Research on the Speciation of Chromium as Relates to CCA

Research Project:Assessing 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

Master of Engineering Thesis Topic:

Chromium Speciation in Environmental Media

Impacted by the Wood Preservative Chromated

Copper Arsenate

Mr. Jin-Kun Song

Background

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) 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

1.2

1.0

0.8

0.6

0.4

0.2

0.0

-0.2

-0.4

-0.6

Cr2O3

CrO4 2-

Cr2O7 -

Cr 3+

Eh

(vo

lts)

0 2 4 6 8 10 12 14

Chromium Eh-pH Diagram

What is Fate of Chromium in CCA Treatment Process

The Treatment Process

CCA

Arsenic Acid(liquid)

Chromic Acid(liquid)

Copper Oxide(solid)

The Treatment Process

CCA

Untreated Wood Product

TreatmentCylinder

Drying and Fixation

To Market

Fate of Chromium in CCA-Treatment Process

– CrAsO4

– Cu(OH)CrAsO4

– CuCrO4

– Cr(OH)3

– Cr6+/wood complexes– Cr3+/wood complexes– Cu2+/wood complexes

•CrO3

•CuO•As2O5

Treating Solution Treated Wood

Cr+6 Cr+3

Fixation of CCA

Cooper and Ung, 1992

Research Objective

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

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

The Method of Analysis

• SW-846 Method 7199

(Determination of Hexavalent Chromium in Drinking Water, Groundwater and Industrial Wastewater Effluents by Ion Chromatography)

0.0 mg Cr+6/l 0.3 mg Cr+6/l 0.6 mg Cr+6/l

Alkaline DigestionSW-846 Method 3060A

IC SystemDetector

IntegratorAnalytical

Column

Guard

Column

LC Pump

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)

Soil Types

Experimental Set Up

50 g of soil

10 ml of 2700 mg/L

Hex. Cr

0

100

200

300

400

500

600

0 5 10 15 20 25

Time (days)

Co

nc

. o

f C

rVI

(mg

/kg

)

Sand CrVI (mg/kg)

Clay CrVI (mg/kg)

Organic CrVI (mg/kg)

Reduction in Total Cr+6 with Time

0

100

200

300

400

500

600

0 20 40 60 80 100 120 140 160 180 200Time (days)

Co

nc

. o

f C

rVI

(mg

/kg

)

Sand CrVI (mg/kg)

Clay CrVI (mg/kg)

Organic CrVI (mg/kg)

Reduction in Total Cr+6 with Time

0

5

10

15

20

25

0 5 10 15 20 25Time (days)

Co

nc

. o

f C

rVI

(mg

/l)

Sand CrVI (mg/l)

Clay CrVI (mg/l)

Organic CrVI (mg/l)

Reduction in Leachable Cr+6 with Time

0

5

10

15

20

25

0 20 40 60 80 100 120 140 160Time (days)

Co

nc

. o

f C

rVI

(mg

/l)

Sand CrVI (mg/l)

Clay CrVI (mg/l)

Organic CrVI (mg/l)

Reduction in Leachable Cr+6 with Time

Experiment 2

How Much CrVI is in CCA-Treated Wood

Results

• Alkaline digestions on CCA-treated wood samples show Cr+6 concentrations in the range 7 to 250 mg/kg (less 3% of all Chromium)

• SPLP leachates have no detectable Cr+6 concentrations

Experiment 3

Under what leaching conditions would Cr+6 form?

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12 14pH

Con

c. o

f C

r (m

g/l)

Cr(mg/l)

CrVI(mg/l)

Chromium Leachability as a Function of Time(weathered wood sample)

0

5

10

15

20

25

30

35

0 2 4 6 8 10 12 14pH

Con

c. o

f C

r (m

g/l)

Cr(mg/l)

CrVI(mg/l)

Chromium Leachability as a Function of Time(weathered wood sample)

TCLP &SPLP

MSW LandfillLeachate

AlkalineLeachate

Experiment 4

Will Cr+6 be formed in simulated alkaline leachate?

Scenario: saturated concrete

0

0.1

0.2

0.3

0.4

0.5

0.6

0.25 pcf CCA Wood 0.6 pcf CCA Wood 2.5 pcf CCA WoodSample

Co

nc

. o

f C

r (m

g/l

)

Cr(mg/l)

CrVI(mg/l)

Chromium Leaching in Concrete Leachate

Experiment 5

Is Cr+6 formed during combustion of CCA-treated wood?

0

5

10

15

20

25

30

35

40

45

Untreatedwood

0.25 pcf CCAWood

0.6 pcf CCAwood

2.5 pcf CCAwood

WeatheredCCA wood

(0.6 pcf)

C&DRecyclingFacility B

C&DRecyclingFacility C

Sample

Co

nc

. o

f C

r (m

g/l

)

Total Cr (mg/kg)

CrVI (mg/kg)

Remaining Tasks

• Data interpretation

• Final report