Dosing and Costing Tools for the Evaluation of ERH

Dosing and Costing
Tools for the
Evaluation of ERH
Treatment of Cr +6
Angus McGrath, PhD (Stantec, Lafayette, California)
David Schroder (Stantec, Sylvania, Ohio)
Daniel Oberle, PE (TRS Group, Sylvania, Ohio)
Presentation Overview
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Properties of Chromium
Problems associated with Cr+6 treatment
Theoretical basis for ERH field observations
Research study hypothesis
Study design
Results
Conclusions and Recommendations
Properties of Chromium
• Commonly found in three oxidation states:
– Hexavalent Chromium (CrO4-2, Cr+6) is an oxidant and
carcinogen
– Trivalent Chromium (Cr+3) is a micronutrient
– Elemental Chromium (Cr0) a metal
• Cr+6 reacts with reducing agents such as ferrous iron,
sulfide, polysulfide, and organic carbon compounds.
• Cr+3 is insoluble at typical ambient pH in
groundwater
Problems with Cr+6 Treatment
• Cr+6 is toxic to most microorganisms above a
concentrations of approximately 30 mg/L
• Certain forms of yeast (30 mg/L), Bacilus (200 to 500
mg/L) and other organism isolated from chromium
waste are more resistant to Cr+6
• Sulfate Reducing Bacteria (SRB) have been isolated
from tannery waste in Pakistan that can tolerate
[Cr+6] up to 45 g/L at temperatures of 35 to 37
degrees C (Shakoori et al., 1999)
• SRB are also extreme thermophiles surviving extreme
temperatures
ERH Field Observations
• Implementation of ERH at a site with
chlorinated VOCs and hexavalent chromium
(Cr+6) at 90 µg/L
• During initial heating Cr+6 concentrations
dropped to below detection limits (5 µg/L)
• What caused the reaction to occur given that
Cr+6 had been in aquifer for decades?
Theoretical Reactions
• Soil contains reduced species such as ferrous
iron (Fe+2), sulfides, natural organic matter
• Reductants such as Fe+2 and sulfides either in
solution, adsorbed on mineral surfaces or
generated by microbes readily react with
dissolved species in saturated soil
• Reaction with organic matter must overcome
an activation energy barrier of ~ 90 kJ/mol &
heat can supply this activation energy
Research Hypothesis
• Elevated temperature provides the activation
energy to facilitate the reaction between
natural organic matter and Cr+6
• Heat stimulates microbial activity which either
directly (Bacillus sp. such as Acinetobacter
haemolyticus and Pannonibacter phragmitetus
BB) or indirectly (through production of
reductive species) reduces Cr+6.
• A combination of the two effects.
Experimental Design
• Soil samples from a depth of 10 to 12 ft,
clayey sand with gravels removed
• ~1.1% TOC as measured by Walkley-Black
Method
– Soil Cr+6 reduction capacity = 32,000 mg/kg
• 1 kg of soil, 200 mL water, Dose of 1, 10, 100
and 500 mg/kg Cr+6
Experimental Design
(cont)
• Incubated at 40oC, 60oC, and 80oC
• Samples were sampled after 7 and 14 days
40oC Results
600
Cr+6 Concentration mg/kg
500
40C @ 1 mg/kg
40C @ 10 mg/kg
40C @ 100 mg/kg
400
300
200
100
0
0
5
Days
10
15
60oC Results
Cr+6 Concentration mg/kg
600
500
60C @ 1 mg/kg
60C @ 10 mg/kg
60C @ 100 mg/kg
400
300
200
100
0
Days
80oC Results
Cr+6 Concentration mg/kg
600
500
80C @ 1 mg/kg
80C @ 10 mg/kg
80C @ 100 mg/kg
400
300
200
100
0
0
5
Days
10
15
Bacteriological Results
Test Sample
4oC
Bacteria Populations – colony forming units per milliliter
(cfu/mL)
Sulfate Reducing
Iron Reducing
Total Heterotrophs
Bacteria
Bacteria
200 – 1,200
2,300 – 9,000
50,000 – 500,000
40oC
5,000 – 18,000
2,300 – 9,000
500,000 – 7,000,000
60oC
5,000 – 18,000
2,300 – 9,000
500,000 – 7,000,000
80oC
5,000 – 18,000
2,300 – 9,000
500,000 – 7,000,000
Observations
• After 7 days little or no reaction was observed
at 40oC for 100 and 500 mg/kg samples
• After 14 days, 40oC 100 mg/kg sample
decreased to 60 mg/kg
• At 60oC 100 mg/kg sample decreased to 80
mg/kg after 7 and 6 mg/kg after 14 days
• At 80oC 100 mg/kg sample decreased to 3
mg/kg after 7 and to 0 after 14 days
• 500 mg/kg sample unchanged at all
temperatures
Observations
• Bacterial numbers increased in the 40oC, 60oC,
and 80oC tests in previous tests – bacterial
counts were not conducted in this study
Conclusions
• In Situ heating can readily reduce mobile
carcinogen Cr+6 to the insoluble micronutrient
Cr+3 at concentrations as high as 100 mg/kg
• Above 100 mg/kg Cr+6 appears to be toxic to
the soil microbes
• Iron reducing bacteria or other heterotrophs
may play a significant role in Cr+6 reduction
• Treatment can be achieved at 40oC or less
when concentrations are below 100 mg/kg
Conclusions
• The required heating to achieve 40oC can be
achieved in 2 to 3 weeks at most sites
• ERH operational costs will be less than $100
per cubic yard in clayey soils that are
otherwise difficult to treat
• Effective treatment of CVOCs and other
contaminants that require microbial reduction
depend on removal of Cr+6 first