Reliable Determination of Cyanide in Treated Water

Abstract

Reliable determination of cyanide in water samples is important for both wastewater and drinking water treatment plant operators and regulatory agencies. However, as a result of cyanide's diverse chemistry, obtaining reliable results has been challenging because several chemical mechanisms can form or destroy cyanide—and some of these can occur within the sample container or during laboratory pretreatment and analysis, leading to biased results. Also, sample matrix constituents or preservation chemicals can interfere with the analytical determination. The US Environmental Protection Agency acknowledged this difficulty for wastewater samples in the 2012 Methods Update Rule by revising the footnote for cyanide preservation to indicate that some interferences may not be mitigated and any technique for removal or suppression of interferences can be used as long as quality control measures are used to demonstrate that the technique worked. Many of the same concerns inherent in testing wastewater apply to testing drinking water. In this study, the effects of holding time, preservation, and on‐line digestion and distillation on cyanide results for wastewater and drinking water were examined, including the use of field dilution as a treatment for interferences and field spikes as a means to gauge whether sample integrity was maintained.

Free Cyanide Forms During Determination of Free Cyanide in Drinking Water

“…From a scientific perspective, the solution to avoiding false positives is to avoid the conditions that cause them. On the basis of this study and a previous study (Delaney & Blodget 2016a), the authors recommend field dilutions to reduce the matrix interference, elimination of NaOH preservation, same‐day analysis, and the use of a field spike to demonstrate sample integrity.…”

“…While the authors cannot unequivocally state that FCN is being detected in this study, the FIA method used is highly selective. Previous investigations using the automated spectrophotometric TCN and ion‐selective electrode FCN analyses lend credence that FCN is being detected (Delaney & Blodget 2016a). While it is possible that these results could be due to an unexpected interference, the issue of false detection remains because the samples have no detectable cyanide at the start, and the preservation and testing are performed according to method requirements.…”

“…“Follow the method.” For regulatory drinking water testing, laboratories are required to “follow the method,” including procedures for how samples are preserved and treated for potential interferences. This regulatory mantra is problematic for cyanide (Delaney & Blodget 2016a), and similar false positives from the sample preservation and testing have been demonstrated (Delaney et al 2007). Cyanide formation during wastewater preservation and testing has also been demonstrated (Stanley & Antonio 2012, Khoury et al 2008, Carr et al 1997, Delaney et al 1997).…”

“…A previous study indicated some FCN formation when dechlorinating drinking water with arsenite (Delaney & Blodget 2016a). Also, the use of sulfamic acid was not included in this study because it is not included as an option in Method OIA‐1677‐DW.…”

“…Cyanide has an additional level of complexity in that it can be formed or destroyed by a variety of chemical reactions (ASTM 2009). Previous studies demonstrated that cyanide can form in the preserved sample container (Delaney et al 2007), or during the sample preservation and analysis (Delaney & Blodget 2016a). This simple but illuminating experiment using deionized water in place of a raw drinking water supply was conducted to verify previous observations regarding FCN formation during drinking water treatment and cyanide testing.…”