HEAVY METALS MOBILIZATION HEAVY METALS MOBILIZATION MONITORED BY A MONITORED BY A

MUSSELS BIOACCUMULATION TESTMUSSELS BIOACCUMULATION TEST

D. Bottos*, S. Manente, A. Mao, D. Manca and G. Perin**Environmental Science Department, Ca’ Foscari University, Venice

*danielabottos@unive.it **guiper@unive.it ABSTRACT

Sediments represent the final fate of most contaminants carried to the sea by rivers, acting as sinking device for these substances. Natural and anthropogenic activities can remobilise hazardous substances, as heavy metals, from sediment to the water column, becoming dangerous for filtering organisms. Mytilus galloprovincialis is commonly used as bioindicator to assess the contamination status in aquatic environment; it is a widespread column filtering organism, with a great filtration capacity, easy to handle and edible.Heavy metals mobilization from sediment was monitored by a Mytilus g. bioaccumulation test, in condition of tangential aeration. An aeration system (MODUS) that can oxygenate the water in proximity to sediment bottom layer without causing resuspension phenomena, was used. A Microcosmo has been recreated with a bottom layer of sediment and artificial sea water. Aeration system worked until a surface oxidized layer formed; then all the water has been substituted with clean water, without perturbation of the bottom sediment. Afterwards, mussels have been introduced into the tank. Bioaccumulation test ran for 28 days: samples of mussels have been collected in order to monitor heavy metals bioaccumulation in soft tissues and hepatopancreas.The results have shown MODUS worked very well. Summarizing: a) aeration of bottom sediment layer permitted biomass-film growth and it confirmed oxygenation event in respect of sediment layer at interface; b) at the same time mussels heavy metal content didn’t raise because MODUS worked without heavy metal bioavailability increasing. An in field application of this system allows a (partial) restoration of polluted areas.

ECOTOXICOLOGYECOTOXICOLOGY

Research done in the frame of the TAGUBAR Project Cooperazione Italiana allo SviluppoMinistry for Foreign Affairs of Italy

Acknowledgements

STALLING: STALLING: total Cr and Cd in Mytilus galloprovincialis

Cr and Cd in hepatopancreas

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Cr and Cd in soft tissues

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EXPERIMENT: EXPERIMENT: total Cr and Cd in Mytilus galloprovincialis

EXPERIMENT:EXPERIMENT: total Cr and Cd in sediment

Cr in hepatopancreas

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Cd in hepatopancreas

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Cr in soft tissues

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ANALYSISANALYSIS

◊ ◊ Sediments:Sediments:

ORIGINAL sediment, OXYDATED (OX) and REDUCED (RED) layers sediment

digested using microwave digester (ETHOS 1600) with 5 mL of milliQ water, 1,5 mL of hydrofluoric acid and 3 mL of Acqua Regia (3:1 HCl:HNO3).

◊ ◊ Mussels:Mussels:

Hepatopancreatic tissues separated from soft tissues and then both digested using microwave digester (ETHOS 1600) with 1 mL of milliQ water, 5 mL of nitric acid and 2 mL of 30% hydrogen peroxide.

ChromiumChromium andand CadmiumCadmium concentrations in each sediment and mussel samples were determined using GF/FL atomic absorption AAS Varian SpecrtAA–250Plus.

EXPERIMENTAL DESIGNEXPERIMENTAL DESIGN

MODUSMODUS (MOdulo di Disinquinamento Unitario di Sedimento) Unit for Sediment Treatment

MODUS is a special aeration system based on air-lift principle, built for water oxygenation by air insufflation without resuspension (Fig 1). Air-lift effect consists in mixing air to water originating a light density water mixture, which rises up along the column and drags up the residual water. The mixture loses air at the open top of the pipe becoming heavier and falling down forced by gravity. The primary characteristic of this system is that MODUS can oxygenate the water in proximity to sediment bottom layer without causing resuspension phenomena (Fig. 2) through a tangential air flow.

Tank with artificial sea water (33‰) and layer of sediment in the bottom

Setting up of the thermostatic system

Substitution of water in the tank with clean

artificial sea water

Formation of surface Formation of surface oxidized layeroxidized layer

Stalling of mussels

Mussels were put into experimental tank

Samples of mussels collected at 1st 3rd 7th 14th 21th 24th 28th day of experiment

28th day: samples of sediment (oxidized and

reduced layers)

CONCLUSIONSCONCLUSIONSMODUS has shown to be an efficient aeration system because it is able to oxygenate the upper sediment layer without sediment resuspension. It allowed the biomass-film growth, able to reduce metals diffusion at interface sediment-water and contribute to oxygenate the bottom, laying the bases for reconstruction of benthic biocenosis (Fig. 6). Algal biomass also probably acts as a further compartment for the free metals’ ions.

During stalling Cr and Cd concentration values decrease, confirming the good practice of stalling (16 days) organisms before in vitro tests.

Cd hepatopancreatic values are high in comparison with soft tissues ones; in fact, the digestive gland is known as the preferential organ for metals accumulation in mussels [1,2] and the first detoxification site, from which metals move to soft body in a second time, in order to affect elimination/storage events [3].

Mussels heavy metals content didn’t raise because MODUS worked without heavy metal bioavailability increasing.

Because of its great capacity on formation Cl-complexes in salt-water, Cd is easy and rapid to remove from sediment and probably remains in solution for a longer time [4,5]; so it is quickly bioavailable to marine organisms. The change of water after the formation of oxidized layer in sediment, has determined a new shifting of equilibrium from sediment to “clean” water, by fugacity principles [6].

MICROCOSMOMICROCOSMOA Microcosmo has been recreated in a glass tank with a bottom layer of lagoon sediment and artificial sea water (33‰), maintained at 12 °C by a thermostatic system (Fig. 3). Tangential aeration system worked two weeks during which a surface oxidized layer was formed; afterwards all the water in the tank has been substituted with clean artificial sea water, taking care of not provoking perturbation of the bottom sediment (Fig. 4). After changing water, almost 50 mussels have been put into the tank and suspended in the water column (Fig. 5). Mussels were previously taken from a mussel farming and put into a stalling tank, during 16 days, with only artificial seawater in order to allow them to acclimatize to experimental condition and clean up digestive tract. Bioaccumulation test run for 28 days: during this period the tangential aeration system worked non-stop and samples of mussels have been collected at 1st, 3rd, 7th, 14th, 21st, 24th, 28th day of experiment in order to monitor heavy metals bioaccumulation in mussels soft tissues and hepatopancreas.

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Cd in soft tissues

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References

[1] Soto M. et al. (1997). The contribution of metal/shell-weight index in target tissues to metal body burden in sentinel marine molluscs. 2. Mytilus galloprovincialis. The Science of Total Environment 198: 149-160.

[2] Cardellicchio N. et al. (1998). The influence of environmental and physiological factors on the accumulation of heavy metals in mussels (Mytilus galloprovincialis). Ann. Chem. 88: 577-584.

[3] Viarengo A. (1989). Heavy metals in marine invertebrates: mechanisms of regulation and toxicity at cellular level. CRC Rev. Acq. Sci. 1: 295-317.

[4] Eggleton J. and Thomas K.V. (2004). A review of factors affecting the release and bioavailability of contaminants during sediment disturbance events. In press.

[5] Caetano M. et al. (2003). Metal remobilization during resuspension of anoxic contamined sediment: short-term laboratory study. Water, Air, Soil Pollution 143: 23-40.

[6] MacKay D. et al. (1992). Generic models for evaluating the regional fate of chemicals. Chemosphere 24(6): 695-717.

IN

OUT Air insufflation

Fig. 1

Fig. 2

Fig. 4

Benthic algal biomass-Benthic algal biomass-film on sediment layer film on sediment layer formed during the testformed during the test

Fig. 6

Fig. 5 OXOX

REDRED

Fig. 3

t0 stall t0 exp

t0 expt0 stall

t0 exp

t0 exp

t0 exp

t0 exp t3t3

t3t3

t7

t7t7

t7 t14

t14

t14

t14

t21

t21

t21

t21 t24

t24 t24

t24

t28

t28

t28

t28

Italian Branch

Total Cd in sediment

0,00,20,40,60,81,01,21,41,61,8

original ox red

Total Cr in sediment

020406080

100120140160180

original ox red