Research on the Speciation of Chromium as Relates to CCA

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Research on the Speciation of Chromium as Relates
to CCA
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Research ProjectAssessing the Impact of
Chromium in the Environment

• Assessing the Impact of Chromium in the
  Environment
• Funding provided by the Florida Department of
  Environmental Protection RCRA Program

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Master of Engineering Thesis TopicChromium
Speciation in Environmental Media Impacted by the
Wood Preservative Chromated Copper ArsenateMr.
Jin-Kun Song
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Background
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Chromium Exists as Several Chemical Species

• Most common oxidation states 0, 3, 6
• 0 Elemental Chromium (Cr)
• 3 Trivalent Chromium
• Species Cr3, Cr2O3
• 6 Hexavalent Chromium
• Species CrO42-, Cr2O7-

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Chromium Speciation Important!

• The characteristics and properties of trivalent
  chromium and hexavalent chromium are greatly
  different.
• Cr(VI) is much more toxic and mobile than Cr(III)

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Difference between Cr(VI) and Cr(III) Factored
into Regulations

• RCRA Regulations
• 40 CFR 261.4(b)(6)(i)
• A solid waste that is a characteristic or listed
  hazardous waste solely because of chromium is not
  hazardous if..

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• (A) The chromium in the waste is exclusively (or
  nearly exclusively) trivalent chromium and
• (B) The waste is generated from an industrial
  process which uses trivalent chromium exclusively
  (or nearly exclusively) and the process does not
  generate hexavalent chromium and
• (C) The waste is typically and frequently
  man-aged in non-oxidizing environments.

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• Ingestion
• Cr(III) 78,000 mg/kg
• Cr(VI) 390 mg/kg

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Cr(III) versus Cr(VI)

• Hexavalent chromium exists in alkaline, strongly
  oxidizing environments
• Trivalent chromium exists in moderately oxidizing
  and reduced environments

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Chromium Eh-pH Diagram
1.2 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6
Cr2O7 -
CrO4 2-
Cr 3
Eh (volts)
Cr2O3
0 2 4 6 8 10
12 14
pH
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What is Fate of Chromium in CCA Treatment Process
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The Treatment Process
Chromic Acid(liquid)
Arsenic Acid(liquid)
Copper Oxide(solid)
CCA
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The Treatment Process
CCA
Drying and Fixation
TreatmentCylinder
To Market
Untreated Wood Product
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Fate of Chromium in CCA-Treatment Process
Treating Solution
Treated Wood

• CrAsO4
• Cu(OH)CrAsO4
• CuCrO4
• Cr(OH)3
• Cr6/wood complexes
• Cr3/wood complexes
• Cu2/wood complexes

• CrO3
• CuO
• As2O5

Cr6
Cr3
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Fixation of CCA
Cooper and Ung, 1992
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Research Objective

• Examine the fate and behavior of hexavalent
  chromium in various environmental media impacted
  by CCA

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Methods of Cr(VI) Analysis

• Typical chromium measurements are total chromium
  (Cr(III) Cr(VI) other)
• Methods have been developed for Cr(VI) analysis
• Sample holding time is minimal

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Methods of Cr(VI) Analysis

• Solvent extraction followed by total chromium
  analysis
• Colorimetric and Ion Chromatography Methods being
  Used

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The Method of Analysis

• SW-846 Method 7199
• (Determination of Hexavalent Chromium in
  Drinking Water, Groundwater and Industrial
  Wastewater Effluents by Ion Chromatography)

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0.0 mg Cr6/l
0.3 mg Cr6/l
0.6 mg Cr6/l
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Alkaline Digestion
SW-846 Method 3060A
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IC System
Detector
Analytical Column
Integrator
Guard Column
LC Pump
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Experiment 1

• Examination of the fate of hexavalent chromium in
  soils spiked with CCA solution
• Cr(VI)/Cr(total) concentrations were measured
  over time
• Leachable Cr(VI) and Cr(III) concentrations
  measured over time (SPLP)

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Soil Types
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Experimental Set Up
50 g of soil
10 ml of 2700 mg/L Hex. Cr
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Reduction in Total Cr6 with Time
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Reduction in Total Cr6 with Time
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Reduction in Leachable Cr6 with Time
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Reduction in Leachable Cr6 with Time
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Experiment 2

• How Much CrVI is in CCA-Treated Wood

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Results

• Alkaline digestions on CCA-treated wood samples
  show Cr6 concentrations in the range 7 to 250
  mg/kg (less 3 of all Chromium)
• SPLP leachates have no detectable Cr6
  concentrations

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Experiment 3

• Under what leaching conditions would Cr6 form?

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Chromium Leachability as a Function of
Time(weathered wood sample)
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Chromium Leachability as a Function of
Time(weathered wood sample)
MSW LandfillLeachate
AlkalineLeachate
TCLP SPLP
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Experiment 4

• Will Cr6 be formed in simulated alkaline
  leachate? Scenario saturated concrete

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Chromium Leaching in Concrete Leachate
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Experiment 5

• Is Cr6 formed during combustion of CCA-treated
  wood?

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Remaining Tasks

• Data interpretation
• Final report