Wastewater treatment issues: Cyanide destruction questions

A. Hi Robert. Myself and most of the regular readers are from the metal finishing industry. We can recommend good books that cover cyanide destruction for metal finishing, like Cherry's "Plating Waste Treatment" ⇨
and Roy's "... Surface Finishing Wastewater Treatment Systems" ⇨
But they probably aren't highly applicable to treatment by artificial wetlands, the reason being that metal finishing wastes contain metals as well as cyanide, and you probably don't want to put those metals out into the environment.

Regards,

A. That's true, Mitchell, thanks! But the readers should know that the thing causing the chlorine to go into solution as sodium hypochlorite versus evolving into the room as poisonous chlorine gas, is the high solubility of chorine at high pH. If they generate too much chlorine or fail to keep the solution sufficiently alkaline, they can soon have a roomful of poisonous, choking chlorine -- so they need to do a bit of study first.

A. Hi Mark. Not to my knowledge. However, excessive chlorine can wreak havoc further along in the treatment process.

Regards,

A. You are correct about the first step: adding bleach at a pH of greater than 10 will oxidize the cyanide to cyanate within just a few minutes. Although if the spill has a lot of cyanide (if it's a stripper rather than a rinse), you probably should dilute it along the way. A very easy test tool for the first treatment step is potassium iodide starch paper [affil links]. As long as it turns dark blue there is excess bleach present, which means the amenable cyanide has been destroyed if the pH was right.

Once the cyanide has been converted to cyanate this way, it will not be reconverted to cyanide.

The chemistry behind your second step is only partially correct, however. First, I don't think you need to reduce the pH to anywhere near 2.0 -- more like 7.0-8.0 should be adequate. Second, you need to add a lot more bleach to complete the oxidation. This reaction is much slower, and you should allow at least an hour, even for treatment of dilute wastes.

I've been in the waste treatment industry from the time when single-stage cyanide treatment was acceptable. Although I'm not personally aware of any cases where acid was added to cyanide-bearing waste before it was inadequately chlorinated, it has almost surely happened, and I certainly share your concern about the inherent danger in prescribing a procedure that has the potential to go so tragically wrong.

So be careful that have a reliable mixing mechanism and the cyanide waste is not somehow stratified away from the bleach.

A. Only one point to add. In my army days, we were taught that immediate first aid for cyanide poisoning was administration of amyl nitrite inhalant. Unfortunately, these can be hard to obtain.

James Totter, CEF
- Tallahassee, Florida

A. In addition to the amyl nitrite, I would be sure that my local emergency room has a cyanide antidote kit. We actually have had one prescribed to us and keep it in our plating office to be sent to the hospital with emergency personnel. Many hospitals do not have a kit readily available so the best way is to have it on hand. Your company physician could help with this. Bill

A. I currently operate a cyanide based plating process with a heavy emphasis on safety. If you have a potential to generate HCN Gas you need detectors (visual and audio). Current advice is to have at hand Drager Tubes, Oxygen cylinders and related breathing apparatus rather than amyl nitrite capsules. Intravenous antidote kit for paramedics when they arrive is also advisable.

A. Thanks Sean, but American readers should remember that U.K. medical opinion may influence, but does not determine, U.S. law :-)

Shops will be fined by OSHA if they don't have the amyl nitrite capsules (which are the abusable drug commonly known as "poppers", so are sometimes troublesome to find legitimately). If you do acquire them, I've heard that a good way to manage them is to keep them behind a "Break Glass" box like sometimes used for fire alarms.

At one time Eli Lilly offered a "cyanide antidote kit" which had the capsules, the intravenous solutions, and everything conveniently packaged. I understand that the kit is now known as the "Pasadena cyanide antidote kit". Your poison control center, emergency management response team, or anti-terrorism response team will know what you are talking about if a local pharmacist gives you a blank stare.

Cyanide detectors [on eBay or Amazon] are CRITICALLY necessary, not only to warn people when there IS cyanide, but more importantly to help assure them when there IS NOT! Why? According to an editorial I read recently, the First Aid Squad was called to a plating shop for a distressed victim who the manager believed was suffering a heart attack. The (perhaps paranoid) Squad saw cyanide warning signs and treated him (or failed to treat him) accordingly. He died and, according to the editorial, no autopsy was ever performed, again for fear of cyanide poisoning. Heart attack is a far more common cause of distress than cyanide poisoning and you absolutely need multiple detectors to demonstrate that the problem is NOT cyanide or your workers may not receive proper medical treatment.

A. I've one other thing to add.

If someone has passed out from cyanide gas, then don't give them the "kiss of life" or you too could be poisoned. There is a method for resuscitation, and I've seen it described in The Canning Handbook [on eBay, Amazon, AbeBooks], I'd take it that other cyanide solution suppliers would give this information also.

I wanted to commend you for bringing that up about the kiss of life. Many people wouldn't necessarily know to think about cross poisoning from CPR and another thing to add to that is CPR would do absolutely no good until the antidote is administered, because cyanide prevents blood vessels from exchanging oxygen.

A. Thermal hydrolysis is an effective method for cyanide destruction. However, the by-products are formaldehyde and ammonia which also have potential disposal problems.

A. While there is a great amount of paperwork with plating and the whole world knows that you use cyanide are good reasons to avoid it if at all possible.
Some of what the prior reply listed is for a certified waste treatment facility. There used to be ways around this classification. Check with the state first and then the EPA region and a good consultant when you get answers that do not seem correct.

A. Hi Nagesh. As above, potassium iodide starch paper [affil links] or an ORP meter can be easily used to quickly determine that essentially all amenable cyanide has been removed. See the above discussion as well as the referenced link for the techniques, indications, required pH for the operation, etc.

But if you are asking how to do a quantitative analysis for total cyanide, this is a rather sophisticated test, and a repetitive complaint has long been that highly skilled labs get differing results on split samples. Please see topic 12425, "Cyanide Wastewater Analytical Result Variations" if that is your interest. Good luck.

Regards,

A. Hi Ernie. lima beans & almonds would not contain cyanide if sunlight reliably destroyed it; and there are analytical methods for cyanide that involve ultraviolet light in the analysis. So moderate amounts of UV do not quickly destroy cyanide.

Regards,

A. Almond and apple pips do not contain cyanide. They contain amygdalin, sometimes called laetrile or vitamin B17 which can break down to produce cyanide.
Ultra violet light has been proposed for the treatment of cyanide containing effluents but I would expect the low UV content of sunlight to have minimal effect.
More information at
https://www.ncbi.nlm.nih.gov/pubmed/15561362
and similar.

A. Hi Ernie,

I would think more of a bacterial catalyzed oxidation of cyanide than a UV-based oxidation. Cyanide is a relatively stable ion, and is very difficult to remove below some concentrations.

There are some bacteria capable of doing that job, and in some gold mines there is a bacterial wastewater treatment for cyanide.

I really hope this operation doesn't leach cyanide into the puddles as you say, because it would be very bad for the animals and it could pollute more than just this random puddle near the dump.

Best regards,

Stoichiometry

A. Hi Saadat. The reaction is:

So, stoichiometrically, one mole of peroxide is needed to reduce one mole of cyanide to cyanate. But obviously there is a lot more involved in dealing with real-world situations; consequently, much more peroxide is probably actually needed.

Luck & Regards,