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Title: Mineral dissolution kinetics at the pore scale

Abstract

Mineral dissolution rates in the field have been reported to be orders of magnitude slower than those measured in the laboratory, an unresolved discrepancy that severely limits our ability to develop scientifically defensible predictive or even interpretive models for many geochemical processes in the earth and environmental sciences. One suggestion links this discrepancy to the role of physical and chemical heterogeneities typically found in subsurface soils and aquifers in producing scale-dependent rates where concentration gradients develop. In this paper, we examine the possibility that scale-dependent mineral dissolution rates can develop even at the single pore and fracture scale, the smallest and most fundamental building block of porous media. To do so, we develop two models to analyze mineral dissolution kinetics at the single pore scale: (1) a Poiseuille Flow model that applies laboratory-measured dissolution kinetics at the pore or fracture wall and couples this to a rigorous treatment of both advective and diffusive transport, and (2) a Well-Mixed Reactor model that assumes complete mixing within the pore, while maintaining the same reactive surface area, average flow rate, and geometry as the Poiseuille Flow model. For a fracture, a 1D Plug Flow Reactor model is considered in addition to quantify themore » effects of longitudinal versus transverse mixing. The comparison of averaged dissolution rates under various conditions of flow, pore size, and fracture length from the three models is used as a means to quantify the extent to which concentration gradients at the single pore and fracture scale can develop and render rates scale-dependent. Three important minerals that dissolve at widely different rates, calcite, plagioclase, and iron hydroxide, are considered. The modeling indicates that rate discrepancies arise primarily where concentration gradients develop due to comparable rates of reaction and advective transport, and incomplete mixing via molecular diffusion. The magnitude of the reaction rate is important, since it is found that scaling effects (and thus rate discrepancies) are negligible at the single pore and fracture scale for plagioclase and iron hydroxide because of the slow rate at which they dissolve. In the case of calcite, where dissolution rates are rapid, scaling effects can develop at high flow rates from 0.1 cm/s to 1000 cm/s and for fracture lengths less than 1 cm. At more normal flow rates, however, mixing via molecular diffusion is effective in homogenizing the concentration field, thus eliminating any discrepancies between the Poiseuille Flow and the Well-Mixed Reactor model. This suggests that a scale dependence to mineral dissolution rates is unlikely at the single pore or fracture scale under normal geological/hydrologic conditions, implying that the discrepancy between laboratory and field rates must be attributed to other factors.« less

Authors:
; ;
Publication Date:
Research Org.:
Ernest Orlando Lawrence Berkeley NationalLaboratory, Berkeley, CA (US)
Sponsoring Org.:
USDOE. Laboratory Directed Research and Development Program; National Science Foundation (NSF)
OSTI Identifier:
929022
Report Number(s):
LBNL-63303
Journal ID: ISSN 0016-7037; GCACAK; R&D Project: 366169; BnR: YN0100000; TRN: US200811%%484
DOE Contract Number:
DE-AC02-05CH11231
Resource Type:
Journal Article
Resource Relation:
Journal Name: Geochimica et Cosmochimica Acta; Journal Volume: 0; Journal Issue: 0; Related Information: Journal Publication Date: 0
Country of Publication:
United States
Language:
English
Subject:
54; ANORTHOSITES; AQUIFERS; CALCITE; DIFFUSION; DISSOLUTION; FLOW RATE; FRACTURES; GEOMETRY; IRON HYDROXIDES; KINETICS; LAMINAR FLOW; REACTION KINETICS; SIMULATION; SOILS; SURFACE AREA; TRANSPORT

Citation Formats

Li, L., Steefel, C.I., and Yang, L. Mineral dissolution kinetics at the pore scale. United States: N. p., 2007. Web.
Li, L., Steefel, C.I., & Yang, L. Mineral dissolution kinetics at the pore scale. United States.
Li, L., Steefel, C.I., and Yang, L. Thu . "Mineral dissolution kinetics at the pore scale". United States. doi:. https://www.osti.gov/servlets/purl/929022.
@article{osti_929022,
title = {Mineral dissolution kinetics at the pore scale},
author = {Li, L. and Steefel, C.I. and Yang, L.},
abstractNote = {Mineral dissolution rates in the field have been reported to be orders of magnitude slower than those measured in the laboratory, an unresolved discrepancy that severely limits our ability to develop scientifically defensible predictive or even interpretive models for many geochemical processes in the earth and environmental sciences. One suggestion links this discrepancy to the role of physical and chemical heterogeneities typically found in subsurface soils and aquifers in producing scale-dependent rates where concentration gradients develop. In this paper, we examine the possibility that scale-dependent mineral dissolution rates can develop even at the single pore and fracture scale, the smallest and most fundamental building block of porous media. To do so, we develop two models to analyze mineral dissolution kinetics at the single pore scale: (1) a Poiseuille Flow model that applies laboratory-measured dissolution kinetics at the pore or fracture wall and couples this to a rigorous treatment of both advective and diffusive transport, and (2) a Well-Mixed Reactor model that assumes complete mixing within the pore, while maintaining the same reactive surface area, average flow rate, and geometry as the Poiseuille Flow model. For a fracture, a 1D Plug Flow Reactor model is considered in addition to quantify the effects of longitudinal versus transverse mixing. The comparison of averaged dissolution rates under various conditions of flow, pore size, and fracture length from the three models is used as a means to quantify the extent to which concentration gradients at the single pore and fracture scale can develop and render rates scale-dependent. Three important minerals that dissolve at widely different rates, calcite, plagioclase, and iron hydroxide, are considered. The modeling indicates that rate discrepancies arise primarily where concentration gradients develop due to comparable rates of reaction and advective transport, and incomplete mixing via molecular diffusion. The magnitude of the reaction rate is important, since it is found that scaling effects (and thus rate discrepancies) are negligible at the single pore and fracture scale for plagioclase and iron hydroxide because of the slow rate at which they dissolve. In the case of calcite, where dissolution rates are rapid, scaling effects can develop at high flow rates from 0.1 cm/s to 1000 cm/s and for fracture lengths less than 1 cm. At more normal flow rates, however, mixing via molecular diffusion is effective in homogenizing the concentration field, thus eliminating any discrepancies between the Poiseuille Flow and the Well-Mixed Reactor model. This suggests that a scale dependence to mineral dissolution rates is unlikely at the single pore or fracture scale under normal geological/hydrologic conditions, implying that the discrepancy between laboratory and field rates must be attributed to other factors.},
doi = {},
journal = {Geochimica et Cosmochimica Acta},
number = 0,
volume = 0,
place = {United States},
year = {Thu May 24 00:00:00 EDT 2007},
month = {Thu May 24 00:00:00 EDT 2007}
}
  • The chief goals for CEKA are to (1) collect and synthesize molecular-level kinetic data into a coherent framework that can be used to predict time evolution of environmental processes over a range of temporal and spatial scales; (2) train a cohort of talented and diverse students to work on kinetic problems at multiple scales; (3) develop and promote the use of new experimental techniques in environmental kinetics; (4) develop and promote the use of new modeling tools to conceptualize reaction kinetics in environmental systems; and (5) communicate our understanding of issues related to environmental kinetics and issues of scale tomore » the broader scientific community and to the public.« less
  • Pore water chemistry and 234U/238U activity ratios from fine-grained sediment cored by the Ocean Drilling Project at Site 984 in the North Atlantic were used as constraints in modeling in situ rates of plagioclase dissolution with the multicomponent reactive transport code Crunch. The reactive transport model includes a solid-solution formulation to enable the use of the 234U/238U activity ratios in the solid and fluid as a tracer of mineral dissolution. The isotopic profiles are combined with profiles of the major element chemistry (especially alkalinity and calcium) to determine whether the apparent discrepancy between laboratory and field dissolution rates still existsmore » when a mechanistic reactive transport model is used to interpret rates in a natural system. A suite of reactions, including sulfate reduction and methane production, anaerobic methane oxidation, CaCO3 precipitation, dissolution of plagioclase, and precipitation of secondary clay minerals, along with diffusive transport and fluid and solid burial, control the pore fluid chemistry in Site 984 sediments. The surface area of plagioclase in intimate contact with the pore fluid is estimated to be 6.9 m2/g based on both grain geometry and on the depletion of 234U/238U in the sediment via a-recoil loss. Various rate laws for plagioclase dissolution are considered in the modeling, including those based on (1) a linear transition state theory (TST) model, (2) a nonlinear dependence on the undersaturation of the pore water with respect to plagioclase, and (3) the effect of inhibition by dissolved aluminum. The major element and isotopic methods predict similar dissolution rate constants if additional lowering of the pore water 234U/238U activity ratio is attributed to isotopic exchange via recrystallization of marine calcite, which makes up about 10-20 percent of the Site 984 sediment. The calculated dissolution rate for plagioclase corresponds to a rate constant that is about 102 to 105 times smaller than the laboratory-measured value, with the value depending primarily on the deviation from equilibrium. The reactive transport simulations demonstrate that the degree of undersaturation of the pore fluid with respect to plagioclase depends strongly on the rate of authigenic clay precipitation and the solubility of the clay minerals. The observed discrepancy is greatest for the linear TST model (105), less substantial with the Al-inhibition formulation (103), and decreases further if the clay minerals precipitate more slowly or as highly soluble precursor minerals (102). However, even several orders of magnitude variation in either the clay solubility or clay precipitation rates cannot completely account for the entire discrepancy while still matching pore water aluminum and silica data, indicating that the mineral dissolution rate conundrum must be attributed in large part to the gradual loss of reactive sites on silicate surfaces with time. The results imply that methods of mineral surface characterization that provide direct measurements of the bulk surface reactivity are necessary to accurately predict natural dissolution rates.« less
  • The authors have studied the thermodynamics and kinetics of hematite dissolution in bicarbonate solutions under constant pCO{sub 2}. The solubility of hematite is increased in the presence of bicarbonate. They have established that the complexes responsible for this increase are FeOHCO{sub 3} (aq) and Fe(CO{sub 3}){sub 2}{sup {minus}}. The stability constants of these complexes at the infinite dilution standard state are log {beta}{sub 11} = {minus}3.83 {plus minus} 0.21 and log {beta}{sub 2} = 7.40 {plus minus} 0.11, respectively (all errors are given at 2 {sigma} confidence level through this work). The rate of dissolution of hematite is enhanced inmore » bicarbonate solutions. This rate of dissolution can be expressed as R{sub diss} = k{sub 1}(HCO{sub 3}{sup {minus}}){sup 0.23} (mol m{sup {minus}2} h{sup {minus}1}), with k{sub 1} = 1.42 10{sup {minus}7} h{sup {minus}1}. The combination of the study of the surface complexation and kinetics of dissolution of hematite in bicarbonate solutions indicate that the dissolution of hematite is surface controlled and bicarbonate promoted. The rate of dissolution follows the expression R{sub diss} = k{sub HCO}{sub 3{sup {minus}}}{l brace}=FeOH {minus} HCO{sub 3}{sup {minus}}{r brace}, where k{sub HCO{sub 3}{sup {minus}}} = 1.1 10{sup {minus}3} h{sup {minus}1}. The implications of these findings in the oxic cycle of iron in natural waters are discussed, most importantly in order to explain the high-Fe(III) concentrations measured in groundwaters from the Pocos de Caldas complex in Brazil.« less