Can redox manipulations remove dissolved arsenic from groundwater

1
Can redox manipulations remove dissolved arsenic
from groundwater?

• Alison R. Keimowitz1,2, Pamela Cole2,3, Brian J.
  Mailloux2, Martin Stute2,3, H. James Simpson1,2,
  Steven N. Chillrud2, Yan Zheng2,4

1 Columbia University Department of Earth and
Environmental Sciences 2 Lamont-Doherty Earth
Observatory 3 Barnard College 4 Queens College
This work was supported by NIH Grant 1 P42
ES10349, NIEHS Grant P30 ES09089, NSF GRT
Traineeship in Hydrology DGE9554573, United
Technologies Corporation, Columbia Earth
Institute, and the Howard Hughes Medical
Institute.
2
Background

• Arsenic most soluble with Ehlt100 mV
• Release of arsenic to anoxic groundwaters
  frequently reported
• Bangladesh, West Bengal India, Taiwan

After Masscheleyn et al., Environmental Science
and Technology, 1991. Activities of As, Mn, Fe
10-4
3
Winthrop, Maine, USA

• Closed, unlined landfill sited in glacial till
  and outwash
• Leachate has shifted groundwater chemistry more
  reducing (ORP -105 mV)
• Sediments contain 7 mg/kg As
• Sediment reducing GW 300 mg/L As
• Groundwater extraction and treatment
  system ineffective
• Other landfills show similar processes

Lackovic et al., Hazardous Industrial Wastes,
1999. Keimowitz et al., Applied Geochemistry, In
prep.
4
If redox is the problem, can redox be the
solution?
After Masscheleyn et al., EST, 1991
5
ORC Pilot Experiment

• ORC Oxygen Release Compound
• Contains magnesium peroxide, phosphate and minor
  components
• Designed to release O2 over 6 months
• 1400 kg ORC injected through aquifer thickness

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ORC Pilot Experiment
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ORC Pilot Experiment Results
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ORC Pilot Experiment Results
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ORC Pilot Experiment

• 1400 kg ORC released 330 kg O2
• But what is the subsurface oxygen demand?

• BOD
• BOD of water satisfied 100 times
• Only 20 of BOD of sediments met

• COD
• COD of water satisfied 18 times
• Only 0.1 of COD of sediments met

10
Arsenic Removal via Sulfide Precipitation?
After Masscheleyn et al., EST, 1991 and DM
Sherman, U of Bristol, Env. Geochem course
website As, Mn 10-4 M, Fe 10-6 M, S 0.01 M
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Proof of Principle

• Previous studies have demonstrated removal of
  arsenic by SRB
• Sulfate is high at Winthrop due to GWETS
• Preliminary studies indicate that native
  microbial communities at Winthrop are
  carbon-limited

e.g. Rittle, Drever, Colberg, Geomicrobiology
Journal 1994 Jong Parry, Water Research 2003
12
Experimental Setup

• Incubate sediment acetate artificial GW for
  7 weeks in reaction vessel and microcosms
• Microcosms permit sediment sampling, replicate
  samples
• Reaction vessel permits electrode monitoring,
  sampling from 1 reservoir
• Monitor changes in water and sediment

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Results Acetate Respiration, Sulfate Reduction
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Results Metals
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Results pH, ORP
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Explanation of Data?
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Ongoing Research

• Characterization of sediment
• Sequential extractions
• XANES/EXAFS
• XRD?
• Characterization of microbial processes
• Phospholipid fatty acids
• DNA

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Conclusions

• Remediation by oxidation
• Opposes natural redox buffering of sediment
• Remediation by further reduction
• Requires C addition
• pH changes reduce effectiveness (?)
• Both difficult due to complex hydrogeological
  flowpaths
• More research is warranted in situ remediation
  may prove very difficult