Adsorption and Transport of Naturally Occurring Radioactive Materials (NORMs) in Clays Atomistic Computational Modeling Approach

Transcription

1 Adsorption and Transport of Naturally Occurring Radioactive Materials (NORMs) in Clays Atomistic Computational Modeling Approach Andrey G. Kalinichev, Brice Ngouana, Iuliia Androniuk Laboratoire SUBATECH, IMT-Atlantique, Nantes, FRANCE 50 nm 1

2 Backgroung and Introduction Shale formations are characterized by a small porosity and low permeability* that trap hydrocarbons in the rocks. Hydraulic fracturing increases the extremely low permeability of shale rocks to enable the economic production of shale gas and shale oil**. - Water What is the fate of NORMs in the environment? in Fracfluid: - Proppant (sand) - Chemicals Schematic representation of shale gas extraction*** Flowback fluid: - Water - Chemicals - Gas/oil Atomistic computational modeling of fluid-rock interactions * Ho TA, Striolo A. AIChE Journal, 61, 2993 (2015) ** Yethiraj A, Striolo A. J. Phys. Chem. Lett., 4, 687 (2013) *** - Naturally occurring radioactive materials (NORMs) The are several environmental issues related to shale gas exploration/extraction, including NOMRs and their fate as they could be released in the process out 2

3 Mineral Composition of Some European Shales Clay Pyrite Feldspar Quartz Carbonates Organic matter Bowland shale (UK) Alum shale (Denmark)* Dotternhausen shale (Germany)* Wickensen shale (Germany)* Harderode shale (Germany)* Haddessen shale (Germany)* Clay minerals are one of the primary components of most shales Because of their adsorption and retention properties, they can play a crucial role in the confinement of NORMs that could be released in the context of fracking * Rybacki et al. J. Petrol. Sci. Eng., 135, 702 (2015) 3

4 Basic Structure and Composition of Clay Minerals Clay minerals are layered aluminosilicates, with properties changing as a function of their atomic structure and composition Clay fractions of shales predominately contain kaolinite, smectite, and illite Montmorillonite (smectite) - moderate layer charge Kaolinite - no layer charge Muscovite (illite) high layer charge O T T O T Si 8 Al 8 O 20 (OH) 16 (Si 7.75 Al 0.25 )Al 3.5 Mg 0.5 O 20 (OH) 4 Na 0.75 (Si 6 Al 2 )Al 4 O 20 (OH) 4 K 2 4

5 Principal Tool: Molecular Dynamics Simulations Numerically solve Newtonian equation of motion for N interacting particles: ri(t+ t) = ri(t) + vi(t) t + ½ ai(t) t2 ; t ~ 1 fs = s ai = Fi/m = [ - U(r1,r2,... rn) / ri ] / m ; i=1, 2,, N (N ~ atoms) U = SSUij = SS(Aij/rij12 - Bij/rij6 + qiqj /e0rij) + S ½kb (rij - r0)2 + S ½kq (qij - q0)2 Short-range repulsion v-d-waals Coulombic bond stretching bond bending Time averaging over a dynamic trajectory of the simulated system Periodic boundary conditions (PBC) 5

6 MD Modeling of Clays and Clay-Related Materials Non-trivial problems 1 m Poorly characterized structure and composition Low symmetry, high degree of compositional disorder Large unit cells, stacking disorder Strong local electrostatic fields due to site substitutions Need for special force-field parameterization for realistic modeling ClayFF - specialized semi-empirical fully flexible force field model Allows for realistic modeling of exchange of momentum and energy among all atoms solid substrate and aqueous solution Based on accurate theoretical models of oxides, hydroxides, silicates, etc. Combines well with other available force fields for organic-inorganic systems Cygan, Liang, Kalinichev (2004) J.Phys.Chem. B, 108, Special focus: real-life complexity of shale rock mineral components on the atomic scale 6

7 Partial Charges based on ClayFF Parameterization Accurate determinations of partial charges are required to represent charge distributions of interlayer and external surfaces where electrostatic forces control sorption and transport processes Atomic charges derived from DFT/GGA calculations for cluster and periodic models of simple oxides and hydroxide phases Allow for charge delocalization among coordinating oxygens for substitutions T Al Al Si Mg O T Cygan, Liang, Kalinichev (2004) J.Phys.Chem. B, 108,

8 MD Modeling of Clay-Solution Interfaces Aqueous solution Classical Newtonian dynamics Ntot ~ 3,000 10,000 atoms; NH2O ~ 0-1,000 mol ClayFF force field (Cygan et al., 2004) a b c ~ nm3 Periodic boundary conditions NVT- or NPT-ensemble T=300K; P =1 bar ~7nm Dt = fs; ttotal ~ ns ~10nm Confined fuid structure: Atomic density profiles ( ) Atomic density surface distributions ( ) Ion adsorption sites Interfacial H-bonding network Clay layers ~3nm Confined fluid dynamics: Diffusion coefficients (longer time scale) Spectra of vibrational and rotational dynamics (shorter time scale) 8

9 Clay Particle Edges: Special Adsorption Sites Aggregate of clay particles Basal (001) surface Interlayer pore Basal (001) surfaces and interlayers are extensively studied and their properties are reasonably well known Clay edges have received much less attention yet Edge surfaces Primary clay particle Ab initio (quantum) MD is a direct answer, but it is very expensive computationally AIMD ~n 100 atoms; ~ Å3; t ~ ps S.V.Churakov, Geochim. Cosmochim. Acta, 71, (2007) X. Liu et al., Geochim. Cosmochim. Acta (2012, 2013, 2014, 2015) S. Tazi et al., Geochim. Cosmochim. Acta, (2012) Inter-particle pore ClayFF Parametrization for Clay Edges New special ClayFF bending terms for Mg-O-H, Al-O-H, and Si-O-H UClayFF-MOH = UClayFF-orig + UM-O-H = = UClayFF-orig + k (q -q0 )² k and q0 have to minimize the differences between DFT and ClayFF-MOH results Pouvreau, Greathouse, Cygan, Kalinichev, J.Phys.Chem.C, 2017, 121, ; J.Phys.Chem.C, 2018 (in preparation) 9

10 Interlayers and (001) Surfaces of Charged Clays Fully octahedral 1/3 tetrahedral Fully tetrahedral and 2/3 octahedral Montmorillonite Layer charge e T = 298 K Muscovite Layer charge -2.0 e T = 363 K Diffusion coefficient m²/s D2w (Sr2+) D2w (Ba2+) D3w (Sr2+) 1.79 D3w (Ba2+) 3.09 FF parameters for Ra2+ have been developed and are currently tested B.F.Ngouana-Wakou, A.G.Kalinichev, A. G. (2014) Structural arrangements of substitutions in smectites: Molecular simulation of the swelling properties, interlayer structure, and dynamics of hydrated Cs-montmorillonite revisited with new clay models. J.Phys.Chem.C, 118,

11 Ion Mobility at Basal (001) Surfaces of Clays Diffusion coefficient ( 10-9 m²/s) from MD simulation Ion Interface Bulk Kaolinite Montmorillonite Muscovite Ba ± ± ± ± 0.2 Na+ 0.5 ± ± ± ± 0.4 Water 1.8 ± ± ± ± 0.1 Increasing layer charge Diffusion coefficient ( 10-9 m²/s) from MD simulation Ion Interface Bulk Kaolinite Montmorillonite Muscovite Sr ± ± ± ± 0.1 Na+ 1.2 ± ± ± ± 0.4 Water 2.2 ± ± ± ±

12 Montmorillonite (010) Edge Surfaces Montmorillonite: 96 ([Si7.75Al0.25](Al3.5Mg0.5)O20(OH)4.5(H2O)10Na0.75) 45 Å 41 Å 81 Å substitutions sites are accessible to solution (surface 1) Substitution sites are not accessible to solution (surface 2) Simplified fluid composition 1480 H2O 6 NaCl ( 0.1M) 4 SrCl2/BaCl2 ( 0.07M) 12

13 Atomic Density Profiles: Montmorillonite Substitutions sites are accessible to solution (surface 1) Substitution sites are not accessible to solution (surface 2) Cation exchange between interfacial Ba2+ and interlayer Na+ ions is quite strong 75% of Ba2+ ions initially in the interfacial region enter the interlayers 13

14 Atomic Density Profiles: Montmorillonite Surface 1 Surface 2 The 25% of Ba2+ ions remaining in the interfacial region are in a outer sphere coordination Most of the Na+ ions leaving the interlayer region accumulate near surface 1, preventing Sr2+/Ba2+ ions to closely approach that surface Two Na+ peaks on surface 1: the 1st around 2.0 Å and the 2nd around 3.0 Å Two types of inner sphere adsorption configurations 14

15 Atomic Density Surface Maps: Montmorillonite Surface 1 Surface 2 Si Al T O T Mg Ooh 1st peak Owa t Na+ 2nd peak Na+ ions in the 1st peak are located within the range of x coordinates corresponding to the edge surface atoms in the octahedral layer Na+ ions in the 2nd peak are located within the range of x coordinates corresponding to the edge surface atoms in the tetrahedral layer 15

16 Ion Coordination to Surface Atoms: Montmorillonite Surface 1 Surface 2 1st peak Na+ Na+ Na+ owat oh obts ob distance (Å) Coordination number Na+ ions in the first peak occupy octahedral vacancies On surface 1, they avoid the Mg/Al substitutions but seat near the Al/Si substitutions (CN 6) owat oh obos distance (Å) Coordination number On surface 2, Na+ ions are mostly found near the internal Mg/Al substitutions (CN 6) 16

17 Ion Coordination to Surface Atoms: Montmorillonite 2nd peak Surface 1 owat Surface 2 oh owat distance (Å) 2.3 Na distance (Å) 2.3 Coordination number oh 2.3 Coordination number Na+ ions in the 2nd peak are found close to the Si-O-H groups (CN 6)

18 Muscovite (010) Edge Surfaces Muscovite: 96 ([Si6Al2][Al4]O19.5(OH)5(H2O)0.5K2) 29 Å 41 Å 103 Å substitutions sites are accessible to solution (surface 1) Substitution sites are not accessible to solution (surface 2) Simplified fluid composition 1480 H2O 6 NaCl ( 0.1M) 4 SrCl2/BaCl2 ( 0.07M) 18

19 Atomic Density Profiles: Muscovite Surface 1 Ba2+ Surface 2 Most of the K+ ions leaving the interlayer region accumulate near surface 1 This prevents Na+ and Ba2+ to closely approach that surface The smaller concentration of K+ ions on surface 2 allows Na+ and Ba2+ to come closer, but Ba2+ is still in OS coordination 19

20 Atomic Density Profiles: Muscovite Surface 1 Sr2+ Surface 2 All K+ ions leaving the interlayer region accumulate near surface 1, this prevents Na+ and Sr2+ to closely approach that surface Hence Na+ and Sr2+ are found near surface 2, in both IS (peak around 2.5 Å) and OS (peak around 3.5 Å) coordination 20

21 Atomic Density Surface Maps: Muscovite Surface 1 Surface 2 Si T O T Al Ooh 1st peak Owa t Na+ Sr2+ K+ 2nd peak Two types of adsorption sites are observed for Sr2+ on surface 2 Similar adsorption sites for Na+ and K+ 21

22 Ion Coordination to Surface Atoms: Montmorillonite Surface 2 Surface 1 1st peak Sr2+ and K+ ions in the first + K owat oh obts ob distance (Å) peak occupy octahedral vacancies, with a little shift towards the tetrahedral layer where they bind to a Si-O-H group owat oh ob distance (Å) Coordination number Coordination number 2.1 Sr2+ Effects of ionic size, and hydration energy? CN (K+) 6.5 CN (Na+) 5.5 CN (Sr2+)

23 Kaolinite (010) Edge Surfaces Kaolinite: 64 (Si8Al8O19.5(OH)17(H2O) 28 Å 41 Å 108 Å Simplified fluid composition 1480 H2O 6 NaCl ( 0.1M) 4 SrCl2/BaCl2 ( 0.07M) 23

24 Atomic Density Profiles: Kaolinite The first Ba2+/Sr2+ peak is in the range 5-6 Å from the surface (around 2.5 Å for muscovite when there is no screening effects of K+ ions) Inner-sphere complexation is observed for Na+ (1st peak around 3 Å) H (edge OH O (edge OH) Na+ x (Å) y (Å) Similar types of adsorption sites as presented for montmorillonite and muscovite 24

25 Next Step: Site-Specific Adsorption Free Energy Profiles and Cation Exchange Equilibria Potential of Mean Force: K+ PMF W ( z ) k B T ln ( z ) const 1a (opposite) 1b (nearby) 1c (single) z - distance from the surface 1b 1c 2a outersphere 2b 1a innersphere 2c Cs+ 1a (opposite) 1b (nearby) 1c (single) N. Loganathan, A.G.Kalinichev (2017) J. Phys. Chem. C, 121, K+surf + Cs+aq K+aq + Cs+surf 25

26 Conclusions and Outlook Significant cation exchange between interfacial Sr2+/Ba2+ and interlayer Na+ ions in the (010) montmorillonite system Effect of substitution location : On surface 1 (substitutions accessible to solution), the adsorbed ions sit near the Al/Si tetrahedral substitutions but avoid the Mg/Al octahedral substitutions (present in montmorillonite) The Na+ and K+ ions dissociating from the interlayers of montmorillonite and muscovite accumulate on surface 1 pushing then the other ions to surface 2 (substitutions not accessible to solution) On surface 2 (substitutions not accessible to solution), Na+ ions are mostly found near the internal Mg/Al substitutions in montmorillonite Two types of inner sphere complexation with the (010) edges of kaolinite, montmorillonite and muscovite: The cation (Na+/K+/Sr2+/Ba2+) occupy an octahedral vacancy, interacting with 2 Al-O-H groups or 2 Al-O-H groups and 1 Si-OH group The cation (Na+/K+/Sr2+/Ba2+) sit in the tetrahedral layer, interacting with 1 Al-O-H group and 1 Si-O-H group Site-specific adsorption free energy profiles are now being calculated for the identified sites to quantitatively evaluate their adsorption strength and cation exchange capabilities 26

27 Acknowledgments N.Loganathan, M.Pouvreau Subatech, IMT Atlantique, Nantes R.T.Cygan, J.A.Greathouse Sandia National Labs, Albuquerque, USA This project has received funding from the European Union's Horizon 2020 research and innovation program under grant agreement No Supercomputing resources allocation under DARI (projects n x , t , and A on the OCCIGEN cluster of CINES CCIPL computer resources allocation on Waves cluster 27