Arsenic incorporation in natural calcite lattice: Evidence from electron spin echo spectroscopy Francesco Di Benedetto a,b, ⁎ , Pilario Costagliola a , Marco Benvenuti a , Pierfranco Lattanzi c , Maurizio Romanelli d , Giuseppe Tanelli a a Dipartimento di Scienze della Terra, Università di Firenze, Italy b Museo di Storia Naturale, Università di Firenze, Italy c Dipartimento di Scienze della Terra, Università di Cagliari, Italy d Dipartimento di Chimica and CSGI, Università di Firenze, Italy Received 12 January 2006; received in revised form 28 March 2006; accepted 31 March 2006 Available online 22 May 2006 Editor: G.D. Price Abstract Quaternary travertines of the Middle Pecora Valley (Tuscany, Italy) contain up to 257 ppm arsenic. Such a content is environmentally relevant, but low enough to make the exact chemical speciation of arsenic difficult by applying conventional investigation techniques. The task was addressed by use of the Electron Spin Echo (ESE) spectroscopy, taking advantage from the modulation by the arsenic nucleus of the decay spectrum of the paramagnetic ion Mn(II), occurring as replacement of Ca in the calcite lattice. Interpretation of the spectra suggests that arsenic occurs in the calcite lattice in the position of C, through the substitution CO 2- 3 ⇔AsO 3- 3 . This mechanism of arsenic incorporation by calcite may be an effective limit of arsenic mobility under conditions where immobilization through sorption by iron and/or manganese oxyhydroxides is not operating. © 2006 Elsevier B.V. All rights reserved. Keywords: arsenic; travertine; electron spin echo spectroscopy; calcite 1. Introduction Arsenic contamination represents a serious environ- mental problem in many parts of the world [1–3]. Specifically, arsenic contamination of ground water has been recognised by far as one of the major threats for public health. The growing concern about the effects on human health of this metalloid has encouraged a wide range of studies on the chemical and physical state of arsenic in natural environment, because this feature controls the mobility of the element and, ultimately, its intake in the human food-chain [4–6]. Much attention has been paid to sorption–desorption reactions between arsenic and mineral surfaces, especially with Fe- and Mn-oxyhydroxides [7,8]. This is considered the most important and diffused mechanism controlling arsenic mobility and determining its release to groundwaters [9]. Comparatively, the influence of calcite on arsenic mobility has received less attention, in spite of the wide diffusion of this mineral at the Earth's surface [10,11]. With few exceptions (e.g. [12]), natural carbonates do not contain appreciable amounts of arsenic, thus limiting or hindering its determination by the most common Earth and Planetary Science Letters 246 (2006) 458 – 465 www.elsevier.com/locate/epsl ⁎ Corresponding author. Museo di Storia Naturale, Università di Firenze, Via G. La Pira, 4 - I 50121, Firenze, Italy. Tel.: +39 055 275 6349; fax: +39 055 284571. E-mail address: dibenefr@geo.unifi.it (F. Di Benedetto). 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.03.047