An experimental study of calcite dissolution rates at acidic conditions and 25 jC in the presence of NaPO 3 and MgCl 2 Marwan Alkattan, Eric H. Oelkers * , Jean-Louis Dandurand, Jacques Schott Laboratoire de Ge ´ochimie, CNRS/UMR 5563-OMP-Universite ´ Paul-Sabatier, 38 rue des Trente-Six Ponts, 31400 Toulouse, France Abstract Dissolution rates of single calcite crystals were determined from sample weight loss using free-drift rotating disk techniques. Experiments were performed at 25 jC in aqueous HCl solutions over the bulk solution pH range  1 to 3 and in the presence of trace concentrations of aqueous NaPO 3 and MgCl 2 . These salts were chosen for this study because aqueous magnesium and phosphate are known to strongly inhibit calcite dissolution at neutral to basic pH. Reactive solutions were undersaturated with respect to possible secondary phases. Neither an inhibition or enhancement of calcite dissolution rates was observed in the presence of aqueous MgCl 2 at pH 1 and 3. The presence of trace quantities of NaPO 3 , which dissociates in solution to Na + and H 2 PO 4  , decreased the overall calcite dissolution rate at pH V 2. This contrasting behavior could be attributed to the different adsorption behavior of these dissolved species. As calcite surfaces are positively charged in acidic solutions, aqueous Mg 2+ may not adsorb, whereas aqueous phosphate, present as either the anion H 2 PO 4  or the neutral species H 3 PO 4 0 , readily adsorbs on calcite surfaces leading to significant dissolution inhibition. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Calcite dissolution rates; NaPO 3 ; MgCl 2 1. Introduction The motivation of this work is the improved under- standing of the rates and mechanism of calcite dis- solution in acidic solutions. Toward this goal, the dissolution rates of calcite crystals have been measured as a function of aqueous NaPO 3 and MgCl 2 at a temperature of 25 jC and  1 < pH < 3. Comparison of these rates with those reported in the literature illustrates the effect of these salts as a function of their concentration and pH. The purpose of this manuscript is to report the results of this experimental and com- parative study. Due to its widespread presence in crustal rocks and its role in CO 2 cycle, the dissolution/precipita- tion kinetics of calcite have been studied more than perhaps any other mineral (c.f. King and Liu, 1933; Tominaga et al., 1939; Weyl, 1958, 1967; Berner, 1967, 1975, 1978; Nestaas and Terjesen, 1969; Griffin and Jurinak, 1973; Berner and Morse, 1974; Plummer and Mackenzie, 1974; Lund et al., 1975; Plummer and Wigley, 1976; Sjo ¨berg, 1976, 0009-2541/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII:S0009-2541(02)00121-3 * Corresponding author. Tel.: +33-561558785; fax: +33- 561558138. E-mail address: oelkers@lmtg.ups-tlse.fr (E.H. Oelkers). www.elsevier.com/locate/chemgeo Chemical Geology 190 (2002) 291 – 302