Optimizing Sample Preservation for HexavalentPreservation for Hexavalent Chromium Analyses in Waters
Jane Timm, James Lovick Jr, Raymond Siery, and Yongtao Liand Yongtao Li
Underwriters LaboratoriesU de te s abo ato es
2011 NEMC, Bellevue, Washington
© 2011 Underwriters Laboratories Inc.
Presentation Outline
IntroductionBackground informationBackground information
Regulatory update
Common analytical methodsCommon analytical methods
EPA Method 218.6 InstrumentationOptimization & PerformanceSample results
ConclusionsConclusions
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Background Information
• Chromium (Cr) is found naturally in rocks, plants, soil and volcanic dust, humans, and animals. Trivalent chromium Cr(III) is an essential nutrient for the bodyCr(III) is an essential nutrient for the body.
• Water sources can be affected by hexavalent chromium Cr(VI) naturally or through contamination from industrialCr(VI) naturally, or through contamination from industrial centers, landfills, and improper discharge of industrial processing streams.
• Cr(VI) can be removed using a handful of proven treatment techniques, e.g. anion exchange, membrane filtration (nanofiltration and reverse osmosis) reduction-filtration (nanofiltration and reverse osmosis), reduction-coagulation and precipitation, adsorption, etc.
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Background Information• Cr(VI) is classified as a known human carcinogen via
inhalation in EPA’s Integrated Risk Information System (IRIS) database (1998) and by the U S Occupational(IRIS) database (1998) and by the U.S. Occupational Safety and Health Administration (OSHA).
• The California Department of Public Health (CDPH) p ( )classified Cr(VI) as an “unregulated chemical requiring monitoring” in 1999.
• National MCL for total chromium = 100 µg/L
• California MCL for total chromium = 50 µg/L
• The National Toxicology Program (NTP) concluded that Cr(VI) is carcinogenic when ingested in drinking water (2007)(2007).
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Regulatory UpdateC f Off f• The California Office of Environmental Health Hazard
Assessment (OEHHA) established a Public Health Goal of 0.02 µg/L Cr(VI) in drinking water (2010).µg ( ) g ( )
• Environmental Working Group reported that 31 out of 35 cities evidenced detectible levels of Cr(VI), with samples from 25 cities exhibiting levels of Cr(VI) >0.06 µg/L.
• USEPA released a draft risk assessment of Cr(VI) and t t d th t it i lik l t h i t dstated that it is likely to cause cancer when ingested over
a lifetime (IRIS, 9/2010).
• USEPA will release the final version of its “Toxicological• USEPA will release the final version of its Toxicological Review of Hexavalent Chromium” in the Summer of 2011, and to determine if additional standards and testing requirements are appropriate.
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Regulatory Update
• USEPA released a Guidance for Public Water Systems on Enhanced Monitoring for Chromium-6 (Hexavalent Chromium) in Drinking Water (01/2011)Chromium) in Drinking Water (01/2011)− EPA Method 218.6
Buffered samples pH = 9 0 9 5− Buffered samples, pH = 9.0 – 9.5
− 5 days of holding time
• USEPA has proposed changes to its Unregulated• USEPA has proposed changes to its Unregulated Contaminant Monitoring Regulation 3 (UCMR 3), which will likely include total Cr and Cr(VI).
• Finally, it is likely that USEPA will tighten drinking water standards to address the health risks posed by Cr(VI) in th f tthe near future.
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Common Analytical Techniques and Methods
Total Cr GFAASICP-MS
EPA Methods 218.2 and 200.9EPA Method 200.8
ICP-AES EPA Method 200.7Dissolved Cr(VI) IC-PCR-UV/Vis EPA Method 218.6
Dionex Application Update144Dionex Application Update144 SM 3500-Cr CASTM D5257-11
Cr(III) and Cr(VI) speciation
IC-PCR-UV/VisLC-ICP-MSIC-ICP-MS
Dionex Application Update 165
IC ICP MSPre-concentration- Sol-gel
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- SPE cartridges
EPA Method 218.6 InstrumentationDionex ICS 5000Dionex ICS 5000
ICS-5000 SP single pump
AS-DV autosampler
ICS Series VWD variable wavelength detector (UV/Vis)
DC ICS 5000 d l CD dDC ICS-5000 dual CD and ECD
PC 10 pneumatic postPC-10 pneumatic post-column reagent delivery system
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EPA Method 218.6 ConditionsAnalytical column Dionex IonPac AS7 (4x 250 mm)
Guard column Dionex IonPac NG1 (4 x 50 mm)
Eluent 250 mM (NH ) SO and 100 mM NH OHEluent 250 mM (NH4)2SO4 and 100 mM NH4OHEluent flowrate = 1.0 mL/minBack pressure = 1200-1300 psiSample loop = 1 mL
PCR 2 mM 1,5-diphenylcarbohydrazide, 10% (V/V) methanol, and 1N H2SO4PCR fowrate = 0.33 mL/minPCR coil = 1 mLPCR coil temperature = 30 °C
Absorbance detection 535 nmAbsorbance detection 535 nm
Noise Auto zero
Run time 10 min
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Sample pH buffer 2.5M (NH4)2SO4 and 1M NH4OH (pH = 9.0-9.7)
Cr(VI) Method Sensitivity
Spiking Conc. MDL
( /L)
Spiking Conc. LCMRL DL Critical
Level(µg/L)
(µg/L)(µg/L) (µg/L) (µg/L) (µg/L)
0 01 0 006 0 01 0 06 0 027 0 009 0 00640.01 0.006 0.01-0.06 0.027 0.009 0.0064
0 02 0 0030.02 0.003
0.02 0.015
0.02 0.016
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Cr(VI) Method Accuracy and Precision (n = 4)
MatrixSpiking Conc.
Mean Recovery RSD
(%)(µg/L) (%) (%)
DI Water 1 0 99 0 6DI Water 1.0 99 0.6
Chlorinated DW 1.0 103 0.7Chlorinated DW 1.0 103 0.7
Chlorinated SW 1.0 103 0.9
Chlorinated GW 1.0 101 1.0
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Water Matrix Characterization
Parameter Drinking Water Surface Water Groundwater
Cr (VI) 0.02 µg/L 0.05 µg/L < 0.02 µg/L
Total Cr 0.17 µg/L 0.21 µg/L < 0.1 µg/L
Nitrate < 0.1 mg N/L 2.5 mg N/L < 0.1 mg N/L
Nitrite < 0 1 mg N/L < 0 1 mg N/L < 0 1 mg N/LNitrite < 0.1 mg N/L < 0.1 mg N/L < 0.1 mg N/L
Ammonia < 0.1 mg N/L < 0.1 mg N/L < 0.1 mg N/L
Total cyanide < 0.02 mg/L < 0.02 mg/L < 0.02 mg/L
Total phosphate < 0.05 mg P/L 0.11 mg P/L < 0.05 mg P/L
TOC 0.5 mg/L 5.8 mg/L < 0.5 mg/L
Turbidity 0 23 NTU 11 9 NTU 0 05 NTUTurbidity 0.23 NTU 11.9 NTU 0.05 NTU
Conductivity 955 umhos/cm 531 umhos/cm 532 umhos/cm
Heterotrophic plate < 2 MPN/mL > 738 MPN/mL 40 MPN/mL
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count (HPC)
Effects of Holding Time and TemperatureBuffered DW SW and GW (Cl2 ≈ 0 2 ppm)Buffered DW, SW, and GW (Cl2 ≈ 0.2 ppm)
2.5 110
2.4
DW-4C-2ppb Cr (VI)DW-20C-2ppb Cr (VI)SW-4C-2ppb Cr (VI)SW-20C-2ppb Cr (VI)GW-4C-2ppb Cr (VI)GW-20C-2ppb Cr (VI) 105
2 2
2.3
100
2.1
2.2
95DW-4C-2ppb Cr (VI)DW-20C-2ppb Cr (VI)SW-4C-2ppb Cr (VI)SW-20C-2ppb Cr (VI)
20 5 10 15 20 25 30
Holding Time (Days)
900 5 10 15 20 25 30
GW-4C-2ppb Cr (VI)GW-20C-2ppb Cr (VI)
H ldi Ti (D )
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Holding Time (Days) Holding Time (Days)
Effects of Holding Time and TemperatureBuffered DW SW and GW (Cl2 ≈ 0 2 ppm)Buffered DW, SW, and GW (Cl2 ≈ 0.2 ppm)
3.5DW-4C-20ppb Cr (III)-2ppb Cr (VI)
1DW-4C-20ppb Cr (III)
3
DW 4C 20ppb Cr (III) 2ppb Cr (VI)DW-20C-20ppb Cr (III)-2ppb Cr (VI)SW-4C-20ppb Cr (III)-2ppb Cr (VI)SW-20C-20ppb Cr (III)-2ppb Cr (VI)GW-4C-20ppb Cr (III)-2ppb Cr (VI)GW-20C-20ppb Cr (III)-2ppb Cr (VI)
0.8
DW-20C-20ppb Cr (III)SW-4C-20ppb Cr (III)SW-20C-20ppb Cr (III)GW-4C-20ppb Cr (III)GW-20C-20ppb Cr (III)
0.4
0.6
2.5
0.2
20 5 10 15 20 25 30
Holding Time (Days)
00 5 10 15 20 25 30
Holding Time (Days)
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Cr(III) Speciation as a Function of pH Ionic strength of ~0 01 M and Cr(III) = 1 0 mg/LIonic strength of 0.01 M and Cr(III) = 1.0 mg/L
(Source: visual MINTEQ program)
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Cr(VI) Speciation as a Function of pH Ionic strength of ~0 01 M and Cr(VI) = 1 0 mg/LIonic strength of 0.01 M and Cr(VI) = 1.0 mg/L
(Source: visual MINTEQ program)
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Formal Reduction Potential pH = 9 0 9 5pH = 9.0 - 9.5
Cl ↑ + 2e 2Cl- E0’ ≈ 1 36VCl2↑ + 2e 2Cl- E0 ≈ 1.36V
O2↑ + 4H+ + 4e 2H2O E0’ ≈ 0 2VO2↑ 4H 4e 2H2O E 0.2V
CrO42- + 8H+ + 3e Cr3+ + 4H2O E0’ ≈ - 0.2V4 2
Cr(OH) → CrO 2-Cr(OH)3 → CrO4
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Effects of Chlorine, Cr(III), and Temperature24 hrs of chlorination buffered prior to analysis24 hrs of chlorination, buffered prior to analysis
1000.2 ppm Cl2, 4C 0.2 ppm Cl2, 20C
80
2 ppm Cl2, 4C4 ppm Cl2, 4C
2 ppm Cl2, 20C4 ppm Cl2, 20C
60
20
40
0
20
18
25 50 75 100
Cr (III) Concentration (ug/L)
Effects of Chlorine, Cr(III), and pH Buffer
1000.25 ppm Cl2, BDI0.5 ppm CL2, BDI0.75 ppm Cl2, BDI
0.25 ppm Cl2, NBDI0.5 ppm Cl2, NBDI0.75 ppm Cl2, NBDI
60
80pp ,
1 ppm Cl2, BDIpp ,
1 ppm Cl2, NBDI
40
60
20
40
02 5 5 7 5 10
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2.5 5 7.5 10
Cr (III) Concentration (ug/L)
Effects of Chlorine, Cr(III), and pH Buffer
100.25 ppm Cl2, BDI0.5 ppm CL2, BDI
0.75 ppm Cl2, BDI1 ppm Cl2, BDI
8
6
2
4
0
2
20
02.5 5 7.5 10
Cr (III) Concentration (ug/L)
Effects of Chlorine and pH BufferAverage conversion rate of Cr(III) (0 25 5 7 5 10 ppb)Average conversion rate of Cr(III) (0.25, 5, 7.5, 10 ppb)
70
BDI Water NBDI Water
50
60
54
62
4045
20
30
18
0
10
0.9 1.2 6.9 8.4
18
21
00.25 0.5 0.75 1
Chlorine Concentration (mg/L)
Effects of Chlorine and pH Buffer (n=4)Cl2 = 1 ppm pH buffered >12 hrsCl2 = 1 ppm, pH buffered, >12 hrs
Cr(III) = 10 ppb, Cr(VI) = 1 ppb, >6 hrs at ambient temperature8
6
7 pH = 8.6
4
5pH = 9.0
pH = 9.3
pH = 9.6
2
3
4pH = 9.4
0
1
2
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010 25 50 75 100
Concentration of Ammonium Hydroxide (mM)Concentration of Ammonium Hydroxide (mM)
Cr(VI) Field Sample Results
T t l Sample # Lowest Median Highest Mean Matrix Total
Sample #Sample #
(≥ 0.02 µg/L)Conc.(µg/L)
Conc.(µg/L)
gConc.(µg/L)
Conc.(µg/L)
DW 307 277 0.020 0.021 38.8 2.32
GW 33 20 0.024 0.088 10.7 1.17
SW 101 87 0.024 0.15 10.2 0.61
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Conclusions• EPA Method 218.6 is applicable for analyzing Cr(VI) with
an MRL of 0.02-0.03 µg/L.
• EPA Method 218.6 can provide good accuracy (±10% recovery) and precision (≤20% RSD).
The presence of Cr(III) may affect sample results• The presence of Cr(III) may affect sample results.
• Cr(III)-to-Cr(VI) conversion is dependent on total Cr(III), Cl and NH OH concentrations temperature andCl2 and NH4OH concentrations, temperature, and holding time.
• When Cl2 concentrations varied from 0.25 mg/L to 1When Cl2 concentrations varied from 0.25 mg/L to 1 mg/L, Cr(III)-to-Cr(VI) conversion rates were 18% to 62% for non-buffered samples and 0.9% to 8.4% for buffered samplessamples.
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Conclusions• The use of (NH4)2SO4−NH4OH buffer to adjust sample
pH to 9.0-9.5 cannot completely stop the oxidation of Cr(III) Cr(VI) formation rates were generally less thanCr(III). Cr(VI) formation rates were generally less than 10%.
• General recommendations for Cr(VI) control in drinking ( ) gwater include:
−Remove Cr(III) before disinfection.
−Use alternative disinfection techniques, such as chloramination instead of chlorination.
−For a chlorination system, consider to reduce Cl2doses.
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