12626 DOI: 10.1021/la9041088 Langmuir 2010, 26(15), 12626–12639 Published on Web 06/30/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Acid-Base Behavior of the Gaspeite (NiCO 3 (s)) Surface in NaCl Solutions Adri an Villegas-Jim enez,* ,† Alfonso Mucci, † Oleg S. Pokrovsky, ‡ and Jacques Schott ‡ † Earth and Planetary Sciences, McGill University, 3450 University Street, Montr eal, Quebec H3A 2A7, Canada, and ‡ G eochimie et Biog eochimie Exp erimentale, LMTG, UMR 5563, Universit e de Toulouse-CNRS, 14, Avenue Edouard Belin 31400 Toulouse, France Received October 28, 2009. Revised Manuscript Received May 17, 2010 Gaspeite is a low reactivity, rhombohedral carbonate mineral and a suitable surrogate to investigate the surface properties of other more ubiquitous carbonate minerals, such as calcite, in aqueous solutions. In this study, the acid-base properties of the gaspeite surface were investigated over a pH range of 5 to 10 in NaCl solutions (0.001, 0.01, and 0.1 M) at near ambient conditions (25 ( 3 °C and 1 atm) by means of conventional acidimetric and alkalimetric titration techniques and microelectrophoresis. Over the entire experimental pH range, surface protonation and electrokinetic mobility are strongly affected by the background electrolyte, leading to a significant decrease of the pH of zero net proton charge (PZNPC) and the pH of isoelectric point (pH iep ) at increasing NaCl concentrations. This challenges the conventional idea that carbonate mineral surfaces are chemically inert to background electrolyte ions. Multiple sets of surface complexation reactions (i.e., ionization and ion adsorption) were formulated within the framework of three electrostatic models (CCM, BSM, and TLM) and their ability to simulate proton adsorption and electrokinetic data was evaluated. A one-site, 3-pK, constant capacitance surface complexation model (SCM) reproduces the proton adsorption data at all ionic strengths and qualitatively predicts the electrokinetic behavior of gaspeite suspensions. Nevertheless, the strong ionic strength dependence exhibited by the optimized SCM parameters reveals that the influence of the background electrolyte on the surface reactivity of gaspeite is not fully accounted for by conventional electrostatic and surface complexation models and suggests that future refinements to the underlying theories are warranted. 1. Introduction Carbonate minerals are of considerable environmental signifi- cance due to their ubiquity and high reactivity in natural aquatic systems. In aqueous solutions, their macroscopic properties are controlled by multiple homogeneous and heterogeneous equili- bria among which surface reactions (i.e., ionization and ad- sorption) are recognized to play a critical role. 1 This realization has stimulated numerous studies on the surface reactivity of hydrated carbonate surfaces. 2-11 Most of these efforts have focused on the derivation of empirical and semiempirical relationships to represent ion partitioning between the aqueous phase and the surface of calcite, aragonite, magnesium-bearing carbonates, and, to a lesser extent, other divalent carbonate minerals. 12 Despite these efforts, the quantitative characterization of the sorptive properties of most carbonate surfaces has lagged behind that of other minerals, such as metal oxides and silicates, 13 because of their higher reactivities (i.e., faster reaction rates and greater solubilities) and the complex interplay of stepwise and/or parallel reactions (e.g., adsorption, ion exchange, surface precipitation, coprecipitation, dissolution). 1 These considerations drove earlier workers to develop a fast flow-through reactor-based titration protocol that minimizes the contribution of dissolution and precipitation during acid-base titrations performed on sparingly soluble carbonates. 2 This protocol was used to obtain surface charge data for siderite, 2,3 rhodochrosite, 2,3 magnesite, 4 and dolomite. 5,6 On the basis of these results, surface complexation models (SCMs) that include ionization (i.e., acid-base) and mineral constituent adsorption reactions were proposed for these minerals. Unfortunately, the application of this approach to highly reactive carbonate minerals, such as calcite or aragonite, is not feasible because their fast dissolution kinetics interferes significantly with the computation of surface charge. 2,3 Conversely, gaspeite, a nickel-bearing rhombohedral carbonate mineral, displays the slowest dissolution kinetics of all naturally occurring calcite-type minerals in aqueous solutions 7 and, thus, is amenable to investigations by experimental protocols commonly applied to metal oxides but unsuitable for other carbonate minerals. Natural gaspeite specimens typically contain intermediate to high amounts of magnesium which closely reflect the physical properties of the hypothetical solid solution: Ni 0.5 Mg 0.5 (CO 3 ). 14 In contrast, laboratory experiments revealed that Ca 2þ substitu- tion by Ni 2þ ions in the calcite structure is very limited, 15 which may explain why a complete NiCO 3 -CaCO 3 solid solution series has not been found in nature. Regardless of their purity, NiCO 3 minerals display a rhombohedral structure predominantly bounded by the (10.4) domain, 16 a common feature of other isostructural carbonates such as calcite, magnesite, and dolomite. This char- *Corresponding author. E-mail: adriano@eps.mcgill.ca. Now at: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, D.C. 20015-1305. (1) Morse, J. W. Mar. Chem. 1986, 20, 91. (2) Charlet, L.; Wersin, P.; Stumm, W. Geochim. Cosmochim. Acta 1990, 54, 2329. (3) Van Cappellen, P.; Charlet, L.; Stumm, W.; Wersin, P. Geochim. Cosmochim. Acta 1993, 57, 3505. (4) Pokrovsky, O. S.; Schott, J.; Thomas, F. Geochim. Cosmochim. Acta 1999a, 63(6), 863. (5) Pokrovsky, O. S.; Schott, J.; Thomas, F. Geochim. Cosmochim. Acta 1999b, 63(19/20), 3133. (6) Brady, P. V.; Papenguth, H. W.; Kelly, J. W. Appl. Geochem. 1999, 14, 569. (7) Pokrovsky, O. S.; Schott, J. Environ. Sci. Technol. 2002, 36, 426. (8) Jordan, G.; Higgins, S. 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