Internal Domains of Natural Porous Media Revealed: Critical Locations for Transport ... ·...

Internal Domains of Natural Porous Media Revealed: CriticalLocations for Transport, Storage, and Chemical ReactionJohn Zachara,*,† Sue Brantley,‡ Jon Chorover,§ Robert Ewing,∥ Sebastien Kerisit,† Chongxuan Liu,†

Edmund Perfect,⊥ Gernot Rother,# and Andrew G. Stack#

†Pacific Northwest National Laboratory, Richland, Washington 99352, United States‡Penn State University, University Park, Pennsylvania 16802, United States§University of Arizona, Tucson, Arizona 85721, United States∥Iowa State University, Ames, Iowa 50011, United States⊥University of Tennessee, Knoxville, Tennessee 37996, United States#Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States

ABSTRACT: Internal pore domains exist within rocks, lithicfragments, subsurface sediments, and soil aggregates. Thesedomains, termed internal domains in porous media (IDPM),represent a subset of a material’s porosity, contain a significantfraction of their porosity as nanopores, dominate the reactivesurface area of diverse media types, and are important locationsfor chemical reactivity and fluid storage. IDPM are key featurescontrolling hydrocarbon release from shales in hydraulicfracture systems, organic matter decomposition in soil,weathering and soil formation, and contaminant behavior inthe vadose zone and groundwater. Traditionally difficult tointerrogate, advances in instrumentation and imaging methodsare providing new insights on the physical structures andchemical attributes of IDPM, and their contributions to systembehaviors. Here we discuss analytical methods to characterize IDPM, evaluate information on their size distributions,connectivity, and extended structures; determine whether they exhibit unique chemical reactivity; and assess the potential fortheir inclusion in reactive transport models. Ongoing developments in measurement technologies and sensitivity, and computer-assisted interpretation will improve understanding of these critical features in the future. Impactful research opportunities exist toadvance understanding of IDPM, and to incorporate their effects in reactive transport models for improved environmentalsimulation and prediction.

■ INTRODUCTION

A common physical feature of the earth surface is the presenceof rock, soil, or sediment fragments separated by gas- and/orliquid-filled pores or fracture space. Advection occurs throughlarger interconnected pores or fractures in response togradients in potential, distributing nutrients in the rootingzone, transporting solutes/contaminants from soil to ground-water, and moving oil to surface seeps. Even smaller pores and/or fractures populate the mineral or organo−mineral matrixbetween advective pathways.1−6 Depending upon the scale ofinspection, these matrix elements may consist of unfracturedbedrock, corestones, lithic fragments, fine-grained sedimentlayers, or soil aggregates (Figure 1). We term these porouszones within matrix elements as internal domains of porousmedia or IDPM.Varied in composition, size, connectivity, and geometric

attributes,7,8 IDPM display three-dimensional hierarchicalstructures over many length scales.9−12 The extended structureof IDPM may be geologically inherited; or result from

weathering, diagenesis, or soil formation.7,13−15 Pores orfractures within the hierarchy display varied connectivitydefining a pore or fracture network.16−19 The network structureand topology control: (1) internal pore volumes, mineralphases, and surface areas accessible to solutes, colloids, ormicroorganisms;20−24 and (2) diffusive path-lengths, tortuosity,permeability, and the predominance of advective or diffusivetransport.25−28 Generally, IDPM are localized zones wheretransport is dominated by slow advection and/or diffu-sion.3,26,29−31

Liquids in IDPM exhibit a high interfacial area to volumeratio, and IDPM are consequently locations of enhancedphysicochemical interactions. Chemical processes within IDPMare controlled by molecular interactions in the liquid phase and

Received: October 12, 2015Revised: February 2, 2016Accepted: February 5, 2016Published: February 5, 2016

Critical Review

pubs.acs.org/est

© 2016 American Chemical Society 2811 DOI: 10.1021/acs.est.5b05015Environ. Sci. Technol. 2016, 50, 2811−2829

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with functional groups exposed on pore or fracture walls.3,32

These processes can modify the pore network by changing orcreating IDPM.33−36 Small-scale chemical processes that occurwithin IDPM influence chemical concentrations and fluxes inlarger environmental systems.1,3,37−40 Unique chemical behav-iors may occur in IDPM as a result of their small size.41−43 Theoften minute dimensions of IDPM, and their extended three-dimensional network structures have challenged directobservations of their physical and chemical properties, as wellas model development to describe their transport behavior andgeochemical function.18,44

Recent capability advances to visualize internal structures andproperties of soil and geologic materials and the developmentof computational techniques to extract quantitative informationfrom these measurements provide new opportunities forphysicochemical characterization of IDPM, including theirsize distributions, network connectivity, and internal chemicalproperties and reactivity.17,19,45−49 Ongoing instrumental andsoftware improvements will further enhance spatial andchemical resolution in the future. Interrogation of certain sizeranges of IDPM in three- (x, y, and z) and four-dimensions(time) is now possible.50−53 However, quantitative analysis ofmultidimensional image data is challenging,46,54 as is theincorporation of its detail into pore-scale and macroscopicreactive transport models for system scale descriptions.18

Resolving these challenges will advance understanding ofreactive transport within IDPM, and their controls onenvironmental chemistry at the field scale.In this critical review we highlight scientific advances in the

understanding of reactive transport in IDPM and identify keyknowledge gaps. We assess analysis methods to determine thephysical and chemical properties of IDPM across multiple

scales, and their limitations. We evaluate how new techniques,and groupings of older ones, allow multidimensional character-izations that were not previously possible. Modeling andexperimental findings on reactivity in pore and fracture spaceare evaluated to determine whether unique chemical behaviorsoccur. Reactive transport is then considered with emphasis onpore connectivity, fluid flow, incorporating IDPM into reactivetransport models, and contributions of IDPM reactions tosystem-scale behaviors. The analysis identifies characterizationchallenges, needed new capabilities, and future researchopportunities on the impact of IDPM on pressing environ-mental science issues.

■ SIZE CLASSIFICATION OF PORESIDPM and pores specifically, are critical features of natural solidmaterials investigated within environmental chemistry, soilscience, and geology. Each of these disciplines has different size-related characterization schemes for pores (Figure 2). The

IUPAC definitions55 originate from a chemical and catalysisperspective56 and are widely used in the materials field. Thedefinitions from soil science57 reflect applied experience in thepore sizes that control advective and diffusive transportprocesses, and field moisture capacity. The geologic defi-nitions58,59 represent an overall characterization scheme fordiverse natural materials. Contrasting definitions of commonterms has contributed to miscommunication in the literature.Here, we use explicit pore size ranges whenever possible. Thegeologic size classification,58,59 however, is adopted wheneverterms such as macropore, mesopore, micropore, nanopore, andpicopore are used.

■ AN EXAMPLE OF IDPM IN THE ENVIRONMENTSince the 1990s, new approaches for extracting natural gas fromshales have enabled huge increases in production.5,60 TheMarcellus shale, for example, could produce as much as 500trillion cubic feet of gas using these new drilling techniques.61

The extraction of natural gas from low permeability shales

Figure 1. Internal domains comprised of pores and fracturesdominated by diffusive transport are ubiquitous within porous mediaof different types including structured soil with aggregates, andweathered, fractured bedrock. Despite varied formational processesand macroscopic appearances, internal domains in different porousmedia types display common physical and chemical attributes andbehaviors, and complex size hierarchies that challenge characterizationand modeling.

Figure 2. Pore size characterization terminology from differentdisciplines. The molecular radii of water and methane are shown forcomparison.

Environmental Science & Technology Critical Review

DOI: 10.1021/acs.est.5b05015Environ. Sci. Technol. 2016, 50, 2811−2829

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involves hydraulic fracturing to develop secondary porositynetworks over significant length scales. It is of great economicinterest to understand the nature of gas-containing pore spacein shales and how hydraulic fracturing enables its release.Characterizing shale pore space is difficult because mineral

particles are micrometer-sized or smaller, and the pores arenanometers to micrometers in size62−65 (Figure 3). Porosity

within the Marcellus shale resides in organic matter andpyrite,66 whereas pores in organic matter dominate in theBarnett shale.67 These intragrain pores host natural gas, andsignificant questions exist on their connectivity. The pores (5−750 nm) are often subspherical to angular, possiblyrepresenting postcompaction genesis during thermal matura-tion of the organic matter. Individual organic domains existwithin these shales with over 20% porosity as defined byscanning electron microscopy. Bedding-parallel pores betweenlaminae containing organic material, and nanopores in fine-grained inorganic material and pyrite framboids function asimportant interconnected permeability pathways in the nativeshale.67

Hydraulic fracturing generates fractures that are millimetersin cross-section and that extend for hundreds of meters.68

Understanding these structures and their hierarchy, rates ofsubsequent physical relaxation, and chemical reactions that mayoccur within them and on newly created surfaces, are key topredicting the stability of the enhanced extraction system andlong-term gas yields.61,69

■ PHYSICAL FEATURES OF INTERNAL POREDOMAINS

Size distribution, surface area, geometry, path lengths,connectedness, and topology are key physical features of theinternal pore system that control liquid and gas movement, therates and extent of intragrain reactions, and solute import andexport.45,46,70 Consequently these properties have been long-standing characterization targets. Connected porosity and itssurface area are most significant because unconnected porositydoes not contribute to transport. Recent attention has focusedon 3-D characterization of internal pore and fracture networksto understand and model the transport of water, gases, andsolutes.53,71,72 Key physical parameters are intrapore residencetimes and fluid velocities, tortuosity and apparent diffusivities,and surface area to liquid ratios.Numerous techniques are used to evaluate the properties of

intragrain pores and fractures (Table 1; and71). The measure-ments include total and interconnected porosity, pore sizedistribution (psd), and internal surface area. Modern targetsinclude the 2-D and 3-D structures of the pore system. Chosenmethods reflect objective, as some applications such as fluidflow through porous media are more sensitive to larger pores,while others including shale gas extraction focus on nanoporeswhere hydrocarbons reside. No one method is able tocharacterize pore space over the large size range of interest ingeologic materials,71,73,74 thus individual methods are selectedfor specific size ranges or multiple methods are applied withoverlapping scale sensitivities. Consequently, porosity isoperationally defined by method, as methods vary in theirability to resolve pores of certain sizes. Sample size,representativeness, and replication are additional considera-tions.75 Measurements that are sensitive to the smallest of pores(<10 nm) are typically restricted to small sample sizes; whilethose appropriate at the core scale (10−100 cm) are lacking inspatial sensitivity (resolution of >10 μm). Porosity measure-ment techniques can be divided into two categories: (1) thosethat measure the bulk properties of powders, or disaggregatedor crushed solids, and (2) those that image the porecharacteristics of intact materials.Physical gas adsorption (PGA; N2, CO2, Ar) and mercury

intrusion porosimetry (MIP) are well-accepted bulk methods tocharacterize intragrain porosity in natural materials (Table1;3,71,76−78). They remain a mainstay in pore characterizationbecause of modest instrumental costs and relative ease in datainterpretation.3,7,62,79,80 These methods use probe molecules orfluids as proxies from which pore properties are derivedthrough isotherm or model application (e.g., BET model81).Models for data interpretation and the physical assumptionsembodied within them (e.g., cylindrical pore shape for MIP oridealized pore shape for PGA81) serve as limitations to accuratepore analysis. BET analysis of gas adsorption data yields theinternal surface area and pore size distribution of accessible,connected pore spaces;3 but adsorption−desorption hysteresiseffects and isotherm nonlinearity can obscure the determinationof pore size distributions in natural materials.3,76,81,82 Nitrogen(at 77 K) has been the most common adsorbate for microporeanalyses on environmental substrates, but it can interact withsurface functional groups leading to artifactual measurementsunder certain conditions.83 Argon (87 K) is the now preferredadsorbate. For nanoporous materials it is beneficial to combineCO2 adsorption at 273 K with argon or nitrogen adsorption at87 K and 77 K, respectively, for robust characterization.83

Figure 3. Backscattered scanning electron microscope (BSEM) imagesof Marcellus shale sampled in central Pennsylvania from 910 ft belowland surface (bls) (A), and from a weathered outcrop of the same shale(B) sampled from 29 ft bls. Samples were prepared using focused ionbeam (FIB) milling. Pore size distribution (psd) (C, D) was obtainedby fitting a polydisperse sphere (PDSP) model to neutron scatteringdata for samples (A) and (B). Water-inaccessible pores weredetermined by scattering in the presence of contrast-matched H2O−D2O. Methodologies are published separately.66 Water-inaccessiblepores dominate the nanopores in the unweathered shale.66 Theporosity of the weathered outcrop sample is higher, and mostly water-accessible.69 Exhumation and weathering has opened microfracturesand pores in organic- and pyrite-containing zones of the shale. (Datafrom X. Gu, Pennsylvania State University).



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Neutron scattering can provide bulk statistical informationon internal pore size distribution over the largest length scalerange of any single method.84 Small-angle neutron scattering(SANS) and ultrasmall angle neutron scattering (USANS)interrogate pores and their connectivity in the size range of 1nm to 15 μm, potentially revealing the full pore size spectrumof a geologic material85(Table 1). Analogous X-ray techniques(SAXS and USAXS) provide similar information over asomewhat smaller scale range.86 Scattering originates primarilyfrom internal and intragranular surfaces and pores. Thescattering signal is a cumulative response that is averagedover a sample size of tens to hundreds of mm3 and ischaracteristic for the internal porosity of the sample. Theaccuracy of the data (momentum transfer and normalizedscattered intensity) is typically better than 5%, but data analysisrelies on model application, and complementary informationfrom gas adsorption and electron microscopy is often utilized toconstrain assumptions. The pairing of scattering measurementswith another technique, such as backscattered electronmicroscopy (BSEM) or computed tomography (CT), canextend the upper measurement range into the domain ofmacropores and larger fractures.11 The analysis of scatteringdata yields the total internal surface area (connected andunconnected) and its roughness, and the pore size distribution(psd).Models based on assemblages of simple scatterers with

tunable size distribution are used for rock pores with highlyirregular pore shapes and wide psd, such as the polydispersehard sphere model.87 While some natural materials contain alarge fraction of spherical pores, others are dominated byfractures and lamellar pores that show preferential orientationwith respect to bedding or jointing. Model assumptions maynot be meet in these systems, and errors in the psd areintroduced which may be similar in magnitude to thoseencountered in PGA. The psd of environmental solids typicallyspan many orders of magnitude, and reveals marked differencesbetween rocks and soils of different provenance and weatheringhistories71,88 (Figure 4A). Scattering data is often describedwith fractal models85,88 providing insights on diagenetic,88

metamorphic,89 and weathering histories.14,90

Neutron scattering is uniquely able to provide bulk statisticalinformation on the entire pore system including connected andunconnected pore fractions.71 By soaking the sample with acontrast-matched H2O/D2O fluid, scattering from connectedpores is eliminated.90 Analysis of SANS from dry and contrast-matched samples allows separation of the volume fraction andproperties of connected and unconnected pores. This can beimportant because unconnected pores can open duringweathering,30,91 hydraulic fracturing, or water rock/sedimentinteraction to become part of the connected pore system. Otherpores that are initially open may close through secondarymineral precipitation,34,92 decreasing intragrain access andaltering transport properties. Following the evolution ofconnected and unconnected porosity with reaction trajectoryhas provided important insight into weathering and diagenicprocesses.33,35,71,90 The combination of bulk neutron scatteringmeasurements with other methods such as MIP, SEM chemicalimaging,93 and/or BSEM74 can yield unique insights on inter-relationships between the pore network, the mineralogicalframework, and its multiscale characteristics.94

Imaging methods utilize electron, or X-ray, or neutronsources to observe pore features in 2- and 3-dimensions fromwhich pore shapes and orientations, and connective networksT

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can be defined. BSEM is the method of choice to determine the2-D characteristics of intragrain pore space11,23,24 in geologicthin sections at relatively high resolution (∼100 nm). BSEM issensitive to mean atomic number (Z), providing gray scalecontrast in mineralogy, organic matter, and pores for mostsamples. The 2-D porosity structure can be determined bycontrast thresholding or segmentation.23 BSEM imaging can becomplemented with energy dispersive X-ray spectroscopy(EDS) to determine mineral phases hosting or bordering thepore space,23,71,75,95,96 or molten alloy intrusion to assessconnected pore structures.29,97,98

The ability to image pores and fractures in 3-D at themicrometer and submicron scale has become possible withelectron microscopy (focused ion beam nanotomography, FIB-nT;17,75) and computed tomography (CT) using neutron-(NCT) or X-ray(XCT) sources.52,53,99 The resulting imagesenable calculation of petrophysical parameters (porosity andconnectivity, surface area), pore network geometries, andliquid/gas-flow parameters (permeability) of intact materials at

the pore scale.19,29,47,100,101 XCT (Table 1) is the most matureand flexible of the methods, seeing widespread applications inthe geological and soil sciences, and water resources.45,46,53,72

XCT is performed with laboratory- and synchrotron-based X-ray sources at comparable spatial resolution. Being a non-destructive technique, XCT is ideal for the study of dynamicprocesses such as fluid flow/displacement and reactivetransport.36,51 High synchrotron X-ray fluxes enable subseconddata collection speeds for in situ studies of transport andreaction.For tomography, a series of 2-D images through the sample

volume are collected (FIB-nT destructively and XCT non-destructively) that may be analyzed individually54,102 or bereconstructed into a 3D representation of the samplevolume51,52 (Figure 4B). The data are processed to removeartifacts, noise, and blur; with subsequent segmentation of grayscale images to discriminate between pore and mineralspace.19,54,102 Image processing and segmentation algorithmsare a source of variability in derived petrophysical parameters,especially for complex samples such as soil or sediment.54,70,102

Digital reconstruction and quantification of the 3D porenetwork and its properties is a more complex proc-ess,7,17,19,28,47,63,103 often requiring subjective choices andassumptions with nonunique results.46 Benchmarking activitiesand round-robin testing are beginning to provide guidance tostandardize image processing, segmentation, and digitalreconstruction efforts to reduce the uncertainty of derivedparameters; their numbers have been limited,54,70,104−106 butgrowing.FIB-nT and XCT differ in their spatial resolution (Table 1).

There are important trade-offs between spatial resolution andfield of view, and consequently the suitability of materials forstudy by these methods. FIB-nT with SEM/BSEM imaging hasthe spatial resolution of ∼10 nm and can be applied to anapproximate 10 μm3 intact cube.27,107,108 The representative-ness of such small samples must be considered.75 This scale ofinterrogation is suitable for fine-grained rocks and sedimentswith small pores; but larger pores and fractures in coarser-textured materials will not be observed. Moreover, many poresin fine-grained sediment are below 10 nm. FIB-nT (with SEManalyses), for example, described only 20−30% of the porespace of argillaceous sediment.27 The resolution of XCT isgenerally larger than 1 μm,108,109 depending on the X-raysource and detector system,106 but new instrumentation hasoptimal sensitivity to 50 nm.110 Comparable to BSEM, XCTcan be performed at dual energies providing information on 3Dmineral distributions.111,112 This, however, is not a simpleapproach for mineralogically complex materials and currentcapabilities are limited. Multiscale XCT analyses99 can beperformed on a range of sample sizes (mm to cm), includingcore size materials,12,113,114 providing information on porespace hierarchies and multiscale physical structures.Resolution effects in 3D pore imaging must be considered in

selecting methodologies, interpreting results, and derivingtransport parameters and fluid flow models,108,114,115 as appliedmethods may only access a fraction of the pore space and/orprovide incomplete measures of pore connectivity. Calculatedparameters for a given material may vary significantly withresolution. The impacts of resolution may be counterintuitive,with important trade-offs to be recognized100,116 depending onmeasurement goal. Lower resolution may yield images thatbetter estimate permeability that is controlled by larger poresand fractures, while higher resolution images capture smaller

Figure 4. (A) Relative cumulative porosity distributions of twodifferent representative rock samples measured using a combination of(ultra) small angle neutron scattering (USANS) and backscatteredelectron microscopy (BSEM) analysis. The sandstone (St. Peterformation) has 50% of its pores over this size range smaller than 54μm.30 In contrast, the shale (Eagle Ford formation) has 50% of itspores smaller than 20 nm.33 The majority of its pores are thereforenanopores, which likely contain different chemical reactivity and fluid/gas transport properties from larger pores. (B) 3-D reconstruction ofpore space (yellow), organic matter (purple), and calcite and quartz(gray) in shale (Opalinus clay) from FIB-nT imaging. Fine-grainedphyllosilicates are clear in this representation. From Keller et al.16



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pores that dominate total porosity and surface area, but thatcontribute little to advection.100,116 In consequence, manypractitioners apply a combination of bulk characterization andmultiple imaging methods,23,109 including those that spandifferent size ranges, to access as much of the pore space as ispossible. This approach requires consideration of theimplications of sample size, and multiscale data fusion andanalysis.12,16,23,117

■ FLUID AND SOLID-PHASE CHARACTERISTICS OFINTERNAL PORE DOMAINS

The interpore chemical environment consists of a solute-containing liquid phase and bounding mineral surfaces, whilepores in gas-bearing shales are generally dry. Organic and/ormineral phases that border the pore or fracture space influenceliquids through surface-mediated reactions including protonexchange, surface complexation, precipitation/dissolution, andion exchange, as well as electro-kinetic and double-layerforces.118,119 Surface functional groups may be intrinsic (e.g.,hydroxyl terminations of mineral phases), or secondaryresulting from alteration, secondary mineral precipitation, oradsorption of inorganic and organic sorbates. Mineral and/ororganic phases bordering pores and fractures may vary in spacewith network hierarchy, and with distance from macroporecontact points as a result of weathering or soil formation.Uranium silicate nanoprecipitates, for example, were formedwithin internal microfractures of granitic lithic fragments invadose zone sediments only in plagioclase-bordered domains,and not those with quartz or K-feldspar,39 implying localizedreaction specificity imparted by pore-water composition or wallsurface structure.The characterization of low-volume, intragrain pore waters

has been a long-standing challenge,120−123 but knowledge ofpore water composition is critical to understand and predict thechemical reactivity of IDPM. Centrifugation is used to extractliquids from generally permeable geologic and soil materi-als,120,122,124,125 with increased speeds used to sample liquids insmaller pores held at higher capillary pressure. An intuitiveparadigm is that higher concentrations of solutes occur inwaters of smaller pores as a result of longer contact times, highsurface area to volume ratios, and approach to equilibrium.124

While recent results for water saturated rhyolite follow thismodel,125 most reported experimental data are not inagreement. Argillaceous sediments or clayrocks are importantas a potential geologic host rocks for radioactive wastedisposal,126,127 and their pore-waters have been well-studiedby mechanical squeezing121,128 and mineralogic analysescombined with geochemical modeling.129 The physical proper-ties of clayrock are similar to shale, and their content of verysmall pores (<100 nm) with negative surface charge densitymakes them exemplary of nanopore systems where membraneand double layer effects on pore water composition, andporosity volumes accessible to anions, cations, and unchargedspecies must be considered.121,128 Extracted waters from Swissclaystones,121 including the well-studied Opalinus clay, werereducing with near neutral pH, an ionic strength of ∼0.35, andion composition dominated by Na+ and Cl−, with lesseramounts of Ca2+, Mg2+, and SO4

2−.For nanopores and pore-throats, the presence of surface

charge may cause compositional changes to pore water withinthe diffuse double layer (DDL), and where DDL’s from porewalls overlap.130 A positive surface charge will lead to a deficitof cations in the DDL, while a negative surface charge will lead

to an excess of cations. The critical descriptive parameter is theDebeye length (D) or double layer thickness which varies withionic strength from approximately 0.96 nm at I = 0.1 to 20 nmat I = 0.0001.130 Thus, water in a 2 nm pore may displaycompositional deficits at I = 0.1, while much larger pores (40nm) will be affected at I = 0.0001. This effect is important forsmall pores (2−50 nm) in shales and argillaceous sediments,where the presence of negative charge density on phyllosilicatesurfaces may create an excess of cations and a deficit of anionsin the adjacent pore water, causing departure of the meansolution composition from electroneutrality.131,132 Such effectshave strong influence on ion diffusion in nanoporedomains.92,133 Models have been developed to describe porewater composition in these nanopore systems that account forsurface charge, ion exchange, and the partitioning of chargecompensating ions in the diffuse double layer.129,131,133−135

The chemical/mineralogic composition of large pores orfractures (e.g., >10 μm) is readily measured in 2-D usingpetrographic thin sections and microbeam techniques such asSEM, BSEM, EMP, XMP, and multienergy XCT analysescombined with image segmentation.10,23,24,99,136,137 Pore-mineral contact boundaries, and hence “reactive surfaceareas”, may be dominated by minor phases in the bulk materialwhich occur as cementing agents or pore surface precipitatesthat may be more- (e.g., chlorite) or less- (e.g., kaolinite)reactive than underlying grains.23,24 Additionally, secondaryphases existing within the pore space may block connectivity,92

or enable pore connectivity through their own intraphase porestructure.23 Precipitated exogenous solutes in larger intragraindomains may also be analyzed by the above techniques,revealing internal channels that connect with macropores39,136

(Figure 5). Microfractures (<100 μm) may form during theweathering of granite through oxidation-driven swelling ofbiotite that subsequently become locations for active secondary

Figure 5. Backscattered electron micrograph (BSEM; left) and X-rayintensity Cu elemental map (right) for a polished section of amillimeter-sized lithic fragment within Hanford subsurface sediment.Cu abundance is denoted by a heat intensity index with red/orangebeing high, green intermediate and light blue low but detectable.Aqueous Cu has entered the basalt clast from the upper interface,moving through and reacting within intergranular domains of differentsize, geometry, connectivity, and pH. Note intraclast hotspot in a clay-rich void, and correlation of physical features, including cracks andvoids, with Cu distribution. Field of view is approximately 100 μm.Thin channels are ≤1 μm. Samples were prepared by embeddingsediment in epoxy, followed by cutting and polishing to produce cross-sectional surfaces of clast interiors. Samples were carbon coated toenhance electrical conductivity, and examined using a JEOL 8530felectron microprobe. The relative elemental Cu abundances werecollected using a wavelength dispersive spectrometer tuned to the CuKa X-ray emission line.



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mineralization.35,91,138 Laser ablation-mass spectrometry108 canprovide multielement chemical composition for pore- andfracture-bounding domains, but the interrogated samplevolumes are small (cubic micrometers).Characterizing the chemical environment of internal pores at

scales <10 μm is especially challenging. Transmission electronmicroscopy combined with electron energy loss spectroscopy(TEM/EELS) is effective for submicrometer 2-D elementalanalysis if fracture or pore space can be sampled by microtome.Excised mm-sized cones of geologic material containing poresand fractures can be analyzed by atom probe tomographyproviding 3-D elemental distributions for the solid phase at thenm-scale.139,140 FIB-nT has been combined with EDS todevelop 3-D chemical distribution maps at submicrometerresolution.75,141 Organic (kerogen) domains with nanoporosityand associated hydrocarbons are common in shales, and havebeen contrast-imaged with BSEM to approximately 300 nm.7,63

These organic domains vary in spatial continuity in shales fromdifferent locations, and display pore-networks that are distinctfrom the mineral matrix.Aggregates are cm-sized structural units in soil13 that are

important domains of chemical reactivity. They contain ahierarchy of pore sizes as imaged by XCT.19,47,142 Establishingthe distribution of organic matter and specific carbon functionalgroups within mineral aggregates is a frontier research challengein the soil sciences, and crucial to understanding the fate of soilcarbon and feedbacks to climate change. Recent advances areallowing access to this complex domain. A chemical contrastingagent (osmium) with high affinity for organic matter was usedwith synchrotron XCT to identify domains of particulate anddiffuse (sorbed) organic matter within soil aggregates and theirrelationships to pore structure.143 Particulate organic matterwas imaged in soil aggregates using XCT and discriminantanalysis.144 Soft X-ray spectromicroscopy (STXM and C-1sNEXAFS;145) was applied to dispersed soil colloids, clay-sizedisolates, and microaggregates (<43 μm) to characterize carbonfunctional groups, inorganic matrix elements, and spatialrelationships involved in intra-aggregate mineral-organicinteractions at the nm-scale.146−148 Similar methods wereapplied to ultrathin sections of soil aggregates revealingdifferences in carbon oxidation state and chemical signatureon external and internal pore surfaces and interiors, and withinsequestered organic matter.48,149−152

■ CHEMICAL REACTIVITY IN INTERNAL POREDOMAINS

Atomistic simulations and X-ray reflectivity measurements haveshown the presence of an ordered interfacial water region onmineral surfaces that differs structurally from the bulkphase153−160 (Figure 6). This phenomenon stems from waterchemisorption and hydrogen bonding to surface sites, leadingto well-defined surface hydration layers that follow thetopography of the surface. The orientation and mobility ofsurface waters are constrained, which, in turn, allows for longerlasting hydrogen bonds with water molecules beyond thehydration layer(s). Overall, a damped, layered structuredevelops in which interlayer interactions diminish graduallyover 1−1.5 nm. Mineral−water interactions affect the dynamicsof interfacial waters; as water molecules approach the interface,their diffusion coefficient decreases rapidly, starting from 2 to2.5 nm from the surface.161−164 As a result, the diffusion ofaqueous species into nanopores is affected by interfacial waterdynamics.163,165 For pore widths of 20 nm or less, the

interfacial region constitutes a significant volume of the pore.As the pore width reduces to 5 nm or less, the interfacialregions of opposing surfaces begin to overlap, reducing thediffusivity of aqueous species in the pore center to values belowbulk. Finally, for pore widths smaller than 2 nm, layered waterstructures overlap strongly and diffusivity becomes highlysensitive to pore size.Interfacial water structure also affects the equilibrium

distribution of electrolyte ions near the surface. Atomisticsimulations and X-ray reflectivity measurements show a clearcorrelation between interfacial water and electrolyte ion densityprofiles,161,166−169 leading to a structured electrolyte distribu-tion with distinct adsorption planes, as predicted by doublelayer models. However, atomistic simulations also predict theformation of a structured electrolyte distribution at neutralsurfaces, a phenomenon not predicted by double layer models.Ion specific effects can arise when electrolyte solutions areconfined in nanopores.170,171 Small cations can penetrate thehydration layer, whereas larger cations remain predominantly inthe pore center displaying higher diffusivity. Anomalousdielectric behaviors have also been reported under cylindricalconfinement.172,173 The water dielectric constant becomesdirection-dependent at the mineral−water interface, andtensorial dielectric profiles calculated from atomistic trajectoriesshow strong anisotropy.174 The opposing effects of confine-ment and ion destabilization of the hydrogen-bond networkcause an increase in the normal dielectric component withincreasing electrolyte concentration. This is contrary to bulkbehavior where dielectric saturation dominates, and thedielectric constant decreases with increasing electrolyteconcentration.It has been hypothesized that reactions in pores ranging

between 1 and 100 nm should be different from larger onesbecause of (1) confinement-induced changes in water proper-ties as described above and their effects on solvation175 and the

Figure 6. (A) Ball-and-stick model of the interface between water andthe (010) surface of forsterite (Mg2SiO4). B) Atomic densitiesprojected on the surface normal and water diffusion coefficient,D(H2O), as a function of distance. The data used to generate thisfigure was published.321



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electrical double layer,42 and (2) surface curvature effects onsite density, surface charge, and counterion condensation.176

However, the question as to whether surface chemicalprocesses (ionization, complexation, or adsorption) in nano-pores differ from those in micro-, meso-, and macropores isunresolved. Associated with this is the formidable difficulty inmeasuring surface site concentrations within internal pore orfracture networks.3

Size effects on the surface chemical reactivity of hydrophilicpores for inorganic ions has been investigated with zeolite,silica, and aluminum oxide sorbents varying in internal poresize.42,177 Contradictory results have been observed on theeffects of nanopore size on surface charge (H+-adsorption/desorption;177−179), ion adsorption capacity,180,181 and ionadsorption thermodynamics42,182 that challenge generalization.Experimental complications are partly responsible, includingsorbent transformations and mass transfer effects.183 Severalstudies, however, provide notable insights both for, and againsta nanopore effect. No effect of small pore diameter (<10 nm)was observed on site density, proton adsorption−desorption, orion adsorption on Al2O3 and SiO2.

177 Anomalously strongsurface complexation of uranyl carbonate complexes wasobserved in nanoporous alumina that resisted bicarbonateextraction and reductive redox reaction [to U(IV)] withreduced quinone.181 Site density and Cd2+ adsorption onnanoporous SiO2 varied with pore size, an apparent effect ofpore surface curvature effects that promoted multiple site-soluteinteractions.42 Small zeolite pores (<0.53 nm) promoted thephysical dehydration of solvated Na ions leading to their strongand selective adsorption.41,184,185 Lastly, numerical calculationsinferred that surface complexation reactions occurring in smallpores with high surface area to volume ratio may alter internalwater pH by surface proton displacement, creating pHgradients that dissipate by diffusion.37

A subset of nanopores (<2 nm) in soils and shales exhibithydrophobic character,184 such as those associated with theuncharged siloxane surfaces of phyllosilicates185,186 and apolarmoieties of organic matter, including kerogen. These lackhydrogen bonding sites, and the free energy of associated wateris higher than bulk.187−189 Hydrophobic nanopore domainsadsorb volatile organic contaminants and hydrocarbons inpreference to water,187 functioning as an important sink fororganic contaminants in the environment. Organic sorbates canbe sequestered in these domains due to slow desorption andhindered molecular diffusion.190,191 Transformation reactionssuch as hydrolysis and dechlorination,192,193 or remediation byvapor extraction may be affected. Micropore volumes ofhydrophobic domains can be measured using trichloroethylene(TCE) as a probe molecule.184,187

Microbial size exclusion may occur where pore throats <220nm prevent ingress. Consequently, pore spaces separated bypore throats below this nominal size are not primary domainsof microbial activity, but may host soluble biogeochemicalproducts. However, the relationships between pore size,geometry, and connectivity; and biogeochemical activity,microbial diversity, and phylogenetic composition are complexand not well understood.21,194−197 Aggregate pore networksregulate organic matter decomposition in soil throughinterrelated effects: (1) microorganism and enzyme sizeexclusion from pore space containing sorbed organiccompounds such as polysaccharides and proteins, (2)organic-mineral interactions in high surface area domains thatreduce bioavailability, (3) tortuous and extended diffusion

pathways limiting oxygen supply for respiration, and (4) wateravailability and potential influencing biological activity.198−203

Quantitative relationships between the statistical properties ofaggregate pore networks and organic matter decompositionrates have not been established because of tomographicinsensitivity to the full pore size spectrum. However, a fractalapproach was effective in predicting the bioavailability oforganic matter within the pore structure of soil aggregates.204

Nanoporous domains of metal oxides are highly effective atstabilizing organic matter against desorption, a result ofpolydentate surface complexation within pores whose sizeapproaches that of the organic matter aggregates themselves.205

Intra-aggregate pore networks are also important locations forinorganic redox processes associated with Fe(III)- and Mn(III/IV)oxides, and soluble polyvalent metals.206,207

A long-standing question is whether precipitation/dissolu-tion reactions display pore size dependence.130 For the limitedcases that have been studied, precipitation appears inhibited innanopores208,209 and preferred in macropores.92,209 Theseobservations support the concept of “pore-size controlledsolubility (PCS)” where precipitates in small pores exhibithigher solubility as a result of surface energy effects caused bypore wall curvature.180,210,211 Under such conditions, higherdegrees of supersaturation are needed to induce precipitation insmall pores than in bulk solution.209 However, Ising modelcalculations suggest that the thermodynamic barrier fornucleation should be reduced in nanopores due to a lowersurface energy to volume ratio for nuclei precipitating in acurved nanopore versus a flat substrate.212 The favorability ofnanopore precipitation is further increased if direct positiveinteraction, such as epitaxy or structural match, occurs betweenthe precipitate and pore wall.32

Understanding the dependence of mineral reactivity on pore-size is important because macroscopic properties such ashydraulic conductivity have a nonlinear dependence on pore-size,213 and nanopores can comprise a significant fraction of thesurface area of soil and rock.88,90,214 Pore clogging, and anassociated reduction of permeability in granular media andfractured rock, occurs when precipitation from aqueoussolution is rapid relative to transport rate.215 Directed mineralprecipitation to sequester contaminant metals is renderedineffective by pore clogging.216−218 A mineral whose precip-itation is independent of pore size, or prefers small pores, mayclog the “throats” between individual pores, allowing a smallamount of precipitated material to block flow.219,220 Alternately,preferential precipitation in large pores may lead to self-sealingof otherwise permeable or fractured rock,221 analogous toquartz or calcite veins, or cements found in sedimentary andother rock types.88,180,209 Pore size distributions can bemodified during dissolution, widening pore throats andincreasing permeability during weathering and carbon seques-tration.222−224 However, when transport rate is balanced againstdissolution rate, larger pores or fractures may becomeprogressively widened at the expense of small pores andultimately channels or wormholes may form.220,225,226

The interplay between mineral precipitation, dissolution, andpore size has profound effects on the transport of fluids andsolutes in porous media. Less clear is a path forward for reliableprediction and control of these phenomena. Pore throatclogging can be explained by a reaction rate that is independentof pore size. But when suppression of precipitation innanopores is observed, it is normally explained by an increasedinterfacial energy due to the small size and curvature of the



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nanopore.180,209,227 However, the interaction energy betweenthe precipitate and pore walls may be equally or moreimportant in determining preferred sites for precipitation.32

Both conceptual models invoke the surface energy between theprecipitating phase and the substrate, a parameter that is readilyestimated on only planar substrates.228 An alternative effectmight be that ions of the same charge as the pore surface areexcluded, changing the cation-to-anion ratio of the pore fluidand, hence, the rate of precipitation.130,216,229 Lastly, theinterplay between transport rate, precipitation/dissolution rate,and mixing may also play a key role in explaining observed poresize dependencies.230

■ FLUID FLOW AND REACTIVE TRANSPORT ININTERNAL PORE DOMAINS

The pore network in granular materials is well connected. Forexample, in a sphere pack, individual pores may, on average, beconnected to 4.3 neighbors.231 However, pore connections areless28,232 in low-porosity intragranular domains. Additionally,the pore connectivity that a solute “sees” maybe less than theconnectivity of the pore network: the connectivity of the solute-accessible pore network decreases with increasing size of thetransporting solute. In IDPM, the accessible pore network maybe below the critical connectivity for molecules greater than agiven size. The critical connectivity is the density ofconnections below which a pathway of arbitrary length doesnot exist. For example, suppose the IDPM of interest resideswithin a soil aggregate. Given sufficient time, a Cl− ion mightdiffuse completely through the aggregate, while a globularprotein may not fit inside the largest pores. An intermediate-sized solute such as a polysaccharide might also diffuse throughthe entire aggregate, but would explore only a subset of thepore volume seen by the Cl− ion. The diffusion of Cl−, itself,may be limited in nanoporous material with negatively chargedpore surfaces92 relative to cations and uncharged solutes.Pore space in many environmental solids displays low

connectivity (<50%; Figure 3C;98). A low-connectivity systemis best described using percolation theory,233 the mathematicsof pathways in unstructured materials. Percolation theorydescribes how macroscopic properties emerge in a systemcomposed of many roughly equivalent parts, given the localdegree of connectivity between those parts.234 For IDPM, theparts represent individual pores within a porous material, andthe macroscopic properties of interest include the connectedporosity and the intragranular diffusion coefficient.26,235−237

Percolation theory has been applied to grain boundary porenetworks in rocks,238 and mass-transfer limited adsorption/desorption in porous aquifer solids.26

Flow and transport within IDPM are currently treatedsimilarly to flow and transport at the Darcy scale. However,practitioners working at specific scales, or with specific fluids,

often ignore certain forces. For example, soil physicistsgenerally ignore air, treating unsaturated flow as being solelyabout water flow. They do this because the viscosity of air is somuch lower than that of water that it rarely affects water flow.At the micro- and nanopore scale, gravity may be negligible,while electrostatic and capillary effects of solid surfaces maydominate. Creeping flow in low-permeability materials may benon-Darcian due to the presence of a threshold gradient.239,240

Working in IDPMs requires that forces and processes that areoften ignored at larger scales be considered, which can lead tocomplication (Table 2).Developments in imaging technologies are facilitating new

discoveries with respect to fluid flow within IDPM. Oneapproach is to perform lattice Boltzmann or Navier−Stokesnumerical simulations of fluid flow within virtual IDPMreconstructed from XCT or FIB-nT images.47,241,242 Suchsimulations can be run at the nanoscale. Alternatively, it is alsopossible to visualize fluid movement within actual IDPM,243 butaccepted methods such as XCT, magnetic resonance imaging(MRI), and positron emission tomography (PET) havesignificant limitations in this context. XCT imaging and PET,for example, rely on the use of tracers,51,244,245 while MRI islimited by iron in the solid matrix246 and has lowerresolution.247 In contrast, neutrons are well suited for imagingwater and oil in geologic and soil materials because of theirstrong attenuation by hydrogen, and their relative insensitivityto air and solid-phase constituents such as iron.52 Neutronimaging (NCT) has been applied to investigate the transientinfiltration of water into packed beds of soil aggregates,248−250

the rhizosphere around plant roots,251−253 oil−water displace-ments in reservoir rocks,254,255 and imbibition of wetting fluidsinto the rock matrix.256−258 However, the spatial resolution ofNCT is inferior to that of XCT, is facility specific, and isgenerally greater than 20 μm.52 Consequently, most neutronimaging facilities cannot currently resolve water or oil withinindividual pores in IDPM.45 Despite the methods limitedspatial resolution, Darcy-scale visualizations of fluid flows in realIDPM can still be valuable.203 Atomic force microscopy (AFM)was applied to map two-dimensional Poiseuille flow through∼100 nm pores in a novel alternative to NCT.204

Two and 3-D imagery of soil and geologic materials is beingused to constrain pore-scale models to provide fundamentalinsights on transport processes in heterogeneous porousmedia.17,18,47,72,75,104,241,259,260 Important parameters for insitu reactivity such as porosity, permeability, mineral distribu-tions and volumes, and pore-exposed mineral surface areas arebeing derived by image analysis to drive microcontinuummodeling of pore scale geochemical processes.44 Dynamicchanges to pore structure and surface area resulting fromdissolution and precipitation during transport are effectivelymonitored in situ by XCT, providing insights on pore-scale

Table 2. Complications Encountered in Studying Flow and Transport in IDPM

property orprocess behavior assumed at the Darcy scale some complications at the IDPM scale

flow Darcy’s law: laminar flow; Newtonian fluid non-Darcy behavior: continuum assumptions are violated;303,304 anion exclusion,305 electrokineticphenomena306,307

capillarity Young−Laplace equation: constant wetting angleand pore shape (circular or slit)

augmented Young−Laplace:308 films and disjoining pressure,309 capillary condensation;310 locallyvariable wetting angle, roughness, and local pore shape311,312

diffusion Fick’s laws surface diffusion,313 Knudsen diffusion,314 steric and double-layer interference causing localosmotic potential gradients,305,315 hindered and single-file transport316,317

poreconnectivity

ignored: assumed high connectivity limiting (at least for large solutes): percolation theory;233,236 connectivity varies withsolute size;291 correlation effects318−320



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factors controlling reaction locations and rates.99,261 Threedimensional imagery from XCT and FIB-nT is providingcrucial data sets to investigate the emergent consequences ofpore network structure and connectivity on transport at higherscales,18 including the definitions of statistical representativeelementary volumes,75,262 and up-scalable values of hydraulicconductivity.100,101 However, the limitations of image-derivedpetrophysical and transport parameters resulting from imageartifacts, segmentation, resolution effects, reconstruction, andsample representativeness54,70,100,263,264 must be evaluated on acase-by-case basis.Dual-porosity models are commonly used to integrate IDPM

into continuum-based reactive transport models.265−269 Theseemploy a subgrid concept within the continuum-basedmodeling framework: each numerical element in the continuummedium is separated into multiple subgrid domains based ontheir physical transport properties (Figure 7). Chemical

connection between the subgrid domains is through eitherdiffusive flux or first-order mass transfer. Because characterizingthe pore structure of the IDPM and its connectivity with theadvection domain can be difficult, the separate domainproperties (e.g., porosity, tortuosity, and diffusivity or masstransfer coefficient) are commonly defined by fitting fluid flowor nonreactive tracer measurements.1,270,271 Assuming thatphysical transport properties in the IDPM are the same fornonreactive and reactive species, measured tracer behavior canbe used to establish reactive transport equations, integratinglocal reactions in IDPM as a source or sink term along physicaltransport paths.Chemical or biogeochemical reactions in individual subgrid

domains are linked by a diffusive flux or mass transfer term atthe subgrid domain interface. When the linkage is diffusive,reaction and transport in the individual subgrid domains in theIDPM are explicitly calculated by solving reactive diffusionequations.272−277 Linkage using a mass transfer term assumesthat each subgrid domain in the IDPM is well mixed andhomogeneous, and a first-order mass transfer model can beused to represent the diffusion-based model. The limitations ofthis assumption are lessened in newer multidomain or multiratemodels, which separate the IDPM into multiple subgriddomains that use multiple first-order models to approximatethe diffusion process.37,278−281

Integrating IDPM into a continuum model requires thescaling of reactive transport properties. IDPM 7are spatiallyheterogeneous in physical transport and reaction properties,but these must be averaged or otherwise up-scaled for modelincorporation. Some properties (microporosity, surface area,

and pore volume) may be linearly averaged282,283 whenconnectivity is not limiting. Others (rate constants) varynonlinearly with scale because transport is coupled with localreactions along transport paths.284−290 These scale-dependentrelationships can be derived using multiscale models thatexplicitly simulate reactive transport in IDPM.285,288,289

While geometry, pore structure, and reaction properties inIDPM are now becoming accessible to characterization, IDPMare often assigned an assumed morphology (spherical,cylindrical, or slab) with constant average properties; or areallowed to vary randomly in space or constrained to followsome theoretical function.273,291,292 These approaches are takenfor computational expediency. The effective parameters used inthese model types are thus no longer intrinsic, because they areconvolved with assumptions of IDPM geometry and properties.This can lead to large variations in key parameter valuesdepending on assumption.293 Scaling must match the computa-tional grid size to the IDPM hierarchy, to the grain sizes withinthe IDPM, and to the relevant scale(s) of reaction (which mayvary with solute size). Scaling of reaction rates within IDPMwith complex pore networks and variable reactive surfaceareas17,19,23 is a major challenge for integrating IDPM processesinto larger scale transport models.In our opening example, we noted that the primary

repository of hydrocarbons in shale is nanopores, and thatchanges to the intrinsic pore network through hydraulicfracturing enables access to and extraction of gaseous andliquid hydrocarbons. Understanding the hierarchical structure,stability, reactivity, and transport properties of natural andengineered pore networks extending to the nanometer domainmay allow the development of improved technologies foroptimal resource recovery. Beyond shale gas and oil, IDPM arewidely significant to other earth science challenges. IDPM, forexample, in basaltic and granitic lithic fragments serve as a long-term repository for adsorbed uranium in a persistent ground-water plume at the U.S. DOE Hanford site.294 Predicting thelong-term behavior of this plume for human health andecosystem protection requires explicit consideration of IDPM,3

and the kinetic mass transfer of reactants and uranium both toand from this pore space to the groundwater plume.37,283,295

Required system scale predictions mandate that currentsemiempirical microscopic models of contaminant releasefrom IDPM be extended to the km scale.288 These predictionscould be improved if more robust models of kineticadsorption/desorption as controlled by IDPM were integratedinto the simulation strategy. And finally, the porosity structureof soil aggregates is now recognized as a critical parametercontrolling organic matter decomposition rates in soil, andhence CO2(g) fluxes from the land surface to the atmospherewith implications to the global carbon budget.143,144,194

■ KEY FINDINGS AND OPPORTUNITIES FOR FUTURERESEARCH

Time-tested and new characterization methods are providingincreasingly detailed insights on the nature of intragrain,hierarchical pore and fracture systems in rock, sediment, andsoil systems. No one method is suitable to characterize the fullspectrum of intragrain space, and each method has limitations.Pore scale hierarchies may span 8 orders of magnitude in lengthscale, with nanopores dominating the internal surface area andintragrain reactivity of many natural materials. Interconnectedporosity, where transport and reactions occur, varies withmaterial (e.g., sandstone > shale) and is typically <50% of total.

Figure 7. Schematic diagram showing a numerical element of dualdomains (advection and IDPM). The dual domains are connectedthrough mass exchange. The IDPM may be further divided intosubgrid IDPM domains.



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Total and interconnected porosity is higher in soil aggregates.Connected porosity exhibits tortuous structural networks thatare now being characterized from submicron to cm scalesthrough imaging and 3-D reconstruction methods leading toimproved prediction of transport parameters and their statistics.An important limitation is that the uncertainties of measuredpore size distributions, connectivity, and extended structuresare difficult to assess, and so far, have not been sufficientlyevaluated. Methods to incorporate highly resolved porenetworks into transport models are in their infancy, withbenefits to describe microscopic and field scale systems to bedetermined in the future.The effects of nanopore confinement on internal water

compositions and chemical reactivity are unresolved. Poresbelow 10 nm may show differences in chemical reactivity frombulk water due to (1) surface curvature effects, (2) ordering ofinterfacial water, (3) overlap of diffuse double layers, and (4)retarded diffusivity. However, predictions on the direction andmagnitude of changes in chemical reactions are not currentlypossible. The pore network may influence chemical reactivity athigher scales by controlling the resupply of oxygen consumedby microbial respiration, and preventing the ingress ofmicroorganisms and secreted enzymes into size-restrictedpore domains. Important effects include control on soil organicmatter decomposition and redox states of Fe and Mn. Intraporechemical reactions may change the pore network producingimportant nonlinear feedback mechanisms: precipitation inpores may reduce permeability and pore network flux, whiledissolution promoted by weathering may increase poreconnectivity, diffusion rates, and subsequent oxidation anddissolution reactions. The development of new data fusionapproaches46 are encouraged to facilitate analysis and under-standing of multiscale pore networks and time-dynamicmeasurements.Advances in imaging and reconstruction methods to define

three-dimensional properties of IDPM are expected in thefuture. Important needs are to bring objectivity to reconstruc-tion approaches, and to reduce errors resulting fromdiscretization effects when analyzing small structures andpores with large surface to volume ratio.46,296 Multiple advanceswill allow mapping of inorganic elemental distributions andorganic matter and its functional groups on pore surfaces atsubmicron pixel scales by synchrotron methods, with higherresolutions possible for soft X-ray microscopy. These analyses,however, are unlikely to be routine because of their complexity.Beamlines under development at third generation synchrotronsources will enhance capabilities for microfluoresence tomog-raphy, including carbon at high resolution, providing improvedopportunities to investigate chemical reactivity in small pores inthe 50 nm size range. New techniques including atom probetomography (APT)297,298 and ptychography299,300 will enablenanoscale chemical mapping of the solid matrix and physicalfeatures around soil and rock nanopores. Meanwhile, improve-ments to the resolution of CT and FIB-nT will push spatialresolutions to even smaller pore sizes (although majorimprovements are not expected because of physical limita-tions), while neutron and X-ray scattering will provide newinsights on transport processes and pathways in complex,interconnected, pore networks. Given these expected pro-gressions, new model development will be needed to utilizedata on multiscale pore network structures and processes forimproved predictions of environmental chemistry in diversesettings.

■ AUTHOR INFORMATION

Corresponding Author*Phone (509) 371-6355; fax (509) 371-6354; e-mail: [email protected].

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This manuscript resulted from a U.S. Department of Energy(DOE), Basic Energy Science (BES) Geosciences CouncilWorkshop on “Internal Domains in Porous Media” held inDecember 2012. The following sources/contracts providedfunding to the authors: J.Z., DOE BER/SBR (54737) and DOEBES Geosciences (56674); S.B., DOE BES (DE-FG02-OSER15675); J.C., DOE BER/SBR (DE-SC0006781); R.E.,DOE BER/SBR (54737); S.K., DOE BES Geosciences(56674); C.L., DOE BER/SBR (54737); E.P., LaboratoryDirected Research and Development Program, ORNL; GR;and A.S., DOE BES Geosciences (ERKCC72). BER/SBR isBiological and Environmental Research, Subsurface Biogeo-chemical Research Program. David Cole, Ohio State University,provided an initial concept for the abstract graphic. The helpfulcomments of three anonymous reviewers are acknowledged.

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