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  • Geochimica Et Cosmochimica Acta
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  • 1
    Language: English
    In: Geochimica et Cosmochimica Acta, May 1, 2013, Vol.108, p.91(16)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.01.010 Byline: Fatemeh Salehikhoo (a), Li Li (a)(c), Susan L. Brantley (b)(c) Abstract: We examined the role of mineral spatial distribution and flow velocity in determining magnesite dissolution rates at different spatial scales. One scale is the column scale of a few to tens of centimeters where dissolution rates are measured. Another is the "local" in situ scale defined as approximately 0.1mm. The experiments used two columns with the same bulk concentration but different spatial distributions of magnesite. In the "Mixed" column, magnesite was evenly distributed spatially within a quartz sand matrix across the whole column, while in the "One-zone" column, magnesite was distributed in one zone in the middle of the column. The two columns were flushed with the same inlet acidic solution (pH 4.0) under flow velocities varying from 0.18 to 36m/d. Columns of different lengths (22, 10, and 5cm) were run to understand the role of length scales. Reactive transport modeling was used to infer local-scale and column-scale dissolution rates. Under the acidic-solution flushing conditions used in this study, local in situ dissolution rates vary by orders of magnitude over a length scale of a few to tens of centimeters. Column-scale rates under different conditions vary between 6.40x10.sup.-12 and 1.02x10.sup.-9 mol/m.sup.2/s. The distribution of local-scale rates, which collectively determine the column-scale rates, depend on flow velocity, column length scale, and mineral distribution. A two orders of magnitude difference in flow velocity results in more than two orders of magnitude difference in the column-scale rates. Under the same conditions of flow velocity and mineral distribution, column-scale rates are higher in short columns and are lower in long columns. Mineral spatial distribution made a maximum difference of 14% in the medium-flow velocity regime where the reaction kinetics of the system operates under mixed-control conditions. Under such mixed-control conditions, the larger difference between the two columns in their spatial variation of pH and saturation state lead to a larger difference in the spatial distribution of local dissolution rates and therefore column-scale rates. In contrast, under slow-flow velocity conditions, the system is mostly at equilibrium without much spatial variation, i.e., the regime of local equilibrium. Under fast-flow velocity conditions, the system is kinetically controlled, the local aqueous geochemistry is everywhere similar to the inlet condition, and is also relatively uniform. Under these two conditions, there is almost no difference between the two columns. Column-scale rates were best understood in terms of the Damkohler number (Da.sub.I) that quantifies the relative dominance of advection and dissolution processes. The observations in this study lead us to surmise that rates of weathering and other natural processes may be similarly affected by chemical heterogeneity in natural systems under conditions where reaction rate and flow rate are comparable. Author Affiliation: (a) John and Willie Leone Family Department of Energy and Mineral Engineering, The Pennsylvania State University, University Park, PA 16802, United States (b) Department of Geosciences, The Pennsylvania State University, University Park, PA 16802, United States (c) Earth and Environmental Systems Institute (EESI), The Pennsylvania State University, University Park, PA 16802, United States Article History: Received 17 January 2012; Accepted 9 January 2013 Article Note: (miscellaneous) Associate editor: Jon Chorover
    Keywords: Carbonate Minerals ; Magnesium Compounds
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 2
    Language: English
    In: Geochimica et Cosmochimica Acta, Feb 1, 2014, Vol.126, p.555(19)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2013.10.051 Byline: Li Li, Fatemeh Salehikhoo, Susan L. Brantley, Peyman Heidari Abstract: We investigate how mineral spatial distribution in porous media affects their dissolution rates. Specifically, we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration (v/v) was held constant. The largest difference was observed between a "Mixed column" containing uniformly distributed magnesite and a "One-zone column" containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid (flow-parallel One-zone column). The columns were flushed with acid water at a pH of 4.0 at flow velocities of 3.6 or 0.36m/d. Breakthrough data show that the rate of magnesite dissolution is 1.6-2 times slower in the One-zone column compared to the Mixed column. This extent of rate limitation is much larger than what was observed in our previous work (14%) for a similar One-zone column where the magnesite was packed in a layer aligned perpendicular to flow (flow-transverse One-zone column). Two-dimensional reactive transport modeling with CrunchFlow revealed that ion activity product (IAP) and local dissolution rates at the grid block scale (0.1cm) vary by orders of magnitude. Much of the central magnesite zone in the One-zone flow-parallel column is characterized by close or equal to equilibrium conditions with IAP/K.sub.eq 0.1. Two important surface areas are defined to understand the observed rates: the effective surface area (A.sub.e) reflects the magnesite that effectively dissolves under far from equilibrium conditions (IAP/K.sub.eq 〈0.1), while the interface surface area (A.sub.I) reflects the effective magnesite surface that lies along the quartz-magnesite interface. Modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of A.sub.e and A.sub.I . Under the conditions examined in this work, the value of A.sub.e varies from 2% to 67% of the total magnesite BET surface area. Column-scale bulk rates R.sub.MgCO.sub.3,B (in units of mol/s) vary linearly with A.sub.e and A.sub.I . Using A.sub.e to normalize rates, we calculate a rate constant (10.sup.-9.56 mol/m.sup.2/s) that is very close to the value of 10.sup.-10.0 mol/m.sup.2/s under well-mixed conditions at the grid block scale. This implies that the laboratory-field rate discrepancy can potentially be caused by differences in the effective surface area. If we know the effective surface area of dissolution, we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems. In this work, approximately 60-70% of the A.sub.e is at the magnesite-quartz interface. This implies that in some field systems where the detailed information that we have for our columns is not available, the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones. Although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport, our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution (chemical heterogeneity) on dissolution. Article History: Received 24 April 2013; Accepted 30 October 2013 Article Note: (miscellaneous) Associate editor: Daniel E. Giammar
    Keywords: Carbonate Minerals -- Analysis ; Magnesium Compounds -- Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 3
    Language: English
    In: Geochimica et Cosmochimica Acta, Sept 15, 2012, Vol.93, p.235(27)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2012.03.021 Byline: Joel Moore (a)(b), Peter C. Lichtner (c), Art F. White (d), Susan L. Brantley (a)(b) Abstract: The reactive transport model FLOTRAN was used to forward-model weathering profiles developed on granitic outwash alluvium over 40-3000ka from the Merced, California (USA) chronosequence as well as deep granitic regolith developed over 800ka near Davis Run, Virginia (USA). Baseline model predictions that used laboratory rate constants (k.sub.m), measured fluid flow velocities (v), and BET volumetric surface areas for the parent material (A.sub.B,m.sup.o) were not consistent with measured profiles of plagioclase, potassium feldspar, and quartz. Reaction fronts predicted by the baseline model are deeper and thinner than the observed, consistent with faster rates of reaction in the model. Reaction front depth in the model depended mostly upon saturated versus unsaturated hydrologic flow conditions, rate constants controlling precipitation of secondary minerals, and the average fluid flow velocity (v.sub.a). Unsaturated hydrologic flow conditions (relatively open with respect to CO.sub.2(g)) resulted in the prediction of deeper reaction fronts and significant differences in the separation between plagioclase and potassium feldspar reaction fronts compared to saturated hydrologic flow (relatively closed with respect to CO.sub.2(g)). Under saturated or unsaturated flow conditions, the rate constant that controls precipitation rates of secondary minerals must be reduced relative to laboratory rate constants to match observed reaction front depths and measured pore water chemistry. Additionally, to match the observed reaction front depths, v.sub.a was set lower than the measured value, v, for three of the four profiles. The reaction front gradients in mineralogy and pore fluid chemistry could only be modeled accurately by adjusting values of the product k.sub.m A.sub.B,m.sup.o. By assuming k.sub.m values were constrained by laboratory data, field observations were modeled successfully with TST-like rate equations by dividing measured values of A.sub.B,m.sup.o by factors from 50 to 1700. Alternately, with sigmoidal or Al-inhibition rate models, this adjustment factor ranges from 5 to 170. Best-fit models of the wetter, hydrologically saturated Davis Run profile required a smaller adjustment to A.sub.B,m.sup.o than the drier hydrologically unsaturated Merced profiles. We attributed the need for large adjustments in v.sub.a and A.sub.B,m.sup.o necessary for the Merced models to more complex hydrologic flow that decreased the reactive surface area in contact with bulk flow water, e.g., dead-end pore spaces containing fluids that are near or at chemical equilibrium. Thus, rate models from the laboratory can successfully predict weathering over millions of years, but work is needed to understand how to incorporate changes in what controls the relationship between reactive surface area and hydrologic flow. Author Affiliation: (a) Department of Geosciences, Penn State University, University Park, PA, United States (b) Center for Environmental Kinetics Analysis, Earth and Environmental Systems Institute, Penn State University, University Park, PA, United States (c) Los Alamos National Laboratory, Los Alamos, NM, United States (d) US Geological Survey, Menlo Park, CA, United States Article History: Received 18 October 2010; Accepted 14 March 2012 Article Note: (miscellaneous) Associate editor: Chen Zhu
    Keywords: Precipitation (Meteorology) -- Usage ; Precipitation (Meteorology) -- Analysis ; Flow (Dynamics) -- Usage ; Flow (Dynamics) -- Analysis
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 4
    Language: English
    In: Geochimica et Cosmochimica Acta, Dec 15, 2012, Vol.99, p.159(20)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.gca.2012.09.019 Byline: J. Donald Rimstidt (a), Susan L. Brantley (b), Amanda A. Olsen (c) Abstract: This paper demonstrates a method for systematic analysis of published mineral dissolution rate data using forsterite dissolution as an example. The steps of the method are: (1) identify the data sources, (2) select the data, (3) tabulate the data, (4) analyze the data to produce a model, and (5) report the results. This method allows for a combination of critical selection of data, based on expert knowledge of theoretical expectations and experimental pitfalls, and meta-analysis of the data using statistical methods. Application of this method to all currently available forsterite dissolution rates (0〈pH〈14, and 0〈 T 〈150[degrees]C) normalized to geometric surface area produced the following rate equations: For pH〈5.6 and 0[degrees]〈 T 〈150[degrees]C, based on 519 data logr.sub.geo=6.05(0.22)-0.46(0.02)pH-3683.0(63.6)1/T(R.sup.2=0.88) For pH5.6 and 0[degrees]〈 T 〈150[degrees]C, based on 125 data logr.sub.geo=4.07(0.38)-0.256(0.023)pH-3465(139)1/T(R.sup.2=0.92) The R.sup.2 values show that [approximately equal to]10% of the variance in r is not explained by variation in 1/T and pH. Although the experimental error for rate measurements should be[+ or -][approximately equal to]30%, the observed error associated with the log r values is [approximately equal to]0.5logunits ([+ or -]300% relative error). The unexplained variance and the large error associated with the reported rates likely arises from the assumption that the rates are directly proportional to the mineral surface area (geometric or BET) when the rate is actually controlled by the concentration and relative reactivity of surface sites, which may be a function of duration of reaction. Related to these surface area terms are other likely sources of error that include composition and preparation of mineral starting material. Similar rate equations were produced from BET surface area normalized rates. Comparison of rate models based on geometric and BET normalized rates offers no support for choosing one normalization method over the other. However, practical considerations support the use of geometric surface area normalization. Comparison of Mg and Si release rates showed that they produced statistically indistinguishable dissolution rates because dissolution was stoichiometric in the experiments over the entire pH range even though the surface concentrations of Mg and Si are known to change with pH. Comparison of rates from experiments with added carbonate, either from CO.sub.2 partial pressures greater than atmospheric or added carbonate salts, showed that the existing data set is not sufficient to quantify any effect of dissolved carbonate species on forsterite dissolution rates. Author Affiliation: (a) Department of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, United States (b) Earth and Environmental Systems Institute, 2217 EES Building, Pennsylvania State University, Univ Pk, PA 16802, USA (c) School of Earth and Climate Sciences, 5790 Bryand Global Sciences, Center, University of Maine, Orono, ME 04469, United States Article History: Received 14 July 2011; Accepted 12 September 2012 Article Note: (miscellaneous) Associate editor: Chen Zhu
    ISSN: 0016-7037
    Source: Cengage Learning, Inc.
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  • 5
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(2), pp.337-351
    Description: We have compiled time-series concentration data for the biological reduction of manganese(III/IV) published between 1985 and 2004 and fit these data with a simple hyperbolic rate expression or, when appropriate, one of its limiting forms. The compiled data and rate constants are available in . The zero- and first-order rate constants appear to follow a log–normal distribution that could be used, for example, in predictive modeling of Mn-oxide reduction in a reactive transport scenario. We have also included details of the experimental procedures used to generate each time-series data-set in our compilation. These meta-data—mostly pertaining to the type and concentration of micro-organism, electron donor, and electron acceptor—enable us to examine the rate data for trends. We have computed a number of rudimentary, mono-variate statistics on the compiled data with the hope of stimulating both more detailed statistical analyses of the data and new experiments to fill gaps in the existing data-set. We have also analyzed the data with parametric models based on the log–normal distribution and rate equations that are hyperbolic in the concentration of cells and Mn available for reduction. This parametric analysis allows us to provide best estimates of zero- and first-order rate constants both ignoring and accounting for the meta-data.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 6
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(23), pp.7644-7667
    Description: Saprolite formation rates influence many important geological and environmental issues ranging from agricultural productivity to landscape evolution. Here we investigate the chemical and physical transformations that occur during weathering by studying small-scale “saprolites” in the form of weathering rinds, which form on rock in soil or saprolite and grow in thickness without physical disturbance with time. We compare detailed observations of weathered basalt clasts from a chronosequence of alluvial terraces in Costa Rica to diffusion-reaction simulations of rind formation using the fully coupled reactive transport model CrunchFlow. The four characteristic features of the weathered basalts which were specifically used as criteria for model comparisons include (1) the mineralogy of weathering products, (2) weathering rind thickness, (3) the coincidence of plagioclase and augite reaction fronts, and (4) the thickness of the zones of mineral reaction, i.e. reaction fronts. Four model scenarios were completed with varying levels of complexity and degrees of success in matching the observations. To fit the model to all four criteria, however, it was necessary to (1) treat diffusivity using a threshold in which it increased once porosity exceeded a critical value of 9%, and (2) treat mineral surface area as a fitting factor. This latter approach was presumably necessary because the mineral-water surface area of the connected (accessible) porosity in the Costa Rica samples is much less than the total porosity ( ). The model-fit surface area, here termed reacting surface area, was much smaller than the BET-measured surface area determined for powdered basaltic material. In the parent basalt, reacting surface area and diffusivity are low due to low pore connectivity, and early weathering is therefore transport controlled. However, as pore connectivity increases as a result of weathering, the reacting surface area and diffusivity also increase and weathering becomes controlled by mineral reaction kinetics. The transition point between transport and kinetic control appears to be related to a critical porosity (9%) at which pore connectivity is high enough to allow rapid transport. Based on these simulations, we argue that the rate of weathering front advance is controlled by the rate at which porosity is created in the weathering interface, and that this porosity increases because of mineral dissolution following a rate that is largely surface-reaction controlled.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 7
    Language: English
    In: Geochimica et Cosmochimica Acta, 2011, Vol.75(2), pp.401-415
    Description: The dissolution–precipitation of quartz controls porosity and permeability in many lithologies and may be the best studied mineral-water reaction. However, the rate of quartz-water reaction is relatively well characterized far from equilibrium but relatively unexplored near equilibrium. We present kinetic data for quartz as equilibrium is approached from undersaturation and more limited data on the approach from supersaturated conditions in 0.1 molal NaCl + NaOH + NaSiO(OH) solutions with pH 8.2–9.7 at 398, 423, 448, and 473 K. We employed a potentiometric technique that allows precise determination of solution speciation within 2 kJ mol of equilibrium without the need for to perturb the system through physical sampling and chemical analysis. Slightly higher equilibrium solubilities between 423 and 473 K were found than reported in recent compilations. Apparent activation energies of 29 and 37 kJ mol are inferred for rates of dissolution at two surface sites with different values of connectedness: dissolution at or silicon sites, respectively. The dissolution mechanism varies with Δ such that reactions at both sites control dissolution up until a critical free energy value above which only reactions at sites are important. When our near-equilibrium dissolution rates are extrapolated far from equilibrium, they agree within propagated uncertainty at 398 K with a recently published model by . However, our extrapolated rates become progressively slower than model predictions with increasing temperature. Furthermore, we see no dependence of the postulated reaction rate on pH, and a poorly-constrained pH dependence of the postulated rate. Our slow extrapolated rates are presumably related to the increasing contribution of dissolution at sites far from equilibrium. The use of the potentiometric technique for rate measurement will yield both rate data and insights into the mechanisms of dissolution over a range of chemical affinity. Such measurements are needed to model the evolution of many natural systems quantitatively.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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  • 8
    Language: English
    In: Geochimica et cosmochimica acta, 2013, Vol.108, pp.91-106
    Description: We examined the role of mineral spatial distribution and flow velocity in determining magnesite dissolution rates at different spatial scales. One scale is the column scale of a few to tens of centimeters where dissolution rates are measured. Another is the “local” in situ scale defined as approximately 0.1mm. The experiments used two columns with the same bulk concentration but different spatial distributions of magnesite. In the “Mixed” column, magnesite was evenly distributed spatially within a quartz sand matrix across the whole column, while in the “One-zone” column, magnesite was distributed in one zone in the middle of the column. The two columns were flushed with the same inlet acidic solution (pH 4.0) under flow velocities varying from 0.18 to 36m/d. Columns of different lengths (22, 10, and 5cm) were run to understand the role of length scales. Reactive transport modeling was used to infer local-scale and column-scale dissolution rates. Under the acidic-solution flushing conditions used in this study, local in situ dissolution rates vary by orders of magnitude over a length scale of a few to tens of centimeters. Column-scale rates under different conditions vary between 6.40×10⁻¹² and 1.02×10⁻⁹mol/m²/s. The distribution of local-scale rates, which collectively determine the column-scale rates, depend on flow velocity, column length scale, and mineral distribution. A two orders of magnitude difference in flow velocity results in more than two orders of magnitude difference in the column-scale rates. Under the same conditions of flow velocity and mineral distribution, column-scale rates are higher in short columns and are lower in long columns. Mineral spatial distribution made a maximum difference of 14% in the medium-flow velocity regime where the reaction kinetics of the system operates under mixed-control conditions. Under such mixed-control conditions, the larger difference between the two columns in their spatial variation of pH and saturation state lead to a larger difference in the spatial distribution of local dissolution rates and therefore column-scale rates. In contrast, under slow-flow velocity conditions, the system is mostly at equilibrium without much spatial variation, i.e., the regime of local equilibrium. Under fast-flow velocity conditions, the system is kinetically controlled, the local aqueous geochemistry is everywhere similar to the inlet condition, and is also relatively uniform. Under these two conditions, there is almost no difference between the two columns. Column-scale rates were best understood in terms of the Damkohler number (DaI) that quantifies the relative dominance of advection and dissolution processes. The observations in this study lead us to surmise that rates of weathering and other natural processes may be similarly affected by chemical heterogeneity in natural systems under conditions where reaction rate and flow rate are comparable. ; p. 91-106.
    Keywords: Models ; Sand ; Quartz ; Magnesite ; Reaction Kinetics ; Observational Studies ; Geochemistry ; Weathering ; Ph
    ISSN: 0016-7037
    Source: AGRIS (Food and Agriculture Organization of the United Nations)
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  • 9
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 May 2013, Vol.108, pp.91-106
    Description: We examined the role of mineral spatial distribution and flow velocity in determining magnesite dissolution rates at different spatial scales. One scale is the column scale of a few to tens of centimeters where dissolution rates are measured. Another is the “local” in situ scale defined as approximately 0.1 mm. The experiments used two columns with the same bulk concentration but different spatial distributions of magnesite. In the “Mixed” column, magnesite was evenly distributed spatially within a quartz sand matrix across the whole column, while in the “One-zone” column, magnesite was distributed in one zone in the middle of the column. The two columns were flushed with the same inlet acidic solution (pH 4.0) under flow velocities varying from 0.18 to 36 m/d. Columns of different lengths (22, 10, and 5 cm) were run to understand the role of length scales. Reactive transport modeling was used to infer local-scale and column-scale dissolution rates. Under the acidic-solution flushing conditions used in this study, local in situ dissolution rates vary by orders of magnitude over a length scale of a few to tens of centimeters. Column-scale rates under different conditions vary between 6.40 × 10 and 1.02 × 10 mol/m /s. The distribution of local-scale rates, which collectively determine the column-scale rates, depend on flow velocity, column length scale, and mineral distribution. A two orders of magnitude difference in flow velocity results in more than two orders of magnitude difference in the column-scale rates. Under the same conditions of flow velocity and mineral distribution, column-scale rates are higher in short columns and are lower in long columns. Mineral spatial distribution made a maximum difference of 14% in the medium-flow velocity regime where the reaction kinetics of the system operates under mixed-control conditions. Under such mixed-control conditions, the larger difference between the two columns in their spatial variation of pH and saturation state lead to a larger difference in the spatial distribution of local dissolution rates and therefore column-scale rates. In contrast, under slow-flow velocity conditions, the system is mostly at equilibrium without much spatial variation, i.e., the regime of local equilibrium. Under fast-flow velocity conditions, the system is kinetically controlled, the local aqueous geochemistry is everywhere similar to the inlet condition, and is also relatively uniform. Under these two conditions, there is almost no difference between the two columns. Column-scale rates were best understood in terms of the Damkohler number (Da ) that quantifies the relative dominance of advection and dissolution processes. The observations in this study lead us to surmise that rates of weathering and other natural processes may be similarly affected by chemical heterogeneity in natural systems under conditions where reaction rate and flow rate are comparable.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
    Source: ScienceDirect Journals (Elsevier)
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  • 10
    Language: English
    In: Geochimica et Cosmochimica Acta, 01 February 2014, Vol.126, pp.555-573
    Description: We investigate how mineral spatial distribution in porous media affects their dissolution rates. Specifically, we measure the dissolution rate of magnesite interspersed in different patterns in packed columns of quartz sand where the magnesite concentration (v/v) was held constant. The largest difference was observed between a “Mixed column” containing uniformly distributed magnesite and a “One-zone column” containing magnesite packed into one cylindrical center zone aligned parallel to the main flow of acidic inlet fluid (flow-parallel One-zone column). The columns were flushed with acid water at a pH of 4.0 at flow velocities of 3.6 or 0.36 m/d. Breakthrough data show that the rate of magnesite dissolution is 1.6–2 times slower in the One-zone column compared to the Mixed column. This extent of rate limitation is much larger than what was observed in our previous work (14%) for a similar One-zone column where the magnesite was packed in a layer aligned perpendicular to flow (flow-transverse One-zone column). Two-dimensional reactive transport modeling with CrunchFlow revealed that ion activity product (IAP) and local dissolution rates at the grid block scale (0.1 cm) vary by orders of magnitude. Much of the central magnesite zone in the One-zone flow-parallel column is characterized by close or equal to equilibrium conditions with IAP/ 〉 0.1. Two important surface areas are defined to understand the observed rates: the effective surface area ( ) reflects the magnesite that effectively dissolves under far from equilibrium conditions (IAP/ 〈 0.1), while the interface surface area ( ) reflects the effective magnesite surface that lies along the quartz–magnesite interface. Modeling results reveal that the transverse dispersivity at the interface of the quartz and magnesite zones controls mass transport and therefore the values of and . Under the conditions examined in this work, the value of varies from 2% to 67% of the total magnesite BET surface area. Column-scale bulk rates (in units of mol/s) vary linearly with and . Using to normalize rates, we calculate a rate constant (10 mol/m /s) that is very close to the value of 10 mol/m /s under well-mixed conditions at the grid block scale. This implies that the laboratory-field rate discrepancy can potentially be caused by differences in the effective surface area. If we know the effective surface area of dissolution, we will be able to use the rate constant measured in laboratory systems to calculate field rates for some systems. In this work, approximately 60–70% of the is at the magnesite–quartz interface. This implies that in some field systems where the detailed information that we have for our columns is not available, the effective mineral surface area may be approximated by the area of grains residing at the interface of reactive mineral zones. Although it has long been known that spatial heterogeneities play a significant role in determining physical processes such as flow and solute transport, our data are the first that systematically and experimentally quantifies the importance of mineral spatial distribution (chemical heterogeneity) on dissolution.
    Keywords: Geology
    ISSN: 0016-7037
    E-ISSN: 1872-9533
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