Determination of Inorganic Anions in Wastewater by Ion Chromatography

Importance of the Topic

The determination of common inorganic anions in wastewater and other environmental waters is critical for ensuring public health and meeting regulatory requirements under the U.S. Safe Drinking Water Act and Clean Water Act. Ion chromatography (IC) has become the preferred technique worldwide due to its ability to simultaneously separate and quantify multiple anions with high specificity and sensitivity.

Objectives and Study Overview

This study evaluates the performance of three anion-exchange columns under conditions consistent with U.S. EPA Method 300.0 for the analysis of inorganic anions in various water matrices. Key goals include:

• Comparing separation efficiency, detection limits, and analysis time for three column types
• Assessing linear dynamic range, precision, and accuracy (recoveries) in drinking water, raw and surface waters, domestic and industrial wastewaters, and soil extracts
• Demonstrating strategies for extended dynamic range using dual detection

Methodology and Instrumentation

Reagents included Type I deionized water and carbonate/bicarbonate eluents at three concentrations. Standards for fluoride, chloride, nitrite, bromide, nitrate, phosphate, and sulfate (1000 mg/L) were prepared or obtained commercially.

• Instrumentation: Thermo Scientific Dionex DX-120 and DX-500 IC systems equipped with GP50 gradient pump, CD20 conductivity and AD20 UV-Vis detectors, ASRS ULTRA suppressor, AS40 autosampler and PeakNet workstation
• Columns and conditions:
  
  • Part A (AS4A-SC): 1.8 mM carbonate/1.7 mM bicarbonate, 2.0 mL/min, run time < 8 min
  • Part B (AS14): 3.5 mM carbonate/1.0 mM bicarbonate, 1.2 mL/min, run time < 14 min
  • Part C (AS9-HC): 9.0 mM carbonate, 1.0 mL/min, run time < 24 min
• Sample preparation: 0.45 µm filtration, C18 cartridge cleanup for organic interferences, ultrasonic extraction of soil samples

Main Results and Discussion

Performance metrics were evaluated in terms of linearity (r² > 0.996), method detection limits (MDLs 2–32 µg/L), retention time and peak area precision (< 0.8% RSD), and spike recoveries (80–120%) across different matrices.

• AS4A-SC column: Fast analysis (< 8 min), excellent precision, but fluoride interference from acetate/formate in complex samples
• AS14 column: Improved fluoride resolution from the void peak and small organic acids, capacity 65 µeq, run time 14 min, consistent recoveries
• AS9-HC column: High capacity 190 µeq, superior resolution in high ionic strength samples, run time 24 min; reduced conductivity response but dual conductivity/UV detection enables nitrite quantification in > 10,000:1 chloride background

Benefits and Practical Applications

IC replaces multiple manual wet-chemistry methods, providing high specificity, automation, and multi-analyte capability. The choice of column allows laboratories to tailor the method to matrix complexity—from low-ionic strength drinking water to highly saline industrial effluents.

Future Trends and Opportunities

Potential developments include higher capacity and efficiency stationary phases, coupling IC with mass spectrometry for trace-level organic anions, real-time on-line monitoring systems, and miniaturization for field applications.

Conclusion

Ion chromatography with AS4A-SC, AS14, or AS9-HC columns offers a robust, flexible platform for inorganic anion analysis in diverse water matrices. Column selection should be based on sample ionic strength and the presence of small organic acids. The method delivers high sensitivity, precision, and accuracy in compliance monitoring and research applications.

References

1. U.S. EPA, Federal Register 1999, 64(230), 67449.
2. U.S. EPA, Federal Register 1995, 60(201), 53988.
3. U.S. EPA Method 300.0: Determination of Inorganic Anions in Water by Ion Chromatography, 1993.
4. A.E. Greenberg, L.S. Clesceri, A.D. Eaton (Eds.), Standard Methods for the Examination of Water and Wastewater, 18th ed., APHA, Washington, DC, 1992.
5. ASTM D4327-97: Standard Test Method for Anions in Water by Chemically Suppressed Ion Chromatography, ASTM International, West Conshohocken, PA, 1999.
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9. ASTM D5905-98: Practice for Preparation of Substitute Wastewater, ASTM International, West Conshohocken, PA, 1999.