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