Cyanide Wastewater Analytical Result Variations

A. I would recommend that they have the POTW analyze the samples for nitrates as well. It has been shown that the presence of nitrates and nitrites can cause a false high total cyanide reading in the SM 4500 analysis method, along with the presence of sulfides and ascorbic acid. Further, it is highly possible that nitrogen in the samples will form higher levels of nitrate compound as the samples sit for longer periods of time.

The addition of sulfamic acid ⇦this on eBay or Amazon [affil links] to the samples, which is described in EPA method 335.2, will usually do the trick. Your client should explain this to the POTW, and ask that they do a companion test using 335.2 as a gauge. This might allow the POTW to better understand the issue of false positives on cyanide.

A. The high total cyanide (but not amenable cyanide) may be due to iron cyanide complexes hat are not easily treated by alkaline chlorination. A simple AA test for total iron will tell if this is a problem. Since each iron ion is complexed by six cyanide ions, a little iron can cause a lot of cyanide to pass through your treatment system.

If it turns out that this is the problem, there are a few, simple ways to deal with it, both at the source (dragin) and in the treatment system.

A. The answer that you are looking for concerning spurious results from total cyanide comes down to multiple chemical interferences. Depending where you take the sample the number of can interferences increase. If there is a nitrogen source which may or may not include nitrate and a carbon source you are likely to get spurious and non reproducible results. Method 335.2 is absolutely subject to error if the POTW is collecting samples from the sewer. Think about it, the sample is digested in 18 Molar or so sulfuric acid for an hour and the analysis collects the off gassing. Odds are that you are creating cyanide in from the soup that's in sewerage. I've been through this with the local POTW, EPA, and the POTW's certified laboratory and shown that method 335.2 analytical results are pure junk when there are multiple contaminates present. As one of the many examples that I have I sent out a sample from my acid gas scrubber that was use to capture NOx from a nitric acid copper etch process. We used sodium hydroxide in the scrubber. The only chemicals present were nitrates, carbonates and some copper and sodium hydroxide. There was no possibility of cyanide in the scrubber solution. We sent a sample of the scrubber solution out to a state certified lab for the cyanide analysis using Method 335.2 and noted that there was nitrate present in the mix so the lab could correct for it. The cyanides results were sky high. There's no correction for multiple interferences and the only God knows what interferences you will find in any particular sewerage sample. The POTW continued to find cyanide long after we stopped using it entirely. I've included the title note from a paper that EPA published concerning this issue. EPA notes in the first paragraph of the paper which I included below the statement concerning Method 335.4 "However, when multiple interferences are present, the total cyanide methods produce questionable analytical results". Note that Method 335.4 and 335.2 are identical in sample preparation, the acid digestion.

If you want a good cyanide sample for analysis take the sample at your treatment tank where there will be minimal chemical inferences not from a sewer line.


THE EFFECT OF MULTIPLE INTERFERENCES ON THE DETERMINATION
OF TOTAL CYANIDE IN SIMULATED ELECTROPLATING WASTE
BY EPA METHOD 335.4

Margaret M. Goldberg# , C. Andrew Clayton#, and Billy B. Potter+
# Research Triangle Institute, Research Triangle Park, NC 27709
+U.S.Environmental Protection Agency, EMSL, Cincinnati, OH

INTRODUCTION
The U.S. Environmental Protection Agency approved test procedures for cyanide
determination are listed in the Code of Federal Regulations 40, Ch.1, Pt.136, Appendix B, Table 1 B. The test procedures are used for the reporting of results of analyses as required by the National Pollutant Discharge Elimination System (NPDES) permits. The methods listed for total cyanide in water all share similar chemistries and interferences. These interferences have resulted in many modifications of the procedures that are included in Standard Methods', ASTM 2, and the EPA methods.3'4 The application of these modifications requires prior knowledge of the interference. Procedures to remove the interferences
generally work well when a single interference is present. However, when multiple interferences are present, the total cyanide methods produce questionable analytical results.

The objective of this study was to evaluate the performance of the method when applied to simulated waste effluent that contained known concentrations of cyanide and multiple interferences. Electroplating waste effluent was studied because there have been many problems reported by analysts for that waste, and because it was known to contain a large number of method interferences. In brief, the problems reported included low recovery of cyanide spikes, suspected false positive results, and poor precision for replicate analyses.

Thiosulphate? I always used to use ascorbic acid. Have they changed the method?

False negatives are much more common than false positives, in my experience.

A. Make up a set of known concentrations and measure them.

Then take your waste water sample and spike it with known concentrations and measure it.

This should lead you down the road to some useful information