Expanded Summary
Reliable Determination of Cyanide in Treated Water MI CHAEL F. DELANE Y A ND C H A RL E S B L O DGE T http://dx.doi.org/10.5942/jawwa.2016.108.0006
Testing drinking water or wastewater samples for free or total cyanide (CN) is fraught with potential problems. CN is ephemeral and has a diverse array of possible chemical reactions, leading to loss of CN or false formation within the sample container or during preservation, storage, or testing. The public knows that cyanide can be highly toxic, and no drinking water or wastewater utility wants to report false detections for CN. Reliable determination of CN in water samples is important for both wastewater and drinking water treatment plant operators and regulatory agencies. However, because of CN’s diverse chemistry, obtaining reliable results has been challenging and can lead to biased results. Also, sample matrix constituents or preservation chemicals can interfere with analytical determination. The US Environmental Protection Agency (USEPA) acknowledged this difficulty for wastewater samples in the 2012 Methods Update Rule. It revised the footnote for CN preservation to indicate that some interferences may not be mitigated and that 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 issues are also of concern for testing drinking water. In this study, the effects of holding time, preservation, and on-line ultraviolet (UV) digestion with amperometric detection on CN results for wastewater and drinking water were examined, including using field dilution as a treatment for interferences and field spikes as a means to gauge whether sample integrity was maintained. In the first experiment, we compared method performance on wastewater samples tested by manual distillation/colorimetry versus flow injection analysis/UV digestion/amperometry. The variability between two approved methods on split wastewater samples was surprisingly high. The second experiment examined the stability of CN in drinking water without dechlorination and found that, in the presence of chloramines, a CN spike degraded over a couple of hours. The third experiment examined the stability of CN over four weeks without sodium hydroxide preservation when dechlorination was not needed. For raw drinking water and secondary treated wastewater before chlorination, more than half of the CN was retained after four weeks. 56
Next, regulatory CN samples from a drinking water treatment plant were tested, using USEPA–approved methods but taking care to avoid regulatory-reportable but false-positive CN results. We found that field dilution could avoid the false CN formation observed with sodium hydroxide preservation or acidic manual distillation. Dilution is commonly used to reduce the effects of the sample matrix on chemical analyses. Although it raises the reporting limit in proportion to the dilution factor, this is acceptable when a sensitive chemical analysis keeps the reporting limit below a regulatory threshold. Field spikes were helpful in demonstrating the stability of CN samples for same-day analyses. Finally, we examined the impact of different dechlorination agents on treated drinking water and found that dechlorination with sodium thiosulfate can cause false CN formation by flow injection analysis/UV digestion/ amperometry and a strong positive interference by ionselective electrodes. This study explored the formation of false CN when sodium hydroxide or strong acid, high-temperature distillation conditions were used and demonstrated the advantage of the gentle nature of the flow injection analysis method, which avoids strongly acidic or basic conditions. However, under these gentle conditions, it is difficult to retain the free CN field spikes when chlorinated samples are dechlorinated. This balance between an effective analysis and maintaining sample integrity is difficult. It was demonstrated that CN can be reliably sampled and tested using a gentle analysis method, testing unpreserved or carefully dechlorinated samples promptly, avoiding sodium hydroxide preservative-caused apparent false detections, using field dilution as a treatment for interferences, and using field spikes to demonstrate acceptable sample integrity. There is a high potential for processes to go wrong in CN testing, so careful attention to sampling, preservation, and testing details is important in avoiding inaccurate and misleading results. Corresponding author: Michael F. Delaney is the director of Laboratory Services, Massachusetts Water Resources Authority, Central Laboratory, Deer Island Treatment Plant, 190 Tafts Ave., Winthrop, MA 02152 USA;
[email protected].
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