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Recovery of silver from radiographic fixer
Recovery of silver from radiographic fixer Taeko Goshima, DMD, Katsuyoshi Hori, DMD, and Akira Yamamoto, Tokyo, Japan DEPARTMENT
OF ORAL RADIOLOGY,
TSURUMI UNIVERSITY,
DDS, PhD,
SCHOOL OF DENTAL MEDICINE
This study examined the silver content of IO brands of dental x-ray film that are commercially available in Japan. Kodak Ektaspeed EP-22 (Eastman Kodak Co., Rochester, N.Y.) was found to have the highest silver content, and SD 100 Gold X-ray Film (San Dental Co., Ltd., Tokyo, Japan) had the lowest content of silver. Electrolysis and ion exchange were used as the methods of silver recovery from films in the general dental office. Ion exchange was more than 99% efficient in silver recovery from the radiographic fixer and resulted in an 80% reduction in biochemical oxygen demand and a 95% reduction in chemical oxygen demand. However, these levels and other environmental pollutants were still above the limits set by the Japanese Pollutant Control law. (ORAL %JRC 0~41 MD ORAL PATHOL 19!M;r/:684-8)
In the late 198Os, Japan adopted an environmental assessment system patterned after the U.S. Environmental Protection Agency. The first Environmental Pollution Control Law passed in Japan was in 1967.’ Several other laws have followed. Oral and maxillofacial radiology results in a variety of wastes that could have an impact on the environment, namely, spent processing chemicals, salivacontaminated intraoral film packet jackets, lead foil, and discarded radiographs. Although new environmentally sound direct digital imaging systems are now available,2‘4 the use of physicochemical radiographic systems still dominates. In Japan, the number of intraoral dental and panoramic radiographs taken in 1984 were 90.878 million and 8.665 million, respectively. By 1989, these figures had increased to 96.63 million for intraoral films and to 11.09 million for panoramic films, s,6 which represent a 6% and a 28% increase, respectively, over a 5-year period. In 1980, the price of silver reached $20.64 per ounce, and there was an impetus to recover silver from fixer solution and discarded x-ray film because of the commercial value of the reclaimed silver.7 Since 1980, however, the price of silver has declined steadily. In 1992, silver had fallen to $3.93 per ounce and has not made appreciable gains since that time.7 The recovery of silver, therefore, is no longer prompted by direct commercial gain. However, because silver is a nuisance as a pollutant that can impair effective sewage treatment, the focus on silver recovery is primarily a concern for the environment.’ Several methods for silver recovery from radiographic fixer are now available (Table I), but most of these are not practical for the general dental office
Copyright @ 1994 by Mosby-Year Book, Inc. 0030-4220/94/%1.00 + .lO T/16/52098 684
Table I. Silver recovery methods Method
Metallic replacement
Advaruages
Low investment Low operating cost
Disadvantages
Effluent high iron content Silver recovered as sludge Effluent
Electrolytic
recovery Precipitation
Silver recovered as pure metal High silver recovery Can attain 0.1 mg silver/L Low investment
Reverse osmosis
Also recovers other chemicals Purified water is recyclable
Ion exchange
Can recover 0.1 to 0.2 mg silver/L
high silver
content unless 2 units used in series Sulfide formation possible Effluent high in content Complex to operate Silver recovered as sludge Treated solution cannot be reused Potential hydrogen sulfide released Concentrate requires further processing High investment High operating cost Only for diluted solutions Complex operation High investment
where costs and convenience must be kept in mind.‘*‘16 Metallic replacement and electrolysis are the most common silver recovery procedures on the market today that are suitable for use in the private office. This article explores the effectiveness of both electrolysis and ion exchange in removing silver and
Goshima,
ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY I/olume 77, Number 6
reducing other environmental polluting factors from spent radiographic fixer. MATERIAL AND METHODS Sensitometric properties of dental x-ray film
Ten brands of x-ray film that are commercially available (for intraoral radiography in Japan) were used in this part of the study (Table II). Shimavision 1400 (Shimadzu Corp., Kyoto Japan) was used (70 kVP, 50 mA, target-to-object distance of I76 cm.) A film was positioned in the center of the beam of radiation and exposed for one impulse; all films were processed in the HOPE Model P-10 automatic dental film processor (Hope Industries, Inc., Willowgrave, Pa.). Densitometric values were measured by PDA-25 (Konica Co., Ltd., Tokyo, Japan) densitometer. Silver content of dental x-ray film
Twenty samples of film from each of 10 different brands (Table II) were used to determine the comparative silver content of each type. Three of the 10 brands were direct-exposure films, and the remaining were indirect-exposure without intensifying screens (Table II). The silver was dissolved with nitric acid, and the amount of silver present in the solution was determined with the use of the Volhalt method.8 Silver recovery
The ion exchange system tested was the Dent-X Quick Silver silver recovery system (Dent-X Corp., Elmsford, N.Y.). The electrolysis system tested was the Silver Magnet AT-2 silver recovery unit (TMW Corp., West Bend, Wise.). Three films, Kodak DF-57, Kodak EP-22 and Hanshin Hi-Fi contained approximately three times as much silver as the others. Of the three, Kodak DF-57 (Eastman Kodak, Rochester, NY) was chosen for the silver recovery phase because it is the direct exposure film most widely used for intraoral radiography in Japan. The radiographs were exposed with the use of a mannequin to simulate the typical clinical situation for a mandibular molar exposure using a Max-R x-ray generator (J Morita Co., Osaka, Japan) (70 kVp, 10 mA, target-to-object distance of 30 cm). In total, 3600 films were exposed for use in the study. Half of these films were processed in Cronex HSF/M (Du Pont Japan Ltd., Tokyo, Japan) and the other half were processed in a Kodak GBX fixer and replenisher (Eastman Kodak Co.) The fixer in each case was divided into three parts. A sample of each was kept as a control for prerecovery silver level assessment. The remainder was divided in half to make four samples, two for each processing chemistry. One sample of each fixer solution was then subjected to silver recovery using each of the two methods.
Hori,
and Yamamoto
685
Table II. Silver content of no. 2 intraoral
radiographic films Silver Manufacturer
Film
Direct-exposure films EKTASPEED EP-22 Ultra-Speed DF-57 Hanshin Hi-Fi
Indirect-exposure films (without intensifying screens) Konica Dental Fuji Dental Periapical Hanshin New Instant
Nix NF-55 SD 100 GOLD X-Ray Yata X-Ray 3TC Beam Dental Roentgen FB-100
(mean
Eastman Kodak Co., Rochester, N. Y. Eastman Kodak Co., Rochester, N. Y. Mfr. Hanshin Technical Lab Ltd., Nishinomiya, Japan
mg)
22.09 19.90 20.12
Konica Co., Ltd., Tokyo, Japan Fuji Photo Film Co., Ltd., Tokyo, Japan Mfr. Hanshin Technical Lab Ltd., Nishinomiya, Japan Nix Company Ltd., Tokyo, Japan San Dental Co., Ltd., Tokyo, Japan Yata Dental Mfg., Ltd., Osaka, Japan 3.T.C. Inc., Tokyo, Japan
7.28 6.96 6.32
9.16 5.60 8.83 6.22
Chemical analyses of fixer solutions
The following parameters were tested for the fixer with each processing chemistry both before and after silver recovery by the two silver recovery methods: 1. A pH-Glass electrode analysis was performed to measure the acidity of the solution as prescribed in the Japanese Environmental Agency standard, JIS Z 8802. 2. Biochemical oxygen demand (BOD). The Winkler sodium azide modifier analysis was carried out as prescribed in the Japanese standard, JIS K 0102, 2 1. This measures the amount of dissolved oxygen consumed by aerobic microorganisms living in water. The sample is diluted in water and kept for 5 days at 20” C. BOD
= (b
- D2) - WI-
P
J32)
xf
686
Goshima, Hori, and Yamamoto
ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY
-June 1994
where
BOD = biochemical oxygen demand (mg oxygen/L) DI = dissolved oxygen in the diluted sample after preparation (mg oxygen/L) D2 = dissolved oxygen in the diluted sampleafterincubation (mgoxygen/L) P = sample dilution ratio (sample/water) BI = dissolved oxygen before incubation of the diluted liquid specimen at the time of measuring BOD (mg oxygen/L) B2 = dissolved oxygen after incubation of the diluted liquid specimen at the time of measuring BOD (mg oxygen/L) 3. Chemical oxygen demand (COD). The potassium permanganate titrimetric analysis (JIS K 0102, 17) CODM,
= (b - a) X f X 7
X 0.2
where CODw,, = oxygen demand by potassium permanganate (mg oxygen/L) b = total amount of N/40 potassium permanganate solution required for titration (mL) a = amount of N/40 potassium permanganate solution required for titration of the blank test (mL) f = factor of N/40 potassium permanganate solution I/ = sample (mL) 4. Total dissolved solids (TDS). The gravimetric analysis following standard JIS K 0102, 14.2. 1,000 R=(a-b)XT where
R = total evaporation residue (mg/L) a = mass of the evaporating dish containing the residue (mg) b = mass of the evaporating glass (mg) V = sample volume (mL) 5. Total suspended solids (T-SS). The gravimetric analysis following standard JIS K 0102, 14.2. 1,000 S=(a-b)XT
where
S = suspended matter (mg/L) a = mass of the filter containing the suspended matter and the watch glass bg) b = mass of the filter and the watch glass (ms) V = sample volume (ml)
RESULTS Properties of film
The characteristic curves for the direct exposure films and also those for the indirect exposure films, Hanshin New Instant, Konica Dental, Nix NF-55, 3TC FB-100, Fugi Dental SDlOO, and Yota are shown in Fig. 1. The exposures needed to produce diagnostic radiographs using indirect exposure film without intensifying screens was considerably greater than that needed using direct exposure film. Silver content of film
The silver contents measured per film are given in Table II. This ranged from 5.6 to 20.12 mg/film. The mean silver content per film for all 10 periapical no. 2 film types was 11.25 mg (standard deviation + 6.64 4. Effluent characteristics of spent fixer
Table III shows the values for the parameters tested for the fixer solution before and after silver recovery. Silver. For CRONEX, silver recovery was only 11% using electrolysis but more than 99.9% with ion exchange. For GBX, silver recovery by electrolysis was only 1 l%, whereas with ion exchange it was 99.7%. pH. The spent fixer was acidic because of its content of acidifier both before and after silver recovery by electrolysis. Silver recovery by ion exchange resulted in a mildly alkaline residual solution. This was a common finding with both processing solutions tested. BOD. BOD values for CRONEX were invariably higher than with GBX solutions both before and after silver recovery. With CRONEX, there was a 34% reduction in BOD after silver recovery by electrolysis, and an 87% by ion exchange. For GBX, there was a 27% increase in BOD after electrolytic removal of silver and an 80% reduction in BOD after silver recovery by ion exchange. COD. For CRONEX, COD was reduced by 23% after electrolysis and 95% after ion exchange. For GBX, the COD value was reduced by only 2% after electrolysis, but by 95% after ion exchange. TDS . For CRONEX, TDS was reduced by 51% and 91% after silver recovery by electrolysis and ion exchange, respectively. For GBX, the TDS remained unchanged after electrolysis, but TDS was reduced by 9 1% after silver recovery using ion exchange. T-SS. For CRONEX, T-SS was reduced by onethird after electrolysis and by more than 30% after ion exchange. For GBX, T-SS doubled after electrolysis but was reduced by two-thirds after ion exchange.
Goshima,
ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY Volume 77, Number 6
/
/f
//
I
2
3
4
5
6 log
7
------
with
-
without
Hanshin’
#Hanshin
8
intensifying
and Yamamoto
687
screens
intensifying
screens
/7
New
9
Hori.
Instant
IO
Exposure
Fig. 1. Characteristic curves for direct- and indirect-exposure radiographic films usedin Japan for intraoral radiography. All films are used with and without intensifying screensfor this purpose; hence, indirect exposure film results in higher radiation exposuresto the patients. Presently more than 50% of intraoral radiographs taken in Japan use indirect-exposure film without screens.
Table III. Spent fixer effluent characteristics CRONEX
Factor
(unit)
Ag (mg/L) PH BOD (mg 02/L) COD (mg 02/L) TDS h/L) T-SS (mg/L)
Prerecovery 3200
Ion exchange recovery of silver
2860
<4.4 8.20 2570 2940 12600
4.95 12900 46700 36000
9
KODAK
Electrolytic recovery of silver
4.25 19400 60900 134000
HSF/M
6
DISCUSSION Ninty-seven percent of dental facilities in Japan are
private practices. More than 50% of the intraoral radiographs used in dental practice are still indirect exposure films used without intensifying screens in the form of specially manufactured “packet-processed” films.5* 6 As the name suggests, developing chemistry is introduced directly into the film packet. After processing the solution is discarded, and therefore it has a potential negative impact on the environment. Given the same speed rating of film, it would seem that the use of the film type with the lowest silver
GBX
Fixer
Electrolytic Prerecovery .3240 5.38 7140 46900 113000 3
and replenisher
recovery silver 2880 5.34
9060 45900 113000 6
Ion Exchange recovery of silver <4.4 8.15 1430 2240 10300 1
content would be one way to reduce the potential pollution of the environment by silver compromised that the obtained image quality is not reduced as a consequence. However, the indirect-exposure films without intensifying screen are much slower than directexposure films. I7 Therefore the patients are unnecessarily overexposed to radiation. Hence, use of direct exposure film is advocated and was used for the silver recovery phase of this study. Although ion exchange recovery is more efficient than electrolysis, the levels of BOD, COD, and other environmental pollutants are still above the limits set
688
Coshima,
Hori. and Yamamoto
by the Japanese government pollution control laws.’ New digital imaging systems for intraoral radiography have the advantages of reduced radiation exposure to the patient, elimination of the need for processing chemistry, and of instantaneous images without environmental pollution.‘8, I9 If these systems become the mode for imaging in dentistry, the need for silver recovery from radiographic fixer would no longer be a concern for our profession. We thank Mr. Makoto Tsuchida for variable suggestions for laboratory works and also Ms. Kiyoe Yamada for typing this manuscript.
ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY June 1994
8. 9.
10.
11. 12.
13. 14.
REFERENCES 1. Agency of Natural Resources and Energy. Power plants and the environment. Tokyo: Ministry of International Trade and Industry 2. Grondahl H-G. Digital radiology in dental diagnosis: a critical review. Dentomaxillofac Radio1 1992;21:198-202. 3. Kashima I, Tajima K, Nishimura K, et al. Diagnostic imaging of diseases affecting the mandible with computed panoramic radiography, ORAL SURC ORAL MED ORAL PATHOL 1990; 70:110-6. 4. Farman AG, Mouyen F, Razzano M. RadioVisioGraphy: concept and applications. In: Preston JD, ed. Computers in dentistry,Chicago:QuintessencePublishingCo. Inc, 1993:12534. 5. Maruyama T, Iwai K, Hashimoto K. Estimation of frequency, population doses, and stochastic risks in dental radiographic examinations in Japan, 1985. Dent Radio1 1987;27:143-53. 6. Mase N, Iwai K, Honjo T, et al. Current status of radiation hygiene on dental radiology in Japan, 1989 [Abstract]. Dent Radio1 1991;31:92. 7. U.S. Department of Commerce. Statistical abstract of the United States 1992. Washington, DC: U.S. Government Printing Office, 1992;112:No.l173.
15. 16.
17.
18.
19.
Day RA Jr, Underwood AL, eds. Quantitative analysis: Asian edition Tokyo: Maruzen Company LTD.. 1959:763. Environmental Protection Agency. Guide to pollution prcvenlion in the photo processing industry. Report No. EPA/625/ 7-91 /012. .Jacobs Engineering Group, Inc. Pasedena, Calif, 1991:13-20. Cooly AC, Dagon TJ. Current silver recovery practices in the photographic processing industry. J Appl Photogr Eng 1976; 2:36-41. Cooly AC, Dagon TJ. Jenkins PW, Robilled KA. Silver and the environment J Imaging Technol 1986;14:183-9. McKinney WE. Silver concentrations in radiographic processing wash water and waste minimization. Materials Evaluation 1991;49:482-6. Kasai K. Silver halide photography and environmental protection, 1991. J Jap Photogr Sci 1991;54:3-9. Polaroid Corporation. A report on the environmental issues and concerns with chemical processing in radiology. Polaroid Publication PID#lB5515, 1991:1350. Charlton AD. Silver-recovery applications for better effluent management. Materials Evaluation 1991;3 1509. Knorre H. Maennig D, Stuetzel K. Chemical treatment of effluent from photo-finishing plants. J Imaging Technol 1988; 14:154-6. Hashimoto K, Thunthy KH, lwai K, et al. Sensitometric comparison of direct and indirect exposure films used in intraoral radiography. J Nihon Univ Sch Dent 1992;34:106-10. Macdonnell D, Price C. An evaluation of the Sens-A-Ray digital dental imaging system. Dentomaxillofac Radio1 1993; 22:121-s. Molteni R. Direct digital dental x-ray imaging with Visualix/ VIXA. ORALSURGORAL MEDORAL PATHOL 1993;76:235-43.
Reprint requests. Taeko Goshima, DMD Tokai Dental Clinic 3-4-14 Minami Shinagawa Shinagawa-ku, Tokyo 140 Japan
OF ORAL RADIOLOGY,
TSURUMI UNIVERSITY,
DDS, PhD,
SCHOOL OF DENTAL MEDICINE
This study examined the silver content of IO brands of dental x-ray film that are commercially available in Japan. Kodak Ektaspeed EP-22 (Eastman Kodak Co., Rochester, N.Y.) was found to have the highest silver content, and SD 100 Gold X-ray Film (San Dental Co., Ltd., Tokyo, Japan) had the lowest content of silver. Electrolysis and ion exchange were used as the methods of silver recovery from films in the general dental office. Ion exchange was more than 99% efficient in silver recovery from the radiographic fixer and resulted in an 80% reduction in biochemical oxygen demand and a 95% reduction in chemical oxygen demand. However, these levels and other environmental pollutants were still above the limits set by the Japanese Pollutant Control law. (ORAL %JRC 0~41 MD ORAL PATHOL 19!M;r/:684-8)
In the late 198Os, Japan adopted an environmental assessment system patterned after the U.S. Environmental Protection Agency. The first Environmental Pollution Control Law passed in Japan was in 1967.’ Several other laws have followed. Oral and maxillofacial radiology results in a variety of wastes that could have an impact on the environment, namely, spent processing chemicals, salivacontaminated intraoral film packet jackets, lead foil, and discarded radiographs. Although new environmentally sound direct digital imaging systems are now available,2‘4 the use of physicochemical radiographic systems still dominates. In Japan, the number of intraoral dental and panoramic radiographs taken in 1984 were 90.878 million and 8.665 million, respectively. By 1989, these figures had increased to 96.63 million for intraoral films and to 11.09 million for panoramic films, s,6 which represent a 6% and a 28% increase, respectively, over a 5-year period. In 1980, the price of silver reached $20.64 per ounce, and there was an impetus to recover silver from fixer solution and discarded x-ray film because of the commercial value of the reclaimed silver.7 Since 1980, however, the price of silver has declined steadily. In 1992, silver had fallen to $3.93 per ounce and has not made appreciable gains since that time.7 The recovery of silver, therefore, is no longer prompted by direct commercial gain. However, because silver is a nuisance as a pollutant that can impair effective sewage treatment, the focus on silver recovery is primarily a concern for the environment.’ Several methods for silver recovery from radiographic fixer are now available (Table I), but most of these are not practical for the general dental office
Copyright @ 1994 by Mosby-Year Book, Inc. 0030-4220/94/%1.00 + .lO T/16/52098 684
Table I. Silver recovery methods Method
Metallic replacement
Advaruages
Low investment Low operating cost
Disadvantages
Effluent high iron content Silver recovered as sludge Effluent
Electrolytic
recovery Precipitation
Silver recovered as pure metal High silver recovery Can attain 0.1 mg silver/L Low investment
Reverse osmosis
Also recovers other chemicals Purified water is recyclable
Ion exchange
Can recover 0.1 to 0.2 mg silver/L
high silver
content unless 2 units used in series Sulfide formation possible Effluent high in content Complex to operate Silver recovered as sludge Treated solution cannot be reused Potential hydrogen sulfide released Concentrate requires further processing High investment High operating cost Only for diluted solutions Complex operation High investment
where costs and convenience must be kept in mind.‘*‘16 Metallic replacement and electrolysis are the most common silver recovery procedures on the market today that are suitable for use in the private office. This article explores the effectiveness of both electrolysis and ion exchange in removing silver and
Goshima,
ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY I/olume 77, Number 6
reducing other environmental polluting factors from spent radiographic fixer. MATERIAL AND METHODS Sensitometric properties of dental x-ray film
Ten brands of x-ray film that are commercially available (for intraoral radiography in Japan) were used in this part of the study (Table II). Shimavision 1400 (Shimadzu Corp., Kyoto Japan) was used (70 kVP, 50 mA, target-to-object distance of I76 cm.) A film was positioned in the center of the beam of radiation and exposed for one impulse; all films were processed in the HOPE Model P-10 automatic dental film processor (Hope Industries, Inc., Willowgrave, Pa.). Densitometric values were measured by PDA-25 (Konica Co., Ltd., Tokyo, Japan) densitometer. Silver content of dental x-ray film
Twenty samples of film from each of 10 different brands (Table II) were used to determine the comparative silver content of each type. Three of the 10 brands were direct-exposure films, and the remaining were indirect-exposure without intensifying screens (Table II). The silver was dissolved with nitric acid, and the amount of silver present in the solution was determined with the use of the Volhalt method.8 Silver recovery
The ion exchange system tested was the Dent-X Quick Silver silver recovery system (Dent-X Corp., Elmsford, N.Y.). The electrolysis system tested was the Silver Magnet AT-2 silver recovery unit (TMW Corp., West Bend, Wise.). Three films, Kodak DF-57, Kodak EP-22 and Hanshin Hi-Fi contained approximately three times as much silver as the others. Of the three, Kodak DF-57 (Eastman Kodak, Rochester, NY) was chosen for the silver recovery phase because it is the direct exposure film most widely used for intraoral radiography in Japan. The radiographs were exposed with the use of a mannequin to simulate the typical clinical situation for a mandibular molar exposure using a Max-R x-ray generator (J Morita Co., Osaka, Japan) (70 kVp, 10 mA, target-to-object distance of 30 cm). In total, 3600 films were exposed for use in the study. Half of these films were processed in Cronex HSF/M (Du Pont Japan Ltd., Tokyo, Japan) and the other half were processed in a Kodak GBX fixer and replenisher (Eastman Kodak Co.) The fixer in each case was divided into three parts. A sample of each was kept as a control for prerecovery silver level assessment. The remainder was divided in half to make four samples, two for each processing chemistry. One sample of each fixer solution was then subjected to silver recovery using each of the two methods.
Hori,
and Yamamoto
685
Table II. Silver content of no. 2 intraoral
radiographic films Silver Manufacturer
Film
Direct-exposure films EKTASPEED EP-22 Ultra-Speed DF-57 Hanshin Hi-Fi
Indirect-exposure films (without intensifying screens) Konica Dental Fuji Dental Periapical Hanshin New Instant
Nix NF-55 SD 100 GOLD X-Ray Yata X-Ray 3TC Beam Dental Roentgen FB-100
(mean
Eastman Kodak Co., Rochester, N. Y. Eastman Kodak Co., Rochester, N. Y. Mfr. Hanshin Technical Lab Ltd., Nishinomiya, Japan
mg)
22.09 19.90 20.12
Konica Co., Ltd., Tokyo, Japan Fuji Photo Film Co., Ltd., Tokyo, Japan Mfr. Hanshin Technical Lab Ltd., Nishinomiya, Japan Nix Company Ltd., Tokyo, Japan San Dental Co., Ltd., Tokyo, Japan Yata Dental Mfg., Ltd., Osaka, Japan 3.T.C. Inc., Tokyo, Japan
7.28 6.96 6.32
9.16 5.60 8.83 6.22
Chemical analyses of fixer solutions
The following parameters were tested for the fixer with each processing chemistry both before and after silver recovery by the two silver recovery methods: 1. A pH-Glass electrode analysis was performed to measure the acidity of the solution as prescribed in the Japanese Environmental Agency standard, JIS Z 8802. 2. Biochemical oxygen demand (BOD). The Winkler sodium azide modifier analysis was carried out as prescribed in the Japanese standard, JIS K 0102, 2 1. This measures the amount of dissolved oxygen consumed by aerobic microorganisms living in water. The sample is diluted in water and kept for 5 days at 20” C. BOD
= (b
- D2) - WI-
P
J32)
xf
686
Goshima, Hori, and Yamamoto
ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY
-June 1994
where
BOD = biochemical oxygen demand (mg oxygen/L) DI = dissolved oxygen in the diluted sample after preparation (mg oxygen/L) D2 = dissolved oxygen in the diluted sampleafterincubation (mgoxygen/L) P = sample dilution ratio (sample/water) BI = dissolved oxygen before incubation of the diluted liquid specimen at the time of measuring BOD (mg oxygen/L) B2 = dissolved oxygen after incubation of the diluted liquid specimen at the time of measuring BOD (mg oxygen/L) 3. Chemical oxygen demand (COD). The potassium permanganate titrimetric analysis (JIS K 0102, 17) CODM,
= (b - a) X f X 7
X 0.2
where CODw,, = oxygen demand by potassium permanganate (mg oxygen/L) b = total amount of N/40 potassium permanganate solution required for titration (mL) a = amount of N/40 potassium permanganate solution required for titration of the blank test (mL) f = factor of N/40 potassium permanganate solution I/ = sample (mL) 4. Total dissolved solids (TDS). The gravimetric analysis following standard JIS K 0102, 14.2. 1,000 R=(a-b)XT where
R = total evaporation residue (mg/L) a = mass of the evaporating dish containing the residue (mg) b = mass of the evaporating glass (mg) V = sample volume (mL) 5. Total suspended solids (T-SS). The gravimetric analysis following standard JIS K 0102, 14.2. 1,000 S=(a-b)XT
where
S = suspended matter (mg/L) a = mass of the filter containing the suspended matter and the watch glass bg) b = mass of the filter and the watch glass (ms) V = sample volume (ml)
RESULTS Properties of film
The characteristic curves for the direct exposure films and also those for the indirect exposure films, Hanshin New Instant, Konica Dental, Nix NF-55, 3TC FB-100, Fugi Dental SDlOO, and Yota are shown in Fig. 1. The exposures needed to produce diagnostic radiographs using indirect exposure film without intensifying screens was considerably greater than that needed using direct exposure film. Silver content of film
The silver contents measured per film are given in Table II. This ranged from 5.6 to 20.12 mg/film. The mean silver content per film for all 10 periapical no. 2 film types was 11.25 mg (standard deviation + 6.64 4. Effluent characteristics of spent fixer
Table III shows the values for the parameters tested for the fixer solution before and after silver recovery. Silver. For CRONEX, silver recovery was only 11% using electrolysis but more than 99.9% with ion exchange. For GBX, silver recovery by electrolysis was only 1 l%, whereas with ion exchange it was 99.7%. pH. The spent fixer was acidic because of its content of acidifier both before and after silver recovery by electrolysis. Silver recovery by ion exchange resulted in a mildly alkaline residual solution. This was a common finding with both processing solutions tested. BOD. BOD values for CRONEX were invariably higher than with GBX solutions both before and after silver recovery. With CRONEX, there was a 34% reduction in BOD after silver recovery by electrolysis, and an 87% by ion exchange. For GBX, there was a 27% increase in BOD after electrolytic removal of silver and an 80% reduction in BOD after silver recovery by ion exchange. COD. For CRONEX, COD was reduced by 23% after electrolysis and 95% after ion exchange. For GBX, the COD value was reduced by only 2% after electrolysis, but by 95% after ion exchange. TDS . For CRONEX, TDS was reduced by 51% and 91% after silver recovery by electrolysis and ion exchange, respectively. For GBX, the TDS remained unchanged after electrolysis, but TDS was reduced by 9 1% after silver recovery using ion exchange. T-SS. For CRONEX, T-SS was reduced by onethird after electrolysis and by more than 30% after ion exchange. For GBX, T-SS doubled after electrolysis but was reduced by two-thirds after ion exchange.
Goshima,
ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY Volume 77, Number 6
/
/f
//
I
2
3
4
5
6 log
7
------
with
-
without
Hanshin’
#Hanshin
8
intensifying
and Yamamoto
687
screens
intensifying
screens
/7
New
9
Hori.
Instant
IO
Exposure
Fig. 1. Characteristic curves for direct- and indirect-exposure radiographic films usedin Japan for intraoral radiography. All films are used with and without intensifying screensfor this purpose; hence, indirect exposure film results in higher radiation exposuresto the patients. Presently more than 50% of intraoral radiographs taken in Japan use indirect-exposure film without screens.
Table III. Spent fixer effluent characteristics CRONEX
Factor
(unit)
Ag (mg/L) PH BOD (mg 02/L) COD (mg 02/L) TDS h/L) T-SS (mg/L)
Prerecovery 3200
Ion exchange recovery of silver
2860
<4.4 8.20 2570 2940 12600
4.95 12900 46700 36000
9
KODAK
Electrolytic recovery of silver
4.25 19400 60900 134000
HSF/M
6
DISCUSSION Ninty-seven percent of dental facilities in Japan are
private practices. More than 50% of the intraoral radiographs used in dental practice are still indirect exposure films used without intensifying screens in the form of specially manufactured “packet-processed” films.5* 6 As the name suggests, developing chemistry is introduced directly into the film packet. After processing the solution is discarded, and therefore it has a potential negative impact on the environment. Given the same speed rating of film, it would seem that the use of the film type with the lowest silver
GBX
Fixer
Electrolytic Prerecovery .3240 5.38 7140 46900 113000 3
and replenisher
recovery silver 2880 5.34
9060 45900 113000 6
Ion Exchange recovery of silver <4.4 8.15 1430 2240 10300 1
content would be one way to reduce the potential pollution of the environment by silver compromised that the obtained image quality is not reduced as a consequence. However, the indirect-exposure films without intensifying screen are much slower than directexposure films. I7 Therefore the patients are unnecessarily overexposed to radiation. Hence, use of direct exposure film is advocated and was used for the silver recovery phase of this study. Although ion exchange recovery is more efficient than electrolysis, the levels of BOD, COD, and other environmental pollutants are still above the limits set
688
Coshima,
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by the Japanese government pollution control laws.’ New digital imaging systems for intraoral radiography have the advantages of reduced radiation exposure to the patient, elimination of the need for processing chemistry, and of instantaneous images without environmental pollution.‘8, I9 If these systems become the mode for imaging in dentistry, the need for silver recovery from radiographic fixer would no longer be a concern for our profession. We thank Mr. Makoto Tsuchida for variable suggestions for laboratory works and also Ms. Kiyoe Yamada for typing this manuscript.
ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY June 1994
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Reprint requests. Taeko Goshima, DMD Tokai Dental Clinic 3-4-14 Minami Shinagawa Shinagawa-ku, Tokyo 140 Japan