Water Online

November 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

Issue link: https://wateronline.epubxp.com/i/896704

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wateronline.com n Water Innovations 26 INSTRUMENTATION electroplating wastewaters, where these ions are usually present in high concentrations, often leading to inaccurate readings. Metal ions, such as copper, also interfere with this method and, in many commercial tests, concentrations as low as 1 mg/L are enough to alter the results. Copper, iron, zinc, and other metal ions can also interfere with the detection of free cyanide when using silver titration methods. In gold-mining processes, where a certain minimal threshold of cyanide is needed to ensure gold extraction from ores, a trustworthy estimation of the free cyanide concentration is essential. Thiosulfate also is known to lead to overestimated concentrations of free cyanide, while the presence of sulfides leads to the appearance of precipitates and renders the detection of endpoints difficult. Both chlorinating agents and silver nitrate titrations are sensitive to the presence of thiocyanate, a major interfering ion in most free cyanide detection methods. For chlorinating agents in particular, thiocyanate reacts with chloramine T, which is subsequently erroneously detected as cyanide. In many commercial tests, concentrations as low as 1 mg/L can already cause interferences. Moreover, thiocyanate also binds to silver ions, leading to false readings if present in concentrations higher than 10 mg/L. Therefore, determining free cyanide in samples known to contain thiocyanate can be challenging when using these methods. A few solutions have been proposed to overcome the previously mentioned limitations, with the removal of nitrites using sulfamic acid being one of them. Yet the addition of supplementary reagents to such complicated matrices brings certain risks and can lead to the formation of new interferences. When facing similar cases, diluting the sample remains the safest solution. This option works only for samples with relatively high concentrations of free cyanide and is not recommended when detection limits as low as 0.1 mg/L must be reached. Corrin-based indicators are a promising new alternative to bypass several limitations of the previously discussed methods. This straightforward technology is rapid, user-friendly, sensitive, and specific for free cyanide. Moreover, it works in both pure water and complex and challenging matrices. The indicator is highly tolerant to elevated concentrations of potentially interfering compounds (e.g., 200 mg/L of nitrite or thiosulfate) and is, therefore, less prone to inaccurate results. Sulfides, the only main interfering ions, can easily be removed through precipitation with ferric chloride (FeCl 3 ) and subsequent filtration prior to free cyanide detection. This technology allows specialized, as well as nonexpert users, to determine free cyanide accurately, using nontoxic materials and minimal equipment. A Framework To Evaluate Your Free Cyanide Detection Method Of Choice A thorough understanding of the advantages and limitations of currently available free cyanide detection methods enables companies in the mining and electroplating industry to manage cyanide more efficiently, reduce operational costs, and safeguard local ecosystems. To facilitate the decision-making process, we propose actual and potential users to compare free cyanide detection methods in terms of their selectivity, speed, simplicity, safety, and equipment requirements. This framework can help users choose and/or reevaluate their free cyanide detection method of choice, taking into consideration their most critical needs and requirements. n References 1. https://www.cyanidecode.org/cyanide-facts/use-mining 2. http://www.toxipedia.org/display/toxipedia/Baia+Mare+Cyanide+Spill 3. Koenig, Robert (2000). Wildlife Deaths Are a Grim Wake-Up Call in Eastern Europe. Science 287(5459):1737-1738. 4. Solutions to analytical chemistry problems with clean water act methods, USEPA Office of Science and Technology (March 2007). 5. ASTM D4282-15. Standard test method for determination of free cyanide in water and wastewater by microdiffusion, ASTM International, West Conshohocken, PA (2015). 6. ASTM D2036-09. Standard test methods for cyanides in water, ASTM International, West Conshohocken, PA (2015). 7. Zelder, F. H., Männel-Croisé, C (2009). Recent advances in the colorimetric detection of cyanide. CHIMIA International Journal for Chemistry 63 (1-2):58-62. 8. Koç, E., et al. (2014). Interference of metals with the determination of free cyanide. Proceedings of 14th International Mineral Processing Symposium, Kusadasi, Turkey 1027-1033. 9. Breuer, P. L., Sutcliffe, C. A., Meakin, R. L (2011). Cyanide measurement by silver nitrate titration: Comparison of rhodanine and potentiometric end-points. Hydrometallurgy 106(3):135-140. 10. Alonso-González, O., et al. (2017). Free cyanide analysis by silver nitrate titration with sulfide ion as interference. Minerals Engineering 105: 19-21. 11. Nagashima, S (1983). Effect of thiocyanate on the pyridine-pyrazolone method for the spectrophotometric determination of cyanide. Analytical Chemistry 55(13):2086-2089. 12. EPA SW-846. Hazardous Waste Test Methods. Method 9014: Cyanide in waters and extracts using titrimetric and manual spectrophotometric procedures (2014). 13. Aebli, Balz; Männel-Croisé, Christine; Zelder, Felix (2014). Controlling binding dynamics of corrin-based chemosensors for cyanide. Inorganic Chemistry, 53(5):2516-2520. Dr. Felix Zelder has been a group leader at the Institute of Chemistry of the University of Zurich since 2006. He specializes in inorganic and bioanalytical chemistry and is the author of more than 11 peer- reviewed papers and review articles on the colorimetric detection of cyanide in various types of samples. He is a scientific partner at CyanoGuard AG. About The Author Mining-impaired water in Romania

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