Speciation of Chromium
Research Project:Assessing the Impact of Chromium in
the Environment
• Funding provided by Florida Department of Environmental Protection
Chromium Exists as Several Chemical Species
Most common oxidation states: 0, +3, +6
0: Elemental Chromium (Cr)
+3: Trivalent ChromiumSpecies: Cr+3, Cr2O3
+6: Hexavalent ChromiumSpecies: CrO4
2-, Cr2O7-
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)
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…..
A solid waste that is a characteristic or listed hazardous waste solely because of chromium is not hazardous if…..
• (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.
• Ingestion:– Cr(III): 78,000 mg/kg– Cr(VI): 390 mg/kg
Cr(III) versus Cr(VI)
• Hexavalent chromium exists in alkaline, strongly oxidizing environments
• Trivalent chromium exists in moderately oxidizing and reduced environments
pH
Eh (V)
10 12 144 6 820
0.0
-0.2-0.4
-0.6
1.0
0.2
0.8
0.4
0.6
1.2
Cr2O3
CrO4 2-
Cr2O7 -
Cr 3+
pH
Eh (V)
10 12 144 6 820
0.0
-0.2-0.4
-0.6
1.0
0.2
0.8
0.4
0.6
1.2
Cr2O3
CrO4 2-
Cr2O7 -
Cr 3+
Chromium in Wood Preservation
• The chromium in CCA preservative solution is hexavalent chromium.
• Upon fixation in the wood, Cr(VI) becomes converted to Cr(III)
When Might Cr(VI) Be Encountered?
• If wood is improperly fixed (as a result, for example, of inadequate fixation time at low temperatures)
• When in contact with oxidizing chemicals such as deck brighteners
Oxidation of Chromium in the Environment
• In the natural environment, chromium tends to exist as Cr(III)
• Oxidation of Cr(III) to Cr(VI) as a result of manganese (hydr)oxides
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
Methods of Cr(VI) Analysis
• Solvent extraction followed by total chromium analysis
• Colorimetric and Ion Chromatography Methods being Used
Tasks
1. Literature Review
2. Assessment of pH and ORP as indicators or Cr speciation
3. Evaluate kinetics of conversion of Cr(VI) to Cr(III) in natural soils
4. Develop guidance document
Kinetic Study
• Three different soil types have been selected• They have beeb spiked with CCA solution to
add a known amount of Cr(VI)• The Cr(VI)/Cr(total) will be measured over
time• Leachable Cr(VI) and Cr(III) will be measured
over time
Soil Types
Experimental Set Up
50 g of soil
10 ml of 2700 mg/L
Hex. Cr
Alkaline DigestionSW-846 Method 3060A
The Method of Analysis
• SW-846 Method 7199
(Determination of Hexavalent Chromium in Drinking Water, Groundwater and Industrial Wastewater Effluents by Ion Chromatography)
Analytical SystemDetector
IntegratorAnalytical
Column
Guard
Column
LC Pump
Calibration• 0.05 ppm CrVI Area: 8773
• 0.1 ppm CrVI Area: 15571
• 0.5 ppm CrVI Area: 80108
• 1 ppm CrVI Area: 157693
Calibration CurveCalibration Curve y = 157436x + 594
R2 = 0.9999
0
20000
40000
60000
80000100000
120000
140000
160000
180000
0 0.5 1 1.5
Concetration
Are
a Series1
Linear (Series1)
Reduction in CrVI with Time
0
50
100
150
200
250
300
350
400
450
500
0 10 20 30 40 50 60
Time (Day)
Con
c. (
mg/
kg)
Clay Soil Sandy Soil Organic Soil
Reduction in Leachable CrVI with Time
Future Schedule
• Begin Kinetic Study – Conduct for 6 months• Measure Cr(VI) in various environmental
matrices as function of ORP and pH– C&D Debris Leachate– SPLP Leachate– Contaminated Soil– Wood Ash
• Prepare Guidance Document
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