Bleach Regeneration

As previously described, bleaches contain oxidizing agents that oxidize metallic silver to silver ion. Today most photographic processing bleaches use the selective oxidizing ability of ferric iron in a chelated form, often as an iron EDTA complex [122]. Other bleaching agents include iron PDTA, ferricyanide, and persulfate. Iron PDTA has replaced iron EDTA in some bleach formulations as a more environmentally acceptable bleaching agent because it is more active; therefore, less is needed to obtain efficient conversion from metallic silver to silver halide in films [123,124].

Iron EDTA and iron PDTA bleaches used in color negative film and some color paper processes are usually regenerated in large photo labs. In most instances, the bleach overflow is collected and the ferrous ion oxidized to ferric by simple aeration, then makeup chemicals are added to bring the solution back to replenisher strength [125].

In the case of persulfate bleach, the overflow is simply restored to replenisher strength by running chemical analyses followed by specific chemical additions. Many persulfate bleaches require an activator or accelerator bath preceding the bleach. Often, the accelerator bath itself may also be reused by collecting the overflow solution and bringing it back to replenisher strength, through specific chemical additions.

Regeneration is attractive to many photofinishing laboratories as a cost-saving measure, and in most cases it will significantly reduce the amount of iron, chelating agent, and COD discharged to the environment.

Ferricyanide Bleach Management. Although most modern processes use alternative bleaches, ferricyanide is still the bleaching agent of choice for a few older processes such as the Kodachrome and Ektachrome Aerial film processes, and in some instances as an option in the Eastman Color motion picture processes. Ferricyanide bleaches underwent intensive study over the years for the development of regeneration and management methods.

In ferricyanide bleaches, ferricyanide ion is the oxidizing agent, which becomes reduced to ferrocyanide upon use. Together, these ions and related forms of the iron cyanide complex are known as hexacyanoferrates. The concentrations of hexacyanoferrates in an effluent can be minimized by a combination of reliable regeneration methods for the bleach overflow and recovery techniques for the fixer and wash waters.

Ferricyanide Bleach Regeneration. Ferricyanide bleach regeneration requires collecting the overflow and treating it with a strong oxidizing agent. Options include persulfate, peroxide, bromine, ozone, and electrolysis [126]. When persulfate (once the oxidant of choice) is used [127], the specific gravity of the solution may build up due to the formation of sulfate byproduct. Eventually, after several regeneration cycles, the sulfate concentration can grow high enough to reduce bleaching activity. This is usually remedied by discarding between 5 and 10% of the overflow. In lieu of hauling away all of the excess bleach, a precipitation technique can be used to prevent the wasted material from entering the sewer (see subsequent section, "Ferrocyanide Precipitation and Recovery"). Note that this same chemical recovery technique can be used to remove ferrocyanide from a fixing bath, when a fixer instead of a water wash step follows the bleach.

An alternative bleach oxidant to persulfate is ozone [128,129]. The use of ozone requires a fairly significant capital investment in equipment, and safeguards to minimize risk to personnel because ozone is a toxic and unstable gas. However, the specific gravity build-up problem attributable to persulfate is eliminated (see previous subsection, "Ozone").

Another technique having many of the advantages of ozone without the risk of a toxic gas is electrolytic bleach regeneration [130]. Ferrocyanide is oxidized to ferricyanide at the anode of an electrolytic cell. Because of the reduction reaction that occurs simultaneously at the cathode, the cell must be divided by some type of semipermeable membrane. Also, since some hydrogen is produced at the cathode, reliable exhaust ventilation is required. Commercial units are available (see previous subsection, "Electro-oxidation"). This is the most widely used method today.

Recovery of Hexacyanoferrate from Washwater. In some cases, it may be necessary to remove hexacyanoferrates from washwaters following a bleach or fixer. Because the complexes will be very diluted, it is not feasible to use precipitation techniques. Two options have been tried to concentrate these salts and allow them to be recovered:

1. Reverse osmosis can be used to concentrate the bleach components in washwater [126]. By using high pressure (300-600 psi), it is possible to produce a permeate stream containing 90% of the volume but only a small quantity of hexacyanoferrate. The smaller brine stream, although only about 10% of the flow, will contain almost all the hexacyanoferrate complex. Although this technique has been demonstrated repeatedly on laboratory and pilot scales, operational problems have limited its use in commercial practice (see previous section, "Reverse Osmosis").

2. Ion exchange has been proven in practice for removing hexocyanoferrate from washwaters [131]. Rohm & Haas Amberlite IRA-68 resin was used successfully. Experiments showed that 50-60 g of hexacyanoferrate could be collected on 1 L of resin before the effluent exceeded 1 mg/L hexacyanoferrate. Following ion-exchange treatment, the resin can be regenerated with a sodium hydroxide solution, producing a solution containing more than 25 g/L hexacyanoferrate (see previous subsection, "Ion Exchange").

Destruction of Hexacyanoferrate. Ferricyanide solutions were also treated by breaking down the hexacyanoferrate to innocuous products through severe oxidation methods. Hendrickson and Daignault [132] discussed the destruction of ferricyanide of hexacyanoferrate solution by chlorination and ozonation. The latter was shown to be a very slow process [63]. In one production system trace levels of hexacyanoferrate in water were eliminated by adding a halogenated compound, bromochlorohydantoin. Finally, although the chemical destruction of hexacyanoferrate solution by oxidation is technically possible, it is generally not economical on a practical basis.

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