Speciation of Chromium

Document Sample
Speciation of Chromium Powered By Docstoc
					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 Chromium
   Species: Cr+3, Cr2O3
+6: Hexavalent Chromium
   Species: CrO42-, 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
       1.2
                             Cr2O7 -
       1.0
       0.8
       0.6
       0.4       Cr 3+                     CrO4 2-
Eh (V)
       0.2
       0.0                        Cr2O3
      -0.2
      -0.4
      -0.6

             0   2       4   6        8   10   12    14
                                 pH
       1.2
                             Cr2O7 -
       1.0
       0.8
       0.6
       0.4       Cr 3+                     CrO4 2-
Eh (V)
       0.2
       0.0                        Cr2O3
      -0.2
      -0.4
      -0.6

             0   2       4   6        8   10   12    14
                                 pH
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 Digestion
 SW-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 System
Detector




                  Analytical
                               Integrator
                   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 Curve
                        Calibration Curve         y = 157436x + 594
                                                      2
                                                     R = 0.9999
       180000
       160000
       140000
       120000
                                                          Series1
Area




       100000
        80000                                             Linear (Series1)
        60000
        40000
        20000
            0
                0      0.5        1         1.5
                        Concetration
                          Reduction in CrVI with Time
                500
                450
                400
                350
Conc. (mg/kg)




                300
                250
                200
                150
                100
                 50
                  0
                      0     10       20      30        40          50      60
                                          Time (Day)
                             Clay Soil    Sandy Soil        Organic Soil
Reduction in Leachable CrVI with Time
               25


               20
Conc. (mg/L)




               15


               10


               5


               0
                    0   5           10       15       20      25      30
                                         Time (Day)
                        Clay Soil        Sandy Soil    Organic Soil
            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

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:39
posted:12/10/2011
language:English
pages:27