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| Titel |
Diffusion, sorption, and retardation processes of anions in bentonite and organo-bentonites for multibarrier systems |
| VerfasserIn |
Birgit Schampera, Stefan Dultz |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250079203
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| Zusammenfassung |
| The low permeability, high cation exchange capacity (CEC) and plasticity of bentonites favor
their use in multibarrier systems of waste deposits [1]. Bentonites have a high CEC
but their ability to sorb anions is very low. There is, however, need for retardation
of anions and organic pollutants in many applications. Bentonites, modified with
certain organic cations, have the capacity to sorb anions and non-polar organic
compounds in addition to cations. Investigations on organically modified clays
address a wide variety of applications including immobilization of pollutants in
contaminated soils, waste water treatment and in situ placement for the protection of ground
water [2]. Many experiments on anion and cation sorption of organo-clays were
conducted in the batch mode which does not reflect solid-liquid ratios and material
densities in barrier systems. Diffusion experiments on compacted clays allow the
evaluation of transport processes and sorption of pollutants at conditions relevant
for repositories. For organo-clays only few diffusion studies are published e.g. [3]
measured the diffusion of tritium and [4] the diffusion of H2O in bentonite and
organo-bentonites.
The organic cation hexadecylpyridinium (HDPy) was added to Wyoming bentonite
(MX-80) in amounts corresponding to 2-400 % of the CEC. The uptake of organic cations
was determined by the C-content, XRD and IR-spectroscopy. Wettability was analyzed by the
contact angle. Physical, chemical and mineralogical properties of clays were characterized.
Diffusion experiments were carried out in situ in a cell attached to the ATR-unit of a
FTIR-spectrometer. For H2O-diffusion the compacted organo-clays are saturated first with
D2O, afterwards H2O is supplied to the surface at the top of the clay platelet. Anion-diffusion
was conducted with NO3--solution instead of H2O only having characteristic IR band
positions at 1350 cm-1. Three different concentrations (0.25M, 0.5M and 1M) were used.
Additional batch experiments with NO3- will support the understanding of sorption behavior
of the anions.
All hydrophilic samples have a higher retardation capacity, indicated by diffusion
coefficients of 2.44 x 10-11 m/s2 for original bentonite and -¤2.1 x 10-11m/s2 for
hydrophilic organo-clays. For hydrophobic organo-clays the H2O diffusion can be
higher and is increased at high bulk density (1-1.5 g/m3) up to 2.76 x 10-10m2/s.
Experiments with NO3- at bulk density of 1.5 g/m3 reveal that the apparent diffusion
coefficients of nitrate are with results up to 5.61 x 1012 m2/s distinctively lower than free
diffusion of nitrate in pure water (6.46 x 1010 m2/s at experimental conditions) and
nitrate diffusion in natural bentonite (2.63 x 1011Â m2/s). The measurements allow
the interpretation of the different sorption mechanisms, retardation capacity and
diffusion behavior of the analyzed clays at different anion concentrations. Ongoing
molecular dynamic simulations will contribute understanding of diffusion processes in
organo-clays including the conditions at the interface of the clay minerals and in
solution.
References:
[1] Shackelford, C.D., Moore S.M. (2013) Fickian diffusion of radionuclides for
engineered containment barriers: Diffusion coefficients, porosities, and complicating issues.
Engineering Geology, 152, 133–147.
[2] Rytwo, G., Nir, S., Shuali, U. (2012) Clay and water treatment. Applied Clay Science,
67–68, 117–118.
[3] Lorenzetti, R.L., Bartelt-Hunt, S.L., Burns, S.E., Smith, J.A. (2005) Hydraulic
conductivities and effective diffusion coefficients of geosynthetic clay liners with
organobentonite amendments. Geotextiles and Geomembranes, 23, 385-400.
[4] Schampera, B., Dultz, S. (2011) H2O self-diffusion in compacted clays as influenced
by surface charge and wettability – obstruction effects of bound H2O layers. Clay and Clay
Minerals,59, 42-57. |
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