In-Situ Chemical Reduction (ISCR) of Cr(VI) “A Belgian Case Study”

1. In-Situ Chemical Reduction (ISCR) of Cr(VI)
“A Belgian Case Study”
Samuel Van Herreweghe,
Joris Nackaerts, Wim Vansina, Mark Van Straaten
Consoil 2010, Salzburg, ThS A17
MAVA www.mava.be
Gorislaan 49 www.EnISSA.com
1820 Steenokkerzeel info@mava.be
Belgium
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2. Site characteristics
• Chrome plating facility near
Brussels
• Several overfillings of chromium
baths <1999
• Geology:
– Quaternary loamy toplayer (2m)
– Tertiary Lede-Brussel Sands
(25 m), calcareous, sandstones
– Groundwater at 8m bgl
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3. Project phases
• Characterization study
– plume delineation (monitoring wells)
– risk assessment
– groundwater model
• Geochemical modeling
• Conceptual site/remediation model
• Bench-scale labtest
• Pilot test
• Full scale remediation plan
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4. Characterization Study
• Source zone: <200 m2; 350 mg/l Cr(VI)
• Plume: 4.000 m2; +/- 350 µg/l Cr(VI)
• minor co-pollution with TRI
• Dispersion risk: remediation obligatory
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5. Geochemical context
• Speciation of chromium
– Cr(VI): toxic & mobile
– Cr(III): low toxicity & insoluble (Cr(OH)3•3H2O or Cr2O3)
• MinteqA2 modelling:
– Input:
• general site parameters (pH, alkalinity, major ions, Eh, …)
• Cr(VI) concentration
– Output vs. varying Eh :
• Cr speciation
• Precipitation
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6. MinteqA2 modelling
– Cr(III) dominant if thermodynamic equilibrium
– Cr2O3 supersaturated (Saturation-index=12,5)
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7. MinteqA2 modelling
Cr2O3 will finally precipitate (~ spontaneous attenuation)
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8. Bench-scale lab test
• In-situ redox manipulation (ISRM) is necessary
• Lab test:
– biological
• molasses
• HRC™
– chemical
• zerovalent iron Fe(0)
• dithionite and Fe(II)sulphate
(divalent iron) mixture
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9. Dithionite
• dithionite creates a progressing plume of soluble Fe(II) by reducing
native iron oxides
6Fe(OH)3 + S2O42- + 10H+ 6Fe2+ + 2SO42- + 14H2O
3Fe+2 + CrO4-2 + 8H2O → 4Fe(0,75)Cr(0,25)(OH)3↓ + 4H+
• aquifer low native “available” iron: extra FeSO4 must be added
• optimal pH for Cr(VI) reduction is between 6 and 8 (site pH: ± 7.5)
• In the presence of dithionite, soluble O2 concentration is low:
4Fe2+ + O2 + 2H2O + 8OH- 4Fe(OH)3 (no clogging)
• Dithionite gradually degrades to sulphite (SO32-), thiosulphate
(S2O3-2), … and finally sulphate (SO42) (harmless)
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10. Remediation concept
> 1%
MnO2
O2
low pH Cr(III)aq Mn2+
complexation
Cr(III) ↓ Cr(VI)aq spill
Fe(II)aq SO42-
Cr(III)aq
Fe(III) S2O42-
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11. Pilot-scale test
In situ chemical reduction (ISCR)
Fe(II)-dithionite (FeSO4-Na2S2O4) pilot test set-up
• getting approval of pilot test by local authorities
(OVAM)
• exact composition injection fluid (fine-tuning)
– soil matrix demand 0,14 M Na2S2O4 +
– available native Fe 0,08 M FeSO4
– spreading (groundwater model): 4m3 injection volume
• direct push not possible (sandstone layers) →
injection via well
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12. pilot-scale test
• setting up monitoring scheme
– installing new monitoring wells
– product decay monitored by SO4 / Stot - ratio
– tracer: Br- was added
– continuous conductivity measurement (downstream well)
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13. pilot-scale test
five months of conductivity measurements in downstream well
groundwater conductivity
vs. time instantaneous
reduction
Cr (VI)
,
140000
120000
concentration (µg/l)
100000
PBM4
80000
60000
40000
20000
0
01-12-08 22-12-08 12-01-09 02-02-09 23-02-09 16-03-09 06-04-09 27-04-09
Tijd
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14. Conclusion pilot test
• fast & full reduction of treated zone (<8 weeks after
product arrival)
• Cr(III) precipitation only slightly slower than reduction
(5 weeks slower)
• long term activity of the product (more then 6 months):
large influential radius (>4m)
• no clogging effects
• sandstone banks hindering product spreading
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15. Full scale remediation
plan
• grid of 56 injection wells
• 10 tons Na2S2O4 and 3
tons of FeSO4
• estimated remediation cost
233.275 € (± 20€/m3)
• plan approved by OVAM
• planning synchronised with
relocating plans of the
plating facility (2011)
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16. Questions?
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