Separation of Chromium (III) and Chromium (VI) by Ion Chromatography

Significance of the Topic

The differentiation of trivalent and hexavalent chromium is vital for environmental monitoring and human health protection. Cr(VI) is highly toxic even at low concentrations and strictly regulated in drinking water, wastewater and industrial materials, while Cr(III) poses minimal risk at typical environmental levels.

Objectives and Overview

This work presents an improved ion chromatography method for simultaneous speciation of Cr(III) and Cr(VI). Key goals include demonstrating separation on a Dionex IonPac CS5A column, optimizing sample preparation to stabilize both species, and evaluating the approach in soil and wastewater matrices.

Methodology and Instrumentation

• Instrumentation: Thermo Scientific Dionex ICS-3000 system with DP dual-gradient pump, DC module, VWD detector and PC10 postcolumn reagent delivery, controlled by Chromeleon 6.8 software.
• Column: Dionex IonPac CS5A analytical column with CG5A guard column.
• Eluent: 2 mM pyridine-2,6-dicarboxylic acid (PDCA), 2 mM Na₂HPO₄, 10 mM NaI, 50 mM ammonium acetate, 2.8 mM LiOH, pH 6.7–6.8.
• Postcolumn reagent: 2 mM 1,5-diphenylcarbohydrazide in 10 % methanol and 0.9 N H₂SO₄; detection at 335 nm (Cr(III)-PDCA) and 530 nm (Cr(VI)-DPC).
• Flow rates: 1.0 mL/min eluent, 0.5 mL/min postcolumn reagent; injection volume 50 µL; reaction coil 375 µL.
• Sample preparation: Add PDCA eluent concentrate and nitric acid, heat 30 min, cool, dilute to volume, 0.45 µm filtration, OnGuard II P cartridge cleanup.

Results and Discussion

• Separation: Retention times ~3.4 min for Cr(III)-PDCA and ~6.0 min for chromate; minor detector baseline change at wavelength switch.
• Calibration: Linear response for Cr(VI) (0.25–0.75 mg/L) and Cr(III) (5–15 mg/L) with r²>0.9993.
• Oxidation artifact: Approximately 8.3 % of Cr(III) converts to Cr(VI) under sample prep conditions; accounted for in quantitation.
• Recoveries: Soil spiked (10 mg/L Cr(III), 0.5 mg/L Cr(VI)) yielded 105 % Cr(III) and 83.5 % Cr(VI); wastewater spiked gave 105.6 % Cr(III) and 94.7 % Cr(VI).
• Method robustness: OnGuard II P cartridge removal of humic acids prevented column fouling; injection volume of 50 µL avoided overloading.

Benefits and Practical Applications

• Single-injection simultaneous speciation reduces analysis time and sample handling.
• Meets stringent regulatory requirements (EPA 218.6, California PHG, EU cement directive).
• Applicable to environmental, industrial and QA/QC laboratories for water, soil and waste analysis.
• Simple sample prep preserves oxidation state distribution and extends column lifetime.

Future Trends and Potential Applications

• Integration with mass spectrometric detection for enhanced sensitivity and isotopic analysis.
• Miniaturized and field-deployable chromatography systems for on-site monitoring.
• Automation and high-throughput platforms for routine environmental testing.
• Expansion to speciation of other redox-active metals (e.g., arsenic, selenium).

Conclusion

The developed ion chromatography method on the Dionex IonPac CS5A effectively separates Cr(III) and Cr(VI), accounts for preparative oxidation artifacts, and delivers reliable quantitation in complex soil and wastewater samples. High recoveries and robust cleanup procedures support routine environmental and regulatory compliance testing.

References

1. Dionex Application Update 144
2. Dionex Application Note 80
3. Dionex Technical Note 26
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