Review of Chromium Speciation Research

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

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)

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

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

Research Objective

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

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

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

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)