ARSENIC UPTAKE BY NATURAL CALCITES: PRELIMINARY RESULTS FROM SEQUENTIAL EXTRACTION OF ITALIAN TRAVERTINES P. Costagliola 1 , V. Rimondi 1 , M. Benvenuti 1 , L. Chiarantini 1 , F. Di Benedetto 2 , M. Gasparon 3 , P. Lattanzi 1 and M. Paolieri 1 1 Department of Earth Sciences, Univ. Firenze, Via G. La Pira, 4, 50121 Firenze, Italy 2 Museum of Natural History, Univ. Firenze, Via G. La Pira, 4, 50121 Firenze, Italy 3 Earth Sciences, The University of Queensland, St Lucia Qld. 4072, Australia Abstract Natural calcite has received little attention as a mineralogical trap for As, despite the abundance of this mineral on the Earth's surface. Recently, Di Benedetto et al. (2006) demonstrated that the CO 3 2− = AsO 3 3- substitution is possible in natural calcite hosted by Quaternary travertine deposits in Southern Tuscany, Italy. Two extraction procedures were compared in this study to quantify the arsenic content in the travertine samples studied by Di Benedetto et al. (2006). The two procedures yielded As values ranging from 109 to 270 mg/kg. These results indicate that the CO 3 2− = AsO 3 3- substitution in natural calcite can be an effective trap for this metalloid. Introduction Arsenic contamination of groundwater and food represents a serious environmental problem in many parts of the world, and has been recognised as one of the major threats to public health (Matschullat, 2000; Smedley and Kinniburg, 2002). The chemical and physical state of arsenic controls its mobility in the natural environment and, ultimately, its intake in the human food-chain. Much attention has been paid to sorption–desorption and coprecipitation reactions between arsenic and mineral surfaces, especially with Fe- and Mn-oxyhydroxides (Lenoble et al., 2002; Ouvrard et al., 2005; Sherman and Randall, 2003). This is considered the most important and common mechanism controlling arsenic mobility and its potential release into the human food-chain (Nriagu, 1994a, b). The uptake of arsenic in solid phases can remove this metalloid from solution, thus retarding its transport and decreasing its bio-availability (Sposito, 1984; Stumm and Morgan, 1996; Langmuir, 1997). The adsorption of arsenic by Fe- and Mn-oxyhydroxides surfaces, although being highly effective in general, represents only a transient immobilization of this element because, in principle, mineral surfaces could exchange it with other chemical species in solution. Recently, Roman-Ross et al. (2006) have demonstrated that it is possible, under laboratory conditions, to synthesize calcite in presence of high As(III) concentrations. The resulting Ca-carbonate may contain, on average, 30±6 mmol/kg (~ 2200 mg/kg) of As(III); the metalloid is incorporated in calcite by a solid solution mechanism, with CO 3 2− ions being replaced by HAsO 3 2− ions, offering a more stable trap than superficial adsorption. Natural calcite has received little attention as a mineralogical trap for arsenic, despite its abundance and widespread distribution on the Earth's surface (Cheng et al., 1999; Le Guern, 2003). This is probably due to the fact that natural carbonates do not contain significant amounts of arsenic, and its detection and quantitative determination by conventional analytical methodologies is therefore difficult. However, in a recent EPR-ESE study of an As-rich natural travertine deposit from Southern Tuscany (Italy), Di Benedetto et al. (2006) demonstrated that arsenic may be trapped also by natural calcite. The EPR-ESE spectra suggest that arsenic enters the calcite lattice through the CO 3 2− = AsO 3 3- substitution, a mechanism that implies a charge imbalance, contrary to what proposed by Roman-Ross et al. (2006). The EPR-ESE spectroscopy does not allow a quantitative determination of arsenic, and therefore the amount of this metalloid that can be trapped by natural calcite is unknown and the actual role that calcite may play in scavenging arsenic from the environment has not been evaluated. To address these questions, we have set up a preliminary experiment aimed at quantifying the amount of arsenic that can be incorporated by natural calcite. The samples were obtained from the travertine sequence studied by Di Benedetto et al. (2006). Two different sequential extraction procedures were tested and compared, because a reliable scheme for arsenic extraction from carbonate rocks is not available in the literature. Sampling area and geological outline Travertines with anomalous arsenic content were sampled in Southern Tuscany in the Pecora River catchment (Fig. 1; Costagliola et al., 2004; Di Benedetto et al., 2006). These travertines are found at the top of a stratigraphic sequence composed mainly of Quaternary clastic sediments filling a basin that formed after the Late Cretaceous-Middle Miocene compressional stages of the Apennine orogeny. They formed in a lacustrine- phytoclastic environment, and are composed by dominant calcite associated with variable amounts of other minerals, including iron oxyhydroxides. Thin centimetric layers of non-carbonate minerals represent cyclic IMWA Symposium 2007: Water in Mining Environments, R. Cidu & F. Frau (Eds), 27th - 31st May 2007, Cagliari, Italy