Combined effects of temperature, mineral type, and surface roughness on chlorite dissolution kinetics in the acidic pH

Highlights

• Dissolution kinetics for a type of Mg-rich chlorite was determined in the acid pH range.

• Effects of temperature and crystal structure on chlorite dissolution congruency were assessed.

• RTM was applied to simulate chlorite dissolution under different pH and SSA.

• Effects of intrinsic factors on chlorite dissolution kinetics in the acidic pH were explored.

Abstract

Many studies reported the chlorite dissolution kinetics in acidic pH—resulting in the development of chlorite dissolution rate law covering the pH and temperature dependence—but they rarely discussed how various intrinsic factors, including chlorite types, structural complexity, and surface roughness (refers specifically to the ratio between Brunauer–Emmett–Teller surface area, BET SA, and geometric surface area, GSA, in this paper), influenced the chlorite dissolution rate or the mechanism for dissolution congruency. In this study, results obtained from mixed-flow reactor experiments performed on a Mg-rich chlorite, over a pH range of 2–6 at 25 °C and 95 °C, indicated that pH and temperature control not only the chlorite dissolution rate but also the congruency of element release. Low pH facilitated the dissolution of Fe from chlorite interlayers and caused incongruent element release rates at 25 °C, while higher temperature experiments at the same pH overcame the differences in surface reactivity between the interlayer and the tetrahedral-octahedral-tetrahedral (TOT) layer, and finally evolved into congruent dissolution. The lower pH also made chlorite dissolution more resistant to varying flow rates. A dissolution rate constant of 10^–10.51 mol/m²/s, a reaction order of 0.32, and activation energy of 42.03 kJ/mol were determined for the Mg-rich chlorite dissolution kinetics at 25 °C, in an acid-enhanced mechanism. Longer-term reactive transport modeling (RTM) on the chlorite dissolution demonstrated that low pH and large specific surface area (SSA) decreased the chlorite percentage, bulk surface area, and saturation index quicker, and in a non-linear fashion. Chlorite with higher SSA lost more bulk surface area (BSA) than that with lower SSA, when the same mass of chlorite was dissolved.

Through tabulating and recalculating a series of data from the literature, the combined effects of intrinsic and extrinsic factors—including chlorite type, BET SA/GSA ratio, and temperature—were explored. Fe-rich chlorite dissolves faster than Mg-rich chlorite in acid and neutral pH, due to the oxidative dissolution mechanism. The chlorite dissolution rate constant is linearly and positively correlated to BET SA/GSA for the same type of chlorite (Fe-rich and Mg-rich). Higher temperatures help reduce the effects of extrinsic and intrinsic factors on chlorite dissolution rates.

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, Fe^II, Fe^III 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 Fe^III for Mg or Fe^II 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 P_O2 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.