I mailed the following to alt-photo-process last November regarding the
disposal of dichromates. Since then, I got some mail from various
chemists who updated this information. (I had also posted this in one
of the rec.photo groups asking for clarification of certain points and
got lots of helpful responses.) I'll summarise the responses at the end.
Also, I'd like to reiterate Steve Avery's excellent mail about taking
all chemical advice posted here with a grain of salt and in light of
local regulations.
So, with that in mind...
----------------------- original message ----------------
Last week, I posted a formula for a chromium intensifier along
with the usual cautions about handling a dichromate. Someone,
Ty Monson I think, asked about the safe disposal of dichromate
solutions. Good question.
As fate would have it, I was cleaning out a bookshelf last night
and came across a copy of the Kodak booklet, "Disposal of
Photographic-Processing Effluents and Solutions" (it is dated
1973, I seem to recall purchasing it in the late 1970's, so no
doubt there are more current editions available.) Here is what
it says about disposing dichromates:
"Dichromate bleaches (which are used to a certain extent in
some black-and-white and color processes) offer potential
toxicity to bacteria in a waste-treatment plant. Thus,
destruction of dichromate ion may be warrented before such
ions leave the processing laboratory. Dichromate ion in
bleach solutions contains hexavalent chromium (that is having a
valence of six), and can be converted into trivalent chromium
(and thus precipitated) by the addition of any of several
chemical reducing agents such as thiosulfate or sulfite. Any
alkaline material (including waste developer) can be added to
neutralize the acidity and cause the chromium to precipitate as
the hydroxide.
The mixture can be filtered so that the solid waste can be
disposed of without discharging soluble dichromate or chromate
ions into the sewer. Note that any silver bearing dichromate
bleach can be desilvered by adding sodium chloride after treatment
with hydroxide [??? I assume this refers to using something like
sodium hydroxide to neutralize acidity]. The silver chloride
can be filtered off along with the chromium hydroxide and sent
out for refining."
---------------- end original message ----------------------
In my original post I also asked for clarification of some of the
ambigous language in the above. From the mail I got, I discovered
the following:
- The above phamplet is indeed outdated wrt current EPA (USA)
regulations, and in fact is no longer available from Kodak, though no
one gave me any specific current guidelines that superceeded the above
for disposing dichromates. Consult your local authorities for the
most up-to-date information!
- Chemically, the following procedure should work. Add used developer,
or even better, a strong alkaline (baking soda, lye [sodium hydroxide],
etc) to the dichromate solution until it becomes alkaline. Determine
this by using litmus paper, which should be availble from many sources,
including Edmund Scientific, if you are in the US. Then filter the
solution to remove the chromium III sludge. I was told that double
layers of coffee filters should work, though lab grade filter paper
would be best. (Do I need to say that you *don't* use you coffee maker
for this step?). At that point, you should be able to dispose of the
liquid which should be chromium free and give the filtered sludge to a
place that handles hazardous waste. The trivalent chromium is less
toxic (according to my sources) but I still wouldn't just toss it down
the drain.
- If the dichromate has been used in conjunction with a silver process
(not an issue with gun printing, but with bleach and intensification
processes it is) then you need to go the extra step of adding table
salt (sodium chloride) after neutralizing the acidity but before
filtering. Unfortunately, I don't have any information about how much
sodium chloride to add.
- Several people warned me not to use thiosulate to neutralize the acidity
of the dichromate solution as any silver in the dichromate won't then
precipitate easily.
That's about it. I offer this in the spirit of information sharing and
NOT as expert advice. If anyone can and wants to add to or correct any
of this, I would welcome it.
Carson Graves
carson@ileaf.com