mixed heavy metal ions in wastewater by natural zeolite 4A and residual products from recycled coal fly ash.Journal of Hazardous Materials,127(1-3),89.

J¨arup,L.(2003).Hazards of heavy metal contamination.British Medical Bulletin,68(1),167.

Kalantzi,I.,Shimmield,T.M.,&Pergantis,S.A.(2013).Heavy metals,trace elements and sediment geochemistry at four Mediterranean fish farms.The Science of the Total Environment,444(2),128-137.

Kwok,C.K.,Liang,Y.,&Wang,H.(2014).Bioaccumulation of heavy metals in fish and ardeid at Pearl River estuary,China.Ecotoxicology and Environmental Safety,106,62-67.

Li,J.,Lu,H.,Zhu,J.,et al.(2009).Aquatic products processing industry in China:Challenges and outlook.Trends in Food Science&Technology,20(2),73-77.

Liu,J.,Feng,X.,&Qiu,G.(2012).Prediction of methyl mercury uptake by rice plants(Oryza sativa L.)using the diffusive gradient in thin films technique.Environmental Science&Technology,46(20),11013.

Liu,Y.,Wang,Y.,&Yu,X.(2010).Investigation and evaluation of heavy metal content in aquatic products of Yangkou Port.Environment and Sustainable Development,35(1),35-37(in Chinese).

Li,J.X.,Wu,L.C.,&Zhang,J.(2015).Research progresses in remediation of heavy metal contaminated soils by biochar.Ecology and Environmental Sciences,24(12),2075-2081(in Chinese).

Montalvo,S.J.,Guerrero,L.E.,Milán,Z.,et al.(2011).Nitrogen and phosphorus removal using a novel integrated system of natural natural zeolite and lime.Journal of Environmental Science&Health Part a,46(12),1385-1391.

Motsi,T.,Rowson,N.A.,&Simmons,M.J.H.(2009).Adsorption of heavy metals from acid mine drainage by natural natural zeolite.International Journal of Mineral Processing,92(1-2),42-48.

Nakhla,G.,Zhu,J.,&Cui,Y.(2007).Liquid-solid circulating fluidized bed waste water treatment system for simultaneous carbon,nitrogen and phosphorus removal:US,US 7261811 B2.

Ngo,L.K.,Pinch,B.M.,&Bennett,W.W.(2016).Assessing the uptake of arsenic and antimony from contaminated soil by radish(Raphanus sativus)using DGTand selective extractions.Environmental Pollution,216,104.

Quan,M.,Gong,Y.L.,&Cui,G.T.(2010).Determination and evaluation of heavy metals in aquatic products in Luoyang.Shaanxi Journal of Agricultural Sciences,(3),36-38(in Chinese).

Roulier,J.L.,Tusseau-Vuillemin,M.H.,Coquery,M.,Geffard,O.,&Garric,J.(2008).Measurement of dynamic mobilization of trace metals in sediments using DGT and comparison with bioaccumulation in Chironomus riparius:First results of an experimental study.Chemosphere,70,925-932.

Sánchez López,F.J.,Gil Garcia,M.D.,Sánchez Morito,N.P.,&Martı nez Vidal,J.L.(2003).Determination of heavy metals in cray fish by ICP-MS with a microwaveassisted digestion treatment.Ecotoxicology and Environmental Safety,54(2),223-228.

Sarwar,N.,Imran,M.,&Shaheen,M.R.(2016).Phytoremediation strategies for soils contaminated with heavy metals:Modifications and future perspectives.Chemosphere,171,710-721.

Simpson,S.L.,Pryor,I.D.,Mew burn,B.,Batley,G.E.,&Jolley,D.F.(2002).Considerations for capping metal-contaminated sediments in dynamic estuarine environments.Environmental Science and Technology,36(17),3772-3778.

Simpson,S.L.,Yverneau,H.,Cremazy,A.,Jarolimek,C.,Price,H.L.,&Jolley,D.F.(2012).DGT-induced copper flux predicts bioaccumulation and toxicity to bivalves in sediments with varying properties.Environmental Science and Technology,46(16),9038-9046.

Tao,Y.,Yuan,Z.,Xiaona,H.,et al.(2012).Distribution and bioaccumulation of heavy metals in aquatic organisms of different trophic levels and potential health risk assessment from Taihu lake,China.Ecotoxicology and Environmental Safety,81(81),55-64.

Tessier,A.,P Campbell,P.,&Bisson,M.(1979).Sequential extraction procedures speciation of particulate trace metals.Analytical Chemisty,51(7),848.

Turkmen,M.,Turkmen,A.,&Tepe,Y.(2009).Determination of metals in fish species from Aegean and Mediterranean seas.Food Chemistry,113(1),233-237.

Wang,Y.,Ding,S.,&Gong,M.(2016).Diffusion characteristics of agarose hydrogel used in diffusive gradients in thin films for measurements of cations and anions.Analytica Chimica Acta,945,47-56.

Wang,L.,Zhao,Z.,&Chen,L.(2007).Survey of enrichment of heavy metals in food chain of quatic animals in Baiguishan Reservoir.China Tropical Medicine,7(11),30-32(in Chinese).

Webb,J.A.,&Keough,M.J.(2002).Measurement of environmental trace-metal levels with transplanted mussels and diffusive gradients in thin films(DGT):A comparison of techniques.Marine Pollution Bulletin,44(3),222-229.

Wu,C.,Maurer,C.,Wang,Y.,et al.(1999).Water pollution and human health in China.Environmental Health Perspectives,107(4),251.

Xu,D.,Chen,Y.,Ding,S.M.,Sun,Q.,Wang,Y.,&Zhang,C.(2013).Diffusive gradients in thin films technique equipped with a mixed binding gel for simultaneous measurements of dissolved reactive phosphorus and dissolved iron.Environmental Science&Technology,47(18),10477-10484.

Yang,Z.F.,Yao,C.Y.,&Xu,L.L.(2012).Study on rural environment problems and countermeasures during China's acceleration of urbanization.Advanced Materials Research,347-353,1836-1846.

Yin,H.,Cai,Y.,&Duan,H.(2014).Use of DGT and conventional methods to predict sediment metal bioavailability to a field inhabitant freshwater snail(Bellamya aeruginosa)from Chinese eutrophic lakes.Journal of Hazardous Materials,264(2),184-194.

Yi,Y.,Yang,Z.,&Zhang,S.(2011).Ecological risk assessment of heavy metals in sediment and human health risk assessment of heavy metals in fishes in the middle and lower reaches of the Yangtze River basin.Environmental Pollution,159(10),2575-2585.

Zhang,C.,&Jiang,Q.J.(2010).Problems in China's aquatic product quality safety and countermeasures.Journal of Shanxi Agricultural Sciences,38(3),61-64.