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PHOTOGRAPHIC WASTE TREATMENT
with hydrogen peroxide
Introduction
Hydrogen peroxide (H2O2) can be used to treat spent photographic solutions to recover silver and to destroy other compounds which can present pollution problems. These other compounds include:
Peroxide reacts rapidly with these compounds, destroying many of them by oxidation and allowing for the recovery of the silver. This Technical Data Sheet summarizes the chemistry for treating photographic wastes and shows how hydrogen peroxide is useful in reducing the Biological Oxygen Demand (BOD) of these wastes.
Silver Recovery
Ionic silver is precipitated by hydrogen peroxide primarily as metallic silver. The reaction is:
The weight ratio of hydrogen peroxide to silver, based on the above equation, is 0.15 to 1.0. Tests have shown that the silver content can be reduced to less than 0.1 microgram/L using hydrogen peroxide treatment.
Sodium thiosulfate
Sodium thiosulfate, the hypo commonly used in photographic processing, can be oxidized by hydrogen peroxide in alkaline solution to produce sodium sulfate. A pH of 8 or higher will yield complete oxidation. The reaction is:
For complete reaction, the weight ratio of hydrogen peroxide to sodium thiosulfate is 0.86 to 1.00. The reaction is 90 percent complete in 1 0 minutes and 98 percent complete within 30 minutes. Because the reaction is exothermic, hydrogen peroxide should be added slowly to control the temperature when the solution contains more than 50 g/L of sodium thiosulfate.
As an example, a photographic waste solution containing 12,740 mg/L of sodium thiosulfate was treated with 11,000 mg/L of H2O2. As a result, the thiosulfate level was reduced by 98%. In addition, the BOD of the solution was reduced by 99% (initial BOD of 4,000 mg/L, final BOD of 40 mg/L). In this case, the BOD was essentially due to the thiosulfate contained in the solution.
Sodium sulfite
Sodium sulfite is oxidized by hydrogen peroxide to produce sodium sulfate, a nontoxic inert salt. The reaction is:
The required weight ratio of hydrogen peroxide to sodium sulfite is 0.27/1.00.
The rate of reaction between sodium sulfite and hydrogen peroxide is pH dependent. For example, when a solution containing 11.4 g/L of sodium sulfite was treated with 3.4 g/L of hydrogen peroxide (10% excess) the following sulfite oxidation was observed:
| pH | Time to Complete 90% Oxidation | Time to Complete 99% Oxidation |
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0.7 8.2 9.4 10.8 |
0.4 minutes 1.4 minutes 5.0 minutes 6.5 minutes |
2.0 minutes 5.0 minutes 20.0 minutes 25.0 minutes |
Formaldehyde
Hydrogen peroxide oxidizes formaldehyde to produce formic acid, which is then further oxidized to carbon dioxide. The overall reaction is:
The reaction can be conducted under acid or alkaline conditions, but proceeds most rapidly at a pH of 9.0 or above.
In theory 2.3 parts by weight of hydrogen peroxide are required to oxidize one part of formaldehyde. However, since the commercial formalin solution used in photographic processing contains 37 percent formaldehyde, one part of formalin requires only 0.84 parts of hydrogen peroxide for complete oxidation.
Tests have shown that after treatment with a stoichiometric amount of hydrogen peroxide, a formaldehyde solution having an initial BOD of 23,000 mg/L had a BOD of 2,000 mg/L - a reduction of 91.3 percent.
Hydroquinone
Hydrogen peroxide oxidizes hydroquinone in the presence of an iron catalyst to form, first, a mixture of dibasic acids and ultimately carbon dioxide and water. The reaction is:
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