Developing Predictive Reactive Transport Models of 99 Tc at Hanford and 137 Cs at Fukushima Using Molecular-Level Spectroscopy and Simulation

Environmental Dynamics and Exposure Pathways of Subsurface Contaminants Developing Predictive Reactive Transport Models of 99 Tc at Hanford and 137 Cs at Fukushima Using Molecular-Level Spectroscopy and Simulation Kevin M. Rosso Pacific Northwest National Laboratory, Richland, Washington, USA Fe(II) Fe(III) Fe(II) Fe(III) Tc(IV) Tc(VII) March 2, 2018

Presentation Outline and Key Messages The subsurface environment is chemically and physically complex, in only rare cases can contaminant reactive transport models truly reliably predictive. Molecular-scale techniques are at the frontier of robust model development. Synchrotron light sources have enabled detailed mechanistic insights 99 Tc at Hanford Molecular simulation and HP computing facilities are an essential complement 137 Cs at Fukushima Striking the best balance between detailed chemical process understanding and useful working models remains subject to practical considerations.

DOE s Hanford Site Iconic Remediation Challenges Produced 67 metric tons of Pu for weapons (1943-1989) >1000 hazardous contaminant plumes (Pu, 137 Cs, 90 Sr, U, 99 Tc) 53 million gallons of HLW stored in 177 huge underground tanks with 67 leakers ( 137 Cs, 90 Sr, U, 99 Tc, Cr) >200-square miles of contaminated groundwater migrating to the Columbia River

1960 Today The Columbia River Driving Risk-Based Decision Making

Highest Mobility Contaminants a Primary Concern

99 Tc Mobility in the Vadose Zone is Poorly Predictable Beta-emitter with t 1/2 ~ 213,000 yrs ~700 Ci released into the Hanford subsurface Redox active: Two stable oxidation states Tc(VII) is a highly soluble and mobile aqueous oxyanion Tc(IV) is sparingly soluble - precipitates as TcO 2 (s) Natural and/or engineered reductive immobilization is a key strategy H 2 O H 2 O HO HO Tc(IV) Tc(IV) OH OH H 2 O H 2 O Tc(VII)O 4- + 4H + + 3e - = Tc(IV)O 2 2H 2 O(s) Pourbaix diagram of [Tc] = 10 5 M (Yalcintas 2015, PhD thesis)

At Depths Where O 2 Recharge is Limited Will Reductive Immobilization Happen Naturally? H 2 O H 2 O HO HO Tc(IV) Tc(IV) OH OH H 2 O H 2 O Tc(VII)O 4- + 4H + + 3e - = Tc(IV)O 2 2H 2 O(s)

Hanford Sediments Contains Ferrous Iron Fine Coarse X-ray fluorescence microprobe analyses of basaltic lithic fragments from the Hanford plateau show crystallites of Fe and Ti Ti Fe Liu, Pearce, Qafoku, Arenholz, Heald, Rosso (2012) Geochim Cosmochim Acta 92, 67-81. Pearce, Qafoku, Liu, Arenholz, Heald, Kukkadapu, Gorski, Henderson, Rosso (2012) J Coll Int Sci 387, 24-38. Pearce, Baer, Qafoku, Heald, Arenholz, Grosz, McKinley, Resch, Bowden, Engelhard, Rosso (2014) Geochim Cosmochim Acta 128, 114-127.

Is this Ferrous Iron Reactive with Tc(VII)? Hanford Fe 3-x Ti x O 4 3Fe 2+ + Tc(VII)O 4 + 4H + 3Fe 3+ + Tc(IV)O 2 + 2H 2 O Magnetite (x = 0.0) Fe 3+ Tet(Fe 2+, Fe 3+ ) Oct O 4 (substitution & reduction) Fe 2+ Tet(Fe 2+, Ti 4+ ) Oct O 4 Ulvöspinel (x = 1.0) Pearce, Qafoku, Liu, Arenholz, Heald, Kukkadapu, Gorski, Henderson, Rosso (2012) J Coll Int Sci 387, 24-38.

Tc(VII) Reduction by Model Ti-Magnetite Nanoparticles is Fast Synthetic Fe 3-x Ti x O 4 3Fe 2+ + TcO 4 + 4H + 3Fe 3+ + TcO 2 + 2H 2 O Liu, Pearce, Qafoku, Arenholz, Heald, Rosso (2012) Geochim Cosmochim Acta 92, 67-81.

Synchrotron X-ray Absorption Spectroscopies a Powerful Tool Interfacial Redox Chemistry H Courtesy Elke Arenholz (ALS)

Courtesy Elke Arenholz (ALS) BL4.0.2 at the Advanced Light Source

Fe L-edge XAS/XMCD Spectra of Magnetite Nanoparticles A Fe 3 O 4 (x = 0)

Fe L-edge XAS/XMCD Spectra of Magnetite Nanoparticles B Fe 3 O 4 (x = 0)

XMCD Enables Quantification of Magnetite Fe(II) Reaction with Tc(VII) XMCD of Fe 2.65 Ti 0.35 O 4 Interaction with Tc(VII) Tet Fe 3+ Intensity (arb. units) 0.0 Loss of Oct Fe 2+ Oct Fe 3+ -- Before -- After 700 705 710 715 720 725 730 Photon energy (ev) Liu, Pearce, Qafoku, Arenholz, Heald, Rosso (2012) Geochimica et Cosmochimica Acta 92, 67-81.

Synchrotron Spectroscopies Enable Mechanistic Insight Fe(II) electrons are mobile

Synchrotron Spectroscopies Enable Mechanistic Insight Fe(II) enriches at surface Fe(II) Fe(III) Fe(II) electrons are mobile Fe(II) Fe(III) H +

Synchrotron Spectroscopies Enable Mechanistic Insight Fe(II) Fe(III) Fe(II) electrons are mobile Fe(II) enriches at surface Protons release Fe(II) Fe(II) Fe(III) Fe(II)

Synchrotron Spectroscopies Enable Mechanistic Insight Fe(II) Fe(III) Fe(II) electrons are mobile Fe(II) enriches at surface Protons release Fe(II) Fe(II) Fe(III) Tc(VII)

Synchrotron Spectroscopies Enable Mechanistic Insight Fe(II) Fe(III) Fe(II) Fe(III) Fe(II) electrons are mobile Fe(II) enriches at surface Protons release Fe(II) Tc(IV) e- supplied by bulk Tc(VII)

Synchrotron Spectroscopies Enable Mechanistic Insight Fe(II) Fe(III) Fe(II) Fe(III) Fe(II) electrons are mobile Fe(II) enriches at surface Protons release Fe(II) Tc(IV) / Fe(III) e- supplied by bulk Tc(VII) Tc(IV) / Fe(III) 1:1

Synchrotron Spectroscopies Enable Mechanistic Insight Fe(II) Fe(III) Fe(II) Fe(II) electrons are mobile Fe(II) enriches at surface Protons release Fe(II) Tc(IV) / Fe(III) e- supplied by bulk Tc(VII) Tc(IV) / Fe(III) 1:1 Fe(III) Vacancies enter 1:3

Synchrotron Spectroscopies Enable Mechanistic Insight Fe(II) Fe(III) Fe(II) Fe(II) electrons are mobile Fe(II) enriches at surface Protons release Fe(II) Tc(IV) / Fe(III) e- supplied by bulk Tc(VII) Tc(IV) / Fe(III) 1:1 Fe(III) Vacancies enter 1:3 Extent depends on x and y and surface condition

At Depths Where O 2 Recharge is Limited Will Reductive Immobilization Happen Naturally? 1 g of Hanford Ti-magnetites can immobilize 99 Tc(VII) at a rate of 0.15 μm / day Fe(III) H 2 O H 2 O + HO HO Tc(IV) Tc(IV) Fe(III) H 2 O H 2 O Fe(III) 3Fe 2+ + TcO 4 + 4H + 3Fe 3+ + TcO 2 + 2H 2 O Liu, Pearce, Qafoku, Arenholz, Heald, Rosso (2012) Geochim Cosmochim Acta 92, 67-81. Pearce, Qafoku, Liu, Arenholz, Heald, Kukkadapu, Gorski, Henderson, Rosso (2012) J Coll Int Sci 387, 24-38. Pearce, Baer, Qafoku, Heald, Arenholz, Grosz, McKinley, Resch, Bowden, Engelhard, Rosso (2014) Geochim Cosmochim Acta 128, 114-127.

March 11, 2011 The Fukushima Dai-ichi Nuclear Accident Japan M9.0 Earthquake Center Fukushima Dai-ichi NPP 50 km Courtesy Masahiko Okumura (JAEA) Fukushima Prefecture

March 11, 2011 The Fukushima Dai-ichi Nuclear Accident Maximum height of the tsunami was 19m! Courtesy Masahiko Okumura (JAEA)

Courtesy Masahiko Okumura (JAEA)

Massive Amounts of 137 Cs/ 134 Cs Aerially Released Air dose rate [μsv/h] (1m height) Courtesy Masahiko Okumura (JAEA)

Contaminated Soil Management is a Challenge Typical temporary storage site for waste soil. Material of this type is planned to be transported, processed, and stored for decades. Courtesy Masahiko Okumura (JAEA)

Contaminated Soil Management is a Challenge Total Waste Soil Awaiting Final Disposition is ~2 10 7 m 3 Typical temporary storage site for waste soil. Material of this type is planned to be transported, processed, and stored for decades. Courtesy Masahiko Okumura (JAEA)

What is the Ultimate Fate of Radiocesium in Native and Waste Soils at Fukushima? Selective Cs + exchange into the interlayer of clay minerals occurs as meteoric input transports exchangeable Cs through soils into regions of unsaturated high-affinity sites. McKinley et al (2004) ES&T 38, 1017. Okumura et al. (2013) J. Phys. Soc. Japan 82, 033802. The exchange process is largely thought to be irreversible and permanent

Density Functional Theory (DFT) Models Can Correctly Describe Single Site Exchange Musc(K) + Cs + (aq) Musc(Cs) + K + (aq) Cesium adsorption 27kJ/mol (*Exp. 23.5) * E. Brouwer et al., J. Phys. Chem. 87, 1213 (1983). Okumura, Nakamura, Machida (2013) Journal of the Physical Society of Japan 82, 033802.

Knowledge Gaps About Long Term Retention Although high-affinity 137 Cs + binding to phyllosilicates is well-known, the long term durability of this association is not: Cs shows a tendency to concentrate by replacing whole interlayers Consequences of progressive radiation damage on structural integrity unknown Structural accommodation of 137 Cs + transmuted to 137 Ba 2+ unknown T. Okumura et al. (2014) Microscopy, 63,65. Activity (Bq/g) 1400 100 1200 1000 80 800 60 600 40 400 200 20 0 0 50 100 150 200 250 300 350 0 400 Time (years) % transmuted Cs

Radiocesium Transmutation Effects on Long- Term Retention is Accessible to Modeling More than Experiment Radioactive isotope Stable daughter isotope High KE electron = stochastic energy losses to ionization, local heating, Bremsstrahlung, and defect creation Anti-neutrino Sassi, Okumura, Machida, Rosso (2017) Physical Chemistry Chemical Physics, 19, 27007.

Radiocesium Transmutation Effects on Long- Term Retention is Accessible to Modeling More than Experiment Total energy of the supercell with the defect D in a charge state q Finite-size correction for charged defect supercell (FNV scheme) Total energy of the perfect supercell Chemical potential (μ i ) of removed or added atoms Chemical potential of added or removed electrons E f [D] for substitutions K + /Cs + Exchange 21.96 kj/mol Ph(K) Ph(Cs K+ ) Cs + /Ba 2+ Transmutation Ph(Ba K 2+ ) 406.2 kj/mol Sassi, Okumura, Machida, Rosso (2017) Physical Chemistry Chemical Physics, 19, 27007.

Modeling Predicts Progressive Weakening of 137 Cs Retention Over Time The large destabilization energy associated with daughter Ba 2+ generation is only partly neutralized by defect formation: Defect formation energy (kj/mol) 0-100 -200-300 -400 Ba K 2+ Ph(Cs K+ ) Ba K 2+ + V K - Ba K 2+ + V H - Ba K 2+ + Al Si 4+ (-138.5) (-191.2) (-357.3) Sassi, Okumura, Machida, Rosso (2017) Physical Chemistry Chemical Physics, 19, 27007.

Modeling Predicts Progressive Weakening of 137 Cs Retention Over Time Now examining the choice of Cs + or K + vacancy creation, including the contribution of cation hydration: Reaction E f (ev) E f (kj/mol) End-state Ph(Ba+K) Ph(Ba 2+ + V - K ) + K + (aq) -1.194-115.196 K + goes into water (μ K+(aq) ) Ph(Ba+Cs) Ph(Ba 2+ + V - Cs ) + Cs + (aq) -1.484-143.199 Cs + goes into water (μ Cs+(aq) ) Defect formation energy (kj/mol) 0-50 -100-150 Ba K 2+ Ba 2+ K + V - K (-115.2) Ba 2+ K + V - Cs (-143.2) 137 Cs transmutation should primarily weaken residual 137 Cs retention because ejecting Cs + to solution is ~0.3 ev more favorable than ejecting K + Sassi, Okumura, Machida, Rosso (2017) Physical Chemistry Chemical Physics, 19, 27007.

Implications and Outlook Molecular-scale experimental and theoretical techniques are at the frontier of robust reactive transport model development. Such techniques have provided useful input for decision-making. Incorporation of detailed chemical process information into models is less of a technical challenge than it is subject to the pressures of finding practical solutions.