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topic 12215

Change Hexavalent to trivalent chrome at high pH using sodium metabisulfite?

A discussion started in 2002 but continuing through 2019


Q. I machine (Electro Chemical) high Nickel/Chromium components and my solutions contains metal hydroxides and chromium VI. I remove them as waste. They have a pH of > 8, (pH is high to aid machining and is essential.) is it possible to use sodium metabisulfite at this high pH to change Hexavalent chrome to trivalent chrome? If it's possible is there market for my waste product?

Peter G [last name deleted for privacy by Editor]
- Middlesbrough, England, UK


A. Hexavalent chrome can be converted to trivalent chrome at a pH range 8.0 to 9.0 using sodium hydrosulfite.

Karl Weyermann
- Lebanon, Kentucky


! It is important to keep hydrosulfite dry and sealed up, it can combust.

Jon Quirt
- Fridley, Minnesota


! Watch out with storing sodium hydrosulfite, though. It's water reactive.

Guy Lester
- Ontario, California

aff. link
Chromium VI Handbook
by Guertin, Jacobs, Avakian
from Abe Books


A. Hi, Peter. There is only "a market" for the sludge in the sense that there may be companies who will recycle it (at greater or less price than landfilling) so you reduce your chance of being stuck with the liability. But I've never heard of anyone paying for this waste product.

Sodium metabisulfite only works at pH of about 5.0 and less (and very slowly at 5.0). Karl's suggestion sounds good to me.

Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha


A. Hi,

You can reduce Cr+6 to Cr+3 with Ferrous Sulfate; the reaction is very fast.

Moshe Yaakov
- Lod, Israel


A. I have used sodium metabisulfite to neutralise hexavalent chromium solutions from a chrome plating bath. This was done to both make the waste bath safer and easier to handle and to neutralise any chromic acid that may have crept into nooks and crannies in the plating jig or the workpiece. All I can say is that the yellow/orange hexavalent chromium solution went green, which suggests it does work. However, metabisulfite gives off sulphurous fumes, so be warned!

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


i. Fumes are a problem only if the pH gets too low! You need to adjust the pH of the treatment tank.

tom & pooky toms signature
Tom Pullizzi
Falls Township, Pennsylvania


A. Both sodium metabisulfite and ferrous sulphate ONLY reduce in acid conditions, pH <3, and will be of no use in this case. Sodium Hydrosulphite is the material of choice but as warned keep absolutely dry. Just dampening Hydros can cause fires. Add only amount required to take yellow colour away. Cr+3 should precipitate as hydroxide but will be of no commercial value.

Geoffrey Whitelaw
Geoffrey Whitelaw
- Port Melbourne, Australia


A. Sodium Metabisulfite does an excellent job of precipitating Cr+3 as the hydroxide. In general, the procedure is:
1. if there is any ferricyanide ion present, add a little laundry bleach - maybe 1 oz./drum of wastewater.
2. Lower pH to about 3 with sulphuric acid.
3. Add some SMBS - use trial and error to find out how much is needed. Agitate by bubbling air.
4. Raise pH with NaOH. Be careful. I use NaOH pellets in a wire basket. When everything goes green, reaction is complete. Problem is that the precipitate is gelatinous and difficult to filter. Just let it settle and decant the supernatant liquid, leaving the precipitate behind. I can't imagine there would be anyone interested in buying the precipitate.

Dennis Kirsch
- San Antonio, Texas


A. At elevated pH only hydrosulfite can reduce the hexavalent chromium to trivalent.

sara michaeli
Sara Michaeli sara michaeli signature
chemical process supplier
Tel-Aviv, Israel


A. The problem with sodium hydrosulfite is that it will only bring the Cr+6 down to about 5 - 10 ppm, under alkaline conditions.

I believe the best solution to treating a hex chrome waste without bringing the pH down into the basement is ferrous sulfate. It will work at pH 5 - 6. The problem with it is that it is only a one electron reductant, so that 3 moles of ferrous is required per mole of hex chrome. This means that it will make a great deal of sludge if your hex chrome is at all high.

Perhaps a combined approach? Use hydrosulfite to reduce the bulk of the chrome, then polish with ferrous.

Hope this is of some help.

dave wichern
Dave Wichern
Consultant - The Bronx, New York


Q. I have a question to ask, if ferric sulphate would be used as the primary precipitant and then the remaining chrome metal be reduced by sodium hydrosulfide, would this work? Has anyone tried using a combination of iron and hydrosulfide?

Rick Fudalewski
- Mississauga, Ontario, Canada


A. Insoluble sulfide technology reduces Hexavalent chrome to the trivalent state at a near neutral pH. This technology produces less sludge than the traditional SMBS and ferrous treatment process and is less costly and safer than the Hydrosulfite method. It also doesn't generate the corrosive SO2 odors that come from the traditional SMBS treatment methods.

Larry B [last name deleted for privacy by Editor]
- Dallas, Texas



Alok kumar jain
- India


Q. Would there be a way to induce compaction of the gelatinous Cr(OH)3 formed? I am trying to find a chemical that can induce the compaction to lower the disposal costs.


Shee Pagsuyoin
- Quezon City, Philippines


A. Hi, Shee.

No chemical can do that to my knowledge. But "difficult to filter" doesn't mean impossible to filter. A filter press preloaded with filter aid is one way to filter it. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

August 8, 2009

Q. We have tried treating hexavalent chromium with SMBS but our problem is the green color of the solution. Is there a way to convert this into a clear solution ?

Teddy Fabic
color coating line - Manila, Philippines

September 21, 2009

A. Hi, Teddy. Hexavalent chromium reduction with SMBS is only the first step in the treatment. After that, the pH must be brought back up and the green trivalent chrome precipitated out.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

November 17, 2010

Q. In Dennis Kirsch's response where would the ferricyanide come from in a chrome solution. We have just recently had this happen and would be interested to know the chemistry.

Phil nelson
- Victoria, BC, Canada

aff. link
"Plating Waste Treatment"
by Kenneth F. Cherry
from Abe Books
info on Amazon

November 17, 2010

A. Hi, Phil. I think Dennis was referring to the treatment of a mixed waste which contained chrome but was not exclusively chrome; rather, it also included some cyanide waste. There is no cyanide in chromic acid, but some older hexavalent chromate conversion coating chemistries included cyanide as an accelerator.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

April 15, 2011

Q. Dear All
I have gone through this conversation of yours, I understand that chrome trivalent sludge is problem for disposal.

I have a application which runs into approximately 1000 mt per month which requires chromium. but the metals that should not be there in the sludge are Ca, Na, K, Zn, Al,Mg.
Metals that are acceptable in the sludge are Fe, Ni, and Co in any percentage levels from 70% to 1% , but Cu, Mn less than 1%

is it possible to have this sludge prepared with ferrous sulphate and use ammonia to increase the pH?

If you all can contribute to this idea I think we can get good environmental solution to this.

Mr Pratik Sanghvi
- Mumbai Ahmedabad, India

December 14, 2012

Q. Hi dear. Please help me to save humans from chrome+6.

I'm try to convert Cr+6 to Cr+3 please give advice and method for my help. Thanks.

muhammad amir
- Karachi, Pakistan

December 14, 2012

A. Hi Muhammad. This entire thread is on that subject, but you need to give specifics if you want to get specifics. We don't understand how your situation is different from everyone else's because you haven't told us yet :-)


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

December 18, 2012

Q. Hi, Ted Mooney.

Dear sir ,
thanks for your answer.
Please give me the procedure to convert Cr+6 to Cr+3. Thanks a lot.

Muhammad Amir
- Karachi, Pakistan

December 18, 2012

A. Hi Muhammad. Sorry, but we may be having a language difficulty.You must explain your actual situation before anyone can help you.

Are you trying to convert a beaker of dilute wastewater, which contains water and hexavalent chromium into trivalent chromium for some reason? That can be done by lowering the pH to below 5.0 and slowly adding a small amount of sodium metabisulfite, under a lab hood, until the faintly amber color of the water turns to faintly blue-green, indicating the conversion is complete.

Are you trying to convert fasteners that already have a hexavalent chromium conversion coating on them to RoHS-compatible trivalent chromium? It is not possible; the old coating must be removed.

Are you trying to convert a hexavalent chromium electroplating bath to a trivalent chromium plating bath? It is not possible; you must discard the old bath and start new.

Are you trying to design an industrial wastewater treatment plant for treating the effluent from a hexavalent chromate conversion coating line or a hexavalent chrome electroplating line? That may involve additions of sulfur dioxide or sodium metabisulfite after lowering the pH, or employing the previously discussed sodium hydrosulphite. But both hexavalent chromium and the sulfur-bearing chemicals like sulfur dioxide, sodium metabisulfite, and sodium hydrosulfite are quite hazardous chemicals. You can't add them to an unknown waste stream without severe hazards. If you over-acidify sodium metabisulfite, for example, it releases hazardous quantities of poisonous gasses.

If you are designing an industrial wastewater treatment plant for plating wastes, please start with a good book on the subject, such as "Operation and Maintenance of Surface Finishing Wastewater Treatment Systems" [affil. link to book info on Amazon]. Best of luck!


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

April 17, 2013

Q. I've taken a new job where the water treatment says that they can't treat chrome with the acid waste stream. At my old shop we treated it all the same first with Sodium Metabisulite at a higher pH then drop pH to 3-4 with Sulfuric Acid. At that pH we would add Zinc Sulfite and Ferrous Sulfite.

Kevin Westbrook
Setting up new plating shop - Portsmoth Virginia

January 8, 2014

Q. Dear friends,

I'm currently working on nickel mine here in Indonesia. The only problem is our waste water that came from run-off's contains Cr6+ (average 0,5 mg/l). We have made a couples of "extra large" sediment basins for precipitating TSS , but we're still searching for the best solution on Cr6+ problem. With daily rainfall 30 to 45 mm, the water discharge is about 471 m3/hour which could potentially pollute the environment when the sediment basin becomes full.

Do you have any suggestion? is it possible using the SMBS method with such big sediment basins?

Thanks in advance, I'm sorry for my bad English :)

Regy Kurniawan
- Kawasi, North Maluku, Indonesia

August 30, 2015

Q. Hi
Dear sir, I am working in an electroplating plant. Here we have already setup an ETP for plant waste water. For chrome neutralization we have passed out this waste water from a process in which chrome is neutralized by SMBS and precipitated by NaOH. Then we do coagulation by alum for reduce the pH to 7.0; than we do flocculation by polyelectrolyte, and after that we do clarification & settling process. Finally we inject the slurry into a filter press for separation of liquid & solid, and than we get the sludge in cake form.

Here my problem is the large amount generation of sludge. I am searching about any procedure for disposing of this sludge. Can we recycle it by chemical reaction and generate potassium dichromate crystal from this sludge

Umesh kaushik
Electroplating on plastic - Chandigarh, Hariyana, India

Hi Umesh. Wastewater treatment and sludge disposal is expensive. It will almost surely prove impractical to make a product from this sludge. The better approach is to focus on retaining/recycling the chromium before it enters the waste stream. Good luck.


Ted Mooney, Teds signature
Ted Mooney, P.E. - Pine Beach, New Jersey
Striving to live Aloha

How to waste treat one gallon of Alodine

October 26, 2015

Q. I have read through the thread several times and find it terrifically helpful and interesting. I am planning to use a small quantity (a gallon of Alodine 1201 to help with the refinishing of about 100 square feet of a 24 ft. aluminum tug style cruiser. I intend to gather as much as possible of the waste water including the rinse in plastic barrels. I will try to keep the rinse to a bare minimum but will still end up with a very dilute waste. Of all the alternatives suggested what would be the best way to treat such dilute waste? Should I keep the concentrated drippings from the initial application separate from the rinse water? obviously much more difficult but possible.

With thanks in advance,

hans elfert
Hans Elfert
- Richmond BC Canada

October 2015

A. Hi Hans. There is no need to separate the more concentrated treatment chemical from the rinse water; you can collect and treat it all together. Reduce the pH of the collected waste water to about 4.5-5.0 with dilute sulfuric acid, slowly add small amounts of sodium metabisulphite powder until all trace of amber/yellow/orange/honey color is gone and you have only a faint blue/green color. Then raise the pH to about 8.5-9.0 with hydrated lime and filter out / settle out any sediment/precipitate before discharging the water.

There may be local or national regulations prohibiting you from doing this, but neutralizing hex chrome with metabisulphite seems far better than discharging hex chrome. There are certainly hazards if you're not familiar with the chemistry, so keep studying. Wear rubber gloves and goggles at a minimum. The reaction will be slow, but safer at pH 4.5-5; lower pH is used in industry, but can release a good amount of sulfur dioxide gas -- better to keep the pH just low enough to reduce the chrome and just take your time. Good luck.

Readers: the internet is a giant one-room schoolhouse where it is impossible to keep people from reading about stuff which they may not be trained & qualified to do. So just because readers may read some steps here does not mean they have the background and skillset to safely and properly follow them :-)


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

October 27, 2015

A. A small amount of Thiol Red will reduce any Chromium VI and precipitate the metals so that they can be settled or filtered.

Lyle Kirman
consultant - Cleveland, Ohio

October 28, 2015

A. Do what Mr. Mooney says. That will work.

Or, since you have so little, you could reduce the Cr6+ to Cr3+ with ferrous sulfate. pH in the neutral range. Then get the pH up to 9 or so, let the solids settle, and dump the free liquid (should be legal) down the drain.

Dewater the settled solids, the filtrate ought to be legal to sewer. Then, put the solids in the trash.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

April 18, 2016

Q. From the above conversation, I understood about the steps to eliminate the Cr+6 to Cr+3. But my problem is that what are the steps to eliminate trichrome (Cr+3) from an effluent?
What are statutory limits designed for trichrome (Cr+3)?
Kindly suggest .

Satish Kasar
- Pune India

January 2017

Hi Satish. I can't tell you the limits because every country has their own, many states and zones within states have their own, and it may also depend on whether you are discharging to surface waters vs. a public treatment works.

But trivalent chrome can be precipitated. That is, at neutral to alkaline pH the chrome will not stay dissolved but will become a floc which you can settle or filter depending on your situation. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

January 8, 2017

A. It is possible to reduce Cr6 in Hyperalkaline conditions.
Please see this video of mine explaining the details --

Regards Prakash

Prakash Vaithyanathan
- Chennai India

May 5, 2017

A. No one has mentioned using calcium polysulfide for Cr(VI) reduction. We use that to treat hex chrome in groundwater in situ. No pH adjustment is required. Sometimes a ferrous iron salt is added to facilitate immobilization of the Cr3+ as the sulfide or with iron oxyhydroxide.

Jim Rehage
- Austin, Texas, USA

High sulfates in wastewater after chrome treatment

January 30, 2018

Q. I am working in effluent treatment plant, this plant is designed to treat chrome water from chrome plating plant.
We are facing a problem of high sulphate content (>1500 ppm) in ETP product water.

shahrukh ahmed
- Karachi,Pakistan

February 2018

A. Hi shahrukh. You are only at about 1% of the solubility of sodium sulfate, so if you use sulfuric acid for pickling and sodium hydroxide for treatment, the problem may not be solvable. But if you use lime for treatment instead of sodium hydroxide, this number might sharply decline. Jar testing is definitely needed. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

February 22, 2018

A. Rather good read on the subject of using lime successfully in Chromium treatment (tannery effluent) done in 2010 by an Egyptian research team:

I love this paper in that it is quite well written in such a manner as to be understandable to non-academics, and found it quite useful some years back while doing a bunch of jar testing on spent trivalent conversion coating bath solution. It's a fantastic resource!

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

Hexavalent chrome from high temperature working of high-chrome steel

November 7, 2018

Q. Great thread thank you. I have a slightly different situation to the others (solids vs. liquid):

In this case while doing maintenance on heavy duty machinery that operates at high temperatures we are discovering Chrome-6 deposits in a couple of areas. The first is in areas where high chrome steels may have been assembled with the aid of some form of anti-seize product and run at high temp. The second is a case where chrome steel is covered with personnel protection insulating blankets (mainly calcium) and were there was contact and high temps and extended time periods

In both cases the material has been tested to contain chrome-6 and is yellowish. They are relatively dry to slightly pasty. Is there a recommendation for treatment to prevent airborne risk in handling and neutralizing directly from a solid (deposits on a metal part) form or do we need to turn the full volume into a liquid?

Jerry Gorski
- Orlando, Florida

November 2018

A. Hi Jerry. I have never heard of that situation and had never even contemplated the possibility, so thanks for this warning! I have seen dried chromate powders and they can certainly be yellow and resemble powdered sulfur. So if it looks like hex chrome and analyzes as hex chrome, I guess it is.

I don't think it will prove practical to treat these dusts and powders in-situ. Dissolving them into water will give you the opportunity for quantitative analysis, reduction & precipitation, and confirmation of what you have accomplished. Further, you will have a widely accepted treatment protocol to follow.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

November 7, 2018

Q. Thank you, I was hoping to find a way to treat in-situ since these machines are distributed all over, and there may be a risk of it getting it locally air bourn.

Thanks again.

Jerry Gorski [returning]
- Orlando, Florida USA

July 18, 2019

Q. We have traditionally used Nitric Acid to lower the pH of the hexavalent chromium solution before adding the reducing agent. Is there another acid that can be used? I know acetic acid is not a good choice.

Chris Dwyer
- Columbus, Nebraska USA

July 2019

A. Hi Chris. Sulfuric acid is the usual choice and almost surely a better choice than nitric acid. Good luck.


pic of Ted Mooney
Ted Mooney, P.E. RET - Pine Beach, New Jersey
Striving to live Aloha

July 20, 2019 (editor's apologies: not posted until Aug. 9)

Ted is correct in that Sulfuric is better than Nitric; this is because Nitric acid is an oxidizer and will absolutely have the opposite effect of what you are trying to achieve in a reduction.

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont is possible thanks to our supporting advertisers, including this one:


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