Environ. Sci. Technol. 1996, 30, 1558-1564
Radiolytic Elimination of Organochlorine in Pulp Mill Effluent FARIBORZ TAGHIPOUR AND GREG J. EVANS* Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 3E5
Chlorinated organic compounds, formed during the bleaching of Kraft pulp with chlorine and chlorine dioxide bleaching agents, are commonly measured in pulp mill effluent as adsorbable organic halogen (AOX). Existing biological treatment methods can typically remove less than 50% of the AOX, which may not be adequate for compliance with future regulations. As a potential treatment method, the radiolytic elimination of organochlorine in pulp mill effluent was investigated in bench-scale experiments. Biotreated and untreated effluent as well as E-stage and C-stage filtrates from a pulp mill were irradiated in a cobalt60 γ-cell. For treated, untreated, E-stage, and Cstage effluent more than 95%, 90%, 70%, and 60% AOX removal was obtained for doses up to 60 kGy, respectively. An increase in organochlorine removal was observed at high pH and in the absence of oxygen. In treated effluent, AOX removal increased from less than 70% in the aerated neutral solution to 80% in the basic media (pH ) 12) and to 96% in the absence of oxygen for a dose of 10 kGy.
Introduction There is considerable concern about the effect of chlorinated organic matter in the environment. Kraft mill bleaching processes represent a significant source of anthropogenic chlorinated organic compounds. In the Kraft process, wood pulp is bleached using plant-specific sequences of chlorination with chlorine and chlorine dioxide and of extraction with alkaline. The filtrates from these chlorination (C) and extraction (E) stages are the dominant sources of organochlorine. Organically bound chlorine is present in bleach plant effluent in a wide range of chemical forms. The quantity and characteristics of the organochlorine depend on some parameters such as the chemicals used and the manner in which they are used during bleaching. The large variety and number of chlorinated organic compounds in pulp mill effluent make the measurement of all the individual compounds unfeasible. Consequently, measurements reflecting the total amount of chlorine in organic forms are * Fax: (416) 978-8605; telephone: (416) 978-1821; e-mail address:
[email protected].
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often used as the indicator of the quality of effluent discharged from a plant. The most common such measurement is the fraction of the chlorine that can be adsorbed onto charcoal, the adsorbable organic halogen (AOX). This technique essentially gives identical results to those obtained if the total organic halogen (TOX) is measured (1). Approximately 4-5 kg of AOX is formed per ton of pulp bleached by chlorine (2-4). Substituting chlorine dioxide for chlorine decreases AOX formation. At 100% substitution, about 1-2 kg of AOX is produced per ton of pulp (5). Chlorinated organic molecules in pulp effluent have a wide range of molecular masses. A mass of 1000 Da is often used as a dividing point to characterize the AOX. The fraction of high molecular weight (HMW) (>1000 Da) in the effluent depends on the chemicals used and the bleaching conditions. In chlorine bleaching, 75-80% of the total AOX is present in HMW forms (6). Substituting chlorine by chlorine dioxide results in a lower proportion of HMW organochlorine, about 50% HMW AOX being observed in effluent from chlorine dioxide bleaching (7). Over 200 different chlorinated organic compounds have been identified in the low molecular weight (LMW) fraction of chlorine bleaching effluent (6, 8). They can be divided into three main chemical groups, which are phenolic, acidic, and neutral compounds (2, 8). The structures of the HMW fraction of organochlorine compounds are not well defined. Some studies have indicated that the major part of these materials consists of cross-linked, probably unsaturated aliphatic compounds with a low content of aromatic nuclei (7). HMW organochlorine has a relatively low chlorine content. Atomic C/Cl ratios of 9-14 for chlorine bleaching (2-4), 45-120 for chlorine dioxide bleaching (3, 4), and 95-260 for chlorine dioxide bleaching of oxygen delignified pulp have been reported (2). Organically bound chlorine on the order of 0.1-0.2 wt % has been found in naturally chlorinated humic material (2). Discharge of chlorinated organic compounds from pulp mills is the subject of environmental and regulatory debate. Since carbon-chlorine bonds are not very common in nature, compounds containing such bonds may be difficult to decompose and, therefore, may have harmful environmental effects (9). Even though there is no hard evidence of a direct relationship between AOX in bleached effluent and long-range environmental effects, increasing concern about the possibility of these effects has resulted in more stringent environmental regulations. Limits based on AOX discharged in pulp effluent have been established in Sweden, Germany, and some provinces of Canada (9). In order to meet these regulations, pulp mills have used combinations of internal process optimization and external effluent treatment facilities. For example, chlorine dioxide, which results in lower AOX formation, has been partially substituted for chlorine in many mills, and most pulp bleaching processes are now moving toward 100% chlorine dioxide substitution (10, 11). The use of total chlorine-free (TCF) bleaching is also being explored. However, the effluents from TCF bleaching have not yet been subjected to extensive ecotoxicological characterization, so their discharge could result in undiscovered effects on the receiving waters, unless they are completely recovered inside the mill (12).
0013-936X/96/0930-1558$12.00/0
1996 American Chemical Society
Biological treatment of bleached Kraft mill effluent is common in many areas. The existing treatment methods generally have less than 50% AOX removal efficiency and may not be adequate for compliance with future regulations. As well, there is no consensus as to how the AOX is removed during the biological treatment. Some studies have suggested that the major mechanism for AOX removal during biological treatment is actually biosorption (13, 14), whereas in others, significant biodegradation has been observed (15). It has recently been suggested that the reduction of AOX during biological treatment is mainly due to nonbiological reactions (16) or chemical and abiotic degradation (17). Novel methods of dealing with bleach plant filtrates such as ultrafiltration and reverse osmosis only separate material in the effluent. Therefore, a major drawback of these methods is that a substantial part of the organochlorine may transfer from the effluent into sludge, causing another disposal problem. Irradiation is a very effective process for the treatment of water, wastewater, and sludge. Free radicals, ions, and other reactive species that are formed due to the interaction of high-energy photons with molecules are the active agents in this process. Studies of the irradiation treatment of water and wastes have indicated that radiation energy can be an important resource in the treatment of water, wastewater, and sludge, both directly and in combination with other treatment methods (18-21). The radiation treatment of wastewater has been shown to remove both organic and inorganic contaminants and to inactivate microorganisms (22). In terms of chlorinated organic compounds, it has been shown that extensive dechlorination of chemicals such as chloroform (19, 23), chlorobenzene (24), chlorophenols (25, 26), chloroethanes (24), and chloroethylenes (19) can be achieved if the radiation dose is high enough. Irradiation of 4-chlorophenol in the presence of scavengers showed that the hydrated electron is the main species responsible for dechlorination of this compound (26). Results from two studies on the radiolytic elimination of AOX were released while the study reported here was in progress. Waite et al. treated a sample of effluent using γ-irradiation and found that a 76% AOX removal could be achieved at doses of 8 kGy (27). In their study of removal of AOX in pulp mill effluent samples using a high-energy electron beam, Berge et al. found that 40 and 70% AOX removal was obtained for doses of 10 and 50 kGy, respectively (28). Surprisingly, chemical pretreatment with acid, base, oxygen, or nitrogen bubbling and hydrogen peroxide addition was found to have no significant effect on overall AOX removal. Some effect would be expected given that these treatments should alter the fraction of the radicals produced by water radiolysis that react with the AOX. In the radiolysis of relatively dilute organochlorine solutions, since water is the major component, almost all the absorbed energy is deposited in water molecules. Therefore, biological and chemical changes in the solution are brought about indirectly by the molecular and radical products of water radiolysis. Incident photons interact with water molecules and transfer part of their energy to the bound electrons through compton scattering. The resulting high-energy electrons lose their energy by ionizing further water molecules and become hydrated once they reach thermal energies. The water molecules undergoing ionization or electron excitation dissociate to produce free radicals such as H and OH, which interact to produce molecular
products such as H2O2 and H2. Hence, the radiolysis of deoxygenated water leads to the formation of the following products:
H2O f eaq-, H, OH, H2O2, H2, OH-, H+
(1)
The radiation chemical yields of these products depend on some parameters such as the pH of the solution and its dissolved oxygen content. In neutral pH deoxygenated water, OH and eaq- are the dominant radicals. In the radiolysis of organochlorine solutions, the molecules are attacked by reactive species, in particular the hydrated electron (eaq-), hydrogen atom (H), and hydroxyl radical (OH), which bring about the release of the chlorine as a chloride ion (29). The radiation chemical yield of each individual species can be represented as a G value with the unit of µmol J-1. In this study, the use of γ-radiation for the destruction of organochlorine in pulp mill effluent was investigated. The primary objective was to measure the dependence of AOX removal on absorbed dose, information needed in order to evaluate the effectiveness of radiolysis as a potential treatment method. The impact of parameters such as pH and dissolved oxygen content was also examined along with the relative contributions of the various free radicals that contribute to the dechlorination process. In a parallel study, reported elsewhere (30), the radiolysis of individual chlorinated organic compounds was investigated.
Experimental Section The pulp mill effluent samples were taken from bleached Kraft mills and were kept refrigerated before use. Samples from the chlorination stage (C-stage), extraction stage (Estage), and untreated effluent (for soft wood) were taken from CDN Pacific Forest Products, Dryden, Ontario. This mill has a bleaching sequence of (DC)EOPDED with 40% ClO2 substitution in the C-stage. Treated effluent was taken from E. B. Eddy Forest products, Espanola, Ontario. This mill has a bleaching sequence of O(DC)EHD with 30% ClO2 substitution for its hardwood line and a sequence of O(DC)EOHD with 60% ClO2 substitution for its softwood line. The mill effluent is treated for 6 days in an aerated basin. The pulp mill effluent was used as received without any additional purification or dilution. Samples, 15 mL in volume, were placed in 25-mL glass scintillation vials and were irradiated in a cobalt-60 γ-cell 220 (Atomic Energy of Canada Limited) with a dose rate of about 10 kGy/h (31). To study the effect of pH on organochlorine removal, the pH of the solution was changed by adding nitric acid or sodium hydroxide. The effect of dissolved oxygen content on organochlorine removal was examined by removing the O2 through bubbling with nitrogen for about 10 min. The relative contributions of the free radicals involved in organochlorine removal was evaluated by adding scavengers to remove selected reactive species. Nitrous oxide and tert-butanol (2-methyl 2-propanol) were used for scavenging the hydrated electron and hydroxyl radical, respectively, while 2-propanol was used to scavenge both the hydroxyl radical and atomic hydrogen. Nitrous oxide was bubbled in samples for about 20 min (the saturated solution concentration is about 25 mol m-3 at standard conditions). The 10 mL of gas space above the solutions in the vials used for irradiation was also filled with nitrous oxide. The concentration of tert-butanol and 2-propanol was 100 mol m-3.
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concentration in effluent is in the range of 80-150 ppm. Of the chlorine used in the bleaching process, only about 10% or less becomes bound to the organic material removed from the pulp and about 90% ends up as chloride (33). All of these chemicals can react with free radicals, decreasing the organochlorine degradation rate. Another contributing factor may be the molecular structure of organochlorine in pulp mill effluent. A large portion of organochlorine in pulp effluent is high molecular weight material containing numerous nonchlorinated sites for attack by free radicals.
FIGURE 1. Dechlorination of AOX in untreated (9) and treated (f) effluent due to irradiation at a dose rate of 10 kGy/h. Error bars represent 95% confidence intervals based on five replicate samples.
For determination of AOX, the organochlorine was separated from the inorganic forms, such as chloride, by adsorption on activated carbon. Air pressure was used to pass samples, typically 15 mL, through 0.3 g of activated carbon in a 5-cm polyethylene tube in about 5 min. Each sample was acidified, using sulfuric acid, to pH 2 before adsorption because organic acids adsorb more readily onto activated carbon at low pH. Following adsorption, the activated carbon was washed for approximately 5 min with 15 mL of 0.08 M potassium nitrate solution to wash off any Cl- retained on the surface of the charcoal. Neutron activation analysis was used to determine the amount of organochlorine adsorbed by the activated carbon. The samples were irradiated in a SLOWPOKE reactor with a flux of 5 × 1011 neutron/cm2s for 300 s and counted for 300 s. The cooling time, the time between irradiation and counting, was between 1 and 20 min for different samples. An EG&G Ortec Model GEM-20190 intrinsic germanium γ-detector with a Canberra Model 8180 MCA multi-channel analyzer was used to count sample activities. The amount of background chlorine present in the activated carbon, polyethylene tube, and plastic vials was measured in order to make necessary measurement corrections. Sodium chloride solutions with concentrations of 100, 200, and 5000 ppm chloride were used as standards.
Results and Discussion Radiation Treatment of Pulp Mill Effluent. Samples of the biologically treated and untreated effluent were irradiated to various doses (Figure 1). Typically only two samples were analyzed for each radiation dose. However, in a few cases up to five replicate solutions were used, allowing the precision to be evaluated. The relative standard deviation was typically within 5%. For this treated and untreated effluent, over 90% and 95% AOX was removed using a dose of 60 kGy; the initial G values for chlorine removal, Gi(Cl-), were 0.035 and 0.017 µmol J-1, respectively. Removal rates were considerably slower than those observed (30) or reported for individual chlorinated organic compounds such as 1,1,1-trichloroethane (24), 4-chlorophenol (26), tetrachloroethene (24), and chloroform (32). The reason for the slower removal may be the existence of many other chemicals such as nonchlorinated organic compounds and chloride in the effluent samples. Acetic acid, formic acid, and methanol may be present at higher concentrations than AOX in effluent. For example, typical methanol
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The degradation rate decreased as the irradiation proceeded. The same behavior was observed in the degradation of chlorinated organic compounds such as dichloroethane, chlorobenzene, and chlorophenol in aqueous solutions (24, 25, 30). However, the elimination rate of AOX in pulp mill effluent decreased much more markedly over the duration of irradiation. This is not surprising, given that organochlorine in pulp mill effluent consists of a very large number of individual compounds, each with its own removal rate. Some, such as those with aliphatic structures, can undergo rapid radiolytic degradation, while others, such as aromatics, may degrade slowly. After a period of time, most of the easily degradable organochlorine will be eliminated, leaving the more slowly degradable compounds. For treated effluent, the initial rate of removal was slower than that for the untreated effluent; however, a high degree of removal was still achieved, indicating that even those types of organochlorine compounds that are persistent against biological treatment can be removed through radiolysis. The character of the organochlorine in treated effluent is different from that in untreated effluent. In most mills, the effluent undergoes treatment prior to discharge in order to reduce the biological oxygen demand and to remove suspended solids through settling. AOX will decrease during this treatment due to mechanisms such as biosorption, biodegradation, and abiotic removal. Some studies have suggested that most of the biodegradable AOX has low molecular weight (16). Another study showed that chlorophenolics, chloroform, and chloroacetone compounds in pulp mill effluent can be very effectively removed through biological treatment methods (15). It may be inferred that the remaining organochlorine in treated effluent is predominantly in forms that are less easily degraded by these mechanisms. However, the majority of the AOX in both treated and untreated effluent is in HMW forms. Hence, it is not surprising that the elimination rate for treated effluent is only slightly slower than that for untreated effluent. It should be noted that a dose of 10 kGy removed approximately 60% and 70% of the AOX initially present in the untreated and treated effluent, respectively. Hence, the fractional rate of removal of AOX from both types of effluent is similar. Since the first chlorination and extraction stages of the bleaching process are the main sources of the organochlorine in mill effluents, C- and E-stage filtrates were studied. The AOX concentration in filtrates of these two stages is much higher than that in the final effluent. The flow rates of C- and E-stage filtrates, however, are correspondingly lower. For C- and E-stage filtrates, about 60% and 70% AOX removal was obtained respectively using a dose of 60 kGy; the initial G values for chlorine removal, Gi(Cl-), were 0.11 and 0.09 µmol J-1, respectively (Figure 2). Higher rates of AOX degradation were observed for C- and E-stage filtrates in comparison with that for final effluent, although
TABLE 1
AOX Dechlorination of Untreated Effluent in Presence of Various Scavengers radiation doses (kGy)
FIGURE 2. Dechlorination of AOX in C-stage (0) and E-stage (*) filtrates due to irradiation at a dose rate of 10 kGy/h. Error bars represent 95% confidence intervals based on five replicate samples.
0 0 0 0 0 10 10 10 10 10 10 10 10 10 10 10 10
scavenger
nitrous oxide nitrous oxide nitrous oxide nitrous oxide nitrous oxide 2-propanol 2-propanol 2-propanol tert-butanol tert-butanol tert-butanol All the above scavengers
AOX concn (ppm) 18.0 17.1 17.1 17.4 18.7 8.0 7.9 11.2 11.3 11.6 9.6 10.0 9.8 8.2 8.8 8.5 12.5
av AOX concn (ppm)
AOX degraded (ppm)
17.6
% AOX removala
0
7.9
9.7
100
11.4
6.2
64
9.8
7.8
80
8.5
9.1
94
12.5
5.1
53
a AOX degradation from the sample without any scavenger is taken to be 100% removal, and AOX degradations of other samples are compared with that.
represented by (35) H2O
eaq- + N2O f N2 + O- 98 N2 + OH + OHk ) 9.1 × 106 m3 mol-1 s-1 (2) FIGURE 3. Dechlorination of AOX in C-stage (0), E-stage (*), untreated (9), and treated (f) effluent versus radiation dose normalized by initial concentration.
the percentage removal of AOX at a given dose was lower. The same behavior was observed in the elimination of individual chlorinated organic compounds (24, 25, 30, 34). In order to allow a proper comparison of the C- and E-stage filtrates with the effluent samples, the percentage of AOX removal was plotted against the absorbed radiation energy divided by the initial concentration (Figure 3). Dividing the radiation dose by the initial concentration is equivalent to normalizing the radiation dose based on the relative flow rates of streams, giving a comparison of total radiation energy required rather than energy per unit volume. It is assumed here that the lower AOX concentration in untreated effluent is only due to the dilution of Cand E-stage filtrates. The results show that the same percentage of AOX removal can be achieved using much less radiation energy by treating C- and E-stage filtrates rather than the entire plant effluent. Evaluation of the Reactive Species. The radiolytic decomposition of chlorinated organic compounds is accomplished through reactions with free radicals such as the hydrated electron (eaq-), hydrogen atom (H), and hydroxide radical (OH). The relative contributions of these free radicals was studied using scavengers to remove selected radicals and measuring the change in the degradation rates. Specifically, nitrous oxide was used to scavenge hydrated electrons, tert-butanol for hydroxyl radicals, and 2-propanol for scavenging both hydrogen atoms and hydroxyl radicals (Table 1). In the presence of nitrous oxide-hydrated electrons are converted to hydroxyl radicals by the sequence of reactions
The rate constant and solubility for nitrous oxide in water (a saturated solution contains about 25 mol m-3 at standard conditions) are sufficiently high so that this reaction should scavenge over 95% of the eaq-, provided the other reactants are present at a concentration of less than 1 mol m-3 (i.e., if k[reactant] is less than about 107 s-1) (35). tert-Butanol (2-methyl-2-propanol) is frequently used to remove OH when investigating the reactions of H and eaq- by pulse radiolysis (36). The scavenging reactions are (36).
OH + (CH3)3COH f H2O + •CH2C(CH3)2OH k ) 6 × 105 m3 mol-1 s-1 (3) H + (CH3)3COH f H2 + •CH2C(CH3)2OH k ) 170 m3 mol-1 s-1 (4) From the rate constants, it can be seen that hydrogen atoms are not scavenged very efficiently by tert-butanol. 2-Propanol scavenges both H and OH, but it does not have any effect on eaq- (36):
H + (CH3)2CHOH f H2 + (CH3)2•COH k ) 7.4 × 104 m3 mol-1 s-1 (5) OH + (CH3)2CHOH f H2O + (CH3)2•COH k ) 1.9 × 106 m3 mol-1 s-1 (6) Samples were saturated with nitrous oxide by bubbling. Measurements confirmed that the bubbling process did not remove organochlorine through volatilization. The extent of AOX removed by irradiation in the presence of nitrous oxide was only 64% of that obtained in the absence
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of any scavenger. In comparison, scavenging of hydrogen atoms and hydroxyl radicals only reduced AOX removal to 80%, while scavenging of hydroxyl radicals alone reduced AOX removal to 94% of that obtained in the absence of any scavenger. The AOX degradation in the presence of 2-propanol, which scavenges both H and OH, was lower than that with tert-butanol, which scavenges only OH. The difference between these two values indicates that H is responsible for a small portion of the degradation of AOX: it can be calculated that if all the hydrogen atoms are scavenged from the solution, the AOX removal will decrease to 86% of that obtained in the absence of scavenger. Based on these results, it appears that the hydrated electron plays a dominant role in dechlorination of AOX while the hydroxyl radical only makes a minor contribution. This conclusion was supported by the observation made when pulp mill effluent was irradiated at different pH and in the absence of oxygen (see below). Curiously, after adding all the scavengers, a substantial degree of AOX removal was still observed. Consequently, experiments were undertaken to verify that this scavenging method was effective. Chlorobenzene was irradiated in the presence of each scavenger and also in the presence of all the scavengers together. Degradation rates decreased in the presence of each scavenger, and no organochlorine degradation was observed in the presence of all the scavengers. The amounts of scavenger added were determined based on calculations that indicated that the rate of reaction of the target radical with the scavenger would be between 2 and 3 orders of magnitude faster than that estimated for the reaction of the radical with AOX. These scavenger concentrations were in fact similar to those used in other studies (26). As an example, a calculation for the reaction of the hydrated electron with scavengers and AOX is given here. The concentration of AOX is 13 ppm in the untreated effluent. If a molecular weight of 35 (chlorine molecular weight) is assumed for AOX, the molecular concentration will be 0.26 mol m-3. The reaction of eaq- with some alkyl halides can be quite rapid, although its reaction with chlorinated phenolic compounds is typically much slower. For example, the rate constant for the reaction with chloroform is 3 × 107 m3 mol-1 s-1 while that with chlorophenol is 4.4 × 105 m3 mol-1 s-1. The rate constant for the reaction with high molecular weight organochlorine compounds, which form AOX (AOX has a molecular weight between 100 and 4000 (33)), would likely be at the lower end of this range. It is assumed for these calculations that the rate constant for the reaction of eaq- with AOX is 106 m3 mol-1 s-1. Based on this assumption, the pseudo-firstorder rate constant (k[solute]) for the reaction of hydrated electron with AOX and N2O are
k(for e + AOX)[AOX] ) 106 × 0.26 ) 2.6 × 105 s-1 k(for e + N2O)[N2O] ) 9.1 × 106 × 25 ) 2.3 × 108 s-1 indicating that the reaction of eaq- with N2O should be almost 3 orders of magnitude faster than its reaction with AOX. However, other nonchlorinated chemicals that react with eaq- may also be present in the effluent. Based on the observed G value of 0.035 µmol J-1 for dechlorination of AOX in the effluent, as compared to that for production of eaq- (0.28 µmol J-1), it appears that up to 12% of the hydrated electrons were consumed through dechlorination reactions. This suggests that the rate of consumption of eaq- through
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FIGURE 4. Effect of pH on AOX removal for treated and untreated effluent irradiated to a dose of 10 kGy.
reactions that did not result in dechlorination was between 1 and 2 orders of magnitude faster than the dechlorination reactions. Hence, the consumption of eaq- by N2O should have been at least an order of magnitude faster than its consumption through reactions with effluent components, indicating that the amount of N2O added should have been adequate. A similar calculation indicated that the amount of tert-butanol should have been adequate to scavenge the hydroxyl radical, even though, for example, approximately 2 mol m-3 methanol was present in the effluent. The 2-propanol should have been able to compete with the AOX for the hydrogen atoms, however, its effectiveness in competing with other chemicals that may have been present in the effluent was not as clear. It is not known why significant removal was obtained when all the scavengers were added to the pulp mill effluent. However, given the range of organic and inorganic chlorine species present in effluent and of nonhalogenated organics, resolution of this question may be very difficult. The possibilities of the complete consumption of the scavengers during irradiation and direct reaction of γ with organochlorine were evaluated and rejected. The possibility of a loss of nitrous oxide from the vial was considered unlikely because of the reproducible results. The presence of many other chemicals in the pulp mill effluent, however, may have been responsible for these results. The concentration of scavengers was adequate to compete with the AOX. However, in the cases of 2-propanol and tert-butanol, the possibility that other materials in the effluent may have reacted at comparable rates to the scavengers cannot be ruled out. Also, it is conceivable that, in complex solutions such as pulp mill effluent, scavengers may react to some extent with species other than the target radicals and, consequently, may not completely remove these radicals. Hence, based on these experiments, it is possible to conclude that at least 40% of the AOX removal is due to eaq-. The radicals responsible for the remaining 60% of the removal cannot yet be clearly demonstrated. Influence of the Effective Parameters. The contribution of each free radical to AOX removal is generally different. This suggests that parameters which can change the yield of reactive species may improve AOX removal. The two main parameters are pH and dissolved oxygen content. The influence of pH on organochlorine removal was studied by irradiation of treated and untreated effluent at pH 2, 7, and 12 to a dose of 10 kGy (Figure 4). It is seen that degradation of organochlorine from both treated and untreated effluent increases at high pH and decreases at
electron is most noticeable in the treated effluent where the organochlorine concentration is lower. Therefore, the removal of oxygen has a greater effect on treated effluent.
Acknowledgments The authors are grateful for the financial support provided by the Ministry of Culture and Higher Education of Iran, Ontario Hydro International, and the research consortium on “Characterization, Treatment and Fate of Bleach Plant Effluents”. The assistance of Luigi Di Pede and Yvonne Ying is also acknowledged.
Literature Cited FIGURE 5. Effect of dissolved oxygen content on AOX removal for treated and untreated effluent irradiated to a dose of 10 kGy.
low pH. For treated effluent, AOX degradation was less than 70% at pH 7, 80% at pH 12, and 50% at pH 2. At pH above 3, hydrogen atoms are converted into hydrated electrons
H + OHaq- f eaq- + H2O k ) 2.5 × 104 m3 mol-1 s-1 (7) in more acidic media; however, the hydrated electron is converted to atomic hydrogen
eaq- + Haq+ f H
k ) 2.3 × 107 m3 mol-1 s-1 (8)
The lower AOX removal in the pH 2 solution indicates that hydrogen atoms are less effective than the hydrated electron. The reason for the higher removal at pH 12 is less clear. At high pH, OH is converted to O-
OH + OH- f O- + H2O k ) 1.3 × 107 m3 mol-1 s-1 (9) and H2O2 is deprotonated
H2O2 T HO2- + H+
Ka ) 1.8 × 10-12
(10)
It is possible that either HO2- or O- contributes to AOX removal. It is also possible that hydrolysis contributed to the difference in the AOX removal between pH 7 and pH 12. The effect of oxygen content is given in Figure 5. For treated effluent, removing the air above the liquid phase and removing dissolved oxygen from the solution by nitrogen bubbling increased AOX degradation. For treated effluent, AOX degradation increased from less than 70% to more than 95% when dissolved oxygen was removed from the solutions. When O2 is present, both hydrated electron and hydrogen atom are scavenged by oxygen to form the superoxide ion and peroxyl radical:
eaq- + O2 f O2H + O2 f HO2
k ) 2.1 × 107 m3 mol-1 s-1 (11) k ) 2 × 107 m3 mol-1 s-1
(12)
The removal of oxygen from the solution by nitrogen bubbling reduces competition, allowing a larger fraction of the solvated electrons to react with organochlorine. Since the oxygen content is similar in both treated and untreated effluent, competition for the reaction with the hydrated
(1) Odendahl, S. M.; Weishar, K. M.; Reeve, D. W. Pulp Pap. Can. 1990, 91 (4), T136-T140. (2) Kringstad, K. P.; Lindstro¨m, L. Environ. Sci. Technol. 1984, 18 (8), 236A-247A. (3) Lindsto¨rm, K.; O ¨ sterberg, F. Holzforschung 1984, 38, 201-212. (4) O ¨ sterberg, F.; Lindstro¨m, K. Holzforschung 1985, 39, 149-185. (5) Chirat, C.; Lachenal, D. Ozone Bleaching Is The Key to Alternative Bleaching Technology. Proceedings of 1993 International Environment Symposium, Paris, 1993; EU CE PA: Paris, 1993. (6) Reeve, D. W.; Earl, P. F. Pulp Pap. Can. 1989, 90 (4), T128-T131. (7) Dahlman, O.; Haglind I.; Mo¨rck, R.; deSousa, F.; Stro¨mberg, L. Chemical Composition of Effluents From Chlorine DioxideBleaching of Kraft Pulps Before and After Secondary Effluent Treatment. Proceedings 1993 International Environment Symposium, Paris, 1993; EU CE PA: Paris, 1993. (8) Suntio, L. R.; Shiu, W. Y.; Mackay, D. Chemosphere 1988, 17 (7), 1249-1282. (9) Reeve, D. W.; Earl P. F. Pulp Pap. Can. 1989, 90 (4), T128-T131. (10) Chirat, C.; Lachenal, D. Ozone Bleaching is The Key to Alternative Bleaching Technology. Proceedings of 1993 International Environment Symposium, Paris, 1993; EU CE PA: Paris, 1993. (11) Mendiratta, S. K. Effluent Free Oxygen Containing Chlorine Dioxide Process and Ultra Low AOX Bleaching Process. Proceedings of TAPPI 1993 Environmental Conference, Boston, 1993; TAPPI: Atlanta, 1993. (12) Folke, J.; Ma¨nnisto¨ E. Chlorine Dioxide in Pulp Bleaching Environmental Influence on Future Use. Proceedings of 1993 International Environmental Symposium, Paris, 1993; EU CE PA: Paris, 1993. (13) Bryant, C. W.; Amy G. L. Organic Halide in Kraft Mill Wastewater: Factors Affecting in-mill Formation and Removal by Biological Treatment. Proceedings of TAPPI 1988 Environmental Conference, Charleston, 1988; TAPPI: Atlanta, 1988. (14) Leuenbreger, C.; Giger W.; Coney, R.; Graydon, J. W.; MolnarKubica, E. Water Res. 1985, 19, 885-894. (15) Gergov, M.; Priha, M.; Talka, E.; Valttila, O.; Kangas, A.; Kukkonen, K. Chlorinated Organic Compounds in Effluent Treatment at Kraft Mills. Presented at The 1988 TAPPI Environmental Conference, Charleston, 1988. (16) Nystro¨m, M.; Svard, S. H.; Skogby, K. Reducing AOX By Nonbiological External Treatment Methods. Proceedings of TAPPI 1992 Environmental Conference, Richmond, 1992; TAPPI: Atlanta, 1992. (17) Ferguson, J. F. Anaerobic and Aerobic treatment for AOX Removal. Proceedings of TAPPI 1993 Environmental Conference, Boston, 1993; TAPPI: Atlanta, 1993. (18) Waite, T. D.; Kroger, P.; Bryan, E.; Swinwood, J. F. Appl. Isot. Radiat. Conserv. Environ. Proc. Int. Symp. 1992, 143-152. (19) Waite, T. D.; Kurucz, C. N.; Cooper, W. J.; Nickelson, M. Appl. Isot. Radiat. Conserv. Environ. Proc. Int. Symp. 1992, 171-181. (20) Bryan, E. H.; Dick, R. I.; Swinwood, J. F.; Kruger, P.; Carlson, D. A.; Hare, G.; Waite, T. Radiation Energy Treatment of Water, Wastewater and Sludge; American Society of Civil Engineers: New York, 1992; p 2. (21) Kurucz, C. N.; Waite, T. D.; Cooper, W. J.; Nickelsen, M. J. High Energy Electron Beam Irradiation of Water, Wastewater And Sludge; Advances in Nuclear Science and Technology, Vol. 22; Lewins, J., Becker, M.,Eds.; Plenum Press: New York, 1991. (22) Singh, A.; Sagert, N. H.; Borsa, J.; Singh, H.; Bennett, G. S. The Use of High-Energy Radiation for The Treatment of Wastewater: A Review. Proceedings of The 8th Symposium on Wastewater Treatment, Montreal, 1985; Environment Canada: Ottawa, 1985. (23) Cooper, W. J.; Cadavid, E.; Nickelson, M.; Lin, J.; Kurucz, C. N.; Waite, T. D. J. Am. Water Works Assoc. 1993, 85, 106-120. (24) Getoff, N. Radiat. Phys. Chem. 1990, 35, 432-439. (25) Getoff, N.; Solar, S. Radiat. Phys. Chem. 1986, 28, 443-450.
VOL. 30, NO. 5, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1563
(26) Getoff, N.; Solar, S. Radiat. Phys. Chem. 1988, 31, 121-130. (27) Wang, T.; Waite, T. D.; Kurucz, C. N.; Cooper, W. J. Water Res. 1994, 28, 237-241. (28) Berge, D.; Rataweera, H.; Efraimsen, H. Degradation of Recalcitrant Chlorinated Organics by Radiochemical and Biochemical Oxidation. Proceedings of 4th IAWQ symposium on Forest Industry Wastewaters, Tampere, Finland, 1993; Pergamon: New York, 1993. (29) Spinks, J. W. T; Woods, R. J. Introduction to Radiation Chemistry, 3rd ed.; John Wiley & Sons: Toronto, 1990; p 225. (30) Taghipour, F.; Evans, G. J. Radiolytic Degradation of Chlorinated Organics. Submitted to Radiat. Phys. Chem. (31) Gammacell 220 Operators Manual; Technical Publications, Atomic Energy of Canada Limited: Katana, 1984; P 1. (32) Rezansoff, B. J.; McCallum, K. J.; Woods, R. J. Can. J. Chem. 1970, 48, 271-276.
1564
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(33) Berry, R. M.; Luthe, C. E.; Voss, R. H.; Wrist, P. E.; Axegard, P.; Gellerstedt, G.; Lindbland, P. O.; Popke, I. Pulp Pap. Can. 1991, 92, 43-53. (34) Gehringer, P.; Proksch, E.; Eschweiler, H.; Szinovatz, W. Radiat. Phys. Chem. 1990, 35, 456-460. (35) Spinks, J. W. T.; Woods, R. J. Introduction to Radiation Chemistry, 3rd ed.; John Wiley & Sons: Toronto, 1990; p 285. (36) Spinks, J. W. T.; Woods, R. J. Introduction to Radiation Chemistry, 3rd ed.; John Wiley & Sons: Toronto, 1990; p 325.
Received for review July 17, 1995. Revised manuscript received December 7, 1995. Accepted December 20, 1995.X ES950526X X
Abstract published in Advance ACS Abstracts, March 1, 1996.
