Silver Recovery 641 Introduction

As previously discussed, silver from photoprocessing operations is much less toxic than free silver ion. Silver is generally removed from photographic products during processing in the form of silver thiosulfate complex, Ag(S2O3)23-. This complex has a dissociation constant of 5x10-14; thus, it is virtually impossible for free silver ion (Ag+) to be present at any significant concentration levels in photoprocessing effluents [10].

In black and white products, because the final image is metallic silver, the amount of silver removed during processing will depend on the amount of exposed image area. In color products, processing removes essentially all the silver from the emulsion. Although primarily found in fixers and bleach-fixes, small quantities of silver will also have seasoned the developer and bleach, and some will be carried over into solutions following the fixer or bleach-fix. (In modern minilabs, the stabilizers or final rinses often contain approximately 25% of the silver available for recovery.)

Even the very smallest processing operations have adequate economic and regulatory justification to recover silver from their exhausted fixers and bleach-fixes, low-flow washes, and stabilizers.

Whether the same justification applies to conventional washwater depends on the size of the operation, treatment alternatives, and applicable regulations. These washwaters usually contain about 2-5% of the potentially recoverable silver in the process. In conventional process machines that utilize multiple-stage, countercurrent, low-flow washes, the amount of silver in final washes may be considerably less.

The two most commonly used methods of silver recovery are electrolytic recovery and metallic replacement using steel wool cartridges [84]. Ion exchange has been used on a limited scale, generally by larger laboratories to treat dilute solutions such as wash waters. Precipitation with sulfide was used in the early part of the 20th century by many laboratories, but because of its handling hazards has largely been superceded by safer processes. It continues to be used in some situations today, particularly in developing countries, because of its low cost and ease of use despite the attendant personal hazards. A more recent development is the TMT (trimercapto-s-triazine) precipitation process, a safe process that can easily be automated, which is finding increasing use in larger U.S. and Canadian laboratories. Other precipitants have been tried by some commercial laboratories but are not widely used for various reasons.

Electrolytic silver recovery can be successfully applied to concentrated fixer and bleachfix solutions in either an in-line or a terminal application. Other solutions that can be terminally desilvered include low-flow washes and plumbingless minilab stabilizer, but generally not highflow washes. This method usually finds application in situations where higher concentrations of silver are encountered and must be removed rapidly, and/or where only a portion of the silver is to be removed continuously so the processing solution can be reused.

Metallic replacement is normally used with fixers and bleach-fixes in a terminal application, known as "tailing," to recover silver from solutions destined for the drain. In the past it was also used for in-line desilvering of bleach-fixes, where the addition of dissolved iron to the reclaimed solution was a benefit. It has also been successfully used to treat low-flow washes that have been combined with fixer and bleach-fix overflows. Metallic replacement has also been used on final washes alone, but with limited success.

Some large photoprocessing installations have used ion-exchange methods to recover silver from washwater or mixtures of washwater, fixers, and bleach-fixes. The destructive oxidation of bleach-fixes and fixers with chemicals such as hydrogen peroxide [68] and chlorine [69,70] also causes silver to precipitate as silver sulfide, which can be removed by settling or filtration.

If electrolytic silver recovery and/or metallic replacement are effectively used, a photoprocessing laboratory should be able to recover on site between 90 and 99% of the potentially recoverable silver. The effluent silver concentration from such a laboratory would be in the 1-5 mg/L range. Even higher concentrations will have no adverse impact on a secondary waste treatment plant or a receiving body of water [9,30,85] after the effluent is biologically treated. If ion exchange is used to treat the silver-bearing effluent, the treated stream would be expected to contain between 0.5 and 2 mg/L silver. If the fixer and bleach-fix overflows are first pretreated by electrolysis followed by ionexchange treatment of the silver-bearing effluent, the final silver concentration can be reduced to the 0.1-0.5 mg/L range. The overall silver recovery efficiency can be 98-99+% of the potentially recoverable silver [86].

The EPA contracted with Versar, Inc., to provide a guidance document describing the control of water pollution in the photographic processing industry. This document, published in 1981, described the results of sampling effluents from 48 photoprocessing laboratories. The maximum silver concentration in the effluents averaged over a 30-day period was 1.1 mg/L, for laboratories using conventional silver recovery methods. Laboratories using conventional silver recovery plus the ion-exchange treatment of washwater averaged 0.4 mg/L over the period. Maximum single-day concentrations were 3.7 and 1.3 mg/L, respectively. The Versar report stated that more than 99% of the photoprocessing facilities it surveyed (over 1100 plants) discharged wastewater to POTWs [7],

In a 1980 study, total silver concentrations in effluents from two POTWs were reported as 0.2 and 0.004 mg/L, respectively, while both effluents contained free ionic silver concentrations of only 4x10-4mg/L [46].

Subsequent studies in the 1990s by Shafer et al. [85,87], Adams and Kramer [88], Rozan and Hunter [89] and others [12], using "clean" analytical techniques (which had not been available previously), also proved conclusively that photographically derived silver (i.e., silver thiosulfate complex) could not survive in a treatment plant nor in a receiving stream, because en route it was always converted to silver sulfide. More than 100 researchers in North America and Europe addressed the subject of silver fate and effects in the environment in a designated silver research program (The Argentum Conferences) between 1989 and 2000 (see previous subsection, "Impact of Biotreatment on Silver") [48-53]. Moreover, a number of these scientists showed it was virtually impossible to measure any significant amount of free silver ion (Ag+) in receiving bodies of water, regardless of the source of the silver and notwithstanding whether that water was aerobic or anaerobic, due to the ubiquitous presence of sulfide in practically all natural waters [54-58]. An entire final conference was devoted to this subject in 1999, with a follow-on meeting in 2000, and the aggregate conclusions of the decade's work published in 2002 as a guidance book for use by scientists and environmental regulatory agencies throughout the world [12]. These revolutionary findings not only eliminate silver as a metal of concern in terms of acute aquatic toxicity, but promise to change past conventional thinking on the toxicity of many other common metals in the natural environment as well.

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