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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

www.EnISSA.cominfo@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?