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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
MAVAGorislaan 491820 SteenokkerzeelBelgium
www.mava.be
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• 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
Site characteristics
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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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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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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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MinteqA2 modelling
– Cr(III) dominant if thermodynamic equilibrium
– Cr2O3 supersaturated (Saturation-index=12,5)
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MinteqA2 modelling
Cr2O3 will finally precipitate (~ spontaneous attenuation)
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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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Dithionite
• dithionite creates a progressing plume of soluble Fe(II) by reducing
native iron oxides 6Fe(OH)3 + S2O4
2- + 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 (SO4
2) (harmless)
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Remediation concept
Cr(VI)aq
Cr(III)aq
Cr(III)aq
Cr(III) ↓↓↓↓spill
Fe(II)aq
Fe(III) S2O42-
SO42-
MnO2
Mn2+
O2
low pH
complexation
> 1%
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Pilot-scale testIn 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
– available native Fe
– spreading (groundwater model): 4m3 injection volume
• direct push not possible (sandstone layers) →injection via well
0,14 M Na2S2O4 + 0,08 M FeSO4
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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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pilot-scale test
five months of conductivity measurements in downstream well
Cr (VI)
0
20000
40000
60000
80000
100000
120000
140000
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
co
ncen
trati
on
(µ
g/l
)
,
PBM4
instantaneousreduction
groundwater conductivity vs. time
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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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Full scale remediationplan
• 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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Questions?