This work is part of a systematic study aimed at the characterization of Mg-silicate mineral dissolution rates. A significant number of past studies have focused on the dissolution rates of the divalent metal silicates, including studies of the dissolution rates of forsterite (Murphy and Helgeson, 1987, 1989; Wogelius and Walther, 1991, 1992; Pokrovsky and Schott, 2000; Rosso and Rimstidt, 2000; Oelkers, 2001b; Golubev et al., 2005; Olsen and Rimstidt, 2008; Prigiobbe et al. 2009; Daval et al., 2011a; King et al., 2014), enstatite (Schott et al., 1981; Oelkers and Schott, 2001), diopside (Petit et al., 1987; Knauss et al., 1993; Brantley and Chen, 1995; Chen and Brantley, 1998; Golubev et al., 2005; Golubev and Pokrovsky, 2006; Dixit and Carroll, 2007; Daval et al., 2011b, 2013) and talc (Lin and Clemency, 1981; Jurinski and Rimstidt, 2001; Saldi et al., 2007).
The motivation for this study is several fold:
The dissolution rates of Ca- and Mg-bearing silicates, such as tremolite, are of particular interest as they are potential sources of the divalent metals required for the mineralogical storage of CO2 (cf. Oelkers and Schott, 2005; Hänchen et al., 2008; Oelkers and Cole, 2008; Oelkers et al., 2008a; King et al., 2010; Ryu et al., 2011; Saldi et al., 2012; Gislason and Oelkers, 2014).
Exposure to tremolite is widely recognized as a cause of asbestos-related diseases (Churg, 1988; Case, 1991; Davis et al., 1991; Robledo and Mossman, 1999; Roggli et al., 2002; Gunter et al., 2007; Pugnaloni et al., 2013). A particle's ability to resist dissolution once submerged in lung fluid (i.e. its biodurability) may be a critical determinant in its toxicity (Addison and McConnell, 2008). This connection has motivated several previous studies aimed at quantifying tremolite biodurability (Oze and Solt, 2010; Rozalen et al., 2013).
The dissolution rates of the Mg silicates appear to exhibit a systematic behaviour as a function of aqueous solution pH and composition (Schott et al., 2009). It is anticipated that this study will provide further insight into this systematic behavior of these minerals.
The goal of this study is to expand our knowledge of tremolite dissolution kinetics at acid to neutral pH. Thus tremolite dissolution rates have been measured at 25 to 150°C, and 1.9 < pH < 6.7 as a function of aqueous fluid concentration at far-from-equilibrium conditions. The results of this study illuminate the durability of tremolite at conditions relevant to mineral carbonation and human health.
To date there have been relatively few studies of the dissolution rates of tremolite reported in the literature. Schott et al. (1981) reported the dissolution rates of tremolite at 22°C at 1 ⩽ pH ⩽ 6 in aqueous HCl solutions measured in batch reactors. Mast and Drever (1987) reported the dissolution rates of tremolite at 22°C and 2 ⩽ pH ⩽ 9 in aqueous solutions containing HCl, KHCO3, H3BO3 and KOH measured in fluidized bed reactors. Measured rates decrease slightly with increasing pH to at least pH 9. In addition, both of these experimental studies observed a preferential release of Mg over Si during the early stages of experiments performed at acidic conditions prior to attainment of stoichiometric steady-state dissolution. This behaviour and similar observations on other divalent metal silicates reported by Oelkers et al. (2009) were attributed to a relatively rapid exchange reaction between H+ and Mg2+ on the mineral surfaces. Similarly, Schott et al. (1981) reported that Ca was preferentially released compared to both Mg and Si during tremolite dissolution at acid conditions. Mast and Drever (1987) performed additional tremolite dissolution experiments at pH 4 and 7 as a function of aqueous oxalate concentrations. In contrast to the effect of the presence of aqueous organic species on Al-silicate minerals (cf. Oelkers and Schott, 1998), no effect of the presence of aqueous oxalate was observed for oxalate concentrations ⩽10−3m. The present study expands upon this past work by determining tremolite steady-state dissolution rates over a wide range of pH at temperatures between 25 and 150°C and by establishing an equation to describe these rates over a variety of environmental conditions.
The degree to which equation 9 provides a quantitative description of tremolite dissolution rates is assessed below.
Methods and samples
Two different tremolite samples were used in this study, providing some variation in tremolite morphology in the starting material. One sample (tremolite A) was a massive sample of bladed to prismatic tremolite crystals from Felch, Michigan, USA, and the other (Tremolite B) was a fibrous tremolite sample from Cannan, Connecticut, USA. Both samples were crushed using a pulverizer and agate mortar and pestle. Selected size fractions of this powder were then cleaned ultrasonically using acetone. The resulting tremolite powders were subsequently dried overnight at 110°C. The mineralogy of these samples was analysed using an INEL CPS 120 X-ray diffractometer (XRD) using CoKα radiation, with a scan speed of 0.02° s−1. Tremolite was the only phase evident in the XRD spectra. The elemental composition of these tremolites was determined using EDS spectroscopy. The resulting compositions were Ca1.99Mg5.02Si8O22(OH)2 and Ca2.10Mg4.97Fe0.07Si8O22(OH)2, respectively, for tremolite A and B when normalized to 8 Si. The surface areas of the resulting tremolite powders, determined by the three-point BET method using a Quantachrome Autosorb-1 together with N2 gas, are listed in Table 1. Images of the tremolite powders were taken prior to dissolution experiments using a JEOL 6360 LV Scanning Electron Microscope (SEM); some examples are shown in Fig. 1a,b. Prior to the dissolution experiments, the tremolite powder was free of fine particles and minerals other than tremolite.
Two distinct open-system reactor systems were used in this study. Experiments were performed at 25, 37 and 50°C in 250 ml Azalon plastic mixed-flow reactors placed in a thermostated bath (see Chairat et al., 2007). These reactors were fitted with Nalgene tubes for supply and recovery of experimental fluids, which were pumped through the reactor with Gilson peristaltic pumps. The reacted fluids were passed through a 0.45 μm Millipore Nitrocellulose in-line filter upon exiting the reactor. Floating teflon-coated stir bars were used to mix the powder/fluid mixture in the reactor and to avoid grinding. Experiments were performed at 80, 100 and 150°C in a Parr titanium mixed-flow reactor system, such as described by Oelkers et al. (2008b). A High Precision/High Pressure Liquid Chromatography Pump provided continuous fluid flow ranging from 1 to 2 g/min during the experiments. The precision of the fluid flow rates was ±4%. The volume of the titanium reactor was 250 ml. The fluid within the reactor was stirred using a Parr magnetically driven stirrer, the temperature controlled by a Parr controlled furnace, and elevated pressure was maintained using a back-pressure regulator.
Experiments were performed in this study either individually or in series, consisting of several experiments performed on a single tremolite powder. Each experimental series is distinguished by the first two entries of its experiment number; the first entry indicates the identity of the tremolite powder used, and the second letter the experimental series run with this powder. For example, series A-a, was performed using tremolite A and consists of experiments A-a-1, A-a-2, and A-a-3. Note that some series consisted of just one experiment. At the beginning of each experimental series the reactor was dismantled at ambient conditions. A specific mass of tremolite powder was placed in the reactor. The reactor was filled with the initial inlet fluid, closed, and placed in a thermostated bath or in a furnace. The temperature, pressure, fluid flow, and stirring rate were adjusted to desired settings. The fluid flow rate and outlet fluid composition were measured regularly. After steady state was verified with a minimum of three constant Si concentrations in the outlet fluid samples obtained over several residence times (defined as the volume of the reactor divided by the reactive fluid flow rate): either (1) the experimental series was stopped; or (2) the inlet fluid composition, fluid flow rate and/or the temperature were then changed to the next desired experimental condition.
Dissolution experiments were performed in fluids consisting of demineralized H2O, and reagent-grade HCl, NaCl, CaCl2 and MgCl2 from Merck, and aqueous SiO2 created from the dissolution of amorphous SiO2 at 80°C. All inlet fluids had stoichiometric ionic strengths of 0.02 or 0.12 mol kg−1. The compositions of all inlet fluids are listed in Table 2. Calcium and magnesium compositions of the inlet and outlet fluids were determined using a Perkin Elmer Zeeman 5000 atomic absorption spectrometer. Silica compositions were measured using the Molybdate Blue method (Koroleff, 1976). The reproducibility of chemical analyses was ±4% for Si, Mg and Ca concentrations >0.5, >0.1 and >0.1 ppm, respectively, but as much as ±10% at lower concentrations. Outlet fluid pH was measured at 25°C immediately after sampling. All outlet fluids were undersaturated with respect to all possible secondary phases as confirmed by PHREEQC calculations.
Photomicrographs of the tremolite surfaces following their dissolution during two representative experiments are shown in Fig. 1c,d. Two observations are apparent. First, dissolution appears to occur primarily on the tremolite prism edges, where the divalent cations can readily move out of the structure. Secondly, a significant number of fine-grained needle-shaped particles are present on the surfaces following the experiments.
Thirty four steady-state dissolution rates were obtained in this study; experimental results and conditions are summarized in Table 3. Two examples of the temporal evolution of instantaneous rates generated from Si, Mg and Ca release are shown in Fig. 2. The first example illustrates rates during experimental series B2-g, performed at pH 4.3 and temperatures of 37 and 50°C. The initial release rates of both Mg and Ca are approximately an order of magnitude faster than that of Si after normalization to the stoichiometry of the dissolving tremolite. The release rates of Mg decrease with time to be stoichiometric with respect to Si after ~25 days of elapsed time, and remain near to stoichiometric with respect to Si until the end of the experimental series. In contrast, the Ca release rates remain ~0.6 orders of magnitude faster until the end of this series. The second example illustrates the instantaneous tremolite dissolution rates in experiment B2-e-1 performed at pH 6.9 at a temperature of 37°C. Initially, Mg and Si are released in stoichiometric proportions, with Ca being released more than an order of magnitude faster. As Si release rates approach steady state, the Mg release rate appears to increase and its release rates are ~0.3 orders of magnitude greater than that of Si at the end of the experiment.
The logarithms of steady-state tremolite dissolution rates based on Mg and Ca release are compared with those based on Si release in Fig. 3. Note that the definition of steady state adopted in this study is that of a near constant Si release rate; Mg and Ca may not have attained a steady-state in these experiments. Nevertheless, as can be seen in Fig. 3, tremolite dissolution rates based on Mg release generally fall within 0.3 log units of the corresponding Si rates. In contrast, steady-state rates based on Ca release are, on average, six times greater than corresponding rates based on Si release. Note that rates based on Ca release measured at the elevated temperature are closer to or even lower than their corresponding Si rates determined at temperatures >80°C
The variation of forward steady-state tremolite dissolution rates measured at 37 and 50°C based on Si release are depicted as a function of pH in Fig. 4 and as a function of in Fig. 5. The general trend of the rates is to decrease with increasing pH with a log rate vs. pH slope near −0.25. Similarly, despite scatter, rates tend to increase with increasing ; the dashed lines drawn in Fig. 5, consistent with the symbols, have a slope of 0.125, consistent with n = 8 in equation 7.
The variation of measured steady-state tremolite dissolution rates at pH ~2 as a function of temperature are quantified in the present study using the empirical Arrhenius equation, equation 8. The activation energy and pre-exponential constant in equation 9 were obtained with the aid of the plot shown in Fig. 6. Regression of these data, as illustrated in this figure, yields EA = 80 kJ/mol.
The degree to which equation 11 describes the rate obtained in the present study can be assessed with Fig. 7 which compares rates determined using equation 11 with their measured counterparts. The average difference between calculated and measured rates is 0.3 log units.
Calcium release and tremolite toxicity
The preferential release of Ca has been observed throughout all experiments performed at T<80°C. Similar to the behaviour of wollasonite during its dissolution (Schott et al. 2012), Ca appears to be removed more quickly than and independently of the other elements. In the case of wollastonite, the Ca-depleted layer does not attain a steady-state thickness at acidic conditions, but grows continuously. Such also appears to be the case for tremolite at least for the duration of the <80°C experiments; the tremolite structure has channels parallel to its Si tetrahedral chains allowing for the ready removal of Ca through proton-exchange reactions.
A significant observation is that the tremolite structure consists of linear talc-like Mg-Si ribbons, with the Mg in 6-fold coordination bound together by adjoining Ca in 8-fold coordination (Warren, 1930; Hawthorne and Oberti, 2007). A schematic illustration of the tremolite structure, projected down its c axis is shown in Fig. 8. It is clear from this image that the removal of Ca will liberate the talc-like ribbons from the bulk tremolite, creating fine needle-like particles. Such particles are evident from the tremolite recovered following the dissolution experiments performed in the present study. It follows that the preferential Ca release in lung fluids could promote the toxicity of inhaled tremolite through the creation of such fine-grained needle-like particles.
Comparison of the dissolution rates of Mg-silicate minerals
The dissolution rates of selected Mg-silicate minerals at ambient temperatures are illustrated in Fig. 9. The rates shown were selected because they were all generated using similar techniques in the same laboratory, facilitating their comparison. Mg-silicate dissolution rates all tend to decrease with increasing pH, though there is some indication that these rates become pH independent at alkaline conditions. Notably, the dissolution rates decrease substantially with the degree of connectiveness of the Si structure. Forsterite, consisting of isolated Si tetrehedra, dissolves faster than enstatite, which is made up of single chains of Si tetrehedra, which itself dissolves faster than talc and tremolite, consisting of sheets of Si tetrehadra, and talc-like double-chained Si tetrehedra. The similar talc and tremolite dissolution rates probably reflect the similarity in their Si-Mg-O framework after the Ca is removed from the latter mineral. Taken together, the comparison shown in Fig. 9 is consistent with a similar dissolution mechanism for these minerals comprising the relatively fast removal of Mg followed by the slower rate-limiting liberation of partially detached Si from the mineral surface.
Experimental uncertainties and data scatter
Uncertainties associated with the rates generated in this study arise from a variety of sources, including the measurement of aqueous solution concentrations, fluid-flow rates, and tremolite surface areas. The uncertainties in the measured values of total aqueous silica, magnesium, and calcium concentration are on the order of ±10% or less. Computational and experimental uncertainties in the pH measured are on the order of ±0.04 pH units. Uncertainties in fluid-flow rate measurements are not more than 4%. The uncertainty associated with the measurement of the surface area of the initial tremolite powder is ±10%. If uncertainties were estimated exclusively from the sum of these contributions, an overall uncertainty of the dissolution rates reported would be of the order of 20%. Nevertheless it is clear from Figs 4, 5 and 6 that other sources lead to the additional scatter in the measured rates. According to mass-balance calculations, <5% of the initial tremolite powder was dissolved in any experimental series. It seems possible, however, that the tremolite surface area present in the reactor evolves significantly during the experiments. Although post-experiment surface areas were not measured, as emphasized above, a significant number of fine, needle-shaped gains were formed during the dissolution experiments performed in this study. As emphasized by a number of studies (Gautier et al., 1994; Luttge et al., 2013; Fischer et al., 2014), the degree to which reactive surface area varies in response to dissolution is currently impossible to define unambiguously, yet consideration of the scatter in the Figs 4, 5 and 6 suggests that such effects may be large. This conclusion is consistent with the rates obtained from experimental series A-a, which dissolved tremolite at pH 2 and 25, 37 and 50°C. This experimental series leads to an anomalously fast rate during the third experiment performed in the series, after it had already dissolved at pH 2 at 25 and 37°C, potentially due to the creation of fine particles related to the preferential release of Ca.
The results and discussion presented above illustrate the distinct dissolution behaviour of tremolite. Tremolite dissolution rates based on Si or Mg release are orders of magnitude slower than corresponding dissolution rates of forsterite and enstatite. As a consequence, the carbonation of tremolite will be far less efficient than the carbonation of these other Mg silicate minerals, limiting its potential in mineral carbon storage efforts. The dissolution of tremolite, however, releases calcium preferentially, at least at temperature >80°C. This preferential release continues for a substantial amount of time, leading to the extensive removal of Ca from the tremolite surface. Due to its mineral structure, the removal of Ca will promote the breaking apart of the tremolite into small needle-shaped particles, which probably will increase its toxicity to human lungs. It seems, therefore, that an improved understanding of the chemical processes controlling the retention vs. the removal of Ca may help to design methods to limit the toxicity of this mineral.
The authors thank Alain Castillo for technical assistance throughout the duration of the experimental work, Carole Causserand for her generous help during the analytical part of the work, and Philippe de Parseval for his assistance with the SEM images. They also thank Stacey Callahan, Oleg Pokrovsky, Pascale Bénézeth, Chris Pearce, Julian Declercq and Morgan T. Jones for helpful discussions during the course of this study. Support from Centre National de la Recherche Scientifique, and the National Science Foundation is gratefully acknowledged.