Dichromate-Waste-Treatment

I posted this text at the altphotolist, too

Even if the amounts of dichromate are very tiny, there is a problem with the dichromate waste which is accumulated during gum printing.

In the literature there are some methods reported to reduce the toxicity of that stuff. One way is reducing the Cr(VI) to Cr(III) with some reduction reagents. In the web you will find protocols for that e.g.
- sodium sulfite http://www.wrc.org.za/Knowledge%20Hu...jul99_p363.pdf
- sodium thiosulfate http://infohouse.p2ric.org/ref/19/18012.pdf
- iron filings http://www.must.edu.ph/mjst/wp-conte...e-Chemical.pdf
- sugar http://inpressco.com/wp-content/uplo...aper524-27.pdf

So far I see all this methods have a few disadvantages:
a) They are working only in a small range of pH. between 1 and 2.
b) To get to this pH you have to use sulfuric acid
c) With the first one, you get toxic gas and with the second the chemical (fixing agent) is expensive.

So I searched further and found this nice paper:

Reduction of hexavalent chromium by ascorbic acid in aqueous solution.
Chemosphere 57:609
https://www.researchgate.net/profile...ication_detail

It works at neutral or even at slight alkaline pH, it goes fast, you see a color change and if you use a excess of ascorbic acid you are in the acid pH-region and it´s cheap because ascorbic acid is a food supplement.

In short for 3 g dichromate you use 10 g of ascorbic acid, which is twice the amount according to the paper above, to be on the safe side and to get the pH more in the acidic direction.

Here you find pictures and a description in German of the experiment what I have done:
http://illumina-chemie.de/dichromatv...ung-t4201.html

Additionally you can use the sodium salt of ascorbic acid, too and you can thread all stuff with it. E.g. I use now a little bit of a ascorbic acid solution for washing my brushes and even here I can see the color change.


Reduction to Cr(III) is a good way to reduce the toxicity of the Cr(VI) compounds and if you use a ascorbic acid solution you can decontaminate your workspace, the bench and other equipment which was in contact with dichromate-containing solutions.


Bj68

I am not a chemist but I do need a simple to understand English plan of action. I have plenty of food grade citric acid crystal. Can that be used to reduce the problem and how exactly?

Thanks for posting!

Thank you for bringing up the documentation that you have researched. I found the South African article on the use of sulfite interesting. But why do you reject reduction by sulfite? I performed a while ago intensification by the dichromate method, reduced the dichromate with sulfite, and the orange solution turned green as expected. I used a sulfite amount stoechiometric + 50% margin. As used, the dichromate solution was already acidic (0.8% concentrated HCl); isn't it always the case when dichromate is used as a bleach? Which takes care of your stated negative (a).

The S.A. article on reduction by sulfite did alert me to the fact that the reaction is not intantaneous (as I believed from watching the color change); see Fig.1 and 2. So, I'll let the dichromate/sulfite stand for... one our or overnight before dumping.

I suggest that you replicate your post on APUG, where a few professional chemists might give their professional opinion.

It should work with citric acid, too

See:
[...]Other work by Hasan and Rocek (1973b) demonstrated that in addition to 2-hydroxy-2-methylbutyric acid, this reaction occurs with other α-hydroxy carboxylic acids, such as maleic, malic, tartaric, and citric acid.
[...]
"USE OF TARTARIC ACID AND ISOPROPYL ALCOHOL IN THE REDUCTION OF HEXAVALENT CHROMIUM AND APPLICATION TO CHROMIUM CONTAMINATED SOILS AND CHROMITE ORE PROCESSING RESIDUE"
http://drum.lib.umd.edu/bitstream/ha...=1&isAllowed=y

But I do not know how fast goes the reduction with citric acid...if you use ascorbic acid you are more or less on the safe side...


Edit:

Quote:

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Originally Posted by Bernard_L

But why do you reject reduction by sulfite?

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I am a little bit allergic against sulfite-vapor (got constricted bronchia), so do not try this method and so far I have seen, this method has to be made in a fume-hood...

Quote:

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Originally Posted by Bernard_L

I suggest that you replicate your post on APUG, where a few professional chemists might give their professional opinion.

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Okay...will do that...


Bj68


Edit the second:

Done: http://www.apug.org/forum/index.php?...atment.135482/

Quote:

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Originally Posted by BJ68

Reduction to Cr(III) is a good way to reduce the toxicity of the Cr(VI) compounds and if you use a ascorbic acid solution you can decontaminate your workspace, the bench and other equipment which was in contact with dichromate-containing solutions. Bj68

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Thanks for posting this. I got all excited about gum over platinum, because of the beautiful results that it could produce. However, I abandoned the whole effort, because of toxicity concerns.

Did you also look into precipitating out the chromium from the Cr(III) to a solid, which can be more safely discarded?

Well. that´s the little problem what I have seen...

I tried this (precipitating) in a few different ways, but with no success:

a) adding sodium carbonate powder
b) adding sodium hydroxide solution
c) adding ammonia solution (drops only) with pH-Control
d) using the sodium salt of ascorbic acid to have more or less a neutral pH ---> reduction worked and then 2 drops ammonia solution 25%

With each of this I got a dark green/blueish solution and no precipitation of Cr(III)hydroxide....

In the dissertation over citric acid as reduction agent I found following phrase: [...]If complexed with organic acids such as fulvic or citric acid, Cr(III) will remain soluble at pH values up to 6.7 depending in part on the pKa of the organic acid and the COOH-Cr(III) ratio (James and Bartlett, 1983b).[...] so my guess is that ascorbic acid is a complexing agent, too.

Eureka...found: http://jcsp.org.pk/ArticleUpload/1935-8634-1-RV.pdf
Ascorbic acid is a good chealting agent...with makes the stuff water soluble....

Additionally:
https://link.springer.com/article/10.1007%2FBF00398817
and the two pictures of the text:
https://static-content.springer.com/...398817/000.png
https://static-content.springer.com/...398817/001.png

Other source:
https://manhattan.edu/sites/default/...eport_2013.pdf

That means you can use ascorbic acid solution to decontaminate your skin if you had contact and even it´s an antidote (see springer link).

Bj68


Edit: Here a patent for soil decontaminating:
http://www.freepatentsonline.com/6221002.html

Excellent post - thanks for sharing your research.

I use a small amount of Ammonium Dichromate in the new cyanotype process. I can keep the chemistry off of me, but I am most concerned for dumping the rinse water down the drain into the septic system. (Rinse water from cleaning brushes/rod/glassware/pipette and exposed prints) Wouldn't want to poison my property over time (I also have a well). I got kicked out of high school chemistry before I learned too much. Can I just stir some ascorbic acid into the rinse water and then no longer have a nasty chemical to go down the drain?

Quote:

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Originally Posted by jp

I use a small amount of Ammonium Dichromate in the new cyanotype process. I can keep the chemistry off of me, but I am most concerned for dumping the rinse water down the drain into the septic system. (Rinse water from cleaning brushes/rod/glassware/pipette and exposed prints) Wouldn't want to poison my property over time (I also have a well). I got kicked out of high school chemistry before I learned too much. Can I just stir some ascorbic acid into the rinse water and then no longer have a nasty chemical to go down the drain?

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Good question.

Now we are getting down to plain speaking.

what is the disadvantage of contracting a wastehauler to just
take it away once a year or 2 years or whatever amount of time it takes to
fill up a 15 gallon drum ?
i've done that with my regular photo chems for 17 years ... it hasn't cost me much $
and i have the peace of mind that i didn't poison someone in the septic plant or turn my yard
into a brownfield/superfund site ... or cause some sort of other reaction that might cause harm &C.

We do that here industrially. The cheap, fast method is reducing with FeCl2. Go buy some clorhidric acid and put some iron in it (nails, etc) fort a couple of days in a ventilated area (outside). You will get a green solution on FeCl2. You can use it to reduce the CrVI to CrIII.

Quote:

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Originally Posted by jp

Can I just stir some ascorbic acid into the rinse water and then no longer have a nasty chemical to go down the drain?

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Yes, that should work, too, but you have to extend the reaction time, because the concentration of the Cr(VI) is very low or you use more ascorbic acid.
I made a test with a big amount of water (10 L) where I put a few grains of potassium dichromate in it. The color of the liquid was slight yellowish. After addition of a bigger amount (about 10 g) ascorbic acid the water was completely decolorized.

So my suggestion where: Collect the first rinse water (where the main part of the dichromate gets washed out) in a bin, add about 1 g ascorbic acid for every liter and let it sit for a while (best would be until your next cyanotype session, but 1 hour should be enough).

One disadvantage or important fact: The treatment changed the toxicity of the chromic compound, but it do not change the fact, that chromium itself is a heavy metal and the cumulation of this stuff e.g. in soil if you have a cesspit (Sickergrube in German) should be avoided. According to this https://en.wikipedia.org/wiki/Chromium_toxicity (the first lines) even Cr(III) has its own problems.

Edit: You can get a side reaction with the other compounds of the cyanotype rinse water...so expect some precipitation....
The other aspect can be how stable is the ferricyanide against the ascorbic acid. From memory I think that there is no risk to produce prussic acid https://en.wikipedia.org/wiki/Hydrogen_cyanide which is very, very toxic, because ferricyanide reacts only with mineral acid to release hydrogen cyanide see https://en.wikipedia.org/wiki/Ferricyanide So I think ascorbic acid is to weak for that reaction.

Bj68

Bj68; thank you for answering! You are great!

From this response I think I will not dump the first rinse down the drain.

Short update: It seems that citric acid is not working as reducing agent see my quick shot at
Minimizing wash water containing dichromate?

Bj68

Quote:

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Originally Posted by BJ68

methods reported to reduce the toxicity

Reduction to Cr(III)

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The common method in darkrooms is (or was...) mixing the dichromate solution with used developer, inmediately the orange shifts to green indicanting the Cr III presence.

It's the way I did it when I was making reversal process with dichromate.

Note: this should be done in open air because some "sulfur dioxide" may be released.

Quote:

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Originally Posted by Pere Casals

The common method in darkrooms is (or was...) mixing the dichromate solution with used developer, inmediately the orange shifts to green indicanting the Cr III presence.

It's the way I did it when I was making reversal process with dichromate.

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Mixing with used developer sounds better.

Do you know if color change indicates toxicity reduction?

Quote:

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Originally Posted by Randy Moe

Mixing with used developer sounds better.

Do you know if color change indicates toxicity reduction?

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Yes... the Cr (VI) solution is orange, the Cr (III) is green, I drop used developer until the the color does not change anymore, so not much Cr III remains, then I add more to be sure that enough developer is added to exhaust the VI as possible.

IIRC there is a "Predominance diagram" that tells the remaining Cr (VI) depending on pH and Cr concentration...

Docs posted by OP are quite interesting...