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The importance of interlayer equivalent pores for anion diffusion in clay-rich sedimentary rocks Cornelia Wigger, and Luc R. Van Loon Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03781 • Publication Date (Web): 16 Jan 2017 Downloaded from http://pubs.acs.org on January 18, 2017
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Environmental Science & Technology
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The importance of interlayer equivalent pores for anion diffusion in clay-rich
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sedimentary rocks
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Cornelia Wigger*, Luc R. Van Loon
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Laboratory for Waste Management, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland
7 8 9 10 11 12 13 14 15 Cornelia Wigger*
22 Luc Van Loon
16 Paul Scherrer Institut
23 Paul Scherrer Institut
17 5232 Villigen PSI
24 5232 Villigen PSI
18 Switzerland
25 Switzerland
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[email protected] 26
[email protected] 20 +41 56 310 5036
27 +41 56 310 2257
21 *corresponding author 28 29
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Abstract
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The anion exclusion behavior in two different clay stones - Opalinus Clay (OPA) and
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Helvetic Marl (HM) - was studied using a well-established experimental through-diffusion
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technique. The ionic strength of the pore water was varied between 0.01 M and 5 M to
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evaluate its effect on the diffusion of HTO and 36Cl-. The total porosity determined by HTO-
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diffusion was independent of the ionic strength, while the anion accessible porosity varies
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with the ionic strength of the pore water. In the case of Opalinus Clay the anion accessible
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porosity increases from 3 % at low ionic strength (0.01 M) up to 8.4 % at high ionic strength
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(5 M), whereas the anion accessible porosity of Helvetic Marl increases from 0.6 % up to only
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1.1 %. The anion exclusion effect in HM is thus more pronounced than the one in OPA, even
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at high ionic strength. This observation can be correlated to differences in mineralogy and to
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the fact that HM has a larger fraction of interlayer equivalent pores. Interlayer equivalent
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pores are small pores in compressed clay stones that are small enough to have – due to
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overlapping electric double layers (EDL) – similar properties as interlayers, and are therefore
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rather inaccessible for anions.
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TOC
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Introduction
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The disposal of radioactive waste in argillaceous sedimentary rocks is considered in various
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countries, e.g. Switzerland, France, Belgium and Canada.1
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properties of these rocks, such as the low hydraulic conductivity (< 10-11 m/s), the strong
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sorption of radionuclides and the slow radionuclide migration dominated by molecular
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diffusion5 make these rocks to ideal candidates for hosting a radioactive waste repository. In
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the case of argillaceous rocks, the pore volume, pore size distribution and pore connectivity
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are dictated by the amount and type of clay minerals building the mineral framework and
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microstructure of the rock. This is due to the fact that the particle size of clay minerals is
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much smaller than the grain size of the other composing minerals such as quartz and calcite,
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the latter having no or only a negligibly small porosity next to the surface.6 Connected pores
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in clay stones are thus mainly associated with the clay matrix7 8 and have equivalent diameters
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in the nanometer range, i.e. clay stones have micropores with a pore diameter < 2 nm and
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mesopores with a diameter between 2 nm and 50 nm.9
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surface charge of the pore walls significantly affects ion migration.11
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positively charged chemical species having small sizes (< 1 nm), the whole pore space of a
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clay rock is available for transport.14 16
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2 3 4
Several advantageous
Besides the pore size, also the 12 13
For neutral and
However, anions are repulsed by the negatively
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charged clay mineral surface.
Hence, only a part of the pore space is available for anion
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transport, i.e. the anion accessible porosity is less than the total porosity.17
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This
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effect is well known as anion exclusion and has been observed in clayey soils,
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compacted single-clay systems25 and in argillaceous rocks.26
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performed aiming to understand the anion exclusion effect in bentonite,28
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clay,30 and in argillaceous rocks such as Opalinus Clay,3 14 31 12 32 Helvetic Marls,33 Callovo-
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Oxfordian argillites and Oxfordian limestones34 19, and Boom Clay35. All these studies show
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that both the diffusive anion fluxes in these clay formations and the measured transport
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porosities were smaller compared to those of HTO because of the anion exclusion effect (Fig.
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1). Although an anion accessible porosity of ca. 50 % ± 10 % of the total porosity was
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observed for a majority of claystones36, lower values down to 10 % of the total porosity were
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also found15
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anion exclusion effect in bentonite29
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studied the effect of varying ionic strength (0.016 - 1 M) on the diffusion of I- in Boom Clay
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and observed a higher accessible porosity with increasing ionic strength. Wittebroodt et al.37
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studied the diffusion of
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and could also observe a significant effect of varying ionic strength (0.012 - 0.12 M) on the
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in
Several studies were 29
in compacted
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. A few studies discuss the impact of ionic strength of the pore water on the
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Cl- and
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and in clay formations35
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, e.g. Moors et al.35
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I in Upper Toarcian argillite samples from Tournemire
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anion accessible porosity. In addition to the experimental studies, several theorical approaches
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exist to model the anion porosity in clay rocks. Muurinen et al.40, Birgersson and Karnland41,
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Appelo and Wersin42, Jougnot et al.43 based their analyses on a Donnan approach, while
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Bourg et al.44, Sposito13, Tournassat et al.45, Greathouse et al.46 interpreted the anion exclusion
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porosity in the framework of the Gouy-Chapman theory. The majority of the experimental
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and modelling studies dealing with compacted bentonites showed that the anion accessible
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porosity increases with increasing ionic strength.17
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compression of the electric double layer (EDL) at high ionic strength. The EDL arises from
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the screening of the negative surface charge of clay minerals by accumulating cations. It
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consists of a Stern layer, devoid of anions, and a diffuse layer where the concentration of
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anions is lower than that of cations.48
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layer is necessary to compensate the negative charge on the surface (Fig. 1).11
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28 38 41 47
This could be explained by a
At high ionic strength a lower volume of the diffuse
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Figure 1: Schematic picture of the pore space of argillaceous rocks and the potential effect of
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ionic strength on the anion accessible pore space: the total pore space comprises the
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interlayer water (2) and interparticle pore water (3+4). The interparticle pore water
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equals the electric double layer pore water (3) and the free uncharged pore water
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(4). Anion diffusion takes mainly place in the free pore water.28
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In the case of natural argillaceous rocks systematic studies on the effect of the ionic strength
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on the anion accessible porosity are scarce and, because of contradictory results it is not clear
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at all whether clay stones behave in a similar way as compacted bentonite. Moreover, the
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broad range of values for anion accessible porosities (10 – 60 % of the total porosity)
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observed for different argillaceous rocks have not been explained properly yet. This prompted
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us to perform a systematic investigation of the anion accessible porosity in natural clay rocks.
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The aim of the work is to study the anion diffusion behavior and the anion exclusion effect in
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clay rocks with different mineral composition in order to better understand the relationship ACS Paragon Plus Environment
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between the ionic strength of the pore water, the mineral composition of the rock and the
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anion diffusion in these clay rocks. This will also enable to better constrain the anion
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accessible porosity w.r.t. the estimation of the pore water composition.
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Materials and Methods
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Samples. Two different clay stone samples were used in this study: Opalinus Clay and
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Helvetic Marl. The Opalinus Clay (OPA) sample originated from the Schlattingen deep
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borehole and was taken at a depth of between -935.14 m and -935.45m. The borehole from
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Schlattingen is located at a distance of ca. 70 km north-northwest of the geological siting
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region Zurich northeast, a potential site for hosting a repository for radioactive waste in
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Switzerland.50 The OPA sample represents a homogeneous clay facies and has a clay content
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of 69 %. The detailed mineral composition, as well as selected sample properties, such as
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cation exchange capacity, total porosity and bulk dry density are given in Table 1. The
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second sample used in this investigation is a Helvetic Marl (HM) sample from a borehole in
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the region Wellenberg at a depth of -475.86 m. The region Wellenberg was proposed as
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potential site for a waste repository for low- and intermediate-level waste in Switzerland,50
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but finally disqualified51. Unlike OPA, HM has a lower clay content of only 26 %. The
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detailed properties of the HM sample are also given in Table 1. The comparison of the two
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samples is interesting because of the significant differences in mineral composition, total
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porosity, cation exchange capacity and bulk dry density.
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Table 1: Mineral composition and physico-chemical properties of Opalinus Clay and Helvetic Marl samples used in this work.
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Parameters Sample 3
Grain density (kg/dm ) 3
Bulk dry density (kg/dm )
Opalinus Clay (OPA)
Helvetic Marl (HM)
SLA -936.25
WLB SB4a/v -475.86
2.70 ±0.0023
2.73 ±0.0013
2.46 ±0.029
2.66 ±0.027
8.89 ±1.18
2.56 ±1.04
1
Total porosity (%)
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CEC (meq/kg sample)
105±0.5
56 ±0.2
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Mineralogy (wt.%)
31.0 ±3
74.0 ±3
Calcite
6.0
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Dolomite
