Home /
Search 🔍
the Site
pub Where the world gathers for
plating, anodizing, & finishing Q&As since 1989


Ferric chloride vs. ferrous chloride

Q. Dave,
Is Ferrous Chloride better than Ferric Chloride for reducing H2S levels in bio-gas from dairy manure digestion?

Lee Lutz
- Crittenden, Kentucky
August 5, 2022

↓ Closely related postings, oldest first ↓

Q. We use ferric chloride in our wastewater plant. We treat mostly copper (copper sulphate) and just a small bit of chrome. I have read a lot of people use ferrous chloride. Is there much difference in the performance or use of these two chemicals when treating copper?

Andrew Burt
plating shop employee - Birmingham, UK
March 2, 2009

A. Hi, Andrew. Whether ferrous or ferric, and whether chloride or sulphate, the basic purpose of these iron chemicals in industrial wastewater treatment is as a co-precipitant. In many cases iron is unregulated in the effluent and can be used to drive some copper (or other heavy metal) out of solution by going into solution itself. Iron also has value as a clarifying agent in water treatment but I don't know that this role is important in your case.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. Ferrous is better than ferric.

Ferric chloride is, in my opinion, of little use in chemical waste treatment. Ferric ion precipitates as a ferrihydrite (commonly called "ferric hydroxide") at a pH of 3 - 4. This is too low a value to achieve effective co-precipitation of most transition metals, like Cu.

If ferrous chloride is added at a fairly low pH, and then, the pH is raised to about 8, three things happen:

1) Ferrous ion becomes a very powerful reducing agent. In waste treatment, this is your friend.

2) Some of the ferrous ion is oxidized to ferric ion, by atmospheric O2.

3) Both ferric and ferrous "hydroxides" form a powerfully co-precipitating "solubility limiting phase" that will trap and drag down numerous regulated metal ions, including Cu, Zn, and Ni.

Do some bench tests.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

thumbs up signThanks, Dave. I have used both ferrous and ferric, mostly on zinc-bearing waste. In either case the pH needs to be reduced before adding the co-precipitant, but I did not realize that you need to get down to 3-4 for the ferric to be most effective. I would not call ferric "of little use" because it cut the zinc in half and turned a "no go" situation into a "go", but I yield to your deeper knowledge of this, appreciate the education, and will certainly use ferrous next time if there is a next time.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

Perhaps, next time I have a Zn problem, I'll give ferric another shot. The experiment/successful result is king.

Ferric chloride is a very effective coagulant. It could be that you were seeing the benefit of this. It is often very difficult to distinguish between coagulant mechanisms, co-precipitation mechanisms, and "sweep floc" mechanisms in figuring out why a particular treatment works. So often in this field, one's opinions are conditioned by the type of waste you've seen in the field. As we are not university researchers, we're happy enough to have something that works.

The magic of iron in mixed states of oxidation is something that's helped me many times. Fe2+ and Fe3+ precipitate together as something I'll call "hydrous magnetite." There are many vacancies of various sizes (Fe3+ is about 0.53 A; Fe2+ is about 0.74 A) in this lattice (reverse spinel structure; it has "A" and "B" sites for cations) that will accept other transition metal ions.

The downside of iron chemistries is the increased sludge volumes.

There's also a similar co-precipitating effect in iron sulfide systems...but I won't go on boring you all with my solution chemistry hobbies. :)

dave wichern
Dave Wichern
Consultant - The Bronx, New York

March 6, 2009

Q. I would like to thank you both. Ted for an excellent website from which I have learnt so much. Ted and Dave for your help and time. As is usual for me one question now has me wanting to ask several. I have found when doing bench tests they are a good way to check if a principle will work, however, when I treat 10000 liters of differing concentration and components I quite often struggle to get a consistent end result as opposed to 1 litre. Plus my boss does not share my enthusiasm for my f--**-g about as he terms it. So, if you will bear with me I have to ask the following: Can I add ferrous chloride at a ph 8-9 or should I always add it at a lower ph? Is 2-3 ok?, I add sodium dithionite at this ph to reduce Cr6. Our system is constant flow and I am currently running it: Adjust pH 2-3, add ferric chloride. Add sodium dithionite. Adjust ph 9-10, add more ferric chloride. Add Polyelectrolyte. I can get clear water but struggle to remove the last 2-3 ppm of copper. Could this be a complex caused by ammonia [affil link] in a product we use? Thanks again!

Andrew Burt [returning]
plating shop employee - Birmingham, UK


A. You have my sympathies regarding your boss. I once had a boss who called doing bench tests "playing in the lab" as opposed to the REAL work.

First, why use sodium dithonite to reduce chrome? The main advantage to this treatment chemical is that it will reduce chrome at higher pH values than sodium metabisulfite. Metabisulfite will work fine at pH 2 - 3 (the closer to 2 the better) and it's lots cheaper.

Second, yes, ammonia [affil link] could indeed be interfering with dropping your Cu. It there a way to segregate it out, possibly by using a batch treatment on the ammonia [affil link] bearing waste?

If not, you might try a bit of sodium sulfide or thiocarbonate along with your ferric chloride as the final step before adding the polyelectrolyte. You might also do better with a final precipitation pH in the low 9's. At this pH, more of the NH3 is present as NH4+, which is not an active complexant.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

A. Andrew, not sure what you mean when you say "struggle to remove the last 2-3 ppm", but in my experience, when you have a strong chelator present like ammonia, it usually takes a precipitant product to achieve compliance. If you find you are using too much ferric chloride or ferrous (if you change to it) try using an organo sulfide like DTC or a more environmentally friendly precipitant called polythio carbonate. These products should help "polish out" the remaining metals without creating significant sludge.

Gordon Djani
- Greenville, South Carolina

A. Andrew,

I have read all conversations.
The best is to use electrical energy with some Iron electrodes.
Try to neutralize the wastewater up to pH about 7.
Bring in some Amperages. The quantity depends of the volume of wastewater.
After the treatment you will have Cu effluent values of 50-100 ppb, and a very little sludge production. About 1/10 of the quantity you have with dosing FeCl3 or other iron salts.
If you have some Chromate in the untreated water, afterwards all chromate disappears in the sludge. Small amounts of ammonia [affil link] have minor effects to the treatment.
The main advantage is you don't need other chemicals as for neutralizing. No polyelectrolyte.
I think with this suggestion you will have very solid wastewater processing, with always good effluent values.

Bert R Snijder
- Zwolle-Hasselt, The Netherlands

March 17, 2009

Q. Hi Dave, I use dithionite basically because it is supplied under a contract. I also feel that I have a peace of mind in that should the first stage rise above the set parameters I will still reduce the chrome. Am I correct in thinking I will need up to twice the amount if the solution should be alkaline? I will try your suggestions for sodium sulfide or thiocarbonate. I had the plant running at low 9's but found the final discharge could easily drop to the 7's as it is a small constant flow plant and mixing can obviously be a problem. By adjusting the position of the ph probe and flow/dosing rate I am running approximately 9.5 at discharge so I will gradually reduce the final dosing ph to keep it consistent and use the discharge ph as a guide. Unfortunately I cannot prevent the addition of ammonia [affil link] so I think this is the best approach?

Gordon, I am not familiar with the chemicals you mention although I have tried calcium chloride successfully to reduce sludge. I seem to use quite a lot of it and when in the quieter hours at night mostly water passed through the polyelectrolyte caused the sludge to float as it effectively overdosed. What are your thoughts?

Bert, your system seems simple enough but I know absolutely nothing about it. Could you recommend some further reading and I could experiment (f**k about)! Thanks everyone for your help.

Andrew Burt [returning]
plating shop employee - Birmingham, UK

March 19, 2009

A. Andrew. The chemicals I refer to are precipitant products used specifically to precipitate out metals in solution, usually complexed. The precipitant chemistries do not aid with coagulation like the calcium chloride would or the iron chemistries will, but they work well in conjunction with any of those chemicals. The precipitants do not add significantly to the sludge volume. If you wish to read further about these products google DTC and/ or polythio carbonates (thio-red) (comparisons) and related articles and websites should come up. Best of Luck

Gordon Djani
- Greenville, South Carolina

March 21, 2009

A. I do not believe that more sodium dithonite is required at higher pH. The reaction may take longer, or not go as far to completion.

The best book I've ever seen on chem waste treatment is Chemical Processes in Waste Treatment, by Eilbeck and Mattock, published by Ellis Horwood Press (all of this is from memory, so pardon me if I made a minor error).
Sadly, it's out of print; but a well stocked university library might have a copy.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

June 27, 2012

Q. I would like to know is there any superiority that I should prefer to use ferrous sulphate instead of ferrous chloride?

Kashmira Tank
- Rajkot, Gujarat, India

June 30, 2012

A. Kashmira, what do you want to use it for?

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

June 24, 2012

Q. Please let me know whether ferric chloride or ferrous sulfate will be a good coagulant chemical in boiler water treatment process?

Thanks with Regards,

Yoga Kannan Indiran
- Trichy, Tamilnadu, India

February 5, 2013

Q. Hello,

Ferrous chloride in atmosphere turns to ferric chloride. What should be done to prevent it from turning into ferric chloride? Can we use any acid to do that, because I need ferrous chloride to work with?

Jalpan Pandya
- Chennai, Tamilnadu, India

March 28, 2013

What chemical will reduce Nitrogen and Phosphorous from process effluent? My process water goes through anaerobic treatment and seems to be fine for BOD, COD, and metals; but P and TKN seem to be high. What can I do?

Artamas Prime
- Jacksonville, Florida, USA

April 26, 2013

A. Hello Artamas,

TKN, I think you must try to reduce the income, so to speak, avoid to use the products that can rise this value above spec. We are actually reducing our use in every product that has phosphorus and nitrogen because of this legal limits...

Phosphorus is a little simpler: You can add some calcium hydroxide water (the supernatant water in a calcium hydroxide-water mix) instead of sodium hydroxide to get the pH high, so the phosphates will be precipitating as calcium phosphate. If you like, you can use a calcium chloride solution, but you will lose some of the calcium as hydroxide at pH 9.

Hope this helps more than it confuses!


Daniel Montañés
- Cañuelas, Buenos Aires, Argentina

Can we use Ferric/Ferrous Sludges to Make and Sell Ferric chloride?

August 13, 2014

Q. Hi, a Client of ours has a lot of Ferric Sludge which they would like removed. Now I know that on some of our Sewerage Treatment plants we use Ferric Chloride.
I was just wondering if there is a possible solution that we could offer in terms of turning the ferric sludge into a product that we can use in our works to make it more cost effective to them.

I have googled "ferric hydroxide into ferric chloride" but no matches come up -- can anyone assist?

shantelle corns
water- waste water industry - UK

August 14, 2014

A. I have never had any luck at all giving away waste or products derived from wastes to anyone.

It's like a used car - you're inheriting a potential problem. Ferric chloride is cheap, and it's just not worth the risk.

dave wichern
Dave Wichern
Consultant - The Bronx, New York

A. Hi. If your client can use their "waste product" in-house; fine, but don't take anyone else's waste! The problem is two-fold: first, accidents can happen (google "Times Beach, MO" -- a ghost town, a whole town permanently evacuated because used oil was spread on the roads but the oil was contaminated).

Secondly, regulatory agencies are rightfully concerned with people trying to get rid of waste products by giving it away or having it processed by the lowest bidder, who may be a fly-by-night -- so they hold the generator of the waste responsible forever, and they view any buyer/user with suspicion. It's probably good that they do this, so that wastes don't get mislabeled or 'lost' without proper treatment -- but everything has unintended consequences, and this regulatory attitude completely killed off America's attempt at an "industrial waste exchange" industry. So now virtually all used chemicals are landfilled; almost never is a chemical recovered/reused/recycled by a second party anymore :-(


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

December 18, 2015

Q. What will excess ferric chloride do in my water treatment plant

foluke Aderinola
- Ikeja, Lagos, Nigeria

April 14, 2016

Q. May I know the price difference between the ferrous chloride and ferric chloride. We are doing a study for metal precipitation in waste water treatment and want to see if the economics is favourable by changing from Ferric to ferrous based salts.

Sabah ul Solim
Oil and Gas - Bangalore India

January 7, 2017

Q. I am dosing ferrous chloride for hydrogen sulfide removal in wastewater but it seem ineffective would using ferric chloride be a better alternative and is is more expensive?

Martyn Burgess
- United Kingdom

January 8, 2017

A. Ferric ion is much more effective than ferrous ion for sulfide removal. The lower required dose should more than compensate for the difference in cost.

Lyle Kirman
consultant - Cleveland Heights, Ohio

February 28, 2017

Q. Is there anything potentially harmful to a Ferrous Chloride solution being added to waste water to clear the odor. Any risks vs benefits?

Jerry Edward
- Luzerne, Michigan, USA

Ferrous Chloride dose rate to remove Sulphides

August 1, 2018

Q. My situation: We currently dose Ferrous Chloride for septicity and sulphide removal. We do not know what dose rate we need to optimise the correct dosing rate as we have extremely low ORP probe readings -350 within our process. I am not convinced either that Ferrous Chloride is the best solution but my manager will not change chemical due to costs. Do you have any ideas please?

Martyn Burgess
- United Kingdom

simultaneous August 4, 2018

A. Hello Mr Burgess,
The place to start is jar-testing your waste stream using varying levels of ferrous, and narrowing down your concentration range until you hit your optimal dosage. Your optimal dosage may not be the same as someone else's, because of the inherent variability amongst processes. There is no one size fits all answer. Find what works for your particular waste:)

Rachel Mackintosh
- Greenfield, Vermont

August 5, 2018

A. I believe that ferric ion would be more effective than ferrous ion for sulfide removal as Fe2S3, a black precipitate. Based on this ratio of iron to sulfide, you would need at least 1.17 mg/L of iron for each mg/L of sulfide. It is also possible to remove sulfides by oxidation.

Lyle Kirman
consultant - Cleveland Heights, Ohio

finishing.com is made possible by ...
this text gets replaced with bannerText
spacer gets replaced with bannerImages

(No "dead threads" here! If this page isn't currently on the Hotline your Q, A, or Comment will restore it)

Q, A, or Comment on THIS thread -or- Start a NEW Thread

Disclaimer: It's not possible to fully diagnose a finishing problem or the hazards of an operation via these pages. All information presented is for general reference and does not represent a professional opinion nor the policy of an author's employer. The internet is largely anonymous & unvetted; some names may be fictitious and some recommendations might be harmful.

If you are seeking a product or service related to metal finishing, please check these Directories:

Chemicals &
Consult'g, Train'g
& Software

About/Contact    -    Privacy Policy    -    ©1995-2023 finishing.com, Pine Beach, New Jersey, USA