Determination of Total Cyanide in Municipal Wastewater and Drinking Water Using Ion-Exclusion Chromatography with Pulsed Amperometric Detection (ICE-PAD)

Significance of the Topic

Cyanide is an acute toxin that inhibits cellular respiration and presents severe health risks in drinking water and wastewater effluents. Regulatory agencies such as the U.S. EPA enforce strict maximum contaminant levels for total cyanide, typically defined as free cyanide and complexed cyanides converted to hydrogen cyanide under strong acid digestion. Wastewater treatment processes, especially chlorination and chloramination, can generate unstable cyanide intermediates, leading to effluent concentrations that exceed influent levels. Accurate, interference-free detection of total cyanide at low microgram-per-liter levels is critical for environmental compliance and public health protection.

Objectives and Study Overview

This study aimed to develop a fast, reliable, and sensitive method for direct determination of total cyanide (µg/L to sub-µg/L) in municipal drinking water and wastewater. Key goals included eliminating interferences from high concentrations of chloride, sulfate, and sulfide, simplifying sample preparation compared to traditional distillation methods, and achieving low detection limits. The authors combined ion-exclusion chromatography (ICE) with pulsed amperometric detection (PAD) on a disposable platinum electrode, paired with the EPA-approved MICRO DIST acid-digestion system, to meet these objectives.

Methodology

• Sample Preservation and Digestion: Water samples were stabilized at collection by adding NaOH, then acid-digested with a magnesium chloride/sulfuric acid mixture at 120 °C in MICRO DIST tubes. Evolved HCN was trapped in 1 M NaOH and diluted to 250 mM for analysis.
• Chromatographic Separation: A Dionex IonPac ICE-AG1 guard and ICE-AS1 analytical column separated cyanide from strong acid anions by Donnan exclusion. Eluent consisted of 50 mM methanesulfonic acid at 0.2 mL/min and 30 °C.
• Amperometric Detection: A pulsed waveform cycled the working electrode through cleaning and detection potentials, using a disposable platinum electrode and a pH–Ag/AgCl reference. This approach prevented fouling by sulfide and avoided chloride detection.
• Calibration and Quantification: Linearity was established from 1–25 µg/L (r2 = 0.9999). The method detection limit was 0.27 µg/L (99% confidence). Accuracy and precision were verified by replicate analyses and spike recoveries in standards and real samples over multiple days.

Used Instrumentation

• Dionex ICS-3000 ion chromatograph with single gradient pump, dual-zone detector, and autosampler with temperature control.
• IonPac ICE-AG1 guard column (4 × 50 mm) and ICE-AS1 analytical column (4 × 250 mm).
• Electrochemical detector with disposable Pt working and pH–Ag/AgCl reference electrodes.
• Lachat MICRO DIST sample digestion system with user-filled collector tubes.

Main Results and Discussion

• Separation and Sensitivity: Cyanide eluted at ~16 min with high symmetry and no overlap with chloride or sulfide peaks. Background noise remained low (20 pC), yielding reliable detection.
• Accuracy and Precision: Triplicate and six-replicate analyses of standards and environmental samples showed recoveries between 97% and 102% and RSDs below 3%.
• Interference Studies: An ASTM challenge matrix containing ammonium chloride, cyanate, thiocyanate, nitrate, and sulfate produced false positives during acid digestion, primarily from cyanate and nitrate conversion to cyanide. Pre-treatment with sulfamic acid or sample screening by IC can mitigate these effects.
• Sample Application: Municipal drinking water contained ~0.7 µg/L total cyanide, while treated wastewater influent spiked with NaOH showed ~6.0 µg/L, confirming cyanide generation in chloramine-treated effluent.

Benefits and Practical Applications

• Direct and selective detection of total cyanide without extensive distillation or spectrophotometric titration.
• Elimination of common interferences from chloride, sulfate, and sulfide thanks to ICE separation and PAD stability.
• Compliance monitoring of drinking water and wastewater treatment facilities with rapid throughput and minimal manual steps.

Future Trends and Possibilities

Advances in miniaturized and reagent-free ion chromatography may further streamline cyanide analysis. Integration of on-line sample pretreatment and automated membrane-based trapping could reduce manual handling and exposure risk. Emerging electrochemical detection waveforms and novel electrode materials may enhance sensitivity and robustness for environmental monitoring of other anionic toxins.

Conclusion

The combined ICE-PAD and MICRO DIST digestion method provides a robust, interference-free approach for low-level total cyanide determination in municipal water samples. It meets regulatory requirements, reduces sample preparation complexity, and offers high sensitivity, accuracy, and reproducibility. This technique supports routine water quality monitoring and can adapt to evolving analytical demands.

Reference

• Simeonova P.; Fishbein L. Hydrogen Cyanide and Cyanides: Human Health Aspects. WHO Concise Int. Chem. Assess. Doc. 61, 2004.
• EPA. National Primary Drinking Water Regulations; 40 CFR 141, 2008.
• EPA. Water Quality Standards; 40 CFR 131, 2008.
• EPA Method 335.2, 335.3, 335.4: Total Cyanide Determination, U.S. EPA Cincinnati, OH, 1980–1993.
• Weinberg H.S.; Cook S.J. Segmented Flow Injection and Amperometric Detection for Total Cyanide. Anal. Chem. 74, 2002, 6055–6063.
• ASTM D19.06 Cyanide Task Group Research Report, 2008.
• Thermo Fisher Scientific. Dionex IonPac ICE-AS1 Product Manual, 2006.
• Thermo Fisher Scientific. Dionex Application Note 188: Glycols and Alcohols by ICE-PAD, 2008.