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topic 6852

Wastewater treatment issues: Cyanide destruction questions


Q. I am looking for information on how to batch treat our rinse water from our Almate zincate process before transferring to our evaporation system. I assume I need to destroy the cyanide component before mixing with the waste stream.

Rinse water contains sodium cyanide and caustic soda.

Can anyone point me in the right direction?


Bruce O'Brien
- PHX, Arizona


A. Hi Bruce. The most economical way is usually with sodium hypochlorite (industrial strength bleach). If you are batch-treating because you have very little such waste water, and you are looking for a minimal system, it can be done manually.

First, you raise the pH to at least 11.5, if necessary, with further additions of an alkali (with good mixing). Since you are already familiar with caustic soda, that could be a good choice for the alkali.

Second, you add the bleach, slowly, with mixing, until there remains an excess of chlorine as indicated by a dark blue to light purple color on potassium iodide starch paper [linked by editor to product info at Amazon].

You have now converted the amenable cyanides to cyanates, and in many cases the waste can now be combined with your other flows for pH adjustment and precipitation. If the waste is to be evaporated and hauled, as I am inferring, this may be all you need to do for cyanide treatment.

You asked to be pointed in the right direction, and I hope we've helped, but you should have a competent consultant or laboratory person help you out and give you hands-on instruction, and you must verify that this idea meets the requirements of your operating permit. Best of luck.

Ted Mooney, Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


A. The most reliable method for treating cyanide is with an ORP (oxygen reduction potential) meter. Some portable pH meters have ORP capabilities. The pH works best between 11.0 and 11.5. The higher the pH, the more bleach it takes to reach the proper ORP reading. At the proper pH, add bleach until the ORP meter reads 450 mV. This will break the cyanide down to the cyanate. You might want to check your permits to make sure it is alright to release cyanates into your rinse water flow. Some regulations require the cyanide to be totally destructed before doing so. To finish the destruction, raise the pH to about 8.5. The ORP reading should be a minimum of 600 mV at this point. Excess bleach may need to be added to achieve this. Good Luck.

Phil Pace
- Tulsa, Oklahoma


Q. Thanks for the informative page, I'm chasing some more detailed information on destruction of NaCN in wastewater. I did see a book on the topic a number of years ago (~5 years)- It may have specifically dealt with its destruction in an artificial wetland. Any assistance greatly appreciated.

Robert Catchpoole

Plating Waste Treatment
Kenneth F. Cherry

O&M of Surface Finishing Wastewater Treatment Systems
Clarence Roy

April 2014

A. Hi Robert. Myself and most of the 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.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


A. Just a thought if you're not aware, you can make sodium hypochlorite in house.You only need a vessel,tank, etc.(sized for your needs), and a electric variac with a anode and the tank for a cathode. Just add salt & water.

Mitchell Lee Shepard
- Gilbert, Arizona, U.S.A.

That's true, Mitchell, thanks! But before messing around, 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 studying first.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


A. The destruction of sodium or potassium cyanate is done with hydrogen peroxide (H2O2). You could also write the reaction that will give you ammonia.

Bouzar Essaidi Samir
- Bouzareah, Algeria

November 20, 2008

Q. Can excessive bleach impair the further reduction of cyanide?

Mark Z [last name deleted for privacy by Editor]
- Santa Ana, California

April 2014

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


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

To minimize searching and offer multiple viewpoints, we've combined multiple threads into the dialog you're viewing. Please forgive any resultant repetition or failures of chronological order.

Cyanide safety, cyanide detectors, antidotes


Q. Dear Metal Finishing team,

Our labs will be dealing with cyanides after Christmas for small scale stripping and plating baths and I have to write a security protocol to make sure that people know what to do if there is any accident with cyanide solutions.

I came up with a scenario where someone would spill a solution of cyanide into the sink drain. What do I do to neutralize cyanides?

As far as I know, you can neutralize cyanide with a Sodium Hypochlorite solution (Bleach) under basic conditions (pH = 9-11) which will oxidize cyanides (CN-) ions into cyanates (CNO-) ions. However cyanate is also toxic (not as much, but still), but I read that you can transform them into NH3 and CO2 by adding a strong inorganic acid to cyanates ( bringing to pH =2). However, my concern is that I'm afraid that by adding acid to the solution, the cyanate will be transformed back into cyanides and will produce HCN (something that we don't really want to happen). Are my concerns right?

Thanks for your help,

Daniel Picard, Chemist
Industrial Materials Institute - National Research Council of Canada - Boucherville, Quebec, Canada


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 [linked by editor to product info at Amazon]. 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.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


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
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

William F. Morgan
- Saratoga Springs, N.Y. USA

Hydrogen Cyanide Detector


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.

Sean O Reilly
- Ireland


A. Thanks Sean, but American readers should remember that U.K. medical opinion may influence, but doesn't decide, 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 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 your local pharmacist gives you a blank stare.

On the matter of cyanide detectors, they are critically necessary, not just 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 having a heart attack. The (perhaps paranoid) Squad believed he was suffering cyanide poisoning and treated him (or failed to treat him) accordingly. He died and, according to the editorial, no autopsy was performed, again for fear of cyanide poisoning. Heart attack is a far more common cause of distress that cyanide poisoning and you absolutely must have multiple detectors as evidence that the problem is NOT cyanide so your workers can receive proper medical treatment.

Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey


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, I'd take it that other cyanide solution suppliers would give this information also.


Ian Brooke
university - Glasgow, Scotland

Ed. note: Please note that the above entries are somewhat old; check the current status of regulations in your area before proceeding.


Today, many companies are very careful of how to handle or store liquid for disposal, that contains "Cyanide". However, as mentioned before by Daniel Picard, and Ted Mooney, I will repeat it too in my post.

For example:
If you want to neutralize acid liquid with caustic liquid, first thing, a person takes a sample from the caustic tank, and use a kit that test for Cyanide. The kit stated that pH must be neutral, so, your first step is to neutralize your sample with acid, until you reach a neutral level. Note, Cyanide is released when the pH is low, so, know what you are doing in term of safety.

Use your Cyanide kit to test for Cyanide. If there is, let say 1 ppm or higher, than you want to find out how much it is needed to neutralize Cyanide.

Add drops of Sodium Hypochlorite (bleach), few drops, and check again. Find out how much you added. Use your potassium iodide (starch paper) as Ted Mooney said.

When you know how much it takes to add bleach, than use that amount to neutralize your caustic in the tank.

After neutralizing is complete, you will be able to neutralize your acid/caustic for disposal..

Shimun Ashurayah
- Hamilton, Ontario, Canada

sidebar July 13, 2011

How is cyanide disposed?

Nicholas K [last name deleted for privacy by Editor]
student - Kenya

July 13, 2011

C'mon now cousin Nicholas.

A good deal of information in response to that issue was already posted on this very page. Just blowing it off and posting a 4-word question, isn't the way to do your science homework :-)

Please do yourself and your teachers proud by saying: "I understood your explanation that cyanide wastes can be disposed of by ..., but what I don't understand yet is ..." :-)


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

2-step Chemical Treatment of Cyanide Waste vs. Thermal Hydrolysis

July 20, 2012

Q. I am to get quotes for a "turnkey" system to destruct cyanide. My search for information suggests the 2-step process: Cyanide to Cyanate using pH10 or higher and ORP sensors (+) 250 mV for around 15-30 minutes with mixing.
Second takes 8.5-9 pH since alkali is lower and NaOH can control the pH. The oxidant is added till ORP reads @(+) 300 mV. However, one website claims the use of Thermal Hydrolysis systems. Is anyone familiar with this?

James Davis
electronics - Pelzer, South Carolina, USA

July 24, 2012

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

Lyle Kirman
consultant - Cleveland, Ohio

July 31, 2012


I thought longer residence times at high temps would kill both of the byproducts that you mentioned. Am I wrong?

James Watts
- Navarre, Florida

August 6, 2012


I don't think the temperatures are high enough to destroy ammonia or oxidizing enough to convert formaldehyde to formate, but I'm not sure about this. Conversion of formaldehyde to formate is a relatively simple reaction with peroxide but ammonia can be a problem if the discharge permit has a limit for it.

Lyle Kirman
- Cleveland, Ohio USA

July 30, 2012

Q. Sounds like even at 80k for a small Thermal Hydrolysis system it just creates different problems to replace the cyanide problem. With low quantities of waste (200 gallons/ month) and having to get permits from DHEC and waste water treatment center approval, paying for consultants, getting drawings approved by PE in the field, getting a certified outside lab to prove your process, having a state certified employee working under our licensed waste facility coordinator, running a process with potential deadly risk all to avoid paying @350/50 gallon drum. Unless I'm being told an exaggerated version of what needs to be done, why would any facility process cyanide waste? Sorry for the rant but too much paperwork to make money.

James Davis [returning]
- Pelzer, South Carolina

July 31, 2012

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.

James Watts
- Navarre, Florida

Ed. note: For more on this topic, please also see letter 40394, "Cyanide plating waste water treatment".

June 27, 2014

Q. Please let me know how to test cyanide concentration in plating waste.

mfr of sodium hypochlorite - Bangalore, Karnataka, India

June 2014

A. Hi Nagesh. As above, potassium iodide starch paper 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 letter 12425, "Cyanide Wastewater Analytical Result Variations" if that is your interest. Good luck.


Ted Mooney,
Teds signature
Ted Mooney, P.E. RET
Pine Beach, New Jersey

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