Minimizing wash water containing dichromate?

Discussion in 'Alternative Processes' started by Steve Goldstein, Oct 10, 2018.

  1. Steve Goldstein

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    Is there anything similar to the sodium-sulfite solution for silver printing that can be used in the wash step for dichromate-sensitized processes? Basically I want to be able to fully wash prints with just a few changes of water instead of using a running-water wash. This would minimize the volume of contaminated water and simplify collecting it for proper disposal instead of washing it all into my septic system.

    Thanks for your thoughts.
     
  2. DennyS

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    Steve Goldstein

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    Thank you!
     
  4. jim10219

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    What kind of dichromate process are you running? For gum bichromates, I just let them sit in a water bath, unagitated for about 20-30 minutes. I might change the water once, depending on the volume I’m using. For casein prints I just hit them with a hard stream of water from a spray nozzle for about 30 seconds. Neither technique uses much water. I use a lot more water when I make silver gelatin prints with my print washer. Then, when I’m finished, I spray a bit of sodium thiosulfate onto the print (after soaking it in water to wet the fibers) for a few minutes to thoroughly wash the dichromate away, and then soak it in a water bath for about 10-15 minutes.

    Sodium thiosulfate is cheap and easy to get. However you have to make sure the print is dry and hardened before using it, otherwise it can lift the softened emulsion.
     
  5. NedL

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    Too bad that didn't get much attention here, maybe it will get some now.

    https://www.photrio.com/forum/threads/dichromate-waste-treatment.135482/

    If there is excess ascorbic acid or sodium ascorbate, does that guarantee complete conversion to trivalent chromium?
    How do you know that the reaction is complete?
    Is trivalent chromium safe to put into Steve's septic system?
    IMHO there are still a few questions....
     
  6. DennyS

    DennyS Member

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    Ned, according to the authors of the original paper, the chromium is fully reduced in 2 hours. Based on their plots, it looks like about 98% or 99% reduction. I have no idea about how safe it is to put into a septic system...
     
  7. NedL

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    Thanks Denny.
    I went and found the iron filings and sulfite papers, and compressed them enough to attach.
    They are attached to this post if anyone is interested.
     

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  8. BJ68

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    Yes, the excess of ascorbic acid shifts the wohle thing (reduction) to the side of the trivalent chromium.
    Under different circumstances the other way (oxidation) from Cr(III) to Cr(VI)is possible e.g. https://www.ncbi.nlm.nih.gov/pubmed/17980482 or https://www.ncbi.nlm.nih.gov/pubmed/21105643
    According to this abstract this oxidation reaction Cr(III) to Cr(VI) is not so unlikely: [...]Because reduced Cr has been considered to be the stable form in soils, we were surprised to find that added trivalent Cr oxidizes readily to the hexavalent form under conditions prevalent in many field soils. The key to the oxidation appears to be the presence in the soil of oxidized Mn, which serves as the electron acceptor in the reaction. The relative ability of a soil to oxidize Cr may be predicted by measuring Mn reducible by hydroquinone, or it may be determined directly by means of a quick test in which Cr(III) is added to a fresh moist soil sample.
    Oxidation of Cr by soils was not discovered earlier because the importance of studying fresh field soils, rather than crushed, dried, stored samples, was not appreciated. Plants were severely damaged by Cr(VI) formed from Cr(III) added to fresh soil samples. Hexavalent Cr still was present in a soil stored moist at 25°C for 5 mo.
    [...]

    Completeness of the reaction: There are colorimetric test kits available see e.g. http://www.hannacan.com/PDF/manHI3846.pdf but the simplest method is time and color (if the solution as no other color e.g. from dyes). See http://www.largeformatphotography.i...te-Treatment&p=1332149&viewfull=1#post1332149 where I have done a experiment with 10 L of water, where I have added a few grains of potassium dichromate. The water was slight yellowish and the addition of 10 g ascorbic acid removed the color completely.

    "Steve's septic system": No (my opinion)... first accumulation of "heavy metals" should be avoided and second (the main reason) there is the possibility that Cr(III) is converted to Cr(VI) back...so if you put somewhere soluble Cr(III) in the environment, than it is very likely that you can measure Cr(VI) after a while. Even if there is more or less a anaerobic surrounding and so the oxidation is very unlikely, I would not join this risk, because the stuff will migrate to other places where it can get oxidized.

    On the other hand there will be a very big diluting effect...here in the EU there are "generic concentration limits" for carcinogenic substances see:
    3.6.3.1.1 and Table 3.6.2 in https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32008R1272&from=en
    In short: if the concentration of Cr(VI) in a mixture is lower as 0.1% the whole mixture has not to be classified as carcinogenic....so at that conclusion it´s likely that the amount what you use have not this impact. But there is the risk of accumulation....

    Bj68
     
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    Steve Goldstein

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    Thank you for this very nice discussion, I'm extremely glad I asked the question!

    It sounds like I should be able to do standing-water washes to minimize the wasted, and then reduce the Cr(VI) in the wash water with Citric [NoNoNo - see below] acid, which is cheap. But following on the comment's in BJ68's post I won't dump the waste into my septic system but will instead take it for disposal.

    So now I can begin seriously thinking about alternative-process printing and will start planning my exposure system.

    EDIT: Yikes, I meant Ascorbic acid, not Citric!!!
     
    Last edited: Oct 12, 2018
  10. NedL

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    Thank you Bj68 for the thoughtful and useful post!
    Steve, I think your plan is very prudent.
     
  11. BJ68

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    Made a "quick shot":

    Used some saturated potassium dichromate solution (which I use for gum prints) added concentrated citric acid solution and nothing happens. No change of color so it seems citric acid does not work. Then I used may pump spray with ascorbic acid (5% m/v) and voila change of color and after adding more, there was no orange color anymore.
    So please use ascorbic acid for that, then you are at the safe side.

    Checked amzon.com 1 kg ascorbic acid $ 21.96 and 1 kg citric acid $ 15.16 so there is not so big difference in price.....

    Here are a few pictures from the "experiment":

    Making the citric acid solution:
    Citric-acid.jpg

    Addition:
    addition.jpg

    Mixing and Waiting time:
    Waiting.jpg


    Adding ascorbic acid 1:
    Ascorbic 1.jpg

    Adding ascorbic acid 2:
    Ascorbic 2.jpg

    Adding more of the 5% solution to be on the safe side, because used a saturated K2Cr2O7 solution; Endpoint:
    Endpoint.jpg

    Bj68
     
    Last edited: Oct 12, 2018
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    Steve Goldstein

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    Yes, my fault entirely for typing Citric acid when I meant Ascorbic acid. Vitamin C was in my mind, and Citric begins with C. I edited my earlier post as well to point out this error.
     
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