Lizardite serpentine dissolution kinetics as a function of pH and temperature, including effects of elevated pCO2
Introduction
Mafic and ultramafic rocks represent important actors of chemical weathering and CO2 consumption (Dessert et al., 2003, Oelkers et al., 2008). Because of this, experimental and field studies aimed at measuring and understanding the high reactivity of (Mg, Fe)-bearing silicates are quite numerous (e.g. Gislason et al., 1996, Rosso and Rimstidt, 2000, Oelkers and Schott, 2001, Wolff-Boenisch et al., 2004, Wolff-Boenisch et al., 2006, Saldi et al., 2007, Kelemen and Matter, 2008, Louvat et al., 2008, Daval et al., 2010b, Apollaro et al., 2011), and recent concerns about rising levels of atmospheric CO2 and their potential impact on climate change has resulted in enhanced efforts dedicated to investigating water-CO2-(Fe, Mg)-bearing silicate interactions (e.g. Giammar et al., 2005, Boschi et al., 2009, Schaef et al., 2010, Hövelmann et al., 2011, King et al., 2011, Kwak et al., 2011, Gysi and Stefansson, 2012, Rosenbauer et al., 2012).
Among (ultra)mafic rocks, serpentinites were recognized early on as one of the most appealing targets for sequestering CO2 by mineral trapping (i.e. carbonation reactions) because of their high MgO content (Goff and Lackner, 1998). The term “serpentine” refers to a group of phyllosilicates with a nominal composition of Mg3Si2O5(OH)4. Their structure consists of alternating tetrahedral Si layers and octahedral (brucite-like) Mg layers. For such 1:1 layered structures, the tetrahedral sheet is connected to the octahedral sheet through the nonshared apical oxygens of the tetrahedra.
The contribution of serpentinites to the global C cycle at the Earth's surface has been suggested for well over 40 years, based on the observation that magnesite often occurs with serpentinized rocks (e.g. Barnes et al., 1973). More recently, Wilson et al. (2006) showed that abandoned serpentine-rich mine tailings at Clinton Creek (Canada) may contain carbonate levels as high as 60 wt.%, and emphasized that the carbon isotopic composition of such carbonates confirmed the atmospheric origin for carbon. Similarly, studies by Wilson et al. (2009) and Pronost et al. (2012) suggested that chrysotile mining wastes could passively capture between 0.6 kt and 6 kt CO2 per year for a given heap, thus providing a process for permanent CO2 sequestration as solid carbonates.
The carbonation reaction between serpentine and CO2, which is spontaneous at standard conditions (see thermodynamic calculations in, e.g. Guyot et al., 2011), can be promoted by elevated pCO2 and T conditions. Accordingly, several studies dedicated to investigating the aqueous carbonation rate of serpentine minerals at elevated pCO2 and temperatures of up to 400 °C have been performed, leading to rather modest results however, due principally to the limited carbonation yields and rates that were measured (e.g. Gerdemann et al., 2007, Dufaud et al., 2009). Several studies reporting results of batch experiments suggested that the consumption of serpentine was ultimately hindered by the formation of protective amorphous silica and/or iron oxyhydroxide coatings (Park et al., 2003, Teir et al., 2007a, Teir et al., 2007b). However, deciphering rate-limiting steps of mineral weathering processes in closed systems remains rather challenging without fully modeling the kinetic reaction paths (e.g. Daval et al., 2009, Zhu et al., 2010, Daval et al., 2011). This requires a detailed knowledge of the dissolution and precipitation kinetic rate laws of the primary and secondary phases. In this respect, it is important to stress that only a very limited number of studies have determined the dependence of serpentine dissolution rates on such fundamental parameters such as surface area, temperature, or pH (e.g. Luce et al., 1972, Lin and Clemency, 1981, Bales and Morgan, 1985). Most of the others aimed at estimating the biodurability of asbestos in lung-like solutions (e.g. Chowdhury, 1975, Jaurand et al., 1977, Churg et al., 1984, Bales and Morgan, 1985, Gronow, 1987, Hume and Rimstidt, 1992).
Both the scarcity of kinetic data and the differences in published results emphasize the need for investigations of the dissolution kinetics of serpentines over a wide range of conditions, which is the primary goal of the present paper. Moreover, none of the abovementioned studies can be used to accurately predict the dissolution and carbonation rates at conditions relevant to in situ CO2 sequestration (e.g. temperatures of ~ 50 °C and pCO2 equal to several tens of bars, relevant for example, for the CarbFix project- see Gislason et al., 2010). In particular, no study has addressed the question as to whether or not CO2 has an intrinsic and direct impact on the dissolution rate of serpentine, other than that due to water acidification. Indeed, only a limited number of studies addressed this question for other silicate minerals. While some of them suggested that the presence of CO2 affected the dissolution rate of silicate minerals through its effect on pH only (e.g. Carroll and Knauss, 2005, Golubev et al., 2005, Prigiobbe et al., 2009, Hellmann et al., 2010), others reported that the dissolution rate was enhanced at high pCO2 (Hänchen et al., 2006), possibly because of the rate-promoting effect of dissolved carbonate species (Berg and Banwart, 2000).
These observations motivated us to undertake a study dedicated to the dissolution of serpentine over an extended range of pH and T conditions, thereby allowing for the determination of the pH- and T-dependences of serpentine dissolution rates, which represent crucial kinetic data for accurately modeling both the weathering and carbonation rates of serpentinite complexes (e.g. Cipolli et al., 2004, Knauss et al., 2005, Apollaro et al., 2011, Orlando et al., 2011). In addition, the intrinsic effect of CO2 on serpentine dissolution rate was deconvoluted from that of pH by comparing results obtained at similar pH in the presence or absence of elevated pCO2. In the last part of this study, the data acquired for lizardite are compared to olivine in order to estimate the degree of CO2 consumption by these two basic silicate minerals under identical conditions.
Section snippets
Starting materials
The starting material was obtained from the mineralogical collection of Université Pierre & Marie Curie (Paris, France). It consists of a piece of serpentinite (~ 100 g) from Buskerud (Norway). The sample is mainly composed of serpentine, with small amounts of iron oxides and dolomite, as identified by scanning electron microscopy (SEM) and qualitative energy dispersive X-ray (EDX) spectroscopy. The dolomite content of the bulk rock was evaluated to be ~ 1.5 wt.% by Rock-Eval measurements (see
Steady-state dissolution stoichiometry
Studying the stoichiometry of silicate dissolution is important, because it sheds light on the reaction mechanisms and the rate laws that are appropriate to describe the dissolution process (Hellmann, 1995). The main results of this study are listed in Table 2. The steady-state behavior of lizardite dissolution can be characterized in terms of the aqueous [Mg]/[Si] ratio. Steady-state lizardite dissolution is stoichiometric only at 90 °C (excepting experiment 90-C) (Fig. 3a). The congruence of
Conclusions
This study reports results from dissolution experiments conducted on serpentine (lizardite) in the acidic pH range and for 27 °C ≤ T ≤ 90 °C, either in dilute HCl solutions or in solutions equilibrated with elevated pCO2. For experiments carried out in H2O–HCl solutions, far-from-equilibrium dissolution kinetics of lizardite based on Si concentrations can be expressed as: , with k0 = 10− 2.27 ± 0.56 mol.m− 2.s− 1; Ea = 42.0 ± 1.5 kJ.mol− 1; n = 0.53 ± 0.08. For experiments carried out in solutions
Acknowledgments
The authors thank F. Molton and S. Chakraborty (ISTerre, Grenoble) for the technical assistance with the experiments. The careful review and comments made by an anonymous reviewer and (A.E.) J. B. Fein were much appreciated and helped improve this manuscript.
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