U mill tailings in northern Saskatchewan, Canada are alkaline (pH 8 to 10) and often contain elevated concentrations of the elements of concern (EOC) As, Se, Mo and Ni. These EOCs are immobilized within the tailings solids by secondary ferrihydrite (FH). Recent analysis of tailings solids (i.e., neutralized mill raffinates and tailings) also showed the presence of a significant reservoir of a secondary Mg-Al hydrotalcite (HTLC) nano-phase which also has been shown to immobilize EOCs. The bonding via EXAFS of Arsenic on HTLC at the final pH 10 stage of the process in the Key Lake mill samples is also not the same as that found for As-FH at lower pH 4-8 found in the Rabbit Lake mill case. Although the tailings are oxic and have remained so for more than 20 years, concern exists as to impact of the development of anaerobic conditions in the tailings and thus the long-term stability of the EOCs. Research suggests ferrihydrite is unstable under moderately reducing conditions (Eh ~ +100 mV) and may undergo phase transformation resuling in redox active species (e.g., Fe, As, and Se) being released into solution. A series of batch abiotic tests were conducted (7 day and 6 months) to investigate the impact of abiotic (via Fe(II)(aq)) reduction on the sequestered EOCs in neutralized U-mill raffinates and tailings (pH 8 and 10).

1. CONTROLS OF TOXIC ELEMENTS IN ABIOTIC
REDUCTIVE DISSOLUTION OF URANIUM MILL
RAFFINATES AND TAILINGS UNDER ALKALINE
CONDITIONS
Mario Alberto Gomez, Jim Hendry, Joseph Essilfie-Dughan, Jian Chen and Samir
Elouatik
Geological Society of America Annual Meeting
Session 190: Trace Elements and Organics in Environmental and Urban
Geochemistry II
October 21, 2014
Vancouver, Canada

2. 1. Introduction and Motivation
2. Experimental Approach
3. Results and Discussion
4. Conclusions
Outline
2

3. • U-mill Tailings in Northern
Saskatchewan, Canada are alkaline (pH
8 to 10) and often contain elevated
concentrations of the elements of
concern (EOC) As, Se, Mo and Ni.
• These EOCs are immobilized within
the Tailings solid phase with
Ferrihydrite (FH) via lime
neutralization of acidic Raffinate
solution to pH 8 to 10.
• Although the Tailings are oxic,
concern exists as to the impact of
anaerobic conditions on the
immobilization of these EOCs in the
long term.
Introduction and Motivation
3

4. • Research suggests FH and EOC-FH (EOC= As, Mo, Se or Ni) may
be unstable under moderately reducing conditions, and may undergo
phase/redox transformation which can release EOCs into the
aqueous phase.
Hansel et al. Environ. Sci. Technol. 2005, 39, 7147-7153
4
Introduction and Motivation

5. Final Neutralized Deilmann and McArthur
Tailings (FNT)
Final Neutralized Raffinates (FNR)
EOC-Ferrihydrite (Synthetic) 5
Experimental Approach to Understanding the Neutralized
Raffinates and Tailings

6. 6
Previous Rabbit Lake in-pit Tailings EXAFS data
Moldovan et al. Environ. Sci. Technol. 2003, 37, 873-873
Rabbit Lake
Tailings
AsO4-Ferrihydrite
Characteristic
As-Fe bond
As-O
Scorodite

7. 7
Previous Rabbit Lake in-pit Tailings EXAFS data
Moldovan et al. Environ. Sci. Technol. 2003, 37, 873-873
• As bonded only to Fe (i.e. ferrihydrite) via Bidentate bridging.
• Al and Mg were considered as amorpohous/crystalline hydro/oxide
phases

8. Ferrihydrite
(FH)
Mg-Al
Hydrotalcite
(HTLC)
8Gomez et al. Environ. Sci. Technol. 2013, 47, 7883-7891.
Experimental Approach to Understanding the Neutralized
Raffinates and Tailings

9. 9
As K-edge EXAFS Data on the McArthur ore
Key Lake Final Neutralized Raffinate at pH 10 Sample
As-O
bond
Characteristic
As-Fe bond
Key Lake Final Neutralized Raffinate
at pH 10
AsO4 adsorbed Hydrotalcite
AsO4 adsorbed Ferrihydrite

10. Final Neutralized Deilmann and McArthur
Tailings (FNT)
Final Neutralized Raffinates (FNR)
EOC-FH
Mg-Al/Fe-SO4/CO3 HTLC
EOC-MgAlFe SO4CO3 HTLC
10
Experimental Approach to Understanding the Neutralized
Raffinates and Tailings
Gomez et al. RSC Advances.
2013, 3, 25812-22.

11. 11
Sample Origin: Key Lake’s Bulk Neutralization
Process (KBNP)

12. Selected Key Lake Bulk Neutralization Process Samples
Gomez et al. Environ. Sci. Technol. 2013, 47, 7883-7891.

13. 13
As K-edge EXAFS Data on the McArthur ore
Key Lake Final Neutralized Raffinates (pH 10) and process step
samples at pH 4 and 6
Key Lake Final Neutralized Raffinate
at pH 10
Key Lake Bulk Neutralization
Process Sample at pH 6
Key Lake Bulk Neutralization
Process Sample at pH 4
Data collection and fitting done by
Dr. Jinru Lin

14. 14
As K-edge EXAFS Data on the McArthur ore
Key Lake Final Neutralized Raffinates at pH 10 and process
sample at pH 10
Key Lake Final Neutralized Raffinate
at pH 10
AsO4 adsorbed Hydrotalcite
Data collection and fitting done by
Dr. Jinru Lin

15. 15
N2(g)
N2(g
)
Raman
XANES EXAFS
TEM
Experimental Approach to Understanding the Neutralized
Raffinates and Tailings

16. Reductive Dissolution of Synthetic Mg-Al/Fe-
SO4/CO3 Hydrotalcites
16
Surface
Bulk

17. 17
McArthur
Derived
Raffinate
pH 8_10mM Fe(II)(aq) pH 10_0.5mM Fe(II)(aq)
McArthur and
Deilmann
Derived
Tailings
Reductive Dissolution of Neutralized Raffinates and
Tailings

18. Reductive Dissolution of Neutralized Raffinates and
Tailings (pH 8 and 10 mM Fe(II)(aq))
18

19. Reductive Dissolution of Neutralized Raffinates and
Tailings (pH 8 and 10 mM Fe(II)(aq))
19
• In both Mg-Al HTLC and FH phases, the EOCs (e.g. Ni and As)
remain in the solid phases after the 7 day and 6 month
reactions.

20. • Phase transformation of Neutralized Raffinates and Tailings to Green
Rust (stable up to 6 months or longer!). 20
Reductive Dissolution of Neutralized Raffinates and
Tailings (pH 8 and 10 mM Fe(II)(aq))
Fe L-edge Green Rust
Ferrihydrite
Tailings
Raffinate

21. 21
As K-edge EXAFS Data of the Reductive Dissolution of FNR
(pH 8 and 10 mM Fe(II)(aq))
Unreacted Key Lake Final
Neutralized Raffinate at pH 10
Reacted Key Lake Final Neutralized
Raffinate at pH 8 and 7 days
Reacted Key Lake Final Neutralized
Raffinate at pH 8 and 6 months
AsO4 adsorbed Green rust (SO4)
Characteristic
As-Fe

22. 22
Fe(III) Fe(II)
Fe(III) Mg(II) Fe(II)
Reductive Dissolution of Neutralized Raffinates and
Tailings (pH 8 and 10 mM Fe(II)(aq))
• Green Rust formation from both Mg-Al HTLC and FH phases.
• Surface and bulk phase transformations observed.
Mg-Al HTLC FH

23. 23
Amorphous
substrate
Another
distinct
particle=>
Occurring
throughout
sample
Reductive Dissolution of Neutralized Raffinates and
Tailings (pH 8 and 10 mM Fe(II)(aq))
Magnetite Formation

24. 24
Reductive Dissolution of Neutralized Raffinates and
Tailings (pH 10 and 0.5 mM Fe(II)(aq))

25. 25
Characteristic
As-Fe
As K-edge EXAFS Data of the Reductive Dissolution of FNR
(pH 10 and 0.5 mM Fe(II)(aq))
Unreacted Key Lake Final
Neutralized Raffinate at pH 10
Reacted Key Lake Final Neutralized
Raffinate at pH 10 and 6 months
Reacted Key Lake Final Neutralized
Raffinate at pH 10 and 7 days
AsO4 adsorbed Green rust (SO4)

26. Global Conclusions
 Under all conditions tested, none of the Neutralized Raffinates,
nor Tailings released any significant EOCs (≤ 0.1 w.t. %) after
7 days or 6 months.
 No EOCs were released when Green Rust was favored @ pH
8 and 10mM Fe(II)(aq) for all samples tested.
 The lack of EOC release occurred in spite of clear phase
transformations (e.g. Green Rust) and even FH/GR
crystallization to Magnetite.
 The abiotic reduction behavior is combination of both individual
phases but also their common origin. 26

27. Future Work, Acknowledgement and Thanks…..
• Future Work: As K-edge EXAFS modelling and manuscript
write up.
• Sponsors: Cameco Corp and NSERC.
• Collaborators: H. Assaoudi (McGill), G. Ventruti
(Università degli Studi di Bari) F. Nelson, J. Koshinsky, J
Robertson, J. Fan, T. Bonli (U of S), S. Eluatik (U of M), Jian
Chen (NINT), M. Celikin (INRS), SGM/SM (Jay Dynes, Tom
Regier), and HXMA (Ning Chen) CLS staff, L. Ribaud (APS),
J. Maley (SSRC), and J. Zimmer (SRC), D.Li and J. De Yoreo
(LBNL-now PNNL).
• Questions or Comments.
• Thank you. 27

28. Appendix
28

29. 29
Sample Origin: Key Lake’s Bulk Neutralization
Process

30. 30
Deilmann Tailings Management Facility

31. +
As
Mo
Se
Ni
Fe(II)(aq)
Anoxic
???? 31
EOC’s-Hydrotalcite
EOC’s-Ferrihydrite
Experimental Approach to Understanding the Neutralized
U-mill and Tailings

32. • Materials tested:
1) Synthetic : FH, Fe/As ~ 4 and 30, Fe/Mo 4 and 30, Fe/Ni 30, All
duplicates or triplicates only short term (7 days). Mg-Fe(II)/Al(III)-
SO4-CO3 HTLCs.
2) Industrial: Unwashed and washed neutralized Raffinate,
unwashed and washed neutralized McArthur derived Tailings,
unwashed Deilmann derived Tailings at 2 distinct depths. All
duplicates or triplicates short (7 days) and long term (~ 6 months).
• Conditions tested for these samples:
1)pH 10 and 0.5mM Fe(II)(aq)
2)pH 8 and 10mM Fe(II)(aq)
3)pH 8 and 0.5mM Fe(II)(aq)
• Techniques applied: ICP-MS, lab and synchrotron based XRD, ATR-
IR, Raman, NEXAFS and EXAFS, EMP, TEM-EDX and ED.
Fe (II)(aq)concentrations
were chosen at the pH of
interest so to avoid
ferrous hydroxide ppt
32
Experimental Approach to Understanding the Neutralized
Raffinates and Tailings

33. Final Neutralized Deilmann and McArthur
Tailings (unleached residue+FNR)
Final Neutralized Raffinates (FNR) made of
EOC – Ferrihydrite and EOC – MgAlFe SO4CO3
Hydrotalcite
EOC-FH
Mg-Al/Fe-SO4/CO3 HTLC
EOC-MgAlFe SO4CO3 HTLC
33
Experimental Approach to Understanding the
Neutralized Raffinate and Tailings

34.  For the synthetic Fe/X and As-FH cases:
 The pH was always lower (never buffered),
 High As on FH prevented phase transformation, but still released As under all
conditions tested, and low As showed phase transformation and As release and
no re-adsorption of As was observed.
 For Mo (high and low) on FH:
 All showed phase inhibition properties [only @ pH 10 and 0.5mM Fe(II)(aq) ] as
well as phase transformation, but released notable concentrations of Mo into
solution that was not re-adsorbed at the end of 7-day reaction period.
 For Ni on FH:
 Phase transformation occurred in all cases (low or high) in addition to Ni
release into solution, however in all cases, the Ni released was, for the most
part, re-adsorbed back into the solid phase at the end of the 7-day reaction.
Summary: Reductive dissolution behavior of synthetic
X-FH
34

35. 0 2 4 6 8 10 12 14 16
0
1800
3600
5400
0
180
360
540
0
180
360
540
0
180
360
540
160
320
480
640
0 2 4 6 8 10 12 14 16
(003)
(006)
(009)
(113)(110)
Two theta ( =0.41A)
Mg-Al HTLC
(OO3)
Washed Raffinate (with Ba)
(OO3)
Washed Raffinate (No Ba)
Reacted Raffinate (No Ba) pH 8 and 10mM
Reacted Raffinate (No Ba) pH 10 and0.5mM
Reductive dissolution behavior of neutralized
Raffinates (pH 8, 7 days, 10 mM Fe(II)(aq))
1000 1500 2000 2500 3000 3500
1000 1500 2000 2500 3000 3500
OH/H2
O
CO3
SO4
Unreacted Reacted Raffinate with Ba
Wavenumber (cm-1)
OH/H2
O
H2
O
Reacted Raffinate (with Ba) pH 8 and 10mM
CO3
SO4
OH/H2
O
H2
OSO4
GR (SO4
)
OH/H2
O
H2
OCO3
SO4
Reacted Raffinate (No Ba) pH 8 and 10mM
OH/H2
O
H2
O
CO3
SO4
Unreacted Reacted Raffinate No Ba
800 900 1000 1100 1200
800 900 1000 1100 1200
CO3SO4
Wavenumbers (cm-1)
Unreacted McArthur derived Raffinate
Raman spectra = Mg-Al HTLC
SO4
CO3
Reacted McArthur derived Raffinate at pH 8 and 10mM
SO4
CO3
Oxidized McArthur derived Raffinate at pH 8 and 10mM
Green Rust (SO4)
SO4
35

36. Unreacted McArthur ore derived Key Lake neutralized
Raffinates
• The FH is doped with Mg and Al.
• Conversely, the Mg-Al Hydrotalcite was doped with Fe.
• Both are also highly doped with Si and S (as SO4). 36

37. Reductive dissolution behavior of neutralized
Raffinates (pH 8, 7 days, 10 mM Fe(II)(aq))
• Both Mg-Al Hydrotalcite and FH are still present along with
the corresponding elements including S (as SO4) and Si.
Mg-Al
Hydrotalcite
FH
37

38. FH FH
Hydrotalcite
38
Hydrotalcite
Unreacted Neutralized Raffinates

39. Mg-Al HTLC
New Hex
phase-
GR is Hex
Globular
FH
39
Reductive Dissolution of Neutralized Raffinates and
Tailings (pH 8 and 10 mM Fe(II)(aq))

40. Tailings 40
Raffinate
Reductive Dissolution of Neutralized Raffinates and
Tailings (pH 8 and 0.5 mM Fe(II)(aq))

41. Reductive dissolution behavior of neutralized mill and
Tailings (pH 8 and 0.5 mM Fe(II)(aq))
41

42. Reductive dissolution behavior of neutralized mill and
Tailings (pH 10 and 0.5 mM Fe(II)(aq))
42

43. pH 8-10
[Fe+2]
pH 8 pH 10 pH 10 OriginalOriginal pH 8 Literature
Raffinate and Tailings component transform to
carbonated- sulfate GR (2012-2013)
43
Mo
Se
Ni
EOC’s-
Ferrihydrite
As
EOC’s-Hydrotalcite
+
Green Rust Magnetite
Time

44. National Institute of Nano-Technology (NINT)
Collaboration
44

45. University of Montreal Collaboration
45

46. Canadian Light Source-SGM, HXMA and SM
Collaboration
46*** Dr. Jinru Lin, HXMA.

47. Reduction solutions preparation
– Preparation of the reacting solutions was conducted (Fe+2
and lime as base) prior to conducting reactions.
– This was important to investigate as it allowed us to
observe the [Fe+2] range that we could use before the
precipitation of any phases in the reacting solution (e.g.
FeOH2, green rust, siderite).
– @ pH 7-8, [Fe+2] ≥ 10mM maybe used.
pH ~
4
pH = 7- pH = 8.5 pH = 10 47

48. – For the target pH of 10, [Fe+2] ≤ 0.5mM may only
be used because the presence of precipitates in
the solution was observed.
10 mM 0.85 mM 0.75
mM
0.5 mM
0.75
mM
0.5 mM
Reduction solutions preparation
48