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Toxicity abatement and biodegradability enhancement of pulp mill bleaching effluent by advanced chemical oxidation
e.>
Pergamon
Wal
Tech. Vol. 40, No. 11-12, pp. 337-342, 1999
PIT: S0273-1223(99)00736-2
ScI
Pnnled in Great Bntaln. All nghts reserved 0273-1223/99 520.00 + 0.00
TOXICITY ABATEMENT AND BIODEGRADABILITY ENHANCEMENT OF PULP MILL BLEACHING EFFLUENT BY ADVANCED CHEMICAL OXIDATION Ma Cristina Yeber*, Jaime Rodriguez*, Jaime Baeza*, Juanita Freer*, Claudio Zaror**, Nelson Duran*** and Hector D. Mansilla*,t * Laboratory 01Renewable Resources. Faculty olChemical Sciences. Universidad de Concepcion. Casilla 160-C. Concepcion. Chile ** Department ojChemical Engineering. Universidad de Concepcion. Casilla 160-C. Concepcion. Chile *** Biological Chemistry Laboratory. Universidade Estadual de Campinas. Brazil ABSTRACT This paper describes the use of photocatalysis to degrade two bleaching effiuents from P. radiara (D.E.,) and E. grandls (CEop) wood processing. Organic matter oxidation was followed by general parameters (TOC, color, AOX). toxiCity and biodegradablhty. Reactions with suspended and supported catalysts, Ti02 and ZnO, were carned out for companson. For both effiuents. photocatalytic treatment led to an improvement in biodegradability and a reduction in acute toxICIty. <1:1 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved
KEYWORDS Advanced oxidation processes; biodegradability; cellulose-bleaching effiuents; photo-catalysis; Ti02; toxicity; ZnO. INTRODUCTION Several attempts to reduce the organic charge, color, AOX and toxicity of bleaching pulp effiuents have been done (Kahrnark and Unwin, 1998). Also, the increase of the biodegradability is a target of the effiuent treatments. In recent years many reports have been focused in the search of new oxidation technologies as pre-treatment for chlorinated effiuents before biological conventional treatments. Advanced Oxidation Processes (AOPs) have been successfully used in the oxidation of several organic compounds, and the efficiency of these systems is'based on the production of strong oxidant moieties, such as hydroxyl radicals. One of the most efficient systems is the photo-assisted catalysis, which is capable to reduce the toxicity and enhance the biodegradability of cellulose effiuents. Photocatalysis involves the irradiation of a semiconductor (Ti02, ZnO) with UV light in the range below 390 om, generating electron-hole pairs on the catalyst surface inducing the formation of radical reduced oxygen species. These radicals are produced in situ and they are highly reactive and unspecific oxidants.
t Corresponding author
337
M. C. YEBER et 01.
338
Reports concerning oxidation using ozone (Mao and Smith, 1995) or advanced oxidation with ozoneIUV and semiconductor photoassisted catalysis applied to different cellulose effiuents have been previously presented (Mansilla, 1997; Yeber, 1998). Also, photocatalytic oxidation has been applied to sulfonated lignins (Kobayakawa et al., 1989) and nonchlorinated cellulose effiuents (Mansilla et al., 1994; Villasenor and Mansilla, 1996). These effluents have been strongly degraded reducing both color and molecular weight of dissolved lignin and lignin derivatives. However, catalyst recovery is a key issue in industrial implementation of such treatment systems. This problem can be solved, in part, if the catalyst is immobilised on inert surfaces without the loss of activity. In this paper we present experimental results on acute toxicity reduction by photocatalysis of cellulose bleaching effluents from wood kraft cooking (DoE.,p and CEop sequences), using two systems: suspended and immobilised catalysts, TiOz and ZnO, activated by UV light (A. > 254 nm).
METHODS CEop effluents were obtained after the first alkaline extraction of a conventional bleaching sequence of Eucalyptus grandis pulping coming from kraft process. DoEop (ECF) effiuents were obtained of the ECF pulp bleaching from Pinus radiata kraft process. These effluents were stored at -4°C to avoid modifications. When reactions were carried out in suspended solution, 2 g of TiOz and ZnO were used for 100 mL ofECF effluent. In experiments with the CEop effluent (150 mL), supported catalysts on glass rashing rings were used. Impregnation of the catalysts we carried out as previously reported (Yeber et al. 1998). Photolysis was 2 performed with a Philips high pressure mercury lamp, 125 watts (A.>254 nm, energy flux 12 mW cm- ) in a glass reactor. Pure oxygen was bubbled at 200 mL mol through a fritted glass placed at the bottom of the reactor. The reactor was refrigerated with water to keep up the temperature at 20°C. The effiuent color was determined at 465 nm and referred to a Pt-Co standard solution (EPA 00080). Chemical oxygen demand (COD) was determined by standard method (EPA 00340) after 2 h of reflux. Adsorbable organic halogen (AOX) was determined using an EUROGLAS ECS-lOOO AOX analyser and the recommended procedure (DIN 38409 HI4). BODs determination was carried out according to EPA 00310. Total organic carbon (TOC) analyses were carried out in a TOC-5000 Shimadzu equipment. Total phenol was determined at 700 nm using the Folin standard method APHA A WWA. Acute toxicity assays were performed by the MICROTO~ determination, which use Photobacterium phosphoreum luminescence to assess the ECso values. Results are expressed as luminescence inhibition (%), measured at 5 and 15 minutes and toxicity units (TUs) were defined as loolECso· The activated sludge inoculum was taken from a laboratory continuous reactor, which was grown in a pulp bleaching effluent. The presence of different bacteria conglomerates, ciliate protozoa, nematodes, vorticella and rotifers in the sludge, was verified by optic microscopy. All samples (40 mL) were inoculated with 1 mL of the sludge. Prior to inoculation the pH was adjusted to 7 and stabilised with 2 mL of phosphate buffer (0.45 mol L- 1). Mineral salts (Fe++, Ca++ and Mg*) were prepared in the same concentration used for BOD analysis, and added to the effiuent as dilution water (1 mL). The culture was grown under agitation (150 rpm, 25°C). The dry weight was determined after 12, 24, 48, 72 and 120 hours by filtering the culrure on a nitrocellulose Millipore filter (45 J.Ull), and drying at 100 °C. RESULTS AND DISCUSSION
Both effluents presented high values of acute toxicity, color, COD, TOC and a great amount of chlorinated compounds, particularly in CE.,p effluents. Table 1 summarises the main features of effluents used in this study.
339
Advanced chemical oxidation
Table 1. Initial general parameters of assayed effluents
CE.,p
Parameter Color (mg Pt L- ) 4510 pH 10.8 COD (mg L- J) 1550 AOX (mg L0 1) 7.7 23 Toxicity· (EC-50) o1 TOC m L 1240 * MICROTOX (vol. %, 5 min)
2933 10.3 2255 73 13 980
In consequence if it was discharged to biological treatment, microorganisms would be inhibited. Photocatalysis was used to reduce this effect. Biodegradability of effluent organic matter was estimated as the BODs/COD ratio. Good biodegradability values were considered in the range over 0.5 (Harmsen and Voortman, 1996). Effluents used here featured ratios below 0.3. Under advanced oxidation conditions with Ti02 and ZnO, the effluent biodegradability rose over 005 within the first minute of reaction, as shown in the Figure I.
006
8 0.4 g oco
0.2
o Eop
02/UVfTi02
02/UV/ZnO
Figure I. ECF Effluent biodegradability (BODs/COD) after I minute of treatment with o2lUVm~ and O,/UV/ZnO systems (initllll value OJ).
To confirm the increase in effluent biodegradability due to photocatalytic treatment, ECF oxidised samples were submitted to activated sludge, adapted to treat pulp mill wastewater. Samples were inoculated and the microorganism's growth was measured as the biomass increase (dry weight) at different periods of time. Figure 2, shows a rapid and continuous increase of biomass after 12 and 24 hour of growth. After that, a variable behaviour in growth was observed as is expected for this kind of cultures. This behaviour is explained by microorganism population changes during culture which are continuously adapting to newly formed compounds (Gray, 1990). This fact was qualitatively observed in cultures by optic microscopy. Figure 2 shows that after one minute of photocatalysis the ECF effluent improves the growth of the microorganisms, in comparison with the untreated sample (Eo). The same was demonstrated by the BODs/COD ratio. However it is not possible to detennine which is the most efficient system since it depends on the nature of sludge. Also, some biorefractory compounds may be fonnedo Similar conclusions were obtained by Stowell et al. (1992) in the ozonationlbiological degradation of2-ehlorophenol. The degradation of the ECF effluent organic matter was also evaluated as the % COD removal according to the formula (I), where CaDi is the initial value (1550 mg 02 LO I) and COOt is the value obtained after each treatment. The total COD removal includes the COD removed after chem/biological treatment (72 h). Table 2 shows the COD removal after each step: chemical, biological and chemicallbiological treatments. COD Removal (%) = [CaDi - COOt] • 100/ CaDi
(1)
M. C. YEBER et at.
340
6
---+- EO -.- 02lLNm02 -.-- 02lLNflnO
o .---,---,---,---,---,---~ o 12 24 36 48 60 72 Time (min) Figure 2. Dry weight mcreasc of activated sludge on untreated pulp bleaching effluent (.), and treated by one systems. minute by 02IUV/ZnO (X) and 02IUVrri02
(*>
Table 2. COD removal after chemical, biological and combined treatment Treatment
COD removal (%) after chemical treatment (l min)
-
Biologic 02/uv/ZnO 02/UVITiOZ
COD removal (%) COD removal (%) Total COD after chem/biol after chem/biol removal (%) treatment (24 h) treatment (72 h) 49 16 14
44 51
58 32 40
58 62 70
Figure 3 shows the initial toxicity of the ECF effluent and is compared with one minute treated sample. It is observed that the acute toxicity was reduced in 50 %, for both catalysts. These results agree with the biomass increase in the activated sludge. It is also indicative of the toxicity reduction. In a previous communication (Peralta-Zamora et aI., 1998) the same trend was found in the toxicity abatement of cellulose and textile mill effluents, in around 50 %, under photocatalytic conditions.
6
O+---l--L----r--'-----'--...,....-~-~___,
Eop
02lUVm02
02lUVIZnO
Figure 3. Microtox acute toxicity of the pulp bleaching ECF effluent treated by I min with AOP treatments.
Effluent coming from conventional pulp bleaching (CEop ) was treated for 2 hours with supported catalysts. Photocatalysis using TiOz reduced the initial TOC (980 ppm) in 55%, whereas with ZnO treatment only reached 31 % reduction. TiO z is more efficient as catalyst to induce the mineralization of the organic matter than ZnO, when immobilised on glass. On the other hand, COD removal reaches 58% for both catalysts after 120 minutes reaction; at longer times the COD and TOC remain approximately constant.
341
Advanced chemical oxidation
AOX is another key parameter, which must be reduced in any effluent treatment, since this is related with the amount of chlorinated compounds and toxicity. Recently, Archibald et al. (1997) reported that TiO z promotes AOX degradation of a bleaching effluent, as compared with the natural sunlight degradation, and this effect is more remarkable after 60 days of exposure. In our case, using immobilised catalysts to treat eucalyptus E1 effluent, the same effect is clearly observed after few minutes of reaction reaching degradation rates of 85% and 90% ofthe initial AOX, after 60 and 120 minutes, respectively. It must be emphasised that the high reduction in a short period of treatment is also a consequence of the high irradiation power of the lamp in the range 300-400 om, as compared with the sunlight. On the other hand, a 2 units pH decrease (from 10.3 to 8.2) was observed after 60 minutes of reaction, which is an indication of the mineralization of organic chlorine to chlorine ions. Photoassisted catalysis with ZnO and TiO z degrades the organic matter dissolved in paper mill effluents, significantly reduced toxicity and improved biodegradability, in assays carried out with the catalyst suspended in the effluent solution. Figure 4 shows that Microtox toxicity was reduced in 50 % after 60 minutes of reaction and remains without changes after that time, when oxidation was carried out using immobilised catalysts. It means that residual lignin derivatives are difficult to be completely degraded. This result is consistent with the COD and TOC reductions at similar extents; however, it can be demonstrated that biodegradability of residual organic matters is enhanced and that photo-catalytic systems could be a good pre-oxidation step before biological treatment. 8
I I
o EoP
Ti02-60 min.
Zn0-60 min.
Figure 4. Acute toxicity removal of eucalyptus CE.. effiuent by supported Ti02 and Zno photocatalysis.
CONCLUSIONS It can be concluded that advanced oxidation process could efficiently degrade effluents with high organic matter charge, improving their biodegradability and reducing toxicity. The photocatalysis with supported catalysts proved to be as efficient as suspended treatment, although required reaction times are different. This may be due to the lower amount of catalyst present in the immobilised system. Further work on immobilisation techniques in studies with lignin model compounds is in order. AKNOWLEDGEMENTS This research was supported by FONDECYT (Grants 1980498, 980459 and 7980035) and Direcci6n de Investigaci6n (U de C, Grants 97.23.14-1 and 97.23.16-1.2). M.C. Yeber thanks to FONDECYT for'the Doctoral Fellowship (Grant 2970096) REFERENCES Archibald, F" Roy-Arcand, L. and Methot, M. (1997), Time, sunlight, and the fate ofbiotreated kraft mill organochlonnes (AOX) in the nature. Wat, Res., 31, 85·94, Kahmarlc, K.A. and Unwin, J.P, (1998). Pulp and paper effiuent management. Water Environ, Res,. 4. 667.690.
342
M. C. YEBER et al.
Gray, N.F. (1990). Activated Sludge: Theory and Practice, ch. 12-16, Oxford University Press, New York. Harmsen, G. and Voortman, B. (1996). Pretreatment of wastewater from tank-c1eanmg industry by chemical oXidatiOn, Proceedings of the International Conference on Oxidation Technologies for Water and Wastewater Treatment (Ed. A. Vogelpohl), Goslar, Germany. Kobayakawa, K., Sato, Y., Nakamura, S. and Fijishima, A. (1989). PhotodecompositIOn of kraft lignin catalysed by titanium dioxide. Bull. Chem. Soc. Jpn., 62, 3433-3436. Mansilla, H. D., Villaseilor, J., Maturana, G., Baeza, J., Freer, J., Duran, N. (1994). ZnO-Catalysed photodegradation of kraft black liquor. J. Photochem Photoblol. A: Chern., 78, 247-256. Mamilla, H.D., Yeber, M.e., Freer, J., Rodriguez, J. and Baeza, J. (1997). Homogeneous and heterogeneous advanced oxidation ofa bleaching effluent from the pulp and paper industry Wat Sci Tech .• 35(4),273-278. Mao, H. and Smith, D.W. (1995). Toward elUCIdating mechanism and kinetics of ozone decolorization and dechlorination of pulp null effluents. Ozone SCI. Engrg., 17(4) 419-448. Peralta-Zamora, P., Gomes de Moraes, S., Pelegrml, R., Freire Jr., M., Reyes, J., Mamilla, H.D. and Duran, N. (1998). Evaluation of ZnO, TiO, and supported ZnO on the photoassisted remediation of black liquor, cellulose and textile effluents. Chemosphere, 36, 2119-2133. Stowell, J.P. Jensen, J.N. and Weber, A.S. (1992). Sequential ChermcalfBiologlcal Oxidation of 2-chlorophenol, Wat. Sci Tech., 26, 2085-2087 Villaseilor, J. and Mansilla H.D. (1996). Effect of temperature on kraft black liquor degradation by ZnO-photoassisted catalySIS. J. Photochem. Photobiol. A: Chern. Yeber, M.C., Freer, J., Baeza, J. and. Mamilla RD. (1998). TiO, and ZnO thin film formation on glass. In The ]'" International Conference on Advanced Wastewater Treatment Recycling and Reuse, Milano, Italy, 907-910.
Pergamon
Wal
Tech. Vol. 40, No. 11-12, pp. 337-342, 1999
PIT: S0273-1223(99)00736-2
ScI
Pnnled in Great Bntaln. All nghts reserved 0273-1223/99 520.00 + 0.00
TOXICITY ABATEMENT AND BIODEGRADABILITY ENHANCEMENT OF PULP MILL BLEACHING EFFLUENT BY ADVANCED CHEMICAL OXIDATION Ma Cristina Yeber*, Jaime Rodriguez*, Jaime Baeza*, Juanita Freer*, Claudio Zaror**, Nelson Duran*** and Hector D. Mansilla*,t * Laboratory 01Renewable Resources. Faculty olChemical Sciences. Universidad de Concepcion. Casilla 160-C. Concepcion. Chile ** Department ojChemical Engineering. Universidad de Concepcion. Casilla 160-C. Concepcion. Chile *** Biological Chemistry Laboratory. Universidade Estadual de Campinas. Brazil ABSTRACT This paper describes the use of photocatalysis to degrade two bleaching effiuents from P. radiara (D.E.,) and E. grandls (CEop) wood processing. Organic matter oxidation was followed by general parameters (TOC, color, AOX). toxiCity and biodegradablhty. Reactions with suspended and supported catalysts, Ti02 and ZnO, were carned out for companson. For both effiuents. photocatalytic treatment led to an improvement in biodegradability and a reduction in acute toxICIty. <1:1 1999 Published by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved
KEYWORDS Advanced oxidation processes; biodegradability; cellulose-bleaching effiuents; photo-catalysis; Ti02; toxicity; ZnO. INTRODUCTION Several attempts to reduce the organic charge, color, AOX and toxicity of bleaching pulp effiuents have been done (Kahrnark and Unwin, 1998). Also, the increase of the biodegradability is a target of the effiuent treatments. In recent years many reports have been focused in the search of new oxidation technologies as pre-treatment for chlorinated effiuents before biological conventional treatments. Advanced Oxidation Processes (AOPs) have been successfully used in the oxidation of several organic compounds, and the efficiency of these systems is'based on the production of strong oxidant moieties, such as hydroxyl radicals. One of the most efficient systems is the photo-assisted catalysis, which is capable to reduce the toxicity and enhance the biodegradability of cellulose effiuents. Photocatalysis involves the irradiation of a semiconductor (Ti02, ZnO) with UV light in the range below 390 om, generating electron-hole pairs on the catalyst surface inducing the formation of radical reduced oxygen species. These radicals are produced in situ and they are highly reactive and unspecific oxidants.
t Corresponding author
337
M. C. YEBER et 01.
338
Reports concerning oxidation using ozone (Mao and Smith, 1995) or advanced oxidation with ozoneIUV and semiconductor photoassisted catalysis applied to different cellulose effiuents have been previously presented (Mansilla, 1997; Yeber, 1998). Also, photocatalytic oxidation has been applied to sulfonated lignins (Kobayakawa et al., 1989) and nonchlorinated cellulose effiuents (Mansilla et al., 1994; Villasenor and Mansilla, 1996). These effluents have been strongly degraded reducing both color and molecular weight of dissolved lignin and lignin derivatives. However, catalyst recovery is a key issue in industrial implementation of such treatment systems. This problem can be solved, in part, if the catalyst is immobilised on inert surfaces without the loss of activity. In this paper we present experimental results on acute toxicity reduction by photocatalysis of cellulose bleaching effluents from wood kraft cooking (DoE.,p and CEop sequences), using two systems: suspended and immobilised catalysts, TiOz and ZnO, activated by UV light (A. > 254 nm).
METHODS CEop effluents were obtained after the first alkaline extraction of a conventional bleaching sequence of Eucalyptus grandis pulping coming from kraft process. DoEop (ECF) effiuents were obtained of the ECF pulp bleaching from Pinus radiata kraft process. These effluents were stored at -4°C to avoid modifications. When reactions were carried out in suspended solution, 2 g of TiOz and ZnO were used for 100 mL ofECF effluent. In experiments with the CEop effluent (150 mL), supported catalysts on glass rashing rings were used. Impregnation of the catalysts we carried out as previously reported (Yeber et al. 1998). Photolysis was 2 performed with a Philips high pressure mercury lamp, 125 watts (A.>254 nm, energy flux 12 mW cm- ) in a glass reactor. Pure oxygen was bubbled at 200 mL mol through a fritted glass placed at the bottom of the reactor. The reactor was refrigerated with water to keep up the temperature at 20°C. The effiuent color was determined at 465 nm and referred to a Pt-Co standard solution (EPA 00080). Chemical oxygen demand (COD) was determined by standard method (EPA 00340) after 2 h of reflux. Adsorbable organic halogen (AOX) was determined using an EUROGLAS ECS-lOOO AOX analyser and the recommended procedure (DIN 38409 HI4). BODs determination was carried out according to EPA 00310. Total organic carbon (TOC) analyses were carried out in a TOC-5000 Shimadzu equipment. Total phenol was determined at 700 nm using the Folin standard method APHA A WWA. Acute toxicity assays were performed by the MICROTO~ determination, which use Photobacterium phosphoreum luminescence to assess the ECso values. Results are expressed as luminescence inhibition (%), measured at 5 and 15 minutes and toxicity units (TUs) were defined as loolECso· The activated sludge inoculum was taken from a laboratory continuous reactor, which was grown in a pulp bleaching effluent. The presence of different bacteria conglomerates, ciliate protozoa, nematodes, vorticella and rotifers in the sludge, was verified by optic microscopy. All samples (40 mL) were inoculated with 1 mL of the sludge. Prior to inoculation the pH was adjusted to 7 and stabilised with 2 mL of phosphate buffer (0.45 mol L- 1). Mineral salts (Fe++, Ca++ and Mg*) were prepared in the same concentration used for BOD analysis, and added to the effiuent as dilution water (1 mL). The culture was grown under agitation (150 rpm, 25°C). The dry weight was determined after 12, 24, 48, 72 and 120 hours by filtering the culrure on a nitrocellulose Millipore filter (45 J.Ull), and drying at 100 °C. RESULTS AND DISCUSSION
Both effluents presented high values of acute toxicity, color, COD, TOC and a great amount of chlorinated compounds, particularly in CE.,p effluents. Table 1 summarises the main features of effluents used in this study.
339
Advanced chemical oxidation
Table 1. Initial general parameters of assayed effluents
CE.,p
Parameter Color (mg Pt L- ) 4510 pH 10.8 COD (mg L- J) 1550 AOX (mg L0 1) 7.7 23 Toxicity· (EC-50) o1 TOC m L 1240 * MICROTOX (vol. %, 5 min)
2933 10.3 2255 73 13 980
In consequence if it was discharged to biological treatment, microorganisms would be inhibited. Photocatalysis was used to reduce this effect. Biodegradability of effluent organic matter was estimated as the BODs/COD ratio. Good biodegradability values were considered in the range over 0.5 (Harmsen and Voortman, 1996). Effluents used here featured ratios below 0.3. Under advanced oxidation conditions with Ti02 and ZnO, the effluent biodegradability rose over 005 within the first minute of reaction, as shown in the Figure I.
006
8 0.4 g oco
0.2
o Eop
02/UVfTi02
02/UV/ZnO
Figure I. ECF Effluent biodegradability (BODs/COD) after I minute of treatment with o2lUVm~ and O,/UV/ZnO systems (initllll value OJ).
To confirm the increase in effluent biodegradability due to photocatalytic treatment, ECF oxidised samples were submitted to activated sludge, adapted to treat pulp mill wastewater. Samples were inoculated and the microorganism's growth was measured as the biomass increase (dry weight) at different periods of time. Figure 2, shows a rapid and continuous increase of biomass after 12 and 24 hour of growth. After that, a variable behaviour in growth was observed as is expected for this kind of cultures. This behaviour is explained by microorganism population changes during culture which are continuously adapting to newly formed compounds (Gray, 1990). This fact was qualitatively observed in cultures by optic microscopy. Figure 2 shows that after one minute of photocatalysis the ECF effluent improves the growth of the microorganisms, in comparison with the untreated sample (Eo). The same was demonstrated by the BODs/COD ratio. However it is not possible to detennine which is the most efficient system since it depends on the nature of sludge. Also, some biorefractory compounds may be fonnedo Similar conclusions were obtained by Stowell et al. (1992) in the ozonationlbiological degradation of2-ehlorophenol. The degradation of the ECF effluent organic matter was also evaluated as the % COD removal according to the formula (I), where CaDi is the initial value (1550 mg 02 LO I) and COOt is the value obtained after each treatment. The total COD removal includes the COD removed after chem/biological treatment (72 h). Table 2 shows the COD removal after each step: chemical, biological and chemicallbiological treatments. COD Removal (%) = [CaDi - COOt] • 100/ CaDi
(1)
M. C. YEBER et at.
340
6
---+- EO -.- 02lLNm02 -.-- 02lLNflnO
o .---,---,---,---,---,---~ o 12 24 36 48 60 72 Time (min) Figure 2. Dry weight mcreasc of activated sludge on untreated pulp bleaching effluent (.), and treated by one systems. minute by 02IUV/ZnO (X) and 02IUVrri02
(*>
Table 2. COD removal after chemical, biological and combined treatment Treatment
COD removal (%) after chemical treatment (l min)
-
Biologic 02/uv/ZnO 02/UVITiOZ
COD removal (%) COD removal (%) Total COD after chem/biol after chem/biol removal (%) treatment (24 h) treatment (72 h) 49 16 14
44 51
58 32 40
58 62 70
Figure 3 shows the initial toxicity of the ECF effluent and is compared with one minute treated sample. It is observed that the acute toxicity was reduced in 50 %, for both catalysts. These results agree with the biomass increase in the activated sludge. It is also indicative of the toxicity reduction. In a previous communication (Peralta-Zamora et aI., 1998) the same trend was found in the toxicity abatement of cellulose and textile mill effluents, in around 50 %, under photocatalytic conditions.
6
O+---l--L----r--'-----'--...,....-~-~___,
Eop
02lUVm02
02lUVIZnO
Figure 3. Microtox acute toxicity of the pulp bleaching ECF effluent treated by I min with AOP treatments.
Effluent coming from conventional pulp bleaching (CEop ) was treated for 2 hours with supported catalysts. Photocatalysis using TiOz reduced the initial TOC (980 ppm) in 55%, whereas with ZnO treatment only reached 31 % reduction. TiO z is more efficient as catalyst to induce the mineralization of the organic matter than ZnO, when immobilised on glass. On the other hand, COD removal reaches 58% for both catalysts after 120 minutes reaction; at longer times the COD and TOC remain approximately constant.
341
Advanced chemical oxidation
AOX is another key parameter, which must be reduced in any effluent treatment, since this is related with the amount of chlorinated compounds and toxicity. Recently, Archibald et al. (1997) reported that TiO z promotes AOX degradation of a bleaching effluent, as compared with the natural sunlight degradation, and this effect is more remarkable after 60 days of exposure. In our case, using immobilised catalysts to treat eucalyptus E1 effluent, the same effect is clearly observed after few minutes of reaction reaching degradation rates of 85% and 90% ofthe initial AOX, after 60 and 120 minutes, respectively. It must be emphasised that the high reduction in a short period of treatment is also a consequence of the high irradiation power of the lamp in the range 300-400 om, as compared with the sunlight. On the other hand, a 2 units pH decrease (from 10.3 to 8.2) was observed after 60 minutes of reaction, which is an indication of the mineralization of organic chlorine to chlorine ions. Photoassisted catalysis with ZnO and TiO z degrades the organic matter dissolved in paper mill effluents, significantly reduced toxicity and improved biodegradability, in assays carried out with the catalyst suspended in the effluent solution. Figure 4 shows that Microtox toxicity was reduced in 50 % after 60 minutes of reaction and remains without changes after that time, when oxidation was carried out using immobilised catalysts. It means that residual lignin derivatives are difficult to be completely degraded. This result is consistent with the COD and TOC reductions at similar extents; however, it can be demonstrated that biodegradability of residual organic matters is enhanced and that photo-catalytic systems could be a good pre-oxidation step before biological treatment. 8
I I
o EoP
Ti02-60 min.
Zn0-60 min.
Figure 4. Acute toxicity removal of eucalyptus CE.. effiuent by supported Ti02 and Zno photocatalysis.
CONCLUSIONS It can be concluded that advanced oxidation process could efficiently degrade effluents with high organic matter charge, improving their biodegradability and reducing toxicity. The photocatalysis with supported catalysts proved to be as efficient as suspended treatment, although required reaction times are different. This may be due to the lower amount of catalyst present in the immobilised system. Further work on immobilisation techniques in studies with lignin model compounds is in order. AKNOWLEDGEMENTS This research was supported by FONDECYT (Grants 1980498, 980459 and 7980035) and Direcci6n de Investigaci6n (U de C, Grants 97.23.14-1 and 97.23.16-1.2). M.C. Yeber thanks to FONDECYT for'the Doctoral Fellowship (Grant 2970096) REFERENCES Archibald, F" Roy-Arcand, L. and Methot, M. (1997), Time, sunlight, and the fate ofbiotreated kraft mill organochlonnes (AOX) in the nature. Wat, Res., 31, 85·94, Kahmarlc, K.A. and Unwin, J.P, (1998). Pulp and paper effiuent management. Water Environ, Res,. 4. 667.690.
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