54 Water / Wastewater ROBUST CYANIDE ANALYSIS IS FUNDAMENTAL FOR SAFE DRINKING WATER
Several regulatory bodies have set guidelines for cyanide levels in drinking water to ensure its quality and to protect consumers. Traditionally, cyanide measurement in drinking water involves colorimetric and spectrophotometric methods, but these suff er the drawbacks of requiring a distillation step and are subject to a range of interferences. Alternative approaches are needed to achieve the measurement sensitivity and robustness required today. Ion chromatography with pulsed amperometric detection (IC-PAD) is proving highly eff ective in this application.
Introduction
Cyanide is well-known as a highly toxic substance. It is present naturally in a variety of foods, including cassava, bamboo shoots and some fruit pits, and is also generated by microorganisms. Industrially, cyanide is used in many processes, including plating and mining, and it may be released into the environment from burning coal and plastics. In the US, drinking water contamination with cyanide typically originates from an industrial source or is the result of leaching from waste sites.
The US government classifi es cyanide as a regulated inorganic contaminant in drinking water. This is set out in US National Primary Drinking Water Regulations, 40CFR 141.62, and enforced by the US Environmental Protection Agency (EPA) and state EPA agencies. Since bottled water is classifi ed as a food, the US Food and Drugs Administration (FDA) Center for Food Safety and Applied Nutrition (CFSAN) is responsible for its regulation. The maximum contaminant level (MCL) for drinking water is 200 µg/L cyanide, as free cyanide, although levels found are typically much lower.
Table 1: Cyanide waveform and experimental conditions
greatest health concern because of its bioavailability and toxicity.
EPA-approved methods for determining free cyanide involve spectrophotometry, colorimetry, or ion-selective electrode detection. Many current methods have inherent drawbacks. The colorimetric and spectrophotometric methods require a distillation step, and they suffer from several interfering factors. These include diffi culties with the high pH solutions used to stabilize water samples, the presence of oxidizers and of sulfur- bearing compounds. The ion-selective electrode method does not require distillation but is highly matrix sensitive.
Ion chromatography (IC) methods for cyanide use either direct current (DC) amperometric detection or pulsed amperometric detection (PAD). No distillations are needed, and there is little or no problem with the interfering substances listed above. The DC method does, however, exhibit electrode fouling over time, whereas the IC-PAD method overcomes all these issues.
Evaluating IC-PAD to Determine Free
Cyanide Levels in Drinking Water Free cyanide analysis of water samples was carried out using an IC system (Thermo Scientifi c Dionex ICS-3000, similar to the current ICS-6000) equipped with columns specifi c for the analysis of trace anions in high-purity water matrices (Thermo Scientifi c Dionex IonPac AS15 and AG15 columns) and an electrochemical detector (Thermo Scientifi c Dionex ICS-3000 ED or ICS-6000 ED) with an amperometric cell containing working, reference, and counter electrodes. This application uses a disposable silver working electrode.
Pulsed amperometric detection involves the brief application of a potential across the working and reference electrodes, followed by higher or lower potentials to clean the working electrode. This avoids the fouling and loss of signal that can occur in DC amperometry. The series of applied potentials is referred to as a waveform. Repeated application of a waveform is the basis of pulsed amperometry. PAD provides a stable and fresh working electrode surface every cycle of the waveform (1 second). For this study, the waveform was optimized for cyanide but can also detect sulfi de, bromide, and thiosulfate. Table 1 shows the waveform and summarizes the measurement conditions used.
Drinking water samples were taken from the City of Sunnyvale, City of San Jose and Twain Harte Valley, CA. The sources of Twain Harte Valley (an old gold mining region) drinking water and Alamitos Creek in Almaden region (old mercury mining) of San Jose, were selected because of their mining history and potential presence of free cyanide. Since cyanide is reactive and unstable, with oxidizing agents causing decomposition and any free cyanide present at neutral pH volatilizing to hydrogen cyanide, samples were stabilized as soon as practicable by treatment with sodium hydroxide solution.
Further details of the experimental setup, eluent, standards and sample preparation are provided in reference 1.
Results Investigating Potential Interfering Factors
Measurement Approaches
and Their Limitations Cyanide can be determined as total cyanide, dissociated cyanide, or free cyanide. Regulatory levels refer to free cyanide which poses the
IET MARCH / APRIL 2023
EPA methods cite sulfi de and sulfi de-generating compounds as potential interferences because sulfi de complexes with free cyanide to form thiocyanate. Nitrate, nitrite, and chlorine are also interfering substances, while copper, iron, and other transition metals complex with free cyanide, making it unavailable for measurement.
Table 3: Recovery of cyanide in treated water samples (Dionex OnGuard II H Cartridges)
In the IC-PAD study reported here, the electroactive ions iodide, thiosulfate, bromide, thiocyanate, and sulfi de are all potential interferences as they are detected using the silver working electrode and cyanide optimized waveform.
The interference effects of non-oxidized and partially oxidized sulfur-containing anions, bromide, and iodide were examined by analyzing solutions of 10 µg/L cyanide and 20 µg/L of each potential interfering anion. Table 2 shows that the resulting free cyanide concentrations were not signifi cantly affected by any of the anions. There was a small decrease with sulfi de and a small increase with thiocyanate, but neither is expected in high concentrations in drinking water since sulfi de is normally removed during sanitation. These experiments confi rm that the cyanide waveform detects thiosulfate, sulfi de and bromide under the conditions used, but does not detect sulfate, thiocyanate, or iodide. Where it is necessary to resolve cyanide from sulfi de, a different set of chromatography columns (Thermo Scientifi c Dionex IonPac AS7) is recommended.
Investigation of the effects of dissolved iron, copper and nickel on free cyanide determinations involved treating a 10 µg/L cyanide standard with each metal. The use of a 600 µg/L solution of iron refl ects expected levels in drinking water, while copper and nickel concentrations were arbitrarily set at 300 µg/L. Further testing included treating the metal solutions with proprietary sample preparation cartridges for matrix clean up (Thermo Scientifi c Dionex OnGuard II H) before adding them to cyanide standards. In the iron solution, the free cyanide concentration loss was comparable to that of the untreated cyanide control over 3 days (Figure 1) whereas after 92 h only 28% of the free cyanide remained in the copper solution (Figure 2), and in the nickel solution free cyanide decreased to 75% within 20 h then stabilized for the remainder of the 3-day experiment (Figure 3).
Table 2: Effect of bromide, iodide, sulfi de, sulfi te, thiocyanate, and thiosulfate on cyanide recovery.
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