Chemical Oxidation Reduction

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Other oxidation techniques, such as the use of hydrogen peroxide [66-68], permanganate, persulfate, bromine, chlorine [69,70], perchlorate, hypochlorite, and numerous other chemical oxidants, have been investigated. Many of these have been used for the regeneration of photographic bleaches, either commercially or experimentally, and in the process were discovered to degrade trace components carried into the bleach solution. Concerns in modern times with these oxidizing agents, in addition to safety and cost of handling as well as general effectiveness, include the possibility of introducing yet another pollutant into the waste stream (e.g., manganese from permanganate), or forming byproducts that are even more toxic if the oxidation reaction is only partial (e.g., forming chloramines by the partial chlorination of organic amines). About the only chemical oxidants used on any significant commercial scale in photographic processing have been hydrogen peroxide [68] and "bromine sticks" (bromochlorodimethylhydantoin), the latter used in a unique application [71] to destroy residual thiosulfate while simultaneously curbing unwanted biological slime during a washwater recycling operation.

Chemical reduction has been commonly used in a number of operations in the past. As previously stated, dithionite and bisulfite have been used to reduce chromium 6+ to chromium 3+ in residues from systems cleaners or black and white reversal bleaches. Sodium borohydride has been used in certain special cases to precipitate silver as elemental silver metal from overflow fixer solutions, or other trace metals from a few processes (see Section 6.4.8, "Other Chemical Precipitants"). Sodium dithionite has been used in combination with ferrous sulfate to precipitate ferrocyanide from bleach wastes as insoluble ferrous ferrocyanide (see subsequent section "Ferrocyanide Precipitation") [72,73].

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