heavy metal partitioning in sediments from the lagoon of venice (italy)

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Heavy metal partitioning insediments from the lagoon of Venice(Italy)Emanuele Argese a & Cinzia Bettiol ba Centro di Studio sulla Chimica e le Tecnologie perl'Ambiente ‐ CNR – Dipartimento di Scienze Ambientali ,Università Ca’ Foscari di Venezia , Calle Larga S. Marta2137, Venezia, 30123, Italy Phone: +390412578602 Fax:+390412578602 E-mail:b Dipartimento di Scienze Ambientali , Università Ca’ Foscaridi Venezia , Calle Larga S. Marta 2137, Venezia, 30123, ItalyPublished online: 19 Sep 2008.

To cite this article: Emanuele Argese & Cinzia Bettiol (2001) Heavy metal partitioning insediments from the lagoon of Venice (Italy), Toxicological & Environmental Chemistry, 79:3-4,157-170, DOI: 10.1080/02772240109358985

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Toxicological and Environmental Chemistry, Vol. 79, pp. 157-170 © 2001 OPA (Overseas Publishers Association) N.V.Reprints available directly from the publisher Published by license underPhotocopying permitted by license only the Gordon and Breach Science

Publishers imprint.Printed in Malaysia.

HEAVY METAL PARTITIONING INSEDIMENTS FROM THE LAGOON OF

VENICE (ITALY)

EMANUELE ARGESEa,* and CINZIA BETTIOLb

aCentro di Studio sulla Chimica e le Tecnologie per l'Ambiente - CNR - Dipartimentodi Scienze Ambientali, Università Ca' Foscari di Venezia, Calle Larga S. Marta 2137,

30123 Venezia, Italy; bDipartimento di Scienze Ambientali, Università Ca' Foscaridi Venezia, Calle Larga S. Marta 2137, 30123 Venezia, Italy

(Received 23 June 2000; Revised 9 August 2000)

In this study heavy metal pollution in surface sediments of the Lagoon of Venice (Italy) wasinvestigated. Attention was focused on the evaluation of the amounts of heavy metals that canpotentially be released into the overlying water column, under particular environmental conditions,thus becoming bioavailable. To this end, the partitioning of heavy metals among different geo-chemical phases was determined by a procedure of sequential selective extractions. This approachcan provide useful information for assessing the risk posed to the Lagoon ecosystem by metalcontaminated sediments and for estimating the anthropogenic heavy metal content. In general,higher levels of contamination were found in sediments sampled in areas situated in the proximity ofsources of pollutants and in degraded sites, which presented limited water circulation and prevailinganoxic conditions. The results of the sequential extraction procedure showed that partitioning isdependent on the metal considered: in particular, Pb, Cu, Zn and Cd were bound mainly to the mostlabile phases of sediments, whereas Cr and Ni were found prevalently in the residual fraction.

Keywords: Heavy metals; sediments; partitioning; sequential extraction; Lagoon of Venice

1. INTRODUCTION

The study of the partitioning of heavy metals in sediments has gainedincreasing attention in an assessment of the risk posed by the presence ofthese pollutants in the aquatic environment.

*Corresponding author. Tel.: +390412578602. Fax: +39041 2578584.E-mail: [email protected].

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158 E. ARGESE AND C. BETTIOL

A number of complex physico-chemical and biological processes can leadto the accumulation of considerable amounts of metals in bottom sediments,which constitute a concentrated pool and a persistent source of these con-taminants. However, the determination of total concentrations provideslittle information on the bioavailability of metals, which is strongly influ-enced by their partitioning among the different components of sediments(Tessier and Campbell, 1987; Calmano et al., 1990).

In fact, heavy metals are distributed among a variety of forms of associ-ation, which differ in the intensity of metal-matrix bonds: as a consequence,each of these forms exhibits a different bioavailability and a differentpotential for remobilization.

Metals dissolved in interstitial waters and absorbed at the surface of thesolid matrix can be easily released into the water column: they constitute theexchangeable fraction of metals. On the contrary, metals bound in the latticestructure of minerals are not releasable in normal environmental conditions;they are commonly referred to as the residual fraction and, in some cases,they can represent a major fraction of the total metal content (Tessier et al.,1979; Chester and Voutsinou, 1981).

Between these two extremes other chemical forms exist, which containmetals that are potentially releasable when variations in physico-chemicalconditions occur in the sediment, such as changes in pH, redox potential andsalinity; in addition, part of the metals bound in these forms can enter thefood chain, following assimilation by benthic detritus-feeding organisms(Aston and Chester, 1976; de Groot et al, 1976; Tessier et al, 1979;Gambrell et al., 1980; Salomons, 1985).

A number of analytical methods have been developed to determine metalpartitioning in sediments. Most of these methods are based on sequentialextraction procedures: the metals are leached selectively from specific formsof association by means of a series of extractions, which are aimed atreproducing the conditions to which sediments may be exposed in theenvironment, leading to the remobilization of metals (Chester and Hughes,1967; Tessier et al, 1979; Rapin et ai, 1983; Weite et al, 1983).

A quantification of the amounts of metals associated with the differentforms can help when estimating and predicting the toxicological risk posedby the slow and continuous release of metals accumulated in sediments.Moreover, it can be useful when evaluating the enrichment in heavy metalcontent due to anthropogenic activities, and the influence of hydrodynam-ical and physicochemical conditions on the processes of metal accumulation.

In this study, heavy metal contamination in the Lagoon of Venicewas investigated. For this purpose, a procedure of sequential selective

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HEAVY METAL PARTITIONING IN SEDIMENTS 159

extractions was set up, on the basis of studies reported in literature (Tessieret al, 1979; Rapin et al., 1983; Weite et al, 1983), and based on the resultsof preliminary investigations - carried out using samples of sediments fromthe Lagoon of Venice - which allowed the identification of the parameterswhich have a major influence on the efficiency, selectivity and repeatabilityof the single extractions.

The adopted extraction sequence provides metal concentrations in fivefractions: (1) exchangeable, (2) bound to carbonates, (3) bound to Fe-Mnoxides/hydroxides, (4) bound to organic matter and sulphides, (5) residual.The analytical procedure was applied to sediment samples collected in threeareas of the Lagoon of Venice, characterized by different morphological,hydrodynamical and physicochemical conditions. Within each area,sampling sites were selected at strategic locations, on the basis of directobservations and of previous studies, which allowed an identification of thesub-areas with different environmental characteristics (Argese et al, 1992;Zonta et al, 1992; Zonta et al, 1994).

1.1. Study Area

The Lagoon of Venice is a wide, shallow, coastal basin, located in north-eastern Italy (Figure 1). It has a surface area of about 550 km2, an averagedepth of about 0.6 m, and is connected to the Adriatic sea by means of threeinlets, which permit tidal exchanges (Donazzolo et al., 1984).

The lagoon is a complex and heterogeneous ecosystem, presenting areaswith different morphological, geochemical and hydrodynamical character-istics and, as consequence, with different environmental conditions.

Pollutants enter the lagoon through a number of point and non-pointsources: it is subject to discharges from the drainage basin and from theindustrial area, where a number of chemical and petrochemical plantsoperate, and domestic sewage from the urban areas is also discharged intoit. Additional sources of contaminants can be represented by boat trafficand by atmospheric depositions (Cochran et al., 1998).

The various areas of the lagoon are affected differently by anthropogenicactivities, depending on their proximity to pollution sources and on hydro-dynamic conditions.

Sediment samples were collected in three different areas of the lagoon: theCona marsh (A), the Giudecca mud flat (B), and an area (C) situated nearthe Malamocco inlet (Figure 1).

The Cona marsh is an estuarine area, characterized by complex morpho-logy and water circulation. In the north of the marsh, the freshwater

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FIGURE 1 Map of the Lagoon of Venice. The three areas under investigation are indicated.

is supplied by rivers and channels from the drainage basin, which receivepollutants from agricultural, municipal and industrial sources. In the south-ern sector, the saltwater enters the marsh as a result of the tide. In thiscomplex system, the fate of metals is related to the processes of fresh andsaltwater mixing and to the transport and sedimentation of fine particlescoming from the drainage basin (Zonta et al., 1994). Three sampling siteswere chosen in this marsh, so that the local effects of pollution sources andhydrodynamic conditions could be investigated (Figure 2).

The Giudecca mud flat is situated behind Giudecca island, which forms apart of the historic centre of Venice, and is surrounded by four channels.

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FIGURE 2 Location of sampling sites 4, 5 and 6, situated in the Cona marsh.

Physicochemical and geochemical studies (Argese et al, 1992; Zonta et al,1992) indicated the existence of sub-areas with different characteristics andenvironmental conditions, due to differences in water movement in the mudflat, induced by the surrounding channels. At the time of this study, the mud

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-ib//' S

FIGURE 3 Location of sampling sites 1, 2 and 3, situated in the Giudecca mud flat.

flat was exposed to an intense macroalgal growth, which caused the devel-opment of anoxic conditions and a drastic deterioration in water qualityin the zones subject to low water exchange. As for the Cona marsh, sedimentsamples were collected in three sites, situated in zones characterized bydifferent hydrodynamical and geochemical conditions (Figure 3).

The remaining two sampling sites were selected near one of the inletsconnecting the lagoon to the sea, relatively far from point pollution sources(Figure 4).

FIGURE 4 Location of sampling sites 7 and 8, situated near the Malamocco inlet.

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2. MATERIALS AND METHODS

The sediment samples were collected in summer 1994 using a syringe-typecorer, designed to keep the sample under N2 atmosphere during bothcollection and transfer to a polyethylene storage vial.

The first 5 cm-section of each core was subjected to a sequential extractionprocedure, following the scheme illustrated in Figure 5. The first two stepswere carried out in a glovebox under a nitrogen atmosphere, to avoid contactwith oxygen, which could cause changes in the metal partitioning. Thesequential leaching procedure was performed on an aliquot of about 2 g ofwet sediment, since drying procedures could alter metal partitioning. Themoisture content was determined on a separate sub-sample by drying at 105 °C.

All the containers and glassware used were soaked for 48 hours in 0.1 MHNO3 and rinsed carefully with Milli-Q water (Millipore). Extracting solu-tions were prepared by using BDH Aristar acids and Carlo Erba analytical

Extraction with 1 M NH4OAc pH=7with continuous agitation for 2h at room T

centrirugationtresidue

Extraction with 1 M NH„OAc pH=4.8 with HOAcwith continuous agitation for 6h at room T

centrirugation

iresidue

Extraction with 0.04 M NH2OHHC1in 25% HOAc for 6 h at 96°C

centrirugationtresidue

Extraction with 30% H2O2 pH=2 + HNO3 for 2 hat 85°C; further H2O2 pH=2 for 3 h at 85OC.

After cooling, 3.2 M NH„OAc in 14% HNO3 withcontinuous agitation for 30 min at room T

centrirugation

tresidue

Digestion with HF/aqua regia 6:1 for 12 hat 180°C. Then, H,BO, for 12 h at 180°C.

I - exchangeablefraction

II-fraction boundto carbonates

III — fraction boundto Fe and Mn

oxides/hydroxides

IV - fraction boundto organic matter

and sulphides

V-residual fraction

FIGURE 5 Scheme of the sequential extraction procedure used to determine metal partitioning.

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grade reagents. The acids used for the dissolution of the residual fractionwere Merck Suprapur and Ultrapur.

The metal concentrations in the extracts were determined by meansof either flame or flameless atomic absorption, depending on the levelof concentration (Varian spectrometer 250 Plus, equipped with a VarianGTA-96 graphite furnace). Calibration was carried out by using the stand-ard addition method, to minimize the matrix effects. The standards wereprepared using 1000mg/l Merck standard solutions. The accuracy of analyt-ical methods was checked by determining total metal concentrations on thereference material BCR 144 (sewage sludge), digested by the procedureindicated in Figure 5 for the dissolution of the residual fraction.

3. RESULTS

The concentrations of heavy metals, determined in each of the five fractionsby means of sequential extractions, are reported in Table I.

In general it can be seen that heavy metal levels are higher in samplescollected in the Cona marsh, in particular in sites 4 and 5, situated respect-ively along the Dese river and at its mouth. Thus, within the marsh, metalcontamination seems to decrease together with the distance from the areainfluenced by the Dese river discharge, as was also found in previous studies(Zonta et al, 1994). These results indicate the important role of this riveras a source of pollutants, as well as the effects of water circulation on theprocesses of transport and accumulation of metals. In this sector of themarsh, in fact, the sediments present a higher content of fine-grained par-ticles, which are known to be very efficient in the scavenging and transportof metals. In site 6, which is situated in the southern sector of the marsh, ina zone characterized by a more active water exchange, induced by the tide,

TABLE I Concentrations of metals (mg/kg dry weight) determined in the five fractions bymeans of sequential extractions. The sum of the concentrations is also reported

/ / / / / / IV V Sum

Iron site 1site 2site 3site 4site 5site 6site 7site 8

2.0111020175.2124.8

31099061012002300820570270

9201100880

390013001300870930

5303800810

45002300130011003500

820010000120001700027000130001200011000

1000016000140002700033000160001500016000

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Manganese site 1site 2site 3site 4site 5site 6site 7site 8

Copper site 1site 2site 3site 4site 5site 6site 7site 8

Zinc site 1site 2site 3site 4site 5site 6site 7site 8

Lead site 1site 2site 3site 4site 5site 6site 7site 8

Nickel site 1site 2site 3site 4site 5site 6site 7site 8

Chromium site 1site 2site 3site 4site 5site 6site 7site 8

n.d.: not determined.

7.26.66.63630

9.85.65.5

n.d.0.17n.d.0.72n.d.n.d.0.32n.d.

1.61.02.75.95.9

0.552.51.7

0.600.710.590.420.370.150.330.32n.d.n.d.0.550.480.240.10n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.n.d.

981201202001701308060

2.12.12.62.72.01.71.51.436476937451916248.99.6111417

8.74.47.6

0.942.01.13.35.51.22.22.7

0.370.800.54

1.01.3

0.590.520.47

3131283132282624

5.13.82.92.54.6

0.463.53.057

1806667793224293.414

5.61110

3.83.93.63.03.11.63.84.72.52.13.34.95.64.04.87.47.94.44.8

1043265929261932

4.225101827

9.44.58.27.43922324518

8.912

0.75.42.54.45.01.41.32.4

0.904.41.86.66.83.01.73.40.51.71.02.83.62.11.11.9

1009089

220230120140180

159.92028401713

6.48.023406885303220

4.48.111182415

8.15.18.2301525192530304552456880465965

250290270550490310270300

2641365274292319

11029020021026010083871838314866291819

13402039363236395160517792576572

and by a grain size distribution in sediments dominated by coarser particles(Zonta et al., 1994), the metal levels are significantly lower.

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In the Giudecca mud flat, the sediments from site 2 present the highest levelsof metal contamination. In this site more degraded conditions can be observed,since it is located in the central part of the mud flat, which is subject to a lowwater exchange and, at the time of sample collection, was affected by algalbloom phenomena; both these features favour the development of anoxicconditions and the deposition of fine particles (Argese et al, 1992). On thecontrary, in site 1, which is situated near a canal that allows an efficient waterexchange, the environmental conditions are better and the sediments have thelowest metal abundances, whereas site 3 presents intermediate levels.

Low concentrations of metals were also found in the sediment samplesfrom sites 7 and 8, and were comparable to those measured in the lesspolluted sites of the other areas investigated, that is sites 1 and 6. Sites 7and 8 are situated in proximity to the Malamocco inlet, which connects thelagoon to the Adriatic sea, and are relatively far from areas in which specificpollution sources may be identified, such as the industrial zone of the cityand the interface land-lagoon, which receives freshwater discharges from thedrainage basin. These sites, in particular, show the lowest levels with respectto the other areas, for Cu, Zn and Pb, whose occurrence in sedimentsis mainly of an anthropogenic origin.

The bar diagrams in Figure 6 show metal concentrations measured in thefive fractions, expressed as percentages of the total content, for the sedi-ments collected in the eight sites under investigation.

The results illustrated in Figure 6 allow us to make some general remarkson the partitioning of metals among the different fractions of the sequentialextraction procedure utilized.

I. Exchangeable fraction: metal concentrations measured in the extractsobtained from the first step were very low and, in many cases, were belowdetection limits. On average, the exchangeable fraction accounts for less than2% of the total content; slightly higher concentrations, which in some samplesreached 3-6% of the total amounts, were observed only for Mn, Zn and Pb.

II. Fraction bound to carbonates: the concentrations of all metals werehigher than those obtained with the former extraction. Particularly highlevels were found for Mn, Pb and, to a lesser extent, for Zn (on average 36,33 and 23% of the total concentrations, respectively). It can be seen thatthe distribution of non-residual Mn and Pb is dominated by the carbonatephase. In most of the samples, the Mn concentrations in this fractionaccounted for about 40% of the total content, whereas the Pb concentra-tions varied from 24 to 45% of the total.

III. Fraction bound to Fe-Mn oxides/hydroxides: in this fraction significantconcentrations were measured for all metals, indicating the high efficiency of

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Fe-Mn oxides and hydroxides as scavengers of trace metals. In particular forZn this fraction was dominant, with concentrations accounting for 25 to 62%of the total. Moreover, most non-residual Cr was found in this fraction.

IV. Fraction bound to organic matter and sulphides: for the majority of thesamples, non-residual Cu was found prevalently in this fraction, with con-centrations ranging from 16 to 61% of the total. Although the extractionprocedure used for the IV fraction doesn't allow a discrimination betweenthe organic and sulphide phases, the high levels of Cu can be ascribed to thewell known special affinity of this metal for organic matter; Cu, in fact,is known to form stable complexes with the functional groups containingoxygen that are present in humic materials (Kerndorff and Schnitzer, 1980).As concerns the other metals, significant concentrations (up to 20%) werefound for Fe; an important fraction of this metal can be bound in the formof authigenic iron sulphides (pyrite, mackinavite, greigite and amorphousmonosulphides) (Morse et al., 1987). Authigenic pyrite, in turn, can be animportant sink for trace metals, which can be incorporated in this phase byco-precipitation (Huerta-Diaz and Morse, 1992; Morse, 1994).

V. Residual fraction: the metal concentrations were generally higher withrespect to those found in the other four fractions, the sum of which repres-ents the non-residual metal content. In particular, Cr, Ni and Fe presentedthe highest levels in the residual fraction, showing that these metals aremainly associated with the most resistant components of the sediments.On the other hand, the lowest percentages were found for Zn and Pb,indicating the prevailing anthropogenic origin of these metals.

As can be seen in Figure 6, the sequential extractions underlined somedifferences in metal partitioning among the sites, which could be relatedto the differences in environmental conditions.

The partitioning of Fe was similar in all samples, except for those col-lected in sites 2, 4 and 8, in which a lower amount was found in the residualfraction, whereas high levels were obtained in the IV fraction; this may beascribed to the more reduced conditions observed in these sites, which canfavour the formation of sulphides. In addition, in site 2, with respect to theother sites, higher concentrations in the IV fraction were determined for allmetals, in particular for Cu. As already mentioned, noticeably degradedenvironmental conditions were observed in this site, where excessive algalgrowth and scarce water exchange cause a depletion of the oxygen in thewater column and the accumulation of decomposing organic material.

The samples from sites 7 and 8, which are situated near the Malamoccoinlet, though presenting similar total metal levels, exhibited different parti-tioning among the five fractions. In the sediments from site 8, which is

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. HEAVY METAL PARTITIONING IN SEDIMENTS 169

situated at a greater distance from the sea and is therefore subject to a lowerwater exchange, higher amounts of metals were found in the non-residualphases, especially in the cases of Cu, Zn and Pb.

In the three sites in the Giudecca mud flat, the percent concentrationsof non-residual Zn were higher with respect to those found in the other sites,suggesting that, in this area, the sediments present an enrichment of Zn of ananthropogenic origin, which could be ascribed to the proximity to theindustrial zone of Porto Marghera, which is relatively near the mud flat,and to the historical centre of the city of Venice. In fact, various investiga-tions have shown that both sediments collected in the area surrounding theindustrial zone and those collected in the inner canals of the city present highlevels of heavy metals, in particular Zn, though this contamination is likelyto be due to different sources (Argese et al., 1997; Donazzolo et al., 1984;Zonta et al., 1995).

4. CONCLUSION

The study of the distribution of heavy metals among different geochemicalphases in sediments is of particular concern in environments characterizedby a high complexity, such as the Lagoon of Venice.

The results of the sequential extraction procedure used in this studyprovided evidence of differences both in total metal content and in parti-tioning, which were related to the geochemical and hydrodynamical char-acteristics of the sites investigated and to their proximity to pollutionsources.

Significant percentages of metals of considerable ecotoxicological concern,such as Cu, Zn and Pb, were found in non-residual forms of association; theycan be released when a significant disturbance of the sedimentary environmentoccurs and can therefore be considered potentially bioavailable. Moreover,these findings indicate that an important amount of these metals in the sedi-ments of the Lagoon of Venice is of an anthropogenic origin.

On the contrary, the distribution of Ni and Cr is dominated by theresidual fraction; these metals are bound mostly to resistant componentsof sediments, from which they are not likely to be released, and thereforeexhibit a poor potential bioavailability.

Acknowledgements

The authors are grateful to Lorena Gobbo for laboratory assistance.

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