WEF Discussion Forums
Laboratory Management and Technical Issues
We have a client with whom we are encountering some issues with cyanide detetion. Up until about 3 weeks ago we were getting non-detect results for their samples. Then, all of the sudden we start getting hits of 0.01-0.025 mg CN/L. We have rerun samples to verify that there were no false positives and all have come back as the same result ± normal sample variance. This is causing some major concern for them and us. They did some random sampling throughout the plant that we tested for CN to see if we can find where it might be coming from. Those results broke down to anywhere there was any type of chlorination, including the city tap water, the samples showed a hit similar to what we got for the routine samples. Any samples that came from an unchlorinated section of the plant were non-detect. None of the samples registered a hit for chlorine during our pre-distillation checks for Cl- and sulfide. As far as they are telling us, they have changed nothing in the plant operation. I believe they told us they are dechlorinating in the plant with sulfur dioxide. Is anyone aware of any kind of reaction with Cl- and something else that would register as CN? They claim that their state regulators told them that Cl- was a positive interferant, but I have always understood it to be a negative one that would decompose the CN. Can someone help me out here? I don't believe that we are detecting native CN, but that we are somehow creating the CN during the distillation process. The sample do have high Nitrates (20-30 mg/L) but we treat with sulfamic acid prior to distilling. We even tried doubling the sulfamic acid concentration with no affect to the CN result. The nitrate concentration has been consistently at this level and we have not had any problems before. As I mentioned earlier, or method is midi-distillation and we follow that with continuous flow analysis via the typical pyridine-barbituric acid analysis. I am out of town currently and don't have good online access, so I might not be able to answer questions untill Monday, but please everyone feel free to brainstorm and ask the questions anyway. I'll try to fill in as much information as I can get to while away. Thank you everyone.
What type of a "plant" is this? What is the source of the waste waters? One typically finds CN in plating shops. As you know, chlorine is the "treatment" for CN.
If the plant operations are NOT using cyanide, it would appear that either the treatment system is creating something that "tests out" as CN, or the sample prep. is doing it? What test method are you using?
If there is no source of CN in the plant it is probably an interferent. I would look for a thiocyanate or nitrile compound such as acrylonitrile. Back in the 70's when I analyzed CN and experimented I found these compound present or formed during treatment to give positive interferences.
Try analyzing samples both for total and CN amenable to chlorination. UV and alkaline chlorination should distroy most CN compounds. I would try chlorinating one of the process samples that previously had a zero CN when analyzed. If CN then shows up then it is being created during chlorination. Then try some dechlorination.
Jeff, this is a wastewater treatment plant, I don't know any more than that as far as type or methods or any of the other technical jargon that you WWTP guys have. We are using 335 for the analysis. Just a standard acid distillation followed by pyridine-barbituric acid detection.
I am 99.99% certain that it is an interferant of some kind. As I mentioned before, we have never had hits from this client in the past. They just started showing up, but not in the influent. I'll try the cyanide amenable to chlorination route. That hadn't occurred to me but I bet it will give us some clues. I remember SM has something to say about CN- being generated during the chlorination/dechlorination process with certain types of waste. That will be our next line of investigation.