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Estuarine, Coastal and Shelf Science 91 (2011) 1e12

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Estuarine, Coastal and Shelf Science

journal homepage: www.elsevier .com/locate/ecss

Assessment of copper, cadmium and zinc remobilization in Mediterranean marinecoastal sediments

Aikaterini Sakellari a, Marta Plav�si�c b, Sotiris Karavoltsos a, Manos Dassenakis a, Michael Scoullos a,*

aUniversity of Athens, Department of Chemistry, Div. III, Laboratory of Environmental Chemistry, Panepistimiopolis Zografou, 157 71 Athens, GreecebRuCer Bo�skovi�c Institute, Center for Marine and Environmental Research, P.O. Box 180, 10002 Zagreb, Croatia

a r t i c l e i n f o

Article history:Received 20 July 2010Accepted 27 September 2010Available online 8 October 2010

Keywords:trace metalsremobilizationorganic mattersedimentspore watercomplexing capacity

* Corresponding author.E-mail address: esakel@chem.uoa.gr (M. Scoullos)

0272-7714/$ e see front matter � 2010 Elsevier Ltd.doi:10.1016/j.ecss.2010.09.008

a b s t r a c t

The remobilization of copper, cadmium and zinc in sediments of three selected coastal microenviron-ments of the Aegean Sea (Eastern Mediterranean) is assessed. Various analytical methods and techniqueswere employed providing concentrations, profiles and forms of metals and organic matter in sedimentsand pore waters.

At Loutropyrgos, a non-industrial site located, however, within an intensively industrialized enclosedgulf, an intense resupply of zinc in pore water from sediment was recorded, correlating with the highestvalue of weakly bound fraction of zinc determined at this area. The comparatively high zinc concentrationsmeasured in the pore waters (394 nM), exceed considerably those in the overlying seawater (12.5 nMdetermined by DGT; 13.5 nM total), resulting in the formation of a strong concentration gradient at thesedimentewater interface. Potential zinc flux at the sedimentewater interface at Loutropyrgos (based on0.4 mm DGT profile) was calculated equal to 0.8 mmol.m�2.d�1. The half lives of trace metals at Lou-tropyrgos site, based on the aforementioned DGT profiles, amount to 0.1 y (Zn), 2.8 y (Cd), 4.5 y (Cu), 2.2 y(Mn) and 0.4 y (Fe) pointing out to the reactivity of these metals at the sedimentewater interface.

Theconcentrationofdissolvedorganic carbon (DOC) inporewatersof the threeselectedsites (2.7e5.2mg/L)was up to four times higher compared to that of the corresponding overlying seawater. Similarly, theconcentrations of carbohydrates in pore waters (0.20e0.91 mg/L monosaccharides; 0.71e1.6 mg/L poly-saccharides) are an order ofmagnitude higher than those of seawater, forming a concentration gradient at thesedimentewater interface. Total carbohydrates contribute between 34 and 48% of the organic carbon of theporewaters, being significantlyhigher than thoseof seawater fromthe corresponding areas,whichwere in therange of 15e21%.

The complexing capacity as for copper ions (CCu) determined in pore water ranges widely, from0.03 mM at Kalamos to 3.76 mM at Molos, whereas the corresponding values for cadmium ions (CCd) werenon detectable, except for Kalamos site, where the value for CCd was equal to 0.03 mM. A significantincrease in the values of CCu, normalized as for DOC, was observed in pore waters in relation to those ofoverlying seawater. This indicates an ‘enrichment’ of pore waters in dissolved organic ligands for copperions per unit of DOC. Up to 72% of DOC could be present as ligands capable to complex copper ions.

� 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Sediments play a major role in the overall fluxes of traceelements in coastal systems, acting occasionally as a source and/orsink. Metals may be recycled several times through the sed-imentewater interface before being permanently stored in sedi-ments or released to the overlying waters. In this context, porewater is an important intermediate in the remobilization of

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metals. This process is greatly influenced by diagenetic mecha-nisms (Shaw et al., 1990; Koschinsky, 2001). Diagenetic changes ofparticulate material deposited to the sea bottom sedimentsinvolve partitioning of trace elements between particles and porewaters and depend critically upon several interrelated factorssuch as the occurrence of suitable ligands, the changing redoxconditions and the decomposition of organic matter due tomicrobial activity and complex geophysicochemical reactions(Santos-Echeandia et al., 2009).

Zhang et al. (1995, 2002) have suggested that the role ofdiagenetic reactions including the (re)supply of metals fromsediments can be evaluated using the vertical profiles of their

A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e122

concentrations in pore waters. High resolution metal profiles canbe obtained by employing the technique of Diffusive Gradients inThin Films (DGT) (Zhang et al., 1995), which is based on masstransport control of metals, through the insertion of a DGT probevertically into the sediment (Zhang et al., 2002; Fones et al., 2004;Gao et al., 2009). It makes use of a layer of Chelex cation exchangeresin placed behind a layer of hydrogel and covered with a 0.45 mmmembrane. With this technique, only the labile metal fraction,determined by the gel layer thickness, which is small enough todiffuse through the gel and capable of binding to the resin layer, isassessed. When two such DGT probes, with different gel thick-nesses, are deployed for the same period of time, it is possible toobtain information about the resupply of the pore water in metalsfrom the solid phase.

The complexing properties of dissolved organic matter (DOM)contained in pore waters are obviously one of the most importantfactors to be taken into account (Burdige, 2002) when discussingthe fate and distribution of metal ions at the sediment/waterinterface. Skrabal et al. (2000) have considered several pathwaysfor the presence of organic ligands in sediments and overlyingwaters, including exudates by bacteria, macrofauna or otherorganisms, other microbial metabolites, including sulfides andpolysulfides, as well as diagenetic alteration of the pre-existingorganic substances. In this context, the colloidal fraction representsan important component in the trace metal pool of pore waters(Huerta-Diaz et al., 2007). The transformation of organic ligands inpore waters through microbial activity and modifications related tothe presence of sulphur species could significantly influence thetype of ligands and their concentrations in pore waters in relationto overlaying waters (Skrabal et al., 1997, 2000, 2006). The changein the redox state of Cu ions (reduction of Cu(II) to Cu(I) state) mayas well influence the complexation of Cu ions in pore waters(Boussemart et al., 1993). For the complexing properties of the porewaters the relevant papers found for the Mediterranean are thoseby Boussemart et al. (1989,1993), Mucha et al. (2008) and Chapmanet al. (2009), focusing on copper.

Fig. 1. Coastal sam

A combination of different analytical methods were appliedaiming to study the remobilization of cadmium, copper and zinc insediments from selected coastal microenvironments of the Aegeansea in relation to the presence of organic matter and sulfur species,taking into account the different physicochemical/geomorpholog-ical characteristics of each site.

2. Materials and methods

2.1. Sampling

The selected sampling sites represent coastal microenviron-ments characterized by different physical, chemical and geomor-phological features and degree of pollution, located in three gulfs ofthe Aegean Sea, namely the gulf of Elefsis, an enclosed polluted areanear Athens (Loutropyrgos site), the Evoikos gulf, an area influ-enced by strong tidal currents (Kalamos site), affected also byindustrialization and urbanization and the Maliakos gulf, an areainfluenced mainly by agricultural activities and the outflow of theriver Sperchios (Molos site) (Fig. 1).

Duplicate sediment cores A and B were collected with plexiglasstubes (B ¼ 8 cm) from each sampling site by scuba diving duringJanuary 2004 and transported to the laboratory under refrigeration.The upper 8 cm of core A was sliced at 1 cm intervals, while forgreater depth 2 cm subsections were taken. The samples werefreeze-dried and sieved. The chemical analyses of heavymetals (Cd,Cu, Zn, Fe and Mn) and organic matter (total organic carbon andtotal carbohydrates) were performed in the silt and clay (<63 mm)fraction. The upper 8 cm of core B was centrifuged under inertatmosphere (N2) for the extraction of an adequate quantity of porewater, following filtration through 0.45 mm.

2.2. Sediment analysis

The total organic carbon (TOC) content of sediment samples wasdetermined by titration (with Fe2þ) of the K2Cr2O7 that had not

pling sites.

A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e12 3

been consumed for the oxidation of the organic compounds of thesediment in strong acidic conditions (Gaudette et al., 1974). Totalcarbohydrates (TCHOs) were determined colorimetrically, accord-ing to the method of Dubois et al. (1956) amended by Liu et al.(1973). The precision of both methods (given as a coefficient ofvariation) was approximately 5e10%.

Fractionation of trace metals in the sediments was carried outwith both single- and multi-step methods.

2.2.1. Single-step method

� Shaking overnight at room temperaturewith dilute (0.5 N) HCl,to extract the weakly bound metal fraction, which according toAgemian and Chau (1976) corresponds broadly with that ofanthropogenic origin.

� Digestion of samples in PTFE beakers on a hot plate (120 �C)with the addition of aqua regia, conc. HF and HClO4, in order todetermine the total metal content (UNEP, 1985, 1986; Marinet al., 1997; De la Torre and Tessier, 2002).

2.2.2. Multi-step methodAdditionally to the single-step schemes, a four-step sequential

extraction procedure for sediment analysis (Rauret et al., 2000)was followed, proposed by the “Standards, Measurements andTesting Programme” (formerly BCR) of the European Commission.The procedure consists of successive extraction with acetic acid(to extract the exchangeable fraction of metals), hydroxylammonium chloride (to extract the reducible fraction; mainlyoxides), hydrogen peroxide and ammonium acetate (to extractthe oxidizable fraction; mainly organometallic complexes andsulphides) and digestion with aqua regia (to extract aqua regiaresidue).

Metal analyses were performed by GFAAS with Zeeman back-ground correction for cadmium (Varian SpectrAA 640Z) (Petit andRucandio, 1999; Giacomelli et al., 2002) and by FAAS (VarianSpectrAA 200) for the rest of the metals studied (Thomas et al.,1994). A solution of 2% NH4H2PO4, 0.4% Mg(NO3)2 in 0.5 M HNO3was used as matrix modifier to determine the total cadmiumcontent as well as cadmium in the aqua regia extract. In the rest ofcadmium determinations a solution of 0.1% Pd(NO3)2, 0.06% Mg(NO3)2 was used. Pyrolysis was performed at 900 �C in the extractof hydroxyammonium chloride and at 800 �C in the rest of casesand atomization at 1900 �C. The reference sediment BCR CRM-601was analysed according to the BCR sequential extraction method(Table 1).

2.3. Pore water analysis

In pore water samples dissolved organic carbon (DOC) wasdetermined by the method of High Temperature Catalytic Oxida-tion (Sugimura and Suzuki, 1988) employing a TOC-5000A

Table 1Measurements of metals in the reference sediment (all values in mg/g).

BCR 601

Metal Experimentalvalue (n ¼ 6)

Indicativevalue

Experimentalvalue (n ¼ 6)

Indicative value

CH3COOH NH2OH$HClCd 4.46 � 0.19 4.45 � 0.67 3.96 � 0.26 3.95 � 0.53Cu 10.3 � 0.4 10.5 � 0.8 70.1 � 1.9 72.8 � 4.9Zn 268 � 6 261 � 13 263 � 4 266 � 17

CH3COONH4 Aqua regiaCd 1.96 � 0.18 1.91 � 1.43 1.2 � 0.1 1.3 � 2.2Cu 75.8 � 3.7 78.6 � 2.6 61.7 � 1.2 60.4 � 4.9Zn 115 � 3 106 � 11 150 � 5 161 � 14

Shimadzu analyzer (Shimadzu Scientific Instruments, Columbia,MD). The precision was estimated as the standard deviationbetween injections and it was less than 2% of the mean. Measure-ments of dissolved mono (MCHOs) and polysaccharides (PCHOs)were performed according to the method of Myklestadt et al.(1997). The precision of the method (given as a coefficient of vari-ation) was 5e10%.

High resolution depth profiles of trace metal concentrations inpore waters of sediments were obtained by employing the in situtechnique of diffusive gradients in thin films (DGT) (Zhang et al.,1995). DGT probes (DGT Research Ltd.) incorporate a layer of Che-lex resin (100 mm bead size) behind a layer of hydrogel throughwhich metal ions can diffuse. The probes locally lower concentra-tions of metal ions in pore water at their surface. Any resultantinduced flux of metals from solid phase to solution is measureddirectly as accumulated metal on the resin and can be interpretedas the concentration on the surface of the probe.

The performance of the DGTassemblies used was checked at thelaboratory according to Zhang and Davison (1995), by replicatedeployments of DGT probes in stirred solutions of knownmetal ionconcentrations added as nitrate salt to Milli-Q-water (Cd 10.7 nM;Cu 31.5 nM and Zn 30.6 nM). The DGT measured concentrationswere <8% of the known concentrations in the solution and therelative standard deviations were <10%. Blank DGT probes werealso prepared and treated as the deployed ones. The accumulatedmass of each metal measured in the blank assemblies was signifi-cantly lower than the mean mass measured in the deployed ones(Cd 0.03 nM; Cu 1.5 nM; Zn 3.8 nM; Fe 2.2 nM; Mn 1.9 nM).

After a fixed time (t) of deployment the mass of accumulatedmetals (M) in the resin layer is measured. Knowing the surface area(A) of the diffusive layer in contact with the sediment, the flux ofmetals (F) from sediment to the DGT probe can be calculated fromequation (1).

F ¼ M=ðt$AÞ (1)

This flux, which is induced by the DGT probe, can be interpreted asthe mean concentration, Ci, at the surface of the probe duringdeployment, according to Fick’s law as outlined by Davison andZhang (1994) (Eq. (2)),

Ci ¼ ðM$DgÞ=ðD$A$tÞ (2)

whereDg is the thickness of the diffusive layer (gel plus filter) andDis the diffusive coefficient of solute in diffusive gel.

According to Zhang et al. (2002) three cases may arise withrespect to the DGT measured flux and interfacial concentration. Thefirst case is the “fully sustained” in which metals removed from thepore water by the DGT probe are rapidly resupplied from the solidphase and where Ci is equal to the bulk pore water concentration, Cb.The second one is the “diffusion only” where no resupply from thesolid phase to pore water occurs andmetals supply to the DGT probeis attributed only to diffusion through the pore waters. The DGTmeasured flux decreases with deployment time due to the depletionof the pore water concentration and in this case Ci z 0.1Cb.The thirdone is the “partially sustained” where the resupply of metals fromsolid phase to pore water in insufficient to maintain the maximumDGT flux, resulting in depletion in pore water concentrations(0.1Cb < Ci < Cb). In practice, the above three cases can be identifiedby deploying two DGT probes with different diffusive gel layerthicknesses, Dg1 (thick) and Dg2 (thin), for the same period of timeand obtainingmeasured interfacial concentrations, C1 and C2. For thefully sustained case, a constant concentration gradient is maintainedacross the diffusive gel. The interfacial concentration equals the bulkconcentration, Cb, and is, therefore, the same irrespective of diffusionlayer thickness (C1 ¼ C2). For the diffusion only case, the

A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e124

concentration gradient is mainly within the sediment, minimizingthe effect of the diffusive gel; then C1/C2 ¼ Dg1/Dg2; that is, theinterfacial concentration will be larger for thicker gels(for theconditions used in this work Dg1/Dg2 ¼ 2.5). For the partially sus-tained case, there will be a difference in measured interfacialconcentration for the two gels, but not so pronounced [C1(Dg2/Dg1) < C2 < C1].

In our case a couple of DGT probes with different diffusive gellayer thicknesses (0.4 and 1.2 mm; DGT0.4/1.2) were deployedsimultaneously, arranged back to back, at each sampling site for24 h. Prior to the immersion the probes were deoxygenated withnitrogen and exposed to air for only a few seconds during theirinsertion into the sediment by scuba diving, taking care tominimizeparticle resuspension. After deployment the resin gel from DGTprobeswas cut into 2.5mm intervals and each gel slicewas eluted ina 1 M nitric acid supra pure solution and diluted for analysis bygraphite furnace atomic absorption spectrometry.

Electrochemical measurements were carried out using an Eco-Chemie (The Netherlands) voltammetric instrument connected toa three-electrode cell (VA 663, Metrohm stand, Switzerland). Theworking electrode was a static mercury drop electrode (SMDE)with a surface area of 0.54 mm2, Ag/AgCl (3 M KCl) was the refer-ence electrode and a carbon rod served as counter electrode.

The differential pulse anodic stripping voltammetry (DPASV)was used for complexing capacity determinations (Plav�si�c et al.,1982). The standard deviation (s) calculated by repeated measure-ments (n ¼ 5) was equal to 10%. The values of the complexingcapacity as well as the corresponding stability constant werecalculated by applying the linear transformation plot (Ru�zi�c, 1982;van den Berg, 1982).

Surface Active Substances (SAS) in pore water samples weredetermined by phase selective alternating current voltammetry(PSACV) (�Cosovi�c, 1985). The surfactant activity is expressed as

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Fig. 2. Vertical distributions of total metals in the sil

equivalent to a model substance, i.e. in mg/L of Triton-X-100. Thereproducibility of electrochemical measurements is 8e10%.

For the determination of the total concentration of sulfur speciesthe method of linear sweep voltammetry (LSV) was employed(Ciglene�cki and �Cosovi�c, 1997).

3. Results and discussion

3.1. Metal fractionation in sediments

According to the single-step technique, the highest value ofweakly bound zinc in the silt and clay fraction of sediments wasfound at Loutropyrgos 52.8 mg/g (50.3e56.5 mg/g), representing 86%(71e97%) of total zinc. The corresponding zinc values for Kalamos11.0 mg/g (9.7e12.3 mg/g) and Molos 9.8 mg/g (5.4e13.1 mg/g) repre-sent respectively 23% (21e25%) and17% (14e21%) of total zinc (Fig. 2).The levels of total zinc in the three areas are directly influenced bytheir weakly bound fraction, which clearly reflects the differentindustrial development of the three sites. The weakly bound fractionof cadmium was found equal to 0.122 mg/g (0.116e0.128 mg/g) atLoutropyrgos corresponding to 62% (59e80%) of total cadmium,0.115 mg/g (0.096e0.162 mg/g) at Kalamos, corresponding to 75%(68e84%) of total Cd and 0.035 mg/g (0.008e0.060 mg/g) at Moloscorresponding to 50% (15e83%) of total Cd (Fig. 2). Regarding copper,the mean concentrations of its weakly bound fraction did not differsignificantly among the study sites with its percentage varying from40% at Molos to 62% at Loutropyrgos. HCl extracted iron andmanganese are not commented here because it is very likely that HCldissolves iron and manganese oxide lattices.

According to the application of the BCR protocol, despite the factthat Znwas found to be lattice-held at Loutropyrgos, its more labilefractions correspond to more than 50% of its total concentration,coinciding with the results obtained by the aforementioned single-

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-- Weakly bound fraction Total metal

t and clay fraction (<63 mm) of sediment cores.

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Fig. 3. Vertical distributions of the DGT-labile fraction of dissolved metals in pore water from the study areas.

A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e12 5

step analytical procedure and a previous study at the same area(Scoullos, 1981). Copper is mainly detected in the lattice-heldfraction of sediments obtained from Molos (74% of total copper)and Loutropyrgos (89%), whereas a significant fraction of copper(25%) is bound to the organic matter in the case of Kalamos.Manganese is mainly found in its more easily exchangeable frac-tions due to its already mentioned tendency to occur in the form ofeasily reducible manganese oxides and carbonate minerals (Tessieret al., 1979; Ujevi�c et al., 2000) whereas iron demonstrates a strongtendency to remain in the lattice.

3.2. DGT profiles, spatial remobilization of metals

Iron and manganese, two redox sensitive elements, are consid-ered as tracers to distinguish the various sedimentary redox layers(van Rysen et al., 1999; Haese, 2006). In oxic environments,concentrations of dissolved manganese and iron are relatively lowdue to the formation of their oxidized solid phases (MnO2 andFeOOH). Suboxic environments are characterized by a peak of dis-solved manganese and iron which are reduced in the absence ofoxygen and thus mobilized to the dissolved phase. In the anoxic

Fig. 3. (continued).

A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e126

zone, a decrease of dissolved iron concentration occurs due to theformation of insoluble iron sulphides. At Loutropyrgos and Kalamosthe oxic layer occurs within 1 cm depth since the pore waterconcentrations ofmanganese and iron at this layer are relatively low(Fig. 3a and b). On the contrary, at Molos (Fig. 3c) the relatively highconcentrations of dissolved manganese and iron detected in thesedimentewater interface indicated that dissolved oxygen wasrelatively low. Concentrations of manganese rise at a shallowerdepth than iron, reflecting reductive dissolution of manganeseoxyhydroxides compared to the iron ones, associated with decom-position of organic matter (Froelich et al., 1979; Naylor et al., 2004).

According to the methodology proposed by Zhang et al. (2002), thesubstantial difference between mean concentrations (CDGT1.2/CDGT0.4) calculated from each profile indicated that manganese(CDGT1.2/CDGT0.4 was calculated equal to 3.6, 3.4 and 3.6 for Lou-tropyrgos, Kalamos and Molos, respectively) and iron (CDGT1.2/CDGT0.4 was calculated respectively equal to 1.9, 2.4 and 6.3) uptakeby DGT is mainly from the pore water with relatively little resupplyof these metals from the solid phase.

Pore water DGT profiles of most of the metals studied demon-strate characteristic maxima at the sedimentewater interface(Fig. 3a, b, c), attributed to the mobilization of metals from particles

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Fig. 3. (continued).

A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e12 7

having recentlyaccumulatedon the sediment surface, through threemainmechanisms: rapid degradationof organicmatter reaching thesediment surface; reductive dissolution of iron and manganeseoxides in environments rich in organic matter; desorption fromparticulates due to gradual change of pH values (Hamilton-Taylorand Davison, 1995; Teasdale et al., 2003). The location of thesemaxima fewmm above themanganese and iron maxima, as well asthe relatively low mean concentrations of organic carbon (from0.14% at Kalamos to 0.94% at Molos) (Fig. 4) determined in thesediment cores of the three high productivity areas (Kormas et al.,2002; Sakellari, 2006), indicate that they are due to the release ofmetals from rapidly decomposing organic material. Total carbohy-drates (TCHOs) demonstrate a similar trend, with their mean

concentrations varying from 0.11% at Kalamos to 0.38% at Molos(Fig. 4). Similarly, Zhang et al. (1995), van den Berg et al. (2000),Naylor et al. (2004) and Tankere-Muller et al. (2007) note thatdegradation of organic matter instead of oxide sources is mainlyresponsible for the release of metals in the sedimentewaterinterface.

3.2.1. LoutropyrgosAt Loutropyrgos site the local maxima of dissolved metals in

pore water recorded near the sedimentewater interface areaccompanied by a maximum of copper and cadmium (DGT0.4) atapproximately 8e12 cm depth. A peak of DGT0.4 iron is observed atthe same depth, whereas a peak of DGT0.4 manganese is recorded

E / V-1.0 -0.8 -0.6 -0.4 -0.2

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A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e128

few cm above, at the 5e7 cm interval (Fig. 3a). It is noteworthy thata small increase of organic carbon is detected at 7e9 cm depth ofthe sediment (Fig. 3). The profiles of total cadmium, copper and zincin the solid phase demonstrate a local enrichment at the surfaceinterval of 3 cm, accompanied by an accumulation of organicmatter in the same section (Figs. 2 and 4). Copper shows alsoa secondary peak at approximately 8 cm depth, which parallelsa similar one of the organic matter (Fig. 4), indicating its well-known tendency to form stable organic complexes (Salomons andForstner, 1984; Rauret, 1998).

3.2.2. KalamosAt Kalamos, an area with comparatively limited organic matter

in sediments (Fig. 4), a small peak of DGT0.4,1.2 cadmium isencountered at 2e4 cm depth, being even smaller in the case of zincand copper (Fig. 3b). At this depth a local maximum of dissolvediron (DGT0.4,1.2) together with a slight increase of organic carbon insediment are also observed. Smaller peaks of all metals in porewater, more significant however for iron and manganese, existbetween 7 and 10 cm depth.

3.2.3. MolosAt the site of Molos local maxima of dissolved zinc and copper

are observed at 3e6 cm depth (DGT0.4,1.2), of cadmium at 4 cm(DGT0.4,1.2), of manganese at 2 cm (DGT0.4,1.2) and 5e6 cm (DGT1.2)and of iron at 6e8 cm (DGT0.4) and 4 cm (DGT1.2) (Fig. 3c). Inparallel, an intense peak of organic matter and %CHOs in sedimentis recorded at 4e7 cm depth (Fig. 4). The significantly increasedamount of organic matter at this depth is related both to manga-nese and iron reduction as well as to the release of these metals inpore water.

In Fig. 5 the LSV voltammetric scans are presented for the Molosand Kalamos pore water samples. Molos sample showed the pres-ence of sulfur species (200 nM) indicated by the peak at a potentialof E ¼ w�0.6 V. Almost all sulfur in this sample is present in theform of organically bound sulfur species. This is indicated by thedifference in the height of the voltammetric peaks obtained byaccumulation at two different potentials: i.e. �0.2 V (scan 1) and�0.4 V (scan 2). The accumulation at �0.2 V determined the totalamount of sulfur species present, while the accumulation at �0.4 Vcollects only inorganic sulfur species (Krznari�c et al., 2001). Thesample fromKalamos did not show the presence of sulfur species atall (Fig. 5, line 3). The sample from Loutropyrgos (like the Molossample) showed the presence of sulfur species but in lowerconcentration (100 nM) (Table 2). The presence of sulfur species inpore water samples from Molos and Loutropyrgos sites indicatesthat the peaks of metals recorded at 8e12 cm at both of these sitesmay be attributed either to the presence of sulfur species andtransformation of oxides to sulfides or to the coexistence of sulphateand iron reducing bacteria in the same zone. Motelica-Heino et al.

-30

-25

-20

-15

-10

-5

00 0.5 1 1.5

%OC

cm

● Loutropyrgos ■ K

Fig. 4. Vertical distributions of organic carbon and total carbohydr

(2003), performing higher resolution DGT and sulfide measure-ments, suggest the latter mechanism.

3.3. Resupply of metals to pore water from sediments

Metal fluxes, calculated according to equations (1) and (2), over2 cm of depth at the sedimentewater interface were obtained bythe 0.4 mm DGT probe, since it corresponds better to the potentialflux (Zhang et al., 1998). The values (in mmol m�2 d�1) obtained forLoutropyrgos are 3.8E-05 (Cd), 4.8E-05 (Cu), 0.8 (Zn), for Kalamos7.9E-06 (Cd), 4.7E-04 (Cu), 3.2E-03 (Zn) and for Molos 2.0E-05 (Cd),9.1E-04 (Cu), 7.9E-04 (Zn).

According to Zhang et al. (2002) when CDGT1.2/CDGT0.4 ¼ 1a substantial resupply of metals from solid phase to solution isindicated. Due to the fact that the metal demand of the DGTsampler is effectively covered by the significantly higher resupplyof pore water by the sediment, the interfacial concentrationsmeasured by DGT represent reasonable estimates of bulk porewater concentrations.

In the present study such a mechanism of constant resupplyoccurs only in the case of zinc at the area of Loutropyrgos (CDGT1.2/CDGT0.4 ¼ 0.9) where, as already mentioned, a significantly higherpercentage of total zinc that is weakly boundwas determined in thesediment (86%). The mean concentration of zinc in pore watersfrom the area of Loutropyrgos (394 nM DGT) is substantially higherin comparison to that of dissolved zinc in the overlying seawatersampled at the same time (12.5 nM DGT, 13.5 nM total) (Sakellari,2006), resulting in the formation of a constant concentration

-30

-25

-20

-15

-10

-5

00 0.2 0.4 0.6 0.8 1

%CHOs

cm

alamos ▲ Molos

ates in the silt and clay fraction (<63 mm) of sediment cores.

Table 2Concentrations of dissolved organic carbon (DOC), monosaccharides (MCHOs), polysaccharides (PCHOs), total carbohydrates (TCHOs), surface active substances (SAS) andsulfur species content (S) of the samples analysed.

Sample DOC (mg/L) MCHOs (mg/L) PCHOs (mg/L) TCHOs (mg/L) %TCHOS/DOC SAS (mg/L eq. T-X-100) SAS/DOC S (nM)

Pore waterLoutropyrgos 5.2 0.42 1.5 1.9 36 0.34 0.065 100Kalamos 2.7 0.20 0.71 0.91 34 0.18 0.066 <1Molos 5.2 0.91 1.6 2.5 48 0.25 0.048 200

Seawatera

Loutropyrgos 0.9 0.06 0.13 0.19 21 0.15 0.166 e

Kalamos 1.3 0.03 0.16 0.19 15 0.10 0.076 e

Molos 1.0 0.03 0.17 0.21 21 0.20 0.200 e

a Scoullos et al. (2006).

A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e12 9

gradient in the sedimentewater interface. Many researchers (vanden Berg et al., 2000; Warnken et al., 2001) argue that the exis-tence of such a concentration gradient is responsible for the flux ofmetals, zinc in the specific case, from the sediment to the overlyingseawater.

It is interesting to calculate the halflife of tracemetals over 2 cmofdepth of the sediment column at Loutropyrgos site, where a mecha-nism of constant resupply of the pore water in zinc occurs. However,sediments are not closed environments since they receive input ofparticulate organic matter and associated elements as well asparticulate material of terrestrial origin. Assuming that the sedimentisa closedsystem, thatoutfluxesobtainedwith the0.4mmDGTproberemain constant with time and a constant grain density of 2.7 g cm3

(typical for terrigenous dominated sediments) (Bielders et al., 1990;Gerland and Villinger, 1995) the half life of metals at Loutropyrgosamounts to 0.1 y (Zn), 2.8 y (Cd), 4.5 y (Cu), 2.2 y (Mn) and 0.4 y (Fe).This indicates the highest reactivity/exchange rate for Zn, followed byFe on the sediment/water interface for Loutropyrgos site.

3.4. Organic matter in pore water

The concentrations of dissolved organic carbon in pore waterswere found to be up to four times higher compared to the corre-sponding of overlying seawater, whereas those of carbohydrateswere even an order of magnitude higher (Table 2). Such an obser-vation which is recorded as well by other researchers is attributedto the formation of dissolved organic carbon during the degrada-tion of particulate organic carbon inside the sediment (Burdige andGardner, 1998; Burdige, 2001; Miyajima et al., 2001; Papadimitriouet al., 2002; Simoneit and Sparrow, 2002; Lahajnar et al., 2005). Asa result, a concentration gradient which appears in the sed-imentewater interface is responsible for the possible supply of theoverlying water with organic matter from the sediment throughpore water (Alperin et al., 1999; Jensen et al., 2005; Lahajnar et al.,2005).

The percentage of organic carbon attributed to total carbohy-drates in pore water ranges between 34 and 48% (Table 2) and issignificantly higher compared to that of corresponding seawaterwhich fluctuates between 15 and 21% (Scoullos et al., 2006).Therefore, through the procedure that precedes the release oforganic matter in the pore water, an increase in the percentage ofcarbohydrates is observed.

Organic matter in the overlying seawater in the same studyareas is characterized by the presence of hydrophilic compoundssuch as polysaccharides, fulvic acids and proteins (Scoullos et al.,2006). Normalizing the concentrations of surface activesubstances (SAS) in the pore water samples as for the corre-sponding of dissolved organic carbon (DOC) (Table 2) demonstratesthat they have similar (SAS/DOC) values to that of the microbialpolysaccharide xanthan (�Cosovi�c and Vojvodi�c, 1998). This points to

the fact that pore water organic matter in the study areas is subjectto a possible ‘aging’ process including microbial activity and mayconsequently demonstrate different characteristics as for metalcomplexation. In addition to organic ligands, sulfide clusters couldalso account for metal binding ligands (Rozan et al., 2000).Recently, thiols are pointed out as possible copper ligands in thepore waters (Chapman et al., 2009). Carbohydrates can react withpolysulfides and produce sulfur rich organic matter (van Dongenet al., 2003). This was demonstrated by the 34S enrichment of thesulfurized carbonyl groups in the paper by Amrani and Aizenshatat(2004). According to other researchers, during remineralizationsediment organic matter generally passes through one or moreDOC intermediates of progressively smaller molecular weights(Henrichs, 1992; Deming and Baross, 1993; Alperin et al., 1994). Inanoxic sediments, where benthic macrofauna is essentially absent,bacteria mediate the remineralization process and extracellularhydrolytic cleavage of particulate biopolymers to high molecularweight DOC compounds is generally thought to be the rate limitingstep in this process (King, 1986; Hoppe, 1991; Meyer-Reil, 1991).

The values of apparent complexing capacity as for copper ions(CCu) determined in pore water range from 0.03 mM at Kalamos to3.76 mM at Molos (Table 3). These values are in accordance withvalues for CCu obtained by Boussemart et al. (1989), for the coastalsites around the city of Nice (Western Mediterranean): estuary ofa small river, urban outfalls and unpolluted bay. Their values werein the range of 0.9e3 mM, although extremely high values for DOCwere quoted for those samples (in the range of 14e100 mg/L). It isnoteworthy that they observed multiple copper peaks (i.e. distor-tions of copper peaks) during the DPASV titrations. Boussemartet al. (1993) speculate that these multiple copper peaks couldoriginate from the oxidation of the adsorbed and stabilized organicCu (I) species. Despite the fact that multiple copper peaks were notobserved in our experiments (although in the samples from Molosand Loutropyrgos the presence of S species was detected), theformation of some Cu(I) organic sulfur species should not beexcluded. Mucha et al. (2008) determined the CCu values in porewater samples by another electrochemical method, that of cathodicstripping voltammetry (CSV), from the lower Douro estuary (NWPortugal). Their values were in the range from 0.132 to 0.734 mM.The values of Skrabal et al. (2000) determined for the ChesapeakeBay porewater samples by DPASVwere in the range 0.135e1.25 mM.

By using the size-exclusion chromatography and mass spec-trometry Vachet and Callaway (2003) found, in Cheasapeake baysamples, two types of organic ligands (regarding their molecularmass) i.e. of 270 and 1600 Da both capable to complex copper.Assuming that the average molecular mass of the ligands in porewaters from the three study areas of the present work is around1000 Da and taking into account that the DOC determined is5.2 mg/L C and the highest CCu value determined is 3.76 mM, it canbe calculated that up to 72% of DOC present could act as ligands

Table 3Apparent complexing capacities of cadmium (CCd) and copper (CCu), normalized values as for DOC and corresponding values of apparent stability constant (Kapp) for porewater samples analysed in the present study.

Sample CCd (mM) CCd/DOC(mmol Cd binding/mg C)

log KCd CCu (mM) CCu/DOC(mmol Cu binding/mg C)

log KCu

Pore waterLoutropyrgos <0.006 e e 0.63 0.12 6.9Kalamos 0.03 0.01 8.3 0.03 0.01 6.2Molos <0.006 e e 3.76 0.72 6.1

Seawatera

Loutropyrgos 45 � 10�3 5.0 � 10�2 8.00 61 � 10�3 0.07 7.15Kalamos 7 � 10�3 0.51 � 10�2 8.27 42 � 10�3 0.03 6.94Molos <0.006 e e 430 � 10�3 0.44 7.32

a Scoullos et al. (2006).

A. Sakellari et al. / Estuarine, Coastal and Shelf Science 91 (2011) 1e1210

capable to complex copper ions. The corresponding values as forcadmium ions (CCd) were non detectable at Loutropyrgos andMolos while at Kalamos the CCd was measured equal to 0.03 mM.With the exception of Kalamos area, a significant increase in thenormalized as for DOC copper complexing capacity values isobserved in pore waters in relation to those of overlying seawater(Table 3). This ‘enrichment’ of pore waters in effective ligands forcopper ions per unit of dissolved organic matter may be attributedtomicrobial and chemical transformations towhich particulate andcolloidal organicmatter is subject until it becomes dissolved in porewaters. The inconsistencies regarding metal complexing capacitiesat Kalamos, combined with the fact that the study site is charac-terized by sediments rich in coarse sand, may be attributed toa relatively easy intrusion of overlying seawater inside the sedi-ment and to the consequent rapid renewal of pore waters.

4. Conclusions

The pore water profiles of copper, cadmium and zinc obtained inthe present study confirm the characteristic surface layer metalmaximum recorded in a number of cases worldwide. This isattributed to the mobilization of metals from particles havingrecently deposited on the sediment surface, mainly through therapid degradation of organic matter accumulated on the surface ofthe sea bottom, particularly in areas of high productivity such as theones studied here.

The pore waters of Loutropyrgos are substantially enriched inzinc, deriving from the sediments, which to a high percentage isweakly bound to the sediment lattice. As a result of the obtainedcomparatively high zinc concentrations in pore waters, whichsignificantly exceed those of the overlying seawater, a consider-able concentration gradient is formed, through which zinc isreleased to the overlying waters. This supply was calculated to be0.8 mmol.m�2.d�1.

The significantly higher concentrations of dissolved organiccarbon and carbohydrates in pore water compared to the corre-sponding in overlying seawater, indicates the existence of a DOCconcentration gradient in the sedimentewater interface, whichcould be responsible for the supply of the overlying water withorganic matter from the sediment through pore water. Thecomparison of copper complexing capacity values for pore water tothat of seawater indicates enrichment in dissolved organic ligands,per unit of dissolved organic matter, for Loutropyrgos and Molos.This enrichment in pore waters may be attributed to complexmicrobial and physicochemical transformations taking place in thewithin the sediments scavenged organic matter until it dissolves inpore waters. This “aging” of organic matter leads to an increase ofavailable effective complexing sites for copper in DOC in the over-lying seawater sampled near the sea bottom of Loutropyrgos andMolos deriving presumably from the pore waters.

The profiles of the studied metals in pore water in combinationwith those of organic matter in sediment provide indications thatin metal mobilization bacterial reduction can occur simultaneouslyor in parallel with the reduction of iron and manganese, wheresulphate and iron reducing bacteria may coexist in the same zone.

Acknowledgements

Marta Plav�si�c acknowledges the financial support of the Croa-tian Ministry of Science, Education and Sport through the project:Nature of the organic matter, interactionwith traces and surfaces inthe environment (098-0982934-2717). We thank the two anony-mous referees for their constructive comments and suggestions.

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