Applied Energy 100 (2012) 187–192

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Applied Energy

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Silver recovery and chemical oxygen demand (COD) removal from wastefixer solutions

Wei-Ting Chen a, Chin-Chi Ma b, Ming-Hsun Lee a, Yung-Chuan Chu a, Lung-Chang Tsai b, Chi-Min Shu b,⇑a Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology (YunTech), Douliou, Yunlin 64002, Taiwanb Department of Safety, Health, and Environmental Engineering, YunTech, Douliou, Yunlin 64002, Taiwan

h i g h l i g h t s

" This study focused on the Ag recovery from waste fixer solutions by electrolysis." 99.55% of silver recovery was obtained with electric current of 3 A during 2.5 h." The purity of silver was 98.62% with electric current of 3 A." Higher removal efficiency of COD by employing distillation was obtained.

a r t i c l e i n f o

Article history:Received 17 February 2012Received in revised form 24 April 2012Accepted 8 June 2012Available online 12 July 2012

Keywords:Silver recoveryWaste fixer solutionsElectrolysisChemical oxygen demand (COD)Distillation

0306-2619/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apenergy.2012.06.026

⇑ Corresponding author. Tel.: +886 5 534 2601; faxE-mail address: shucm@yuntech.edu.tw (C.-M. Shu

a b s t r a c t

This study focused on the silver recovery from waste fixer solutions by electrolysis under different exper-imental conditions. During an 8 h electrolysis procedure, the silver recovery efficiencies were 10.57%,11.75%, 45.5%, 62.68%, and 83.88% for electric currents of 0.1 A, 0.3 A, 0.5 A, 0.7 A, and 0.9 A, respectively.Thus, the most efficient recovery factor of silver was found for a current of 0.9 A. Additionally, the pHvalues of the waste fixer solutions were adjusted prior to treatment by electrolysis, and the silver recov-ery was 100% for a solution with pH 8 that underwent electrolysis for 8 h at an electric current of 0.9 A.The recovery efficiency of silver was 88.64% for an electric current of 3 A and a duration of 3.5 h, but therecovery efficiency of silver decreased to 83.72% for an electrolytic time of 6 h. Furthermore, the recoveryefficiency of silver increased to 99.55% with a decreased electrolysis time of 2.5 h when the solution wasadjusted to pH 8 at an electric current of 3 A. These results suggest that the most suitable experimentalconditions were an electric current of 3 A, pH 8, and 2.5 h of electrolysis. The treated waste fixer solutionsstill had high concentrations of chemical oxygen demand (COD) after electrolysis. The addition of 30 mLof sulfuric acid to 1000 mL of waste fixer solution resulted in a COD removal efficiency of 42%. Further-more, when the waste fixer solutions were treated by distillation to remove COD, the removal efficiencyof COD was 99.3%. Although a higher COD removal efficiency was obtained by employing distillation, thecost of the distillation treatment was more expensive.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The demand for energy is increasing because of rapid industri-alization and societal growth. Conventional primary energysources, such as coal, oil, and natural gas, have limited reservesthat are not expected to last for an extended period of time. Addi-tionally, the environmental problems associated with these con-ventional energy sources continue to affect the global ecosystem.To reduce fossil fuel consumption and alleviate environmentalproblems, renewable energy resources are expected to play a keyrole in future energy consumption [1]. There are many previous

ll rights reserved.

: +886 5 531 2069.).

studies of energy recovery from waste sources and materials, suchas waste heat [2,3], waste tyres [4], and waste solutions. For eachof these processes, waste energy can be recovered and reused,and this study evaluates a recovery process to recycle silver fromwaste fixer solutions.

Because of their high photosensitivity, beginning in 1837, silverhalides were employed in the photographic fixing process usingammonium thiosulfate (ATS) and sodium thiosulfate solutions(STSs) as fixing baths [5]. Because of the extensive usage of silverhalides, photographic manufacturing processes resulted in enor-mous amounts of waste fixer solutions. According to the disposalestimation of waste fixer solutions from the Environmental Protec-tion Administration, Executive Yuan (Taiwan), approximately36,310 tons of waste fixer solutions are generated per year from

188 W.-T. Chen et al. / Applied Energy 100 (2012) 187–192

photographic manufacturing [6]. These waste fixers contain highlevels of silver (Ag), where concentrations are typically in the rangeof 5000–10,000 mg/L. Furthermore, the waste fixer solutions con-tain substantial quantities of components that can be both toxicand valuable depending on the quality of the raw materials [7].Thus, for both economic and environmental reasons, it is importantto recover valuable components (such as Ag) from waste fixersolutions. Conventional fixing of silver compounds (AgX) in photo-graphic and medical X-ray films has been extensively used by thephoto-industry for more than two centuries. The fixing of silverhalides by thiosulfates and thiocyanates is usually rapid and doesnot need any catalytic influence. Silver halides are highly solublein these lixiviants and are considered to be thermodynamicallystable at a slightly alkaline pH. The simplified mechanisms forthe dissolution of AgCl in thiosulfate solutions are shown in Eqs.(1) and (2) [8].

AgClðsÞ þ 2ðNH4Þ2S2O3ðaqÞ () ðNH4Þ3½AgðS2O2Þ�ðaqÞ þ NH4ClðaqÞ ð1Þ

90

100

3,000

3,500

pH3 SIC

pH4 SIC

pH5 SIC

)

AgClðsÞ þ 2Na2S2O3ðaqÞ () Na3½AgðS2O2Þ�ðaqÞ þ NaClðaqÞ ð2Þ

ATS and STS are commonly used as preservatives, hardeners,and buffers in fixing baths, which perform various functions inthe photographic fixing process [8]. This process leads to wastefixer solutions that have high levels of silver, which can be recov-ered using many conventional methods, including electrolysis [9],oxygenation reduction [10,11], and sulfide precipitation [12]. Thetreated waste fixer solutions contain high concentrations of chem-ical oxygen demand (COD) after electrolysis, so it is necessary toremove the high COD concentrations before disposing of the solu-tions. This study evaluates the recovery of Ag from waste fixersolutions, as well as the removal efficiency of COD from the treatedwaste fixer solutions.

0

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Silv

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(mg/

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Rec

over

y ef

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(%)

Fig. 2. Silver recoveries with an electric current of 0.9 A at various pH values.

2. Experimental

2.1. Materials

The waste fixer solutions, which were obtained from a photo-graphic shop (Kodak) in Douliou, Yunlin, Taiwan, did not receiveany pre-treatment, such as electrolysis [9], oxygenation, reduction[10,11], sulfide precipitation [12], or ion exchange [13], and theywere not mixed with any other chemicals. The concentration ofAg in the waste fixer solutions was measured by InductivelyCoupled Plasma-Atomic Emission Spectroscopy (ICP-AES) (Optima,5100 DV) or Atomic Absorption Spectroscopy (AA) (Perkin Elmer,5100), and the concentrations of Ag were found to be in the rangeof 2700–3300 mg/L. Additionally, the waste fixer solutions con-tained a variety of organic and inorganic materials, including aschelating-agents such as EDTA, acetate, thiosulfate, metabisulfite,

0102030405060708090100

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0 1 2 3 4 5 6 7 8

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0.3 A SIC

0.5 A SIC

0.7 A SIC

0.9 A SIC

0.1 A SRE

0.3 A SRE

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0.7 A SRE

0.9 A SRESilv

er io

n co

ncen

trat

ion

(mg/

L)

Electrolysis time (h)

Rec

over

y ef

fici

ency

(%)

Fig. 1. The silver recovery from waste fixer solutions by electrolysis with lowelectric currents.

and ammonium compounds. These materials account for the highCOD concentration of 88,654 mg/L in the solution, excluding theconcentration of Ag. EDTA and acetate were the primary cause ofthe acidic pH of the waste fixer solutions, which had a pH of6.09.

2.2. Analysis of component elements

The component elements in the waste fixer solutions were ana-lyzed by ICP-AES or AA, which were used to determine the metal ionconcentration in the aqueous phase and to analyze the ores. Equalvolumes (10 mL) of the aqueous and organic phases were shakenat room temperature (25 ± 3 �C) for 5 min to ensure complete equil-ibration. The two phases were separated, and a suitable aliquot ofthe aqueous phase was assayed for the metal ion concentration.Three tests indicated that the value of 95% extraction for the metalion was associated with a variation coefficient of ±3% [14].

2.3. Electrolysis procedures

Electrolysis was used to recover silver from the waste fixer solu-tions, where the cathode plate and the anode plate were made ofstainless steel and graphite, respectively. To determine the mostsuitable electrolysis efficiency, the electrolysis process was evalu-ated over a range of settings for a variety of parameters, such aspH, electric current, and electrolysis time.

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0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

SIC

SRE

Electrolysis time (h)

Rec

over

y ef

fici

ency

(%)

Silv

er io

n co

ncen

trat

ion

(mg/

L)

Fig. 3. The silver recovery with an electric current of 3 A.

010

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SIC

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(mg/

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Electrolysis time (h)

Rec

over

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fici

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(%)

SRE

Fig. 4. The silver recovery with an electric current of 3 A at pH 8.

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Electrolysis time (min)

Silv

er io

n co

ncen

trat

ion

(mg/

L)

Fig. 5. The purification of silver by electrolysis with an electric current of 0.9 A.

Fig. 6. The appearance of the silver metal.

010,00020,00030,00040,00050,00060,00070,00080,00090,000

100,000

0 1 2 3 4 5

CO

D c

once

ntra

tion

(mg/

L)

Mass volume (%)

HCl

H2SO4

HNO 3

Fig. 7. The COD removal efficiency of waste fixer solutions with 1–5% of variousacids.

Table 1The COD removal efficiency of waste fixer solutions with 1–5% of various acids.

Acidvolume(%)

COD concentrationafter mixed withH2SO4 (mg/L)

COD concentrationafter mixed withHNO3 (mg/L)

COD concentrationafter mixed withHCl (mg/L)

0 88,654 88,654 88,6541 81,693 87,166 83,9482 65,667 80,130 79,8463 51,205 74,267 78,8974 54,332 64,495 77,3075 53,550 57,850 76,717

W.-T. Chen et al. / Applied Energy 100 (2012) 187–192 189

2.4. Removal efficiency of COD

The COD test is commonly used to indirectly measure the con-centration of organic compounds in a water sample, and it providesa useful index of the amount of organic pollutants that are present.Most applications of the COD test are intended to determine theamount of organic pollutants found in a surface water (e.g., lakesand rivers), which makes COD a useful measure of water quality.COD is typically expressed in milligrams per liter (mg/L), whichindicates the mass of oxygen consumed per liter of solution,although older references may express COD as parts per million(ppm) [15]. We did not use conventional COD removal methods,such as activated sludge processes [16], aerobic and anaerobictreatment processes, or coagulation treatment. Instead, we usedacid leaching and distillation treatments to remove COD.

3. Results and discussion

3.1. Electrolysis of waste fixer solutions at a low electric current

To obtain the proper electric current for electrolysis, 1000 mL ofwaste fixer solution was treated at a low electric current. We eval-uated the process at different electric currents (0.1 A, 0.3 A, 0.5 A,0.7 A, and 0.9 A), and samples were collected at hourly intervalsthroughout the duration of the electrolysis procedure. Followingthis procedure, we used the proper electric current to determinethe most suitable pH value for electrolysis by varying the pH ofthe waste fixer solution (with set pH values of 3, 4, 5, 7, 8, 9, and10), where the original pH of the waste fixer solution was 6.Fig. 1 shows that the recovery of silver increases for a specificelectric current as the electrolysis time is increased. The recovery

efficiencies of silver during the 8 h electrolysis procedure at0.1 A, 0.3 A, 0.5 A, 0.7 A, and 0.9 A were 10.57%, 11.75%, 45.50%,62.68%, and 83.88%, respectively. These results show that the mostsuitable electric current is 0.9 A. We obtained the most suitable pHvalue for the waste fixer solutions from the experiment shown inFig. 2, which used an electric current of 0.9 A for solutions at variedpH values. The recovery efficiencies of silver at pH 3, 4, 5, 6, 7, 8, 9,and 10 were 83.65%, 84.66%, 81.54%, 83.88%, 99.09%, 100%, 98.63%,and 81.20%, respectively. These results show that the most suitablepH value is pH 8. These experiments demonstrate that the mostsuitable recovery efficiency of silver is 100% at 0.9 A and pH 8.The recovery efficiency of silver at pH 9 or pH 10 was less thanthe recovery efficiency at pH 8 because a portion of the metal ionswere deposited while the pH of the waste fixer solutions was ad-justed to 9. Furthermore, the phenomenon of metal ion depositionbecomes obvious when the pH value is adjusted to pH 10. Theco-precipitation of metal ions during electrolysis resulted in a

Fig. 8. The deposits from waste fixer solutions mixed with H2SO4.

Table 2The COD removal efficiency of waste fixer solutions mixed with H2SO4 at an electriccurrent of 10 A.

Electrolysistime (h)

Without H2SO4 With H2SO4

CODconcentration(mg/L)

CODrecoveryefficiency(%)

CODconcentration(mg/L)

CODrecoveryefficiency(%)

0 90,313 0 84,196 01 65,330 27.6 44,459 47.22 58,830 34.8 44,259 47.13 55,320 38.7 44,065 47.64 52,130 42.2 33,442 60.35 49,250 45.4 33,836 59.86 45,810 49.2 32,655 61.27 46,520 48.5 33,049 60.78 47,320 47.6 33,442 60.3

0

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D c

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tion

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Electrolysis time (h)

CO

D re

mov

al e

ffic

ienc

y (%

)

COD with H2SO4

COD RE

COD without H2SO4

COD RE without H2SO4

with H2SO4

Fig. 9. The COD removal efficiency of waste fixer solutions mixed with H2SO4 withan electric current of 10 A.

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COD 7A

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COD RE 15A

Electrolysis time (h)

CO

D re

mov

al e

ffic

ienc

y (%

)

CO

D c

once

ntra

tion

(mg/

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Fig. 10. The COD removal efficiency of waste fixer solutions mixed with H2SO4 withvarious electric currents.

190 W.-T. Chen et al. / Applied Energy 100 (2012) 187–192

decrease in the recovery efficiency of silver. Conclusively, we canobtain the most suitable recovery efficiency of silver by employingthese suitable experimental parameters for the electrolysisprocedure.

3.2. Electrolysis of waste fixer solutions at a high electric current (3 A)

We employed a high electric current (3 A) instead of the lowelectric currents previously evaluated (0.1 A, 0.3 A, 0.5 A, 0.7 A,and 0.9 A) to lessen the electrolysis time, as shown in Figs. 3 and4. A maximum silver recovery efficiency of 88.64% was obtained

for the electrolysis procedure with a current of 3 A and a durationof 3.5 h. The recovery efficiency did not increase with additionalelectrolysis time, and the proper electrolysis time is therefore3.5 h, as shown in Fig. 3. Fig. 4 shows that when we adjusted thewaste fixer solutions to pH 8 and then used an electric current of3 A, the electrolysis time decreased from 3.5 h to 2.5 h, and therecovery efficiency of silver increased from 88.64% to 99.55%. Addi-tionally, the purity of silver was 98.31% and 98.62% for an electriccurrent of 0.9 A and 3 A, respectively.

3.3. The purification of silver

The silver in the waste fixer solutions formed a metal coating onthe stainless steel plate during the electrolysis procedures, and wescraped this silver metal from the plate in order to obtain purifiedsilver after an additional purification procedure. The silver metalthat was scraped off of the plate was dissolved in a high concentra-tion solution of nitric acid, and we applied an electric current of0.9 A to obtain high purity silver via electrolysis. Fig. 5 shows thatas the electrolysis time increased, the silver concentration in thewaste fixer solution decreased and the silver recovery efficiency in-creased. The recovery efficiency of silver reached 98.85% after 3 hof electrolysis, but then decreased for durations of electrolysisgreater than 3 h. The reason for this decrease in recovery efficiencyis that the stainless steel plate was coated with too much silvermetal to allow for additional silver metal to coat the plate. The pur-ity of the silver metal reached 99.9% from the purification process,and the appearance of the silver metal is shown in Fig. 6.

Fig. 11. The distillation water and waste sludge following distillation.

W.-T. Chen et al. / Applied Energy 100 (2012) 187–192 191

3.4. The efficiency of COD removal

Continuous-flow activated sludge reactors were used to treat thephotoprocessing wastewaters from two commonly used photopro-cesses: Kodak Flexicolor Process C-41 (film processing) and KodakEktacolor Process RA-4 (paper processing). Simulated wastewaterswere fed to laboratory-scale reactors at a loading level of 10% (v/v)along with a synthetic base feed. The biodegradation of the base feedwas not affected by the photoprocessing wastewaters. In addition,more than 36% of the photoeffluent-derived COD was destroyed,primarily as a result of sulfite and thiosulfate oxidation. Therefore,the efficiency of COD removal by the continuous-flow activatedsludge reactors was not suitable [17]. Furthermore, Lin and Yang[18] found that photoprocessing wastewaters that were treated byan electrochemical method resulted in a high efficiency of CODremoval (approximately 95% of COD) but that the cost of this electro-chemical treatment was too expensive to use. However, we usedacid leaching and distillation for COD removal and found this meth-od to be an ideal approach to remove COD.

Table 3The COD removal efficiency by distillation of the waste fixer solution.

COD concentration (mg/L)

COD removal efficiency(%)

Waste fixersolution

88,657.3 0

Cooling water 579.9 99.3

3.4.1. Acid leaching for COD removalSmall volumes (1–5 mL) of various inorganic acids (hydrochlo-

ric acid (HCl), nitric acid (HNO3), and sulfuric acid (H2SO4)) wereadded to 100 mL of waste fixer solution in order to remove COD.Fig. 7 and Table 1 show that the efficiency of COD removal wasthe best for the H2SO4 treatment. In addition, the COD concentra-tion decreased from 88,654 mg/L to 81,693 mg/L after being mixedwith 1 mL of H2SO4 and from 88,654 mg/L to 51,205 mg/L afterbeing mixed with 3 mL of H2SO4. The suitable efficiency of COD re-moval was found to be 42.24% for the case where 3 mL of H2SO4

was used. A 1000 mL waste fixer solution that received the addi-tion of 30 mL of H2SO4 resulted in the deposits shown in Fig. 8,and the deposits were assessed to be thiosulfate or organic sulfide.We separated the deposits and the waste fixer solutions beforeconducting electrolysis at an electric current of 10 A. Table 2 andFig. 9 show that the efficiency of COD removal from a mixture withH2SO4 is better than the efficiency of COD removal without H2SO4

because the hydrogen ions in the mixing solutions are affected byelectrolysis. Therefore, the waste fixer solutions mixed with H2SO4

resulted in proper efficiency of COD removal. Furthermore, variouselectric currents (5 A, 7 A, 8 A, 10 A, and 15 A) were applied viaelectrolysis to remove COD. The proper efficiencies of COD removalunder 5 A, 7 A, 8 A, 10 A, and 15 A were 26.94%, 27.12%, 35%,32.52%, and 11.45%, respectively (see Fig. 10). Most of the wastefixer solutions would vaporize under an applied current of 15 A,resulting in the concentration of the organic materials present inthe waste fixer solution. This occurrence resulted in an abnormalphenomenon, where the efficiency of COD removal did not in-crease with the increased electric currents.

3.4.2. Enhancement of concentration by distillation for COD removalAfter the silver recovery processes, 1000 mL of treated waste

fixer solution was distilled and concentrated using a condenserfor COD removal, and the distillation water and waste sludge wereseparated, as shown in Fig. 11. The COD concentration of the wastefixer solutions decreased from 88,657.3 mg/L to 579.9 mg/L, andthe efficiency of COD removal was 99.3%, as shown in Table 3.

4. Conclusions

� Samples of waste fixer solution (1000 mL) with a silver concen-tration of 3000 mg/L underwent an 8 h electrolysis procedure atlow electric currents. For an electric current of 0.9 A, the silverrecovery was 83.88%, which was better than the silver recoveryfor any of the other electric currents that were studied (0.1 A,0.3 A, 0.5 A, and 0.7 A). In addition, when the solution wasadjusted to pH 8 and then underwent 8 h of electrolysis at anelectric current of 0.9 A, the silver recovery increased from83.88% to 100%. When the electric current was increased to3 A for a 3.5 h period, the silver recovery was 88.64%. Addition-ally, when the solution was adjusted to pH 8 and an electric cur-rent of 3 A was applied for 2.5 h, the silver recovery reached99.55% purity. The purity of silver was found to be 98.31% and98.62% for an electric current of 0.9 A and 3 A, respectively, not-withstanding the recovered silver, which could obtain 99.9%purity after being dissolved in nitric acid and undergoing elec-trolysis. The proposed flow chart for the treatment of wastefixer solutions is shown in Fig. 12.� The efficiency of COD removal for waste fixer solutions mixed

with H2SO4 (100 mL of waste fixer solution and 1 mL ofH2SO4) is better than the efficiency of COD removal of wastefixer solutions mixed with other acids. The COD concentrationdecreased from 88,654 mg/L to 51,205 mg/L after being mixedwith 3 mL of H2SO4. The proper efficiency of COD removal forthe 3 mL H2SO4 mixture was 42.24%.� The waste fixer solutions mixed with H2SO4 resulted in a proper

efficiency of COD removal. Electric currents at 5 A, 7 A, 8 A, 10 A,and 15 A were applied during an electrolysis procedure toremove COD, and an electric current of 8 A provided the mostsuitable COD removal efficiency.� Higher COD removal efficiencies were obtained by employing

distillation, but the cost of the distillation treatment is moreexpensive.

Waste fixer solutions

Electrolysis (Deposition: Ag2S)

pH=6.09, Ag=2,700–3,300 mg/L,

COD=88,654 mg/L

Annealing process Waste solution after electrolysis

Dissolved in HNO3

Crude Ag (Purity: 98.32%)

Rising to 1,000 oC,

during 3 h Distillation

(The removal efficiency of COD: 99.3%)

Current: 3 A

Purified Ag (Purity: 99.9%)

Electrolysis

(Current: 0.9 A)

Fig. 12. The proposed flow chart for the treatment of waste fixer solutions.

192 W.-T. Chen et al. / Applied Energy 100 (2012) 187–192

Acknowledgements

This study was guided by the late Professor Hung-Yuan Fangand was carried out by his team of environmental and microbio-logical researchers. This study could not have been accomplishedwithout his meticulous instruction and positive feedback.

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