Environ. Sci. Technol. 1996, 30, 3084-3088
Removal of Silver in Photographic Processing Waste by TiO2-Based Photocatalysis MIN HUANG, ERWIN TSO, AND ABHAYA K. DATYE* Center for Microengineered Ceramics and Department of Chemical & Nuclear Engineering, University of New Mexico, Albuquerque, New Mexico 87131
MICHAEL R. PRAIRIE AND BERTHA M. STANGE Solar Thermal Technology Department, Sandia National Laboratories, P.O. Box 5800, Albuquerque, New Mexico 87185-0703
Treatability data on actual waste show that titaniabased photocatalysis can be used to remove silver ions from black and white photoprocessing waste. The silver ion is reduced to its metallic form producing particles comparable in size to the TiO2 catalyst particles. The mass of silver recovered approaches three times that of the titania. Thiosulfate (the predominant chemical in spent fixer) plays a complicated role in the process of silver ion reduction: (1) as a hole scavenger, it can increase the silver reduction rate; (2) as the chemical that stabilizes silver ion in solution, thiosulfate hinders photocatalysis when present at high concentration. Metallic silver can be separated from titania by the physical process of sonication. We also show that sunlight can be used directly to power the photo-electrochemical silver removal process.
Introduction During photographic processing, silver ions end up in the used fixer at concentrations up to 6000 ppm. In addition, the discharge of wastewater from silver jewelers may contain Ag+ generated from etching operations. Although the harmful effects of silver are not completely understood, it is known to be an effective bactericide and can seriously impair biological systems (1). Approximately half of the municipal sewer codes in the United States limit the silver discharge from 0.05 to 5.0 mg/L (ppm) (2). Because of rising public environmental concerns, however, more stringent regulations on the discharge of silver are expected. Photographic processing generates small amounts of waste: a typical black and white photolab produces about 1 gal of used fixer/day. Furthermore, the discharge of wastewater is periodic due to batch operation. Therefore, * Corresponding author telephone: 505-277-2833; fax: 505-2771024.
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small-scale treatment that is inexpensive, easy to use, and effective enough to meet applicable standards is highly desirable. The commonly used methods for removal of silver ions are metallic replacement, ion exchange, and electrolysis, although each has its limitations (3). In metallic replacement, iron goes into solution and is discharged as a cation, being replaced in the solid phase by silver. However, discharge of Fe2+ in the effluent may be unacceptable and is a potential nuisance. Silver concentrations in the treated effluent can remain high if the unit is not operated properly and the spent fixer is passed through the unit without adequate residence time. The second method, ion exchange, has other operational difficulties such as biological growth, resin fouling, and stripping of silver by the thiosulfate. A method that is widely used is electrolysis where silver is removed in the form of metallic deposits on the cathode. However, this method also has its problems. The lowest achievable Ag+ concentration in the effluent typically ranges from 100 to 200 ppm, well above the 5 ppm discharge limit. The lower limit is determined, in part, by the rate at which thiosulfate can redissolve the deposited silver (4). As Ag+ concentration is reduced, the current density has to be limited otherwise reduction of the thiosulfate to sulfide can occur, and this places a further limitation on the amount of silver that can be recovered under practical operating conditions (5). Since the commercially available methods all have potential drawbacks, we have investigated the use of TiO2 photocatalysis as an alternative approach for removal of silver from photoprocessing wastes. When a photocatalyst (e.g., TiO2) is exposed to light having energy greater than the band gap, electron-hole pairs are generated. The conduction band electrons can reduce metal ions in solution while the holes can react with hydroxyl ions to yield hydroxyl radicals, which can mineralize organic species to CO2 and H2O. The ability of UV-irradiated TiO2 to reduce Ag+ has been reported previously by Hermann et al. (6). Most of their experiments were performed with AgNO3 dissolved in deionized water, from which they proposed kinetic models to describe the deposition of Ag on TiO2. They demonstrated total removal of Ag+, even in the presence of thiosulfate ions, which were added to deionized water at a concentration of 0.02 M. However, the concentration of thiosulfate in a photofixing bath is about 1 M, in addition to the presence of other chemicals that also serve to stabilize Ag+ in solution. While the experiments of Hermann et al. (6) hinted at the possibility of treating Ag+ in photographic waste, no experiments were performed on solutions derived actually from photoprocessing. In view of the potential for photocatalysis in this application, we have investigated the treatment of wastes generated during black and white photoprocessing. The effluent from black and white photoprocessing, unlike its color counterpart, transmits the ultraviolet light necessary for activating the catalyst. The use of sunlight as a source of UV photons was also explored for reduction of Ag+.
Experimental Section Spent fixer solutions were obtained from commercial photoprocessing laboratories or from photoprocessing operations at the campus printing shop and transmission
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1996 American Chemical Society
electron microscopy laboratory. The solutions were used as received without pH adjustment. The pH of fixer is between 4 and 4.5. Photographic fixer solutions contain sodium thiosulfate, sodium sulfite, and acetic acid as the major constituents (7). The photoreactor was a 1.5-L glass vessel with a water jacket for maintaining ambient temperature. The reactor has an inner diameter 13.5 cm and is 6.5 cm tall. A quartz plate sits on top of the reactor to prevent splashing of the reactor contents while allowing transmission of ultraviolet light. The reactor contents were stirred with a magnetic stirrer. A similar setup was used for the solar experiments except that the jacketed reactor was replaced with a 3-L beaker; sunlight entered the beaker from the top and sides. In all tests, the reactor was open to the atmosphere. A 100-W Hg spotlight was used as the UV light. In one experiment, we made use of ambient solar irradiation over a period of several days. All experiments were performed using 300 mL of photoprocessing waste, normally containing the catalyst at the concentration of 1 g/L. Some experiments were performed to explore the role of various chemicals present in used fixer solutions and to determine if the use of hole scavengers might be advantageous. The chemicals used were sodium thiosulfate, ethylenediaminetetraacetic acid (EDTA), and salicylic acid (all reagent grade). One set of experiments was performed on simulated fixer using reagent-grade silver nitrate and sodium thiosulfate to arrive at the desired silver concentrations. The catalyst used was Degussa P25 titania. About 80% of P25 has the anatase crystal structure while 20% has the rutile structure. The primary particles are single crystals around 25 nm in diameter (8) and are agglomerated into particles of about 0.3 µm. The catalyst suspension was allowed to equilibrate for 15 min while stirring in the dark, then at t ) 0, illumination was initiated. Samples were removed throughout the course of the reaction for silver analysis by atomic absorption (AA) spectroscopy. AA is very sensitive for silver, with the lowest detectable concentration being 10 ppb (µg/L). Transmission electron microscopy (TEM) was used to study the microstructure of the spent catalyst and the amount of silver deposited on the TiO2. The effect of catalyst weight loading was also investigated. Under our conditions, the TiO2 photocatalyst is stable in solution and does not photocorrode.
Results Control Experiments. Figure 1 shows the concentration of Ag+ as a function of time for the two control experiments (light alone and catalyst alone) along with the results obtained when UV light was allowed to excite P25 TiO2 in the pot reactor. The sample came from a batch of used fixer that contained about 1800 ppm of Ag+. The pH of the fixer is between 4 and 4.5 and does not change subtantially during the course of the experiment. For Ag+ reduction to Ag0, the overpotential of the TiO2 conduction band electrons is large enough that any small change in flat-band potential induced by pH changes would have a negligible effect on the reduction of Ag+. In these experiments, we also found that the amount of dark adsorption of the Ag+ ions on TiO2 was negligible. The results in Figure 1 show that light and the TiO2 catalyst must be present together for the Ag+ in solution to be removed. Removal of Ag from Spent Fixer. For the data shown in Figure 2, used fixer containing about 6000 ppm of Ag+
FIGURE 1. Removal of Ag+ from used fixer by UV light illumination alone, TiO2 alone, and UV light + TiO2. It is evident that light + TiO2 is necessary for the removal of Ag+.
FIGURE 2. Removal of Ag+ from fixer containing 6000 ppm of Ag+. The rate of Ag+ removal declined when a Ag/TiO2 weight ratio of 3:1 was reached. Addition of fresh TiO2 increased the rate of Ag+ removal but the final concentration could not be lowered below 200 ppm.
was used. The concentration of Ag+ was higher than that of the fixer solution used for the experiments reported in Figure 1. This represents the highest concentration of Ag+ we found among the samples of used fixer we studied. The white, milky suspension of TiO2 turned dark within 1 h. After 15 h, the Ag+ concentration had been reduced by 50%, and the weight ratio of reduced silver to catalyst was 3:1. By this time, the rate of Ag deposition on TiO2 had slowed down considerably. At this point, we let the slurry settle, decanted the clear liquid, and added more catalyst, resulting in an increase in Ag+ reduction rate as shown in Figure 2. A second addition of fresh TiO2 and removal of used catalyst at the end of 30 h of irradiation caused only a marginal increase in silver removal rate. The final Ag+ concentration achieved in this experiment was about 200 ppm. A similar experiment was performed (not shown), but this time instead of filtering the spent catalyst, we simply added another aliquot of TiO2, bringing the total TiO2 concentration to 0.2 wt %. We found that the reaction rate increased as soon as the fresh TiO2 was added even though the spent catalyst had not been removed from the suspension. These results suggest that the darkened catalyst does not prevent the UV light from entering the solution, rather the deposited Ag metal may block light from entering the TiO2 and, therefore, making it incapable of accepting any more Ag. It is also possible that at this loading, the rate of Ag redissolution becomes equal to the rate of deposition
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FIGURE 3. Influence of TiO2 weight loading on rate of Ag+ removal.
FIGURE 5. Electron micrograph of spent TiO2 catalyst after sonication. The large Ag particle has separated from the TiO2 catalyst.
FIGURE 4. Electron micrograph of spent TiO2 catalyst containing a Ag/TiO2 weight ratio of 3.4:1. Note that the particles of Ag metal are considerably larger in diameter than the crystallites of TiO2 on which they are deposited.
and no further removal of Ag+ is possible, similar to that seen for electrolysis (4). Effect of Catalyst Weight Loading. If silver reduction rate is a function of available TiO2 surface area, then observed silver removal rates should depend on catalyst loading. The experiments reported in Figure 3 were performed with a batch of used fixer. As shown in Figure 3, there was no difference in reaction rate for TiO2 loadings of 0.1 and 0.2 wt %. It has been previously suggested that the rate of reaction versus loading approaches a broad maximum at a loading of approximately 0.1 wt % with Degussa TiO2 (9). Thus, it seems reasonable to conclude that the 0.1 wt % loading is sufficient to harvest all of the incident light and that there is no advantage in going beyond the 0.1 wt % catalyst loading. Separation of Ag from the Spent Catalyst. The spent catalyst after Ag deposition was examined by TEM, and a representative image is shown in Figure 4. In this experiment, used fixer containing 6000 ppm of Ag+ was contacted with 0.05 wt % of TiO2 for 16 h. The average Ag/TiO2 weight ratio for this sample was 2.4:1. The actual Ag/TiO2 ratio varies considerably from one grain to another. The figure shows a region having a Ag/TiO2 ratio of 3.4:1 as measured
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by energy dispersive spectroscopy (EDS) within the TEM. It can be seen that the metallic particles deposited on the TiO2 are significantly larger than the 25-nm particle size of the TiO2 catalyst. The high loading of the metal on the TiO2 could easily block light from entering the titania and therefore cause a decrease in the rate of Ag+ removal as seen in Figure 2. Our experiments suggest that as Ag loading is increased, the Ag particles grow in size, and the large particle size of Ag in the spent catalyst suggests that physical means could be used to separate the Ag from TiO2. We found that sonication of the spent catalyst (Figure 5) allowed the Ag particles to be separated from the TiO2. In view of the significant density difference between silver (10.5 g/mL) and anatase titania (3.85 g/mL), sedimentation may allow separation of these phases after the Ag has been dislodged from the TiO2 by sonication. Use of Solar Radiation for Ag Removal. To investigate the possibility of using sunlight directly to drive the photoreduction of Ag+, a batch reactor containing spent fixer and TiO2 was placed outdoors for a few sunny days in late January in Albuquerque, NM, USA. The reactor was exposed to sunlight from about 8:30 AM to 5:00 PM. Since night time temperatures were below freezing, the reactor was taken indoors overnight. Results from this experiment (Figure 6) showed that sunlight is quite effective for the removal of Ag+ from used fixer. However, whereas in Figure 2 we added three aliquots of TiO2 each corresponding to 0.1 wt %, in Figure 6 we added 0.3 wt % TiO2 at the start of the experiment. The lowest Ag+ concentration being attained was about 200 ppm, which was similar to that reported in Figure 2. Therefore, the capacity of titania to reduce Ag+ is similar regardless of whether the titania is all added at one time or whether a portion is first added and then removed from the reactor as it gets loaded with Ag. The rate of Ag+ reduction slowed down with increasing amount of Ag loaded on the TiO2.
FIGURE 6. Removal of Ag+ from spent fixer using sunlight. It can be seen that sunlight is effective for this process, but the rate of Ag+ removal is slower than with lamp illumination.
FIGURE 7. Influence of hole scavengers on Ag+ reduction rate. The hole scavengers were all used at a concentraion of 0.72 mM while the TiO2 loading was maintained at 0.1 wt %.
Effect of Added Hole Scavengers. During steady-state photocatalysis, both holes and electrons need to be consumed at equal rates. If the hole transfer process is slow, the hole concentration will build up leading to increased recombination rates and lowered efficiency. The hole transfer rate can be varied using reagents that are known to act as effective hole scavengers: salicylic acid, EDTA, and citric acid (10). The influence of each of these is shown in Figure 7. No significant increase in silver reduction was caused by any of the added hole scavengers. Since the experiment was conducted with a batch of used fixer, it is possible that other reagents present in the photographic fixer already act, to some extent, as hole scavengers so additional quantities of hole scavengers have no effect. Indeed, used fixer contains a total organic carbon content of 4500 ppm, considerably greater than the amounts of added hole scavengers in Figure 7. Effect of Thiosulfate Ions on the Removal of Ag+. Sodium thiosulfate [Na2(S2O3)] is the major chemical used in photographic fixer to stabilize silver ions in solution. In a typical black and white fixer solution, the thiosulfate concentration is about 1 M. In this experiment, we studied the role of sodium thiosulfate on the reduction of Ag+ in solution. The initial silver ion concentration was 200 ppm, and the TiO2 weight loading was 0.1 wt %. The silver was added to DI water in the form of AgNO3. As shown in Figure 8, the silver can be rapidly removed when no thiosulfate is present and the final concentration
FIGURE 8. Influence of sodium thiosulfate on the reduction of Ag+ from aqueous solutions.
of Ag+ is below the detection limits of AA spectroscopy. In this case, we suspect that the hole scavenger could be water or some other organic impurities in solution. As small amounts of thiosulfate are added, there is a significant increase in the reaction rate. It is possible that thiosulfate when present in small amounts is acting as a hole scavenger and getting oxidized to sulfate or sulfite ions. However, at higher concentrations, the presence of thiosulfate decreases the rate of Ag+ removal by TiO2 photocatalysis. This inhibiting effect could result from the tendency of thiosulfate to complex with the Ag ions and stabilize them in solution. It is known that the silver-thiosulfate complex is negatively charged, and a high rate of agitation has to be used, during electrolysis, to cause reduction of the Ag+ to metal (4). The silver-thiosulfate complex may be limiting the reduction rate in a similar fashion during photocatalysis. Effect of Dilution on Ag Reduction from Spent Fixer. In our preliminary work with spent fixer containing a high concentration of Ag+ (6000 ppm), we had observed that the final Ag+ concentration at a point when the rate of Ag+ removal had slowed considerably was about 200 ppm. It was felt that the level of thiosulfate in solution may be responsible for slowing the rate of Ag removal as lower concentrations of Ag+ are approached. Even though Hermann et al. (6) demonstrated complete removal of Ag+ from solutions containing 0.02 M thiosulfate, the thiosulfate concentration in fixer is about 1.0 M. At this level, the thiosulfate clearly inhibits silver removal as shown in Figure 8. To explore this concentration effect further, we studied the treatment of a sample of black and white photographic processing waste that had been treated by electrolysis in a standard silver recovery unit. The Ag+ concentration in this sample was about 300 ppm after electrolysis and represented the lowest concentration that could be achieved. Catalyst weight loading in this series of experiments was maintained at 0.1 wt %. The change in concentration of Ag+ after 30 min irradiation with a 100-W UV lamp was monitored and is reported in Figure 9. Without dilution, no silver removal was observed after 30 min of irradiation. In contrast, 2-fold dilution led to nearly 75% reduction of silver concentration in 30 min, suggesting that dilution may be helpful for reducing the inhibitory effects of thiosulfate. A total of 100% removal of Ag+ was achieved only when the amount of water added was nine times that of the original fixer. In this case, the concentration of Ag+ in the effluent was below the detectable limit of atomic absorption spectroscopy.
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FIGURE 9. Effect of fixer dilution on the extent of Ag+ removal after 30 min of irradiation with a 100-W lamp.
Discussion Our experiments have shown that the TiO2-based photocatalysis is an effective method for reducing Ag+ from used photoprocessing waste. While the process can handle spent fixer, which contains as much as 6000 ppm of Ag+, the rate of reaction decreases as the ratio of Ag metal deposited to TiO2 approaches 3:1. Additional injections of catalyst are necessary to reduce the Ag+ concentration to a few hundred ppm which, however, is the lowest concentration that can be achieved. Ag+ concentrations that are below the 5 ppm EPA discharge limit could not be achieved in this case due to the high concentration of thiosulfate. As in the case of electrolysis (4), we suspect that the attack by thiosulfate on the reduced silver determines the lowest concentration that can be achieved. Diluting the fixer with clean water by a factor of 2 or more helps improve performance, presumably by lowering the thiosulfate concentration. Under our conditions, increasing the weight loading of catalyst (beyond 0.1 wt %) had only a marginal effect on reaction rate. Since thiosulfate is the major chemical in the fixer whose function is to keep silver ions in the solution, complexes of silver with thiosulfate such as Ag(S2O3)-, Ag(S2O3)23-, and Ag(S2O3)35- can be formed (11). Since complete removal of silver ion in fixer solutions seems inhibited by the presence of thiosulfate, we suspect that the negatively charged complexes such as the ones described above may not be able to overcome the repulsive space charge barrier on the negatively charged TiO2 for Ag+ to be reduced on the TiO2 surface. The intensity of mixing may become important in allowing the Ag+ ions to be reduced by the TiO2. At low concentrations, thiosulfate appears to play a second role, as a hole scavenger. By being oxidized to (SO3)2-, it can effectively consume the holes in UV-activated titania particles, reducing the extent of electron-hole recombination and improving photoefficiency. Our results show that the ratio of weight of silver recovered to TiO2 could be as high as 3:1. The Ag deposits as metallic particles that are weakly held to the TiO2 and can even be dislodged by sonication. Therefore, it should
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be feasible to process the spent catalyst and to recover the Ag for reuse. Since the catalyst costs approximately $0.02/g while silver costs about $0.14/g, the recovery of Ag metal from the spent catalyst should be economically feasible. Solar radiation can also effectively drive the process provided that longer reaction times are permissible. Since the lamp has (5.6 × 10-4 Ei/min) of UV photon flux that can photo-excite the TiO2, we estimate the efficiency of reduction of Ag+ in Figure 3 to be 1.7%. When comparing the TiO2 photocatalysis with processes currently in use, we find that the metal replacement process is the only one that can lower the Ag+ concentration to the EPA discharge limit. The problem with this method is that the effluent comes out colored and contains a high concentration of Fe3+. Electrolysis shares the same disadvantage with TiO2 photocatalysis in not being able to reduce the Ag+ concentration to below a few hundred ppm. If the process is designed mainly to reduce the Ag+ content in the fixer, it is possible that treated fixer could be reused in the photographic process, reducing markedly the amount of liquid waste generated. However, if complete removal of Ag+ from the photoprocessing effluent is desired, dilution of the used fixer is necessary.
Acknowledgments The research performed at the University of New Mexico was supported by a grant from the Waste Management Education Research Consortium (WERC) and also by National Science Foundation Grant HRD 93-53208. The research performed at Sandia National Laboratories is supported by the U.S. Department of Energy under Contract DE-AC04-94AL85000. Transmission electron microscopy was performed at the Electron Microscope Laboratory at the University of New Mexico, Department of Geology. We thank Mr. John Hussler in the Geology Department for the AAS measurements.
Literature Cited (1) Wang, W. C. J. Environ. Sci. Health, Part A, Environ. Sci. Eng. 1992, A27, 1313. (2) U.S. Environmental Protection Agency. Guides to Pollution Prevention: the photoprocessing industry; U.S. EPA: Cincinnati, OH, Oct 1991. (3) Cooley, A. C. J. Imag. Sci. Technol. 1993, 37, 374. (4) Levinson, G. I. P.; Sharpe, C. J. J. Photogr. Sci. 1975, 23, 216. (5) Levinson, G. I. P.; Sharpe, C. J. J. Photogr. Sci. 1981, 29, 16. (6) Herrmann, J.; Disdier, J.; Pichat, P. J. Catal. 1988, 113, 72-81. (7) Brennan, S. J. The Compact Photo-Lab-Index; Morgan & Morgan, Inc.: New York, 1983. (8) Datye, A. K.; Reigel, G.; Bolton, J. R.; Huang, M.; Prairie, M. R. J. Solid State Chem. 1995, 115, 236. (9) Turchi, C. S.; Ollis, D. F. J. Catal. 1989, 119, 483. (10) Prairie, M. R.; Evans, L. R.; Stange, B. M.; Martinez, S. L. Environ. Sci. Technol. 1993, 27, 1776. (11) Hubin, A.; Vereecken, J. J. Appl. Electrochem. 1994, 24, 396.
Received for review February 22, 1996. Revised manuscript received May 28, 1996. Accepted June 4, 1996.X ES960167L X
Abstract published in Advance ACS Abstracts, August 15, 1996.
