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Environmental Pollution 159 (2011) 1123e1128

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Environmental Pollution

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In situ measurements of concentrations of Cd, Co, Fe and Mnin estuarine porewater using DGT

Zhihao Wu*, Mengchang He, Chunye LinState Key Laboratory of Water Environment Simulation, School of Environment, Beijing Normal University, No. 19, Xinjiekouwai Street, Beijing 100875, China

Vertical profiles of metals were measured by the in situ application of

DGT (diffusive gradients in thin films).

a r t i c l e i n f o

Article history:Received 3 September 2010Received in revised form25 December 2010Accepted 8 February 2011

Keywords:In situPorewaterDGT (diffusive gradients in thin films)SedimentChelex resin

* Corresponding author.E-mail addresses: wzh7185@sohu.com (Z. Wu), he

0269-7491/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.envpol.2011.02.015

a b s t r a c t

Vertical profiles of metals were measured by the in situ application of DGT (diffusive gradients in thinfilms). Well-defined laboratory systems ensured the good reproducibility and precision DGT accumulatedmetals on a chelex resin after diffusional transport through a layer of hydrogel. Three kinds ofDGT probes (with three thicknesses of diffusive gel: 0.40 mm, 0.80 mm and 1.92 mm) measured inter-facial concentration and induced flux from solid to solution phase which had intricate variations withdepth. The DGT induced flux and interfacial concentration of four metals belonged to “partially sus-tained” state, indicating a labile equilibrium of metals between solid phase and porewater. Theconcentration profiles showed the good correlation between Mn and Co and peak concentrations of Mnand Fe between �2.00 cm and �5.25 cm with depth.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Estuary is an important stage in the global sedimentary cycle,while estuarine sediments are often the sinks of metal contami-nants in aquatic system and are characterised by a sequence ofchanges in porewater chemistry, which arise due to microbialrespiration and result in the well known vertical sequence ofbiogeochemical zones (Chapam and Wang, 2001). Cd, Co, Fe andMn are four important elements involved in the biogeochemicalprocesses, precipitation, absorption, diffusion, mineralization,reactivity, mobility, toxicity, bioavailability in the sediment/waterinterface.

Davison and Zhang (Davison and Zhang, 1994) developeda simple in situ procedure for measuring concentrations of dis-solved metal species simultaneously and induced fluxes of severalmetals in porewater at high spatial resolution, which preceded theconventional methods of squeezing (Bender et al., 1987), coring andcentrifugation (Hamilton-Taylor andMorris, 1985), sippers (Watsonand Frickers, 1990) and in situ dialysis peepers (Hesslein, 1976).DGT was operated by inducing a controlled perturbation into thesediment and the resultant measurements reflect the response ofthe sediment to that perturbation. DGT accumulates metals ona chelating resin after their diffusive transport through a layer of

mc@bnu.edu.cn (M. He).

All rights reserved.

hydrogel (Fig. 1). The concentration of metal at the surface of thedevice during its deployment is derived from the measured mass ofmetal accumulated in a known time (Davison et al., 1997). DGTmeasurements have been interpreted to provide in situ informationon labile metal species in seawater (Davison and Zhang, 1994);remobilisation fluxes and concentration profiles at high resolution(1 mm) in surficial freshwater sediments (Zhang et al., 1995); ultra-high resolution (100 mm) profiles in microbial mats (Davison et al.,1997); and remobilisation fluxes in soils (Zhang et al., 1998). High-resolution measurements using the in situ diffusive gradients inthin films (DGT) have shown that surface sediment may be highlystructured vertically (Zhang et al., 1995) and horizontally (Davisonet al., 1997) at a scale of millimetre or less.

Net Fe production throughout the upper sub-oxic and anoxicbiogeochemical zones can be determined by DGT. The results areused to determine the changes of Fe(Ⅱ) production at different depthintervals which provide information about the major pathwaysinvolved. Fe recycling in sediment may be driven by the oxidationand precipitation caused by dissolved oxygen or other oxidants insurface sediment. Based on the last reason, Fe and the other metal(Mn) which have the similar chemical character are called redox-sensitive metals (Hyacinthe et al., 2001; Fones et al., 2004).

This oxidation of organicmaterial in sediment is accompanied bythe reduction of electron acceptors such as: O2, NO3

�, Mn(Ⅳ), Fe(Ⅲ)and SO4

2- (Tessier, 1992; Zhang et al., 1995). The conversion of solidphase Fe and Mn to their reduced, dissolved form of Mn(Ⅱ) andFe(Ⅱ) may liberate other trace metals absorbed or bound to them.

Fig. 1. Representation of the concentration of an ionic species in a DGT probe andadjacent porewater during deployment is: fully sustained (case a); partially sustained(case c); or diffusion only (case b) by resupply from the solid phase in sediment.

Fig. 2. Picture and schematic drawing of a DGT probe. A: Diffusive gel. B: Resin gel. C:Membrane filter. D: Front window plate.

Z. Wu et al. / Environmental Pollution 159 (2011) 1123e11281124

424FeðOHÞ3þOrganic Matterþ 704CO2/424Fe2þ

þ 846HCO�3 þ 16NH3 þ 320H2Oþ HPO2�

4 (1)

It has also been demonstrated that hydrogen sulphide producedduring bacterial sulphate reduction (S-R) can react with iron(oxyhydr)oxides minerals to form Fe2þ and oxidised sulphurproducts (Pyzik and Sommer, 1981; Yao and Millero, 1996):

8FeðOHÞ3þ16Hþ þ 8HS�/8Fe2þ þ S8 þ 24H2O (2)

Subsequent reaction with additional dissolved sulphide resultsin the formation of iron monosulphides, which are importantprecursors to pyrite formation (Rickard, 1997):

Fe2þ þ HS�%FeS�s�þHþ (3)

The labile organic fraction decomposes, ultimately to yieldinorganic N, P and trace metals. Thus, the process of organicdecomposition induces a wide range of interrelated chemicalgradients in the immediate vicinity of the interface associated withredox processes. Remobilisation of trace metals (Fe, Mn, Co and Ni)may be important in determining porewater concentrations andfluxes.

In this work, DGT is used tomeasure concentrations of Cd, Co, Feand Mn in porewater of surface sediment of the estuary of DaliaoRiver in order to provide in situ data of the induced flux andconcentration profile. Three kinds of DGT probes (with diffusivegel: 0.40 mm, 0.80mm and 1.92 mm) were used. The article reportsthe determined results of the deployment, describes the deploy-ment method and discusses the practical utility of DGT. The aim ofthis experiment is to investigate complex profiles of the interfacialporewater concentration, the induced flux from porewater toprobes, the reproducibility and accuracy of the operation, thecapacity of the solid phase to resupply solute to porewater and therelease of metals from sediment.

2. Materials and methods

2.1. Sample collection, conventional procedure for porewater and sediment

The sediments of the estuary of Daliao River and Yingkou Bay in the eastern partof Liaodong coast (Northeast China) are fine-grained and organic rich (silty mud)and the pollution level of the four metals is relatively low. The concentrationof metals in surface sediment is: C(Mn) ¼ 108 mg/kg; C(Fe) ¼ 176 mg/kg; C

(Co)¼ 12.78 mg/kg; C(Cd)¼ 1.23 mg/kg. The sampling site (Latitude N: 40�40033.800;longitude E:122�08054.300) is at a cay near the estuary. The sampling operation wasexecuted on July 25, 2009. Before the ebb-tide, the sampling boat with DGTinstallation and other apparatus arrived at this site. Three kinds of DGT probes (withdiffusive gel: 0.40 mm; 0.80 mm; 1.92 mm) were inserted into the sediment andoperated to measure. After 24 h, the probes were retrieved and determined inonshore labs. The detailed deployment of probes would be described in, Section 2.3.

At the same time of the deployment of DGT probes, the core samples wereprocessed for conventional extraction (centrifugation) of porewater and the deter-mination of the overlying water and sediment were also conducted (Zhang et al.,2002). Exposure to atmospheric condition was kept to an absolute minimum at alltimes during the processing of the core. The 1-m core tube was inserted intoa specially designed glove box purged with nitrogen. Sliced samples were trans-ferred to centrifuge tubes and sealed under a nitrogen atmosphere. After the corehad been processed, the samples were centrifuged at 3000 rev/min for 10 min toseparate out the porewater into vials containing the fixing reagents. The interstitialwater was recovered with 30-mL syringes attached with 4-cm lengths of siliconetube. The tube was replaced with a 25-mm, 0.45-mm disposable filter and theporewater was filtered into reservoir vials, from which appropriate aliquots werepipetted. Then, the analysis of metals, sulphate and sulfide in porewater was pro-cessed according to the method (Zhang et al., 2002). Sulphate concentrations weremeasured using a Dionex DX100 ion chromatograph. Sulfide was fixed andmeasured on a LKB Ultrospec 4050 spectrophotometer using the methylene bluemethod (Cline, 1969). Vertical distribution of the concentration of sulphate andsulfide and EH was shown in Fig. S1 (Supporting information).

2.2. Preparation of DGT probe

The perspex DGT probe (supplied by DGT Research Ltd, UK) was 240 mm �40 mm � 5 mm, with a 150 � 18 mm window open to the sediment (Fig. 2). Anagarose-polyacrylamide hydrogel comprising 15% acrylamide (Merch) and 0.3%agarose derived cross-linker (supplied by DGT Research Ltd, UK) was used as the,diffusive gel. Ion-exchange resin (suspended particulate reagent-iminodiacetate,(SPR-IDA)) with a bead size of 0.2 mm impregnated in a thin gel (0.40 mm thickness),was used as the binding layer. The preparation of both the resin and diffusion gel hasbeen previously described in detail (Davison et al., 2000). During casting, the chelexbeads became concentrated due to gravitational settling on one side of gel. This sidewas arranged to face upwards, and a layer of polyacrylamide gel was placed on top ofit. A Millipore 0.45 mm filter (thickness: 0.13 mm) was placed on top of this beforefinally it was covered with 1 mm perspex plate having a 150 � 18 mm2 window. Thewhole assembly was held together by plastic screws at 2.5 cm interval. Three kindsof DGT probes (with three diffusive gel: 0.40 mm; 0.80 mm; 1.92 mm) wereprepared. To avoid any introduction of oxygen to the sediment during operation, theprobes were deoxygenated by immersing DGT in solution with 0.01 mol L�1 NaCland Chelex-100 and bubbling with nitrogen for 24 h. Then they were stored ina clean Ziplock plastic bag at �5 �C before experiment. After deoxygenization, the

Z. Wu et al. / Environmental Pollution 159 (2011) 1123e1128 1125

probe was exposed to air for only a few seconds before the insertion into thesediment.

2.3. DGT operation and retrieval

An detailed description of DGT operation and retrieval was given by Zhang et al.(1995). Diagram and picture of this experiment in relation to the insertion of DGTprobes in sediment are shown in Fig. S2. Three core tubes were pushed into thesediment (horizontal distance of 2 cm) in order to avoid horizontal heterogeneity ofsediment (Zhang et al., 2002). Three probes (with 0.40 mm diffusive gel) were takenfrom the Ziplock plastic bag and amark (a fine line) was made on the side at 2.00 cmbelow the top of the window. The first probe was pushed gently and smoothly intothe sediment of the tube until the mark was in line with the sediment/waterinterface. During the insertion, the probe was kept as straight as possible in order toavoid the disturbance in sediment/water interface. The next two probes wereoperated in the same way as the previous one. Then, a cover was put over the firsttube. Other two kinds of probes (with the 0.80 mm and 1.92 mm diffusive gel) wereoperated at other two tubes in the same way as the operation of DGT0.40. Thetemperature (23 �C) and the time of deployment were noted down. After 24 h, threekinds of probes were pulled out the sediment and the surface of them were rinsedwith MilliQ water to avoid preservation of the particles on the surface of probesespecially the window area. The probes were put in a cleaned plastic bag and werekeep in a refrigerator (�5 �C) before analysis.

2.4. Sample treatment and analysis

DGT gels were cut at the mark of sediment/water interface using a Teflon coatedblade. Along the window edges without disassembling the probe, DGT gels were cutagain and the gels together with the filter membranes were lift out of the windowsand lay them on a clean Perspex sheet. The filter membrane and the diffusive gelwere pealed off and the resin gel was left on the sheet. The resin gel was cut into5.0 mm (vertical) � 18 mm (horizontal) segments. Each gel strip was put intoa centrifuge vial (0.50 mL) and 0.20 mL of 1 mol/L HNO3 solution was added (User’guide to DGT technique). After 24 h, the samples were diluted ten times and ana-lysed. Co, Mn and Cd can be determined by Inductively coupled plasma massspectrometry (ICP-MS, X Series Ⅱ) using a direct injection nebulizer. 103Rh was usedfor internal calibration and the certified reference material (GSD-9) was supplied bythe Institute of Geophysical and Geochemical Exploration of China. The concentra-tion of Fe was determined by using Inductively Coupled Plasma Optical EmissionSpectrometry (ICP-OES, IRIS Instrepid Ⅱ).

2.5. Calculation procedures

The mass of metals (M) accumulated on the resin gel can be calculated from theconcentration of metals in the elution solution (Ce) using the equation:

M ¼ Ce�Vgel þ Vacid

�.fe (4)

Vgel is the volume of the gel strip, Vacid is the acid for elution and fe is the elutionfactor. Some researchers have investigated the elution factor and chosen HNO3 aselution solution with the volume of 0.5e2.0 mL and concentration of 1M. Stableelution (fe ¼ 0.80) can be attained for elution (Zhang et al., 1995; Jansen et al., 2001).In this research, the value of elution factor was chosen as 0.80.

The fluxes (F) induced by DGT can be calculated using the equation:

F ¼ M=ðtAÞ (5)

where t is deployment time (in second) and A is the area of the gel strip (in cm2). Theconcentration of metal measured by DGT (CDGT) can be calculated using theequation:

CDGT ¼ FDg=D (6)

Where Dg is the thickness of the diffusive gel (0.40 mm, 0.80 mm or 1.92 mm)plus the thickness of the filter membrane (0.13 mm), D is the diffusion coefficientof metal in the gel recommended by Zhang et al. (1995, 2002). At 23 �C,DCd ¼ 5.77 � 10�6 cm2/s, DCo ¼ 5.63 � 10�6 cm2/s, DFe ¼ 5.79 � 10�6 cm2/s,DMn ¼ 5.54 � 10�6 cm2/s.

Table 1Mean interfacial concentrations and accumulated mass per unit area measured by DGTAccumulated mass per unit area measured by blank DGT probe (with the 0.80 mm diffu

C0.4 C0.8 C1.92 M/A0.4

Cd 0.27(mg/L) 0.31(mg/L) 0.95(mg/L) 2.50(ng/cm2)Co 0.48(mg/L) 0.60(mg/L) 0.82(mg/L) 4.41(ng/cm2)Fe 1.78(mg/L) 2.10(mg/L) 2.74(mg/L) 16.8(mg/cm2)Mn 0.52(mg/L) 0.90(mg/L) 1.80(mg/L) 4.71(mg/cm2)

Three cases may arise with respect to the DGT induced flux and interfacialconcentration (Fig. 1)

a Fully sustained. Metals removed from the porewater by the DGT probe arerapidly resupplied from solid phase; the flux to the device is effectivelymaintained at its maximal value throughout the operation. Ci is equal to thebulk porewater concentration, Cb. In this experiment, the average concentra-tions of the bulk porewater in sediment have been determined: ci(Cd) ¼ 1.16 mg/L, ci (Co) ¼ 0.98 mg/L, ci (Fe) ¼ 3.52 mg/L, ci (Mn) ¼ 2.43 mg/L.

b Diffusion only. There is no resupply from the solid to porewater. The supply ofmetals to the DGT probe is solely by diffusion through the porewater. The fluxinduced by DGT decreases with operation time due to the depletion of theporewater concentration. For a probe with 0.80 mm thick diffusive gel, theaverage flux for 24 h deployment is approximately 10% of the initially estab-lished flux and Ci z 0.1Cb (Harper et al., 1998, 1999).

c Partially sustained. There is some resupply of metals from solid phase toporewater, but it is insufficient to maintain the maximum flux induced by DGT.Depletion in porewater concentrations occurs, but to a lesser degree than forcase b (0.1Cb < Ci < Cb).

Clearly, depending on the rate and extent of resupply from the solid phase, Cishould be similar to or a little less than bulk porewater concentration. In practise, theabove three cases can be identified by deploying two DGT probes with differentdiffusive gel layer thicknesses,Dg1 (thick) and Dg2 (thin), for the same period of timeand obtaining interfacial concentration, C1 and C2. For the fully sustained case,a constant concentration gradient is maintained across the diffusive gel, as for DGTin well-stirred solutions. The interfacial concentration equals the bulk concentra-tion, Cb and is therefore, the same irrespective of diffusion layer thickness (C1 ¼ C2).For the “diffusion only” case, the concentration gradient is mainly within the sedi-ment, minimizing the effect of the diffusive gel; then, F1 ¼ F2 and C1/C2 ¼ Dg1/Dg2;that is, the interfacial concentration will be larger for thicker gel (for the conditionsused in this work Dg1/Dg2 ¼ 2.5). For the partially sustained case, there will bea difference in measured interfacial concentration for the two gels, but sopronounced [C1(Dg2/Dg1) < C2 < C1].

2.6. Accuracy

The fundamental accuracy and precision of the DGT procedure can be judged byreplicate operation of standard DGT devices in known solutions, which was �4% forthis lab. Other labs have reported �3% (Sangi, 1998) and �10% (Denney et al., 1999).All this variability is associated with the reproducibility of gel casting and thickness,the elution procedure, the instrumental analysis and operation in the stirred solu-tion. Blank DGT probes were prepared and subjected to the same sample pre-treatment and post-treatment procedures as the deployed probes. The resultsdetermined by DGT with 0.80 mm diffusive gel for porewater are stable and thisprobe is often chosen as the deployment probe in practise (Zhang et al., 1995;Do�cekalová and Divi�s, 2005). So, the investigation of blank DGT probe with0.80 mm diffusive gel is important. The accumulated mass per unit area of eachmetal measured in the blank assembly (with the 0.80mmdiffusive gel) (Table 1) wasmuch lower than the mean measured mass per unit area measured in the deployedprobe. Eluent concentrationwas well within the acceptable sensitivity range for ICP-MS and ICP-OES. Analytical quality control was ensured by measurement of thesame standard solution after every 18th sample. The percentage relative standarddeviations of these standards for the analysis were less than 4% for all metals(Table 1).

3. Results and discussion

3.1. Concentrations and induced fluxes of Fe

The mean interfacial concentration, induced flux and verticalconcentration of Fe are shown in Table 1 and Fig. 3. As a whole, forthe three profiles, the major increase occurred at a depth ofapproximately �2.25 cm, with maximum between �3.75 cm

probes (with the 0.40 mm, 0.80 mm and 1.92 mm diffusive gel) for the sediment;sive gel); The result of quality control of the analysis.

M/A0.8 M/A1.92 M/A(blank) RSD(QC)

1.66(ng/cm2) 2.31(ng/cm2) 0.01(ng/cm2) 2.87%3.16(ng/cm2) 1.95(ng/cm2) 0.04(ng/cm2) 1.76%11.3(mg/cm2) 6.70(mg/cm2) 0.06(mg/cm2) 3.03%4.66(mg/cm2) 4.21(mg/cm2) 0.09(mg/cm2) 3.65%

-14

-12

-10

-8

-6

-4

-2

0

2

4

0.0 0.8 1.6 2.4 3.2 4.0

Concentration determined by DGT(0.40mm)

Dep

th(c

m)

Cd(µg/L)

Co(µg/L)

Fe(µg/mL)

Mn(µg/mL)

-14

-12

-10

-8

-6

-4

-2

0

2

4

0.0 0.8 1.6 2.4 3.2 4.0 4.8 5.6

Concentration determined by DGT(0.80mm)

Dep

th(c

m) Cd(µg/L)

Co(µg/L)

Fe(µg/mL)

Mn(µg/mL)

-14

-12

-10

-8

-6

-4

-2

0

2

4

0.0 0.8 1.6 2.4 3.2 4.0 4.8 5.6 6.4 7.2

Concentration determined by DGT(1.92mm)

Dep

th(c

m)

Cd(µg/L)

Co(µg/L)

Fe(µg/mL)

Mn(µg/mL)

C

B

A

Fig. 3. Vertical concentration of Cd, Co, Fe and Mn in sediment of estuary of Daliao River in July 2009 measured by three DGT probes (with three thicknesses of diffusive gel:0.40 mm, 0.80 mm and 1.92 mm) [A:0.40 mm; B:0.80 mm; C:1.92 mm].

Z. Wu et al. / Environmental Pollution 159 (2011) 1123e11281126

and �5.25 cm, and then declined until the bottom of the DGT.For the DGT0.40, the increase of concentration occurred at a depthof approximately �2.25 cm, then the curve continued to increasewith one peak at �3.75 cm (3.66 mg/L) and then decreased grad-ually until the bottom of the DGT (1.59 mg/L). There wasa systematic decrease of Fe concentration (2.21mg/L at�6.25 cm to1.59 mg/L at �13.00 cm). The pattern for DGT0.80 and DGT1.92was somewhat complicated. For the DGT0.80, the concentrationof Fe increase occurred at a depth of approximately �1.75 cm,then the curve continued to increase with two adjacent peaks: oneat �4.25 cm (5.26 mg/L), the other at �5.25 cm (4.71 mg/L). Forthe DGT1.92, the concentration of Fe increased at a depth of

approximately�1.75 cm, then increased to two adjacent peaks: oneat �3.75 cm (6.55 mg/L), the other at �5.25 cm (6.37 mg/L),afterwards, the concentration decreased gradually until the bottomof the probe (1.74 mg/L). Based on the theory reported in Section2.5, it can be inferred that the DGT induced flux and interfacialconcentration of Fe belong to “partially sustained” state.

3.2. Concentrations and induced fluxes of Cd

The mean interfacial concentration, induced flux and verticalconcentration of Cd are shown in Table 1 and Fig. 3. For the DGT0.40,the concentration of Cd increased gradually from 0.16 mg/L (the

Z. Wu et al. / Environmental Pollution 159 (2011) 1123e1128 1127

sediment/water interface) to 0.74 mg/L at �4.75 cm with a subor-dinate peak of 0.48 mg/L at �3.75 cm, afterwards, the concentrationdecreased gradually until the bottom of the DGT (0.17 mg/L). For theDGT0.80, the concentration of Cd decreased gradually from 0.10 mg/L(the interface) to reach two different peaks: 0.56 mg/L at �5.75 cm,0.66 mg/L at �9.8 cm, afterwards, the curve gradually reduced tolow value at the bottom of the DGT (0.23 mg/L). For the DGT1.92, theconcentration of Cd increased gradually from 0.51 mg/L (the inter-face), then the curve appeared with twomaximum peaks: 2.54 mg/Lat �4.25 cm, 3.22 mg/L at �5.75 cm, afterwards, the concentrationdecreased gradually and fluctuated until the bottom of the DGT(0.27 mg/L). Based on the theory reported in Section 2.5, it can beinferred that the DGT induced flux and interfacial concentration ofCd belong to “partially sustained” state.

3.3. Concentrations and induced fluxes of Mn

The mean interfacial concentration, induced flux and verticalconcentration ofMn are shown in Table 1 and Fig. 3. For the DGT0.40,the concentration of Mn increased gradually from 0.31 mg/L (theinterface) with a peak: 0.85 mg/L at �2.75 cm, afterwards, thevalues oscillated in a sinusoidal way and decreased to reach a lowvalue at the bottom of the DGT (0.44 mg/L). For the DGT0.80, theconcentration of Mn increased gradually from 0.22 mg/mL (theinterface) to a peak (2.07 mg/L at �3.25 cm), then the curve quicklydecreased to 0.67 mg/L (at �5.25 cm) and fluctuated until thebottom of the DGT (0.79mg/L). For the DGT1.92, the concentration ofMn increased gradually from 0.49 mg/L (the interface) to a peak(3.35 mg/L at �2.25 cm), then the curve quickly decreased to1.93 mg/L at �5.75 cm and fluctuated until the bottom of the DGT(1.81 mg/L). Based on the theory reported in Section 2.5, it can beinferred that the DGT induced flux and interfacial concentration ofMn belong to “partially sustained” state. As a whole, the broadfeatures of the three DGT profiles of Mn were similar.

3.4. Concentrations and induced fluxes of Co

The mean interfacial concentration, induced flux and verticalconcentration of Co are shown in Table 1 and Fig. 3. For the DGT0.40,the concentration increased gradually from the top of the DGT gel(0.18 mg/L), then the curve went through inflexion under the sedi-ment/water interface and appeared double peaks: 0.78 mg/Lat �2.75 cm; 0.76 mg/L at �3.75 cm; then the curve decreased,fluctuated, came forth two lower peaks: 0.70 mg/L at �6.25 cm and0.68 mg/L at�7.25 cm, and then declined to low value at the bottomof the probe (0.27 mg/L). For the DGT0.80, the concentrationincreased gradually from 0.15 mg/L at the interface to a very sharppeak (2.67 mg/L at �3.25 cm), then the curve decreased gradually,waved and appeared a secondary peak of 0.90 mg/L at �9.25 cm,finally it went down to low value (0.12 mg/L) at the end of the probe.For the DGT1.92, the concentration of Co was low (0.06 mg/L) at thetop of the gel, increased gradually to a very sharp peak (2.28 mg/Lat �2.25 cm), then the concentration of Co decreased rapidly toa low value (0.64 mg/L at�5.75 cm), and then appeared a secondarypeak of 1.49 mg/L at �6.75 cm, after that value, the curve undulatedand decreased until the bottom of the DGT (0.70 mg/L). Based on thetheory reported in Section 2.5, it can be inferred that the DGTinduced flux and interfacial concentration belong to “partiallysustained” state. The broad features of the Co and Mn profiles(Fig. 3) obtained by three DGT probes were similar. The goodcorrelation between the concentration profiles of Co and Mn isshown in Fig. S3. So, it can be concluded that Co was to some extentcoupled with the redox cycle of Mn (Shaw et al., 1990; Santos-Echeandía et al., 2009). The maxima of Mn and Co appeared ata depth from �2.25 cm to �3.75 cm in the sub-oxic zone. In this

zone, Mn oxyhydroxides became the dominant electron acceptorsand engendered the sorption of Co. Lienemann et al. (1997) haveverified this conclusion by elemental analysis and electronmicroscopy.

3.5. Redox processes in sediment and concentration profiles

Through the analysis of Eh and the character of sediment, fourredox character can be differentiated to oxidation (400 w 550mv),weak oxidation (200 w 400mv), weak reduction (0 w 200mv) andreduction (0 w �200mv) (Vershinn and Rozanov, 1982). Eh main-tained comparatively low (Fulton et al., 2004; Yu et al., 2010) in thevertical profile (�154.68 w 94.45mv) (Fig. S1). The sediment withthe Eh value (�200 w þ200mv) was the main zones controlled byFe, Mn, C and S. So, the sediment belonged to two zones: weakreduction (þ2 w �4 cm) and reduction (�4 w �13 cm). Based onthe theory in Section 1, the sequence of biogeochemical zones wasused in this research as a framework to discuss changes inconcentration profiles. The sulphate concentration was almostconstant throughout the Fe-R zone (down to �4.50 cm), but thendecreased below �4.50 cm (top of the S-R zone) (Jorgensen, 1997;Aller and Rude,1988). The concentration of sulphide in the S-R zonewas very low (0.045 mM at �4.50 cm to 0.054 mM at �10.00 cm)and was slightly high (0.054 mM at �10.00 cm to 0.067 mM at thebottom of the DGT) (Fig. S1). Such a profile could be due toproduction of Fe(Ⅱ) in the Fe-R zone and subsequent transport andremoval of dissolved Fe by precipitation as iron monosulphide atthe boundary within the S-R zone where free dissolved sulphidefirst appears (Zhang et al., 2002; Krom et al., 2002). The verticalconcentration of Fe in sediment has verified this theory. The sharppeaks of Fe between �3.75 cm and �5.25 cm validated microbialiron reduction (Fe-R) and the reaction of sulphide with iron (oxy-hydr)oxides (Hyacinthe et al., 2001; Pyzik and Sommer, 1981;Rickard, 1997). Mn is another redox-sensitive metal and its reduc-tion is favoured thermodynamically (Santos-Echeandía et al.,2009). Vertical concentration of Mn rised at a shallower depththan that of Fe, which reflected the more reductive dissolution ofMn oxyhydroxides through the next equation: CH2Oþ2MnO2þ6Hþ ¼ CO2þ2Mn2þþ3H2O (Doulglas and Adeney, 2000).

4. Conclusions

The DGT technique provided an in situ mean of perturbing andmeasuring the chemical equilibrium in a sediment/water system ina controlled, quantitatively defined way. Well-defined laboratorysystems were used to ensure the reproducibility and accuracy ofthe DGT technique. It measured, at fine spatial interval, theconcentration and induced flux of four metals: Fe, Mn, Co and Cdafter diffusional transport through a layer of gel of known thick-ness, which was operated in estuary sediment environment. It canbe judged that the DGT induced fluxes and interfacial concentra-tions of Fe, Mn, Co and Cd belong to “partially sustained” state,indicating a labile equilibrium of these metals between solid phaseand porewater. The fine distribution of the concentration of fourmetals in the sediment/water interface was shown in curves(fluctuation, sharp peak, secondary peak, increment, diminution,minimum, maximum and similarity) determinated by three kindsof DGT probes (with three thicknesses of diffusive gel: 0.40 mm,0.80 mm and 1.92 mm). The vertical concentrations of Co and Mnhad good correlation. There were peak concentrations of Fe andMnbetween �2.00 cm and �5.25 cm in the concentration profiles. Forother redox-sensitive metal (Mn), the broad features of the threeDGT profiles were similar, concentration of Mn rised at a shallowerdepth than that of Fe, which reflected the more reductive dissolu-tion of Mn oxyhydroxides. Based on the good correlation between

Z. Wu et al. / Environmental Pollution 159 (2011) 1123e11281128

the concentration profiles of Co and Mn, it can be concluded thatCo was to some degree coupled with the redox cycling of Mn.

Future multielement analysis could profoundly improve ourunderstanding of sediment geochemistry.

Acknowledgements

The author acknowledges the financial support from theNational Key Basic Research and Development Plan of China (973Program, 2004CB418502)

Appendix. Supplementary material

Supplementary data related to this article can be found online atdoi:10.1016/j.envpol.2011.02.015.

References

Aller, R.C., Rude, P.D., 1988. Complete oxidation of solid phase sulfides by manga-nese and bacteria in anoxic sediments. Geochimica et Cosmochimica Acta 52,751e765.

Bender, M., Martin, W., Hess, J., Sayles, F., Ball, L., Lambert, C., 1987. A whole coresqueezer for interfacial pore-water sampling. Limnology and Oceanography 32,1214e1225.

Chapam, P.M., Wang, F., 2001. Assessing sediment contamination in estuaries.Environmental Toxicology and Chemistry 20, 3e22.

Cline, J.D., 1969. Spectrophotometric determination of hydrogen sulfide in naturalwaters. Limnology and Oceanography 14, 454e458.

Davison, W., Zhang, H., 1994. In situ speciation measurements of trace componentsin natural-waters using thin-film gels. Nature 367, 546e548.

Davison, W., Fones, G., Grime, G.W., 1997. Dissolved metals in surface sediments anda microbial mat at 100 mm resolution. Nature 387, 885e888.

Davison, W., Fones, G., Harper, M., Teasdale, P., Zhang, H., 2000. Dialysis, DET andDGT: in situ diffusional techniques for studying water, sediments and soils. In:Buffle, J., Horvai, G. (Eds.), In Situ Monitoring of Aquatic Systems e ChemicalAnalysis and Speciation. IUPAC, Wiley, Chichester, Great Britain, pp. 495e569.

Denney, S., Sherwoos, J., Leyden, J., 1999. In situ measurements of labile Cu, Cd andMn in river waters using DGT. The Science of the Total Environment 239, 71e80.

Doulglas, G., Adeney, J., 2000. Diagenetic cycling of trace elements in the bottomsediments of the Swan River Estuary, Western Australia. Applied Geochemistry15, 551e556.

Do�cekalová, H., Divi�s, P., 2005. Application of diffusive gradient in thin films tech-nique (DGT) to measurement of mercury in aquatic systems. Talanta 65,1174e1178.

Fones, G.R., Davison, W., Hamilton-Taylor, J., 2004. The fine-scale remobilization ofmetals in the surface sediment of the North-east Atlantic. Continental ShelfResearch 24, 1485e1504.

Fulton, J.R., Mcknight, D.M., Foreman, C.M., Cory, R.M., Stedmon, C., Blunt, E., 2004.Changes in fulvic acid redox state through the oxycline of a permanently ice-covered Antarctic lake. Aquatic Sciences 66, 27e46.

Hamilton-Taylor, J., Morris, E.B., 1985. The dynamics of iron and manganese insurface sediments of a seasonally anoxic lake. Archiv für Hydrobiologie e

Supplement 72, 135e165.

Harper, M.P., Davison, W., Zhang, H., Tych, W., 1998. Kinetics of metal exchangebetween solids and solutions in sediments and soils interpreted from DGTmeasured fluxes. Geochimica et Cosmochimica Acta 62, 2757e2770.

Harper, M.P., Davison, W., Tych, W., 1999. Estimation of porewater concentrationsfrom DGT profiles: a modelling approach. Aquatic Geochemistry 5, 337e355.

Hesslein, R.H., 1976. An in situ sampler for close interval porewater studies.Limnology and Oceanography 21, 912e914.

Hyacinthe, C., Anschultz, P., Carbonel, P., Jouanneau, J.M., Jorissen, F.J., 2001. Earlydiagenetic processes in the muddy sediments of the Bay of Biscay. MarineGeology 177, 111e128.

Jansen, B., Kotte, M., van Wijk, A.J., Verstraten, J.M., 2001. Comparison of diffusivegradients in thin films and equilibrium dialysis for the determination of Al, Fe(III) and Zn complexed with dissolved organic matter. The Science of the TotalEnvironment 277, 45e55.

Jorgensen, B.B., 1997. The sulphur cycle of a coastal marine sediment (Limfjorden,Denmark). Limnology and Oceanography 22, 814e832.

Krom, M.D., Mortimer, R.J.G., Poulton, S.W., Hayes, P., Davies, I.M., Davison, W.,Zhang, H., 2002. In-situ determination of dissolved iron production in recentmarine sediments. Aquatic Sciences 64, 282e291.

Lienemann, C.P., Taillefert, M., Perret, D., Gaillard, J.F., 1997. Association of cobalt andmanganese in aquatic systems: chemical and microscopic evidence. Geochimicaet Cosmochimica Acta 61, 1437e1446.

Pyzik, A.J., Sommer, S.E., 1981. Sedimentary iron monosulfides: kinetics andmechanism of formation. Geochimica et Cosmochimica Acta 45, 687e698.

Rickard, D., 1997. Kinetics of pyrite formation by the H2S oxidation of iron(II)monosulfide in aqueous solutions between 25 and 125 �C: the rate equation.Geochimica et Cosmochimica Acta 61, 115e134.

Sangi, M.R., 1998. Trace metal determination in river water by diffusive gradients inthin-films. Ph.D. Thesis, The University of Otago, New Zealand.

Santos-Echeandía, J., Prego, R., Cobelo-García, A., Millward, G.E., 2009. Porewatergeochemistry in the Vigo Ria (NW Iberian Peninsula): implication for Benthicfluxes of dissolved trace elements (Co, Cu, Ni, Pb, V, Zn). Marine Chemistry 117,77e87.

Shaw, T.J., Gieskes, J.M., Jahnke, R.A., 1990. Early diagenesis in differing depositionalenvironments e the response of transition metals in pore water. Geochimica etCosmochimica Acta 54, 1233e1246.

Tessier, A., 1992. Sorption of trace elements on natural particles in oxic environ-ments. In: Buffle, J., van, H.P., Leeuwen (Eds.), In Environmental Partiles, vol. 1,pp. 425e453. Lewis.

User’ guide to DGT technique, DGT Research Ltd, http://www.dgtresearch.com/.Vershinn, A.V., Rozanov, A.G., 1982. On the problem of Eh measurement by the Pt

electrode and estimation of oxidationereduction conditions in marine media.Geokhimiya 1, 121e128.

Watson, P.G., Frickers, T.E., 1990. A multilevel, in situ pore water sampler for use inintertidal sediments and laboratory microcosms. Limnology and Oceanography35, 1381e1389.

Yao, W., Millero, F.J., 1996. Oxidation of hydrogen sulfide by hydrous Fe(III) oxides inseawater. Marine Chemistry 52, 1e16.

Yu, R.L., Hu, G.R., Wang, L.J., 2010. Speciation and ecological risk of heavy metals inintertidal sediments of Quanzhou Bay, China. Environmental MonitoringAssessment 163, 241e253.

Zhang, H., Davison, W., Miller, S., Tych, W., 1995. In situ high-resolution measure-ments of fluxes of Ni, Cu, Fe, and Mn and concentrations of Zn and Cd inporewaters by DGT. Geochimica et Cosmochimica Acta 59, 4181e4192.

Zhang, H., Davison, W., Knight, B., McGrath, S., 1998. In situ measurement of solu-tion concentrations and fluxes of trace metals in soils using DGT. EnvironmentalScience Technology 32, 704e710.

Zhang, H., Davison, W., Mortimer, RJ.G., Krom, M.D., Hayes, P.J., Davies, I.M., 2002.Localised remobilization of metals in a marine sediment. Science of the TotalEnvironment 296, 175e187.