Research PaperCombined effects of temperature, mineral type, and surface roughness on chlorite dissolution kinetics in the acidic pH
Introduction
The chlorites, a group of phyllosilicates with Mg, Al and Fe present in varying proportions, were estimated to be the fourth most abundant clay mineral in the earth's crust after illite, montmorillonite and mixed-layer illite-montmorillonite (Weaver and Pollard, 1973). It is most commonly found as the primary mineral in metamorphic or igneous rocks, and can be easily weathered in tropical or temperate zones where chemical weathering is intense (Barnhisel and Bertsch, 1989). Chlorite weathering is also regarded as an important precursor for soil clays, such as vermiculite, especially during the early stages of pedogenesis (Righi et al., 1993).
The chlorite structure comprises of a 1.0-nm-thick, negatively charged tetrahedral-octahedral-tetrahedral (TOT) layer, and a 0.4-nm-thick, positively charged interlayer octahedral sheet (Brigatti et al., 2013). The TOT layer consists of a central octahedral sheet sandwiched between two silicon tetrahedral sheets, and the interlayer hydroxide sheet separates TOT layers. Atoms in octahedral sites are usually Mg, FeII, FeIII and Al, or rarely, Cr, Mn, Ni, V, Cu, Zn, and Li. The Si and Al occupy the tetrahedral sites. Extensive substitutions of the Si by Al or Fe (occasionally) cause negative charges in the TOT layers, and substitutions of Al or FeIII for Mg or FeII in the interlayers balance the charges (Barnhisel and Bertsch, 1989).
Because of their complex structures and various chemical compositions, chlorites are less stable than most clays, especially in acidic environments, which has made them the focus of many mineral weathering studies. Many mineralogists studied its weathering pathways under different geochemical environments. One common pathway was the dissolution of chlorite to leave either no solid products or only noncrystalline silica (Adams et al., 1971; Bain, 1977; Berthelin and Belgy, 1979), another route was its alteration to vermiculite accompanied by the oxidation and release of Fe (Gilkes and Little, 1972; Ross, 1975; Murakami et al., 1996). The latter reaction may in turn result in an alteration to other minerals, such as montmorillonite (Buurman et al., 1976) or kaolinite (Banfield and Murakami, 1998; Aspandiar and Eggleton, 2002).
Many laboratory studies were also carried out to estimate the chlorite dissolution kinetics under different geochemical conditions, including under acid, near-neutral or alkaline pH (May et al., 1995; Malmström et al., 1996; Brandt et al., 2003; Gustafsson and Puigdomenech, 2003; Lowson et al., 2005, Lowson et al., 2007; Critelli et al., 2014; Smith and Carroll, 2016), oxic conditions (Krawczyk-Barsch et al., 2004), CO2-saturated conditions (Smith et al., 2013; Black and Haese, 2014), low PO2 conditions (Sugimori et al., 2008), variable saturation states (Zhang et al., 2015), etc. In these studies, an important way used for quantifying the mineral dissolution rate was the mixed-flow reactor experiment.
However, crushing the material in the laboratory usually results in different degrees of reactive surface area exposure, which in turn leads to inconsistent clay mineral dissolution rate measurements (Köhler et al., 2005). The dissolution rates reported for laboratory-weathered silicates are always much higher than that of the field, and one of the reasons is the larger reactive surface areas of freshly prepared minerals (White and Brantley, 2003). On the other hand, the reactive surface area of clay minerals has been in discussion especially in the 1990's. The detachment of specific surface functional groups is regarded as the rate-limiting step of the overall mineral dissolution, and clay minerals as sheet silicates are always anisotropically dissolved because the most reactive surface sites are unevenly distributed between basal and edge surfaces (Bosbach et al., 2000). The real-time, in-situ observations of clay particle dissolution from atomic force microscopy (AFM), and combination of AFM and macroscopic dissolution methods help the researchers characterize more precise reactive surface area and close the gap between experimental and field clay dissolution rates (Bosbach et al., 2000; Brandt et al., 2003).
Many studies reported the chlorite dissolution rates in acid pH ranges and developed the rate law relating chlorite dissolution to pH and temperature dependence (Brandt et al., 2003; Gustafsson and Puigdomenech, 2003; Lowson et al., 2005, Lowson et al., 2007; Critelli et al., 2014), but they rarely discussed how chlorite types, crystal structure or surface roughness influenced the chlorite dissolution rate. In addition, although both stoichiometric and nonstoichiometric chlorite dissolution rates in terms of Mg, Fe, Al, and Si were reported in previous studies (Ross, 1967; Malmström et al., 1996; Brandt et al., 2003; Gustafsson and Puigdomenech, 2003; Hamer et al., 2003; Lowson et al., 2005), it remains unclear how pH, temperature and chlorite structure contribute to the congruency or incongruency of chlorite dissolution, and what mechanisms are involved.
In this work, the effects of temperature, pH, and varying flow rates on the dissolution rates of Mg-rich chlorite, and on the congruency of dissolution, were investigated using mixed-flow reactor experiments conducted through a pH range of 2–6, and at 25 °C and 95 °C. The samples were then observed using scanning electron microscopy (SEM) to characterize the topography of chlorite samples, and X-ray diffraction (XRD) analyses were conducted before and after the dissolution experiments. The reactive transport modeling (RTM) method was also used to simulate the effects of flow rates, pH, and specific surface area (SSA) on chlorite dissolution parameters over a relatively long duration (50,000 h), using kinetic parameters determined through laboratory experiments. Most importantly, through integrating and recalculating chlorite dissolution data reported in the literature and this work for both 25 °C and 95 °C, we explored the combined effects of temperature (an extrinsic factor), and chlorite type and surface roughness (intrinsic factors), on chlorite dissolution rates.
Section snippets
Sample preparation and characterization
The chlorite samples used in this study (Mg-rich variety, clinochlore, obtained from a talc-chlorite ore deposit in Haicheng, Liaoning Province, China) were crushed using a jaw crusher and an agate mortar. The fine-grained material was then dry sieved to yield the 80–154 μm size fraction (Fig. S1 in Supplementary Material). This size fraction was selected as it was greater than the scales of chlorite grains in this research as observed from SEM. Based on Holdren and Speyer (1985), additional
Temperature, pH, and chlorite dissolution rates
As the system only consisted of chlorite and quartz, any element release of Mg, Al and Fe must have come from the chlorite, while Si could be released by both two minerals. Since chlorite dominated the samples used in this study (~80 wt%), and the literature-reported quartz dissolution rates ranged from 10−12.5 to 10−13.5 mol/m2/s over a pH of 2–7 at 25 °C (Bennett, 1991; House and Orr, 1992; Crundwell, 2017), which were more than an order of magnitude smaller than the reported chlorite
Effects of pH and temperature on congruency of chlorite dissolution
Both incongruent and congruent steady-state chlorite dissolution were reported in previous studies (Brandt et al., 2003; Gustafsson and Puigdomenech, 2003; Hamer et al., 2003; Lowson et al., 2005). In this research, Fe release rates were higher than expected throughout the experiment of 25-2 given such a low Fe content in the chlorite (Fig. 1, Table 1). In contrast, although Fe was also preferentially released in the initial dissolution stage (0−100 h) of the experiment 95-2 (Fig. 1), its
Conclusions
Mixed-flow reactor experiments demonstrated that pH and temperature control not only the chlorite dissolution rate but also the congruency of element release. Low pH facilitates the preferential release of Fe from the interlayers of chlorite, however, high temperature can help overcome the different dissolution susceptibilities of TOT layers and interlayers, and facilitate congruent dissolution. The special interlayer configurations of chlorite have been observed to have different reactivity
Declaration of Competing Interest
None.
Acknowledgments
This work was supported by the Key Science and Technology Foundation of Gansu Province, China (18ZD2FA001). We are grateful to the workshop staff at the Key Laboratory of Mineral Resources in Western China (Gansu Province), for helping with the ICP-OES and XRD instruments.
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