CHIMICA DELL AMBIENTE E DEI BENI CULTURALI

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1 XIV CONGRESSO NAZIONALE DI CHIMICA DELL AMBIENTE E DEI BENI CULTURALI La chimica nella società sostenibile Rimini, 2-5 giugno 2013 ATTI DEL CONGRESSO

2 La chimica nella società sostenibile Atti del XIV Congresso Nazionale di Chimica dell'ambiente e dei Beni Culturali Rimini, 2-5 Giugno 2013 A cura di: Divisione di Chimica dell Ambiente e dei Beni Culturali Società Chimica Italiana ISBN:

3 Caratterizzazione e valutazione di fenomeni di contaminazione ambientale: acqua e suolo Mercury isotope ratios as contamination markers: procedure development and applications Alex Berni 1 alex.berni@unimore.it, Carlo Baschieri 1, Caterina Durante 1, Andrea Marchetti 1, Lucia Bertacchini 1, Stefano Covelli 2, Riccardo Petrini 2, Andrea Emili 2, Daniela Manzini 3 1 Department of Chemical and Geological Sciences, University of Modena and Reggio E., Italy 2 Department of Mathematics and Geosciences, University of Trieste,, Italy 3 Centro Interdipartimentale Grandi Strumenti, University of Modena and Reggio E., Italy 1. Introduction Mercury, Hg, is one of the most harmful elements present in the Earth, and has both natural and anthropological sources. Moreover, Hg undergoes many different transformation pathways during its biogeochemical, or industrial, cycles which in general involve redox reactions, both abiotic and biotic, and phase changes [1]. Despite the toxicity of this pollutant, there is still a lack in the knowledge about its biogeochemistry in the ecosystem and, therefore, it is of utmost relevance to develop new scientific approaches to deepen its transformation mechanisms and to identify the contamination sources. In this context, the determination of mercury stable isotopes ratios seems to be an extremely interesting and challenging application to verify the provenance of the element, i.e. to identify whether it is of natural or anthropogenic source. This approach can be useful in case of polluted areas where many are the contamination sources in order to plan an environmental requalification, as in the present case study represented by the National Interest Site of the lagoon of Marano-Grado (Trieste, Italy), which is the main object of the present investigation. 2. Relation This work deals with the setup of an analytical methodology for the determination of the isotopic ratios of mercury in sediments samples, performed by means of HR-MC-ICP/MS instrumentation [2]. Owing to the difficulties in the mercury ICP determination, due to the high number of the acquisition parameters to be standardized, the optimization and the validation of the analytical procedure was mandatory in order to obtain highly accurate and precise data. The Experimental Design technique, DoE, was applied in this work, allowing the maximum yield of information with a reasonably low number of experimental runs (see 2.1). After the optimization procedure, the analytical method was applied for the determination of the Hg isotopic fingerprints in environmental samples collected from: the Marano-Grado area, nearby Idrijia-Isonzo drainage basin and the Adriatic Sea Design of Experiment (DoE) Because of the relatively high atomic masses of its isotopes, Hg undergoes fractionation processes in the environment of very low magnitude. Thus a reliable determination of mercury isotopic ratios requires high precision measurements. The five factors, namely instrumental parameters, supposed to mostly affect the inner precision of the instrumental data are: a) the concentration of Hg in the sample solution; b) the concentration of the external standard solution of thallium used for mass-bias correction; c) the signal integration time for each measured cycle; d) the number of cycles per sample run and e) the uptake flow of sample solution. The influence of these variables on the inner precision of the analytical measurements, expressed in terms of relative standard error (RSE%), was evaluated by means of a Central Composite Face-centered Design (CCF), considering three different levels (i.e. values) for each variable. To build up the entire data set for the CCF design, 29 experimental runs were performed. Tab. 1 Conditions selected by DoE Parameter Hg Tl Int. time No. Cycles flow Value 5 ppb 10 ppb s μL/min XIV Congresso Nazionale di Chimica dell Ambiente e dei Beni Culturali COMUNICAZIONI ORALI

4 Caratterizzazione e valutazione di fenomeni di contaminazione ambientale: acqua e suolo The best analytical condition selected by the DoE, reported in table 1, allows a good isotope ratio determination for sample solutions with mercury concentrations down to 5 ng g Isotopic characterization of samples Sample pretreatment and analysis procedure Hg was extracted from sediments by means of a microwave-assisted digestion procedure carried out with an HCl/HNO 3 acidic mixture. The relative abundance measurements for the Hg isotopic pattern were determined by using a Thermo Finnigan Neptune MC-ICP/MS spectrometer. Mercury was introduced in the plasma as Hg 0 by means of a cold vapor generator apparatus (CVG). About 80 samples were analyzed Results Because of the absence of an isotopic SRM, mercury isotopic ratios are expressed as δ-values with respect to the SRM NIST 3133 reference. In particular, δ 202 Hg and δ 201 Hg are usually taken as indicators of mass-dependant (MDF) and mass-independent fractionation (MIF) processes. Most of the samples do not show significant MIF effect but show meaningful MDF values: lagoon and riverine samples lye in the range -1< δ 202 Hg < 0 while, on the contrary Adriatic sea samples are characterized by lower MDF data (-2< δ 202 Hg < -0.7). Although the measured values are in good agreement with literature data [3], this experimental approach highlights a quite complex situation between the Isonzo drainage basin (higher δ 202 Hg values) and the Gulf of Trieste (more enriched in lighter Hg isotopes). In fact, Isonzo River carries cinnabar and waste debris from the Idrija mining region, so its sediments fingerprint may be affected by the different sources of Hg or extraction technologies exploited trough five centuries of excavation activity. In the Grado and Marano Lagoon, Hg isotopic signature is the consequence of the mixing of two Hg contamination sources: sediments coming from the Idrija mine, transported by the Isonzo River and carried by tidal currents into the Lagoon and Hg discharged by the Torviscosa chlor-alkali plant Conclusions This study, the first entirely performed in Italy, highlighted the potentiality of Hg isotope ratio determination to distinguish the extension of environmental pollution derived from different sources. Hg contamination of the Grado and Marano Lagoon is due to sediments transported by riverine and tidal fluxes more than the contribution of Torviscosa chlor-alkali plant. Future developments should be directed both to assess the robustness of the analytical method in a longer period of time and to gain knowledge about accumulation history of Hg in sediments and biota. Bibliografia [1] J. Faganeli, M.E. Hines, S. Covelli, A. Emili, M. Giani, Mercury in lagoons: An overview of the importance of the link between geochemistry and biology. Estuar. Coast. Shelf Sci., 13, , 2012 [2] D. Foucher, H. Hintelmann, High-precision measurement of mercury isotope ratios in sediments using cold-vapor generation multi-collector inductively coupled plasma mass spectrometry, Anal. Bioanal. Chem., 384, , [3] D. Foucher, N. Ogrinc, H. Hintelmann, Tracing mercury contamination from the Irija mining region (Slovenia) to the gulf of Trieste using Hg isotope ratio Environ. Sci. Technol., 43, 33-39, 2009 XIV Congresso Nazionale di Chimica dell Ambiente e dei Beni Culturali COMUNICAZIONI ORALI

5 XIV CONGRESSO NAZIONALE DI CHIMICA DELL AMBIENTE E DEI BENI CULTURALI La chimica nella società sostenibile In collaborazione con: Con il patrocinio di: Università degli Studi di Modena e Reggio Emilia Comune di Rimini Con il contributo di: