Environ. Sci. Technol. 2003, 37, 811-814
Carbohydrate-Derived Chlorinated Compounds in ECF Bleaching of Hardwood Pulps: Formation, Degradation, and Contribution To AOX in a Bleached Kraft Pulp Mill CARMEN S. R. FREIRE, ARMANDO J. D. SILVESTRE, AND CARLOS PASCOAL NETO* Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
Five monochlorinated compounds derived from glucuronoxylan were identified in the liquid process streams of a kraft pulp mill producing hardwood pulp with ECF bleaching, representing ca. 15-20% of the AOX of the bleaching filtrates. The environmental risk of such compounds is negligible because a major fraction (70-80%) is degraded during effluent mixing and neutralization, and about 20-30% is degraded during the biological treatment of the mixed effluent. Only less than 3.5% (0.009-0.017 kg/ tAD) of the compounds formed in the bleaching leave the mill in the final effluent.
Introduction Historically, substantial environmental problems have been associated with bleached pulp production, particularly when bleaching was performed by sequences using elemental chlorine (Cl2) where highly chlorinated organic compounds were formed in considerable amounts (1,2). Polychlorinated compounds, such as the well-known dibenzodioxins, dibenzofurans, and phenols, have great potential for bioaccumulation and are persistent toxics responsible for severe environmental problems (1-4). Following the recognition of large-scale environmental contamination by organochlorines, during the last twenty years the pulp industry has implemented extensive process changes. As far as the bleach plant is concerned, the major changes have been the substitution of elemental chlorine by chlorine dioxide (ClO2) (1,5), resulting in elemental chlorine free (ECF) bleaching processes. In such processes, the unbleached pulp is sequentially treated with acidic aqueous ClO2 solutions (D stages) and aqueous NaOH (E stages); the DEDED sequence is one of the most used worldwide (6). The introduction of ECF bleaching has drastically reduced both the amount of chlorinated compounds discharges and their degree of chlorination, resulting in effluents with lower adsorbable organic halogens (AOX) and toxicity. The total amount of chlorinated compounds in bleaching effluents resulting from ECF bleaching processes is only 10-20% of that found in chlorine based bleaching effluents (7). To predict and understand the environmental impact associated with the bleaching technologies, and thereby to contribute to the search for new solutions to decrease effluent toxicity, a significant number of studies concerning the * Corresponding author phone: +351 234 370693; fax: +351 234 370084; e-mail:
[email protected]. 10.1021/es0200847 CCC: $25.00 Published on Web 01/17/2003
2003 American Chemical Society
chemical composition of bleaching effluents have been published (8,9). More than 500 low-molecular-weight organic compounds have now been identified in bleaching effluents. Most of the compounds identified in bleaching effluents are degradation products of lignin (1,8) or are derived from other wood components, such as extractives or carbohydrates. Lignin degradation products are also commonly considered as the major precursors of chlorinated compounds (10). However, three monochlorinated compounds derived from glucuronoxylan were recently identified as the major components of the ethyl acetate extract of chlorine dioxide bleaching filtrates of Eucalyptus globulus kraft pulps (11). These compounds were further identified in similar amounts in D filtrates from DEDED bleaching of both Eucalyptus globulus and birch (hardwoods) kraft pulps (12). Even these chlorinated compounds represent an important fraction of the total AOX of D filtrates, they are easily degradable considering they were not present, at least in detectable amounts, in the final mill effluents (10). The purpose of the present work was to assess the behavior of these carbohydrate-derived chlorinated compounds produced in a hardwood (Eucalyptus globulus) bleaching plant, from their formation till the final effluent discharges, and therefore to evaluate their degradability and contribution to the effluents AOX.
Experimental Section The bleaching effluents were obtained in a Eucalyptus globulus bleached kraft pulp mill using a D0E1D1E2D2 bleaching sequence (E1 stage reinforced with oxygen and hydrogen peroxide) and counter-current washing. The mill is equipped with wastewater primary and secondary (biological) treatment systems. Samples were taken at different points of the bleached pulp mill and wastewater treatment plant as shown in Figure 1. Specific sample locations and types are as follows. Sample A: Acidic D0 bleaching effluent (pH ∼3), collected after the first chlorine dioxide stage, at the bleaching plant. Sample B: “Neutral” effluent (pH ∼7), resulting from the mixing of D0 effluent with the alkaline effluent (pH ∼11) from the pulp mill (AEPM), collected at the entrance of the primary treatment plant (PTP). Sample C: “Neutral” effluent (pH ∼7), collected after the primary treatment plant. Sample D: Mixed effluent (pH ∼7), resulting from the mixture of “neutral” effluent with the effluent from E1 bleaching stage (pH ∼7), collected before the secondary treatment. Samples E and F: Final effluent (pH ∼7); collected after the secondary treatment plant (STP) with 24 h delay between them. All the samples investigated are mixtures of several samples collected periodically during 1 day of mill production, representing the average composition of the mill liquid streams. After collection, samples were purged with nitrogen and kept at 5 °C until solvent extraction. A 50-mL portion of each filtrate was acidified at pH ∼2 with 5% HCl and then extracted with ethyl acetate (3 × 50 mL); the solvent was evaporated to dryness and the extracts were weighed. The extraction yields were in the range of 0.01-0.05% (m/V), decreasing along the treatment plant. Before GC-MS analysis, each dried sample was trimethylsilylated as previously described (13). To study the influence of effluent neutralization on the amount of chlorinated compounds, three samples of an industrial D0 filtrate were neutralized with 10% NaOH and heated at 40, 50, and 70 °C for 1 h. After that, the neutralized filtrate samples were left overnight at room temperature. Then, half of each sample was filtered in a Millipore 0.22-µm VOL. 37, NO. 4, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Concentrations (mg/L) of Chlorinated Compounds 1-5 (see Figure 2; 4 and 5, isomers of 3) in Samples Studied chlorinated compounds sample
1
2
3
4
5
A B C D E F
22.5 2.39 3.42 2.82
45.3 0.87 1.30 0.60 0.20 0.34
7.08
4.29
1.89
0.07
0.63
FIGURE 1. Schematic diagram showing the liquid streams of the bleaching plant and the wastewater treatment plant, with the sampling points (A-F). Abbreviations used: D, chlorine dioxide bleaching stages; E, alkaline extraction stages; AEPM, alkaline effluent from pulping mill; PTP, primary treatment plant; STP, secondary treatment plant.
FIGURE 2. Chemical structures of the chlorinated compounds identified in chlorine dioxide bleaching effluents. filter. Filtered and unfiltered neutralized filtrates were acidified at pH ∼2 and then extracted with ethyl acetate as described above. The derivatized extract was also analyzed by GC-MS. GC-MS analyses were performed using a Trace Gas Chromatograph 2000 Series equipped with a Finnigan Trace MS mass spectrometer, using helium as carrier gas (35 cm/ s), equipped with a DB-1 J&W capillary column (30 m × 0.32 mm i.d., 0.25 µm film thickness). The chromatographic conditions were as follows: initial temperature 80 °C for 5 min; temperature increase rate 4 °C/min; final temperature 285 °C for 10 min; injector temperature 290 °C; transfer-line temperature 290 °C; and split ratio 1:100. For quantitative analysis, GC-MS was calibrated with pure compound 1 (isolated and purified in our previous studies (11)) relative to 1-eicosanol, the internal standard used. The respective multiplication factor needed to obtain correct quantification of the peak areas was calculated as an average of six GC-MS runs. The AOX measurements were done according to the ISO 9562 method.
Results and Discussion Formation and Degradation. The ethyl acetate extracts of the effluent samples collected at different points of the bleached kraft pulp mill and wastewater treatment plant (Figure 1) were analyzed by GC-MS after derivatization to study the behavior of the three monochlorinated compounds, namely 2-chloro-3,4-dihydroxypentanedioic acid 1, 3-chloro2,4,5-trihydroxy-2-hexenedioic acid 1,4-lactone 2, and 3-chloro-2,4-dihydroxytetrahydrofuran-2,5-dicarboxylic acid 3 (Figure 2), recently reported as the main components of the ethyl acetate extracts of both laboratorial and industrial Eucalyptus globulus chlorine dioxide bleaching filtrates (11). Compounds 1 and 2 were identified in all investigated samples (Figure 2), but compound 3 was detected only in the acidic (D0) effluent (sample A). Additionally, two isomers of compound 3 (compounds 4 and 5) (Figure 2) also 812
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FIGURE 3. Variation in the amounts of chlorinated compounds 1-5 (kg per ton of air-dried pulp, kg/tAD) in the samples studied. previously identified in Eucalyptus globulus bleaching filtrates (11) were only detected in the D0 filtrate. The concentrations of those chlorinated derivatives in the different samples studied are shown in Table 1. As expected, the most concentrated effluent is that issued from the first chlorine dioxide stage (Sample A). The amounts of compounds 1-5 in this acidic effluent, 0.477 kg/tAD (Figure 3) are in the range of those previously reported (11). After the mixing with alkaline effluent coming from the pulp mill AEPM (Sample B, Figure 1), yielding a “neutral” effluent, a decrease of ca. 80% of the total amount of carbohydrate-derived chlorinated compounds 1-5 is observed (Figure 3). Compounds 3-5 completely disappear while the amounts of compounds 1 and 2 are reduced by 50 and 90%, respectively (Figure 3). It is well-known that AOX in acidic bleaching effluents is considerably reduced when the effluents are neutralized by mixing with alkaline effluents or by alkali addition before conveyance to the wastewater treatment system (14-16). Aiming to understand the behavior of compounds 1-5 during the effluent mixing and neutralization, we simulated in the laboratory the neutralization of an industrial D0 effluent. The effluent was neutralized to pH 7 with alkali, heated during 1 h to temperatures in the range of 40-70 °C, left overnight at room temperature, and analyzed before and after filtration. A strong decrease in the contents of compounds 1-5 is observed after neutralization, particularly at 70 °C (Figure 4). When the neutralized effluent is analyzed after filtration, the decrease is slightly higher than that in unfiltered effluent. Thus, the great decrease in compounds 1-5 occurring during the industrial effluent mixing is assigned to its chemical degradation, although the coprecipitation with colloidal substances (lignin and carbohydrate fragments) may also slightly contribute to this reduction, as shown by the analysis of filtered D0 effluent samples (Figure 4). The main chemical degradation pathway seems to be by chlorine substitution, as suggested by the appearance of trihydroxypentanedioic acid in the neutralized effluents (Figure 4). After the primary treatment of the “neutral” effluent (Sample C) there was an unexpected increase (around 30%)
FIGURE 4. Effect of neutralization of D0 filtrates on the concentration of carbohydrate-derived chlorinated compounds 1-5: a, unfiltered effluent; b, filtered effluent.
FIGURE 5. AOX (kg per ton of air-dried pulp, kg/tAD) of samples studied and contribution of chlorinated compounds 1-5 to the total AOX. in the total amount of carbohydrate-derived chlorinated compounds (Table 1, Figure 3), also observed for the total AOX of the effluent (Figure 5), as discussed below. The amounts obtained for compounds 1 and 2 were 0.094 and 0.036 (kg/tAD), respectively. Compound 3 was also detected in this effluent, however it was present in lower amounts (0.017 kg/tAD). The primary treatment is just a physical process that promotes clarification of the effluent by removing solid particles (sludge). Therefore, it is not expected that this treatment would cause an increment in those chlorinated compounds; on the contrary, a fraction of chlorinated organic compounds (including, eventually, compounds 1-5), may be retained by primary sludge (17). A tentative explanation deals with a punctual unsteady-state regime in the running of the bleaching or wastewater treatment processes when sample collection was carried out. In the mixed effluent (Sample D, Figure 1), resulting from the mixture of “neutral” effluent with the effluent coming from the E1 bleaching stage, the total amount of carbohydratederived chlorinated compounds is practically the same as that in the “neutral” effluent (Sample C) (Figure 3). This shows that, unexpectedly, the content of compounds 1-5 in the effluent coming from E1 bleaching stage (and previously from D1 stage, Figure 1) is negligible, although this effluent presents an AOX level of 0.13 t/AD (determined from the difference between AOX in sample D and that in sample C, Figure 5). In fact, our previous results (12) showed that about 35% of carbohydrate-derived chlorinated compounds in a DEDED sequence are formed in the D1 stage. The disappearance of compounds 1-5 formed in the D1 stage may be explained by alkaline degradation, as discussed above, because in this specific mill the D1 filtrate is used to wash the pulp in the alkaline E1 bleaching stage (Figure 1). The chlorinated compounds 1 and 2 entering the biological treatment (STP, Figure 1) are almost completely
degraded during this effluent treatment stage, being identified only in small amounts (0.0087 and 0.017 kg/tAD, respectively) in the final mill effluents (Samples E and F). Thus, less than 3.5% of compounds 1-5 coming from the bleaching plant in the acidic (D0) effluents leave the wastewater treatment plant in the final effluent; about 70-80% are destroyed during effluents mixing/neutralization and about 20-30% are degraded in the biological treatment (Figures 1 and 3). The decrease observed in the secondary treatment is consistent with the small resistance of weakly chlorinated compounds to biodegradation (7). The main low-molecular-weight components present in the ethyl acetate extract of the final mill effluents are saturated fatty acids such as tetradecanoic (myristic), hexadecanoic (palmitic), and octadecanoic (stearic) acids (results not shown). Contribution to AOX. The total AOX of the samples studied and the contribution of the chlorinated compounds 1-5 to this parameter are summarized in Figure 5. The variation of total AOX of the effluents studied present a pattern similar to that observed for carbohydrate-derived chlorinated compounds (Figures 3 and 5), even when the unexpected increase in compounds 1-2 occurred after the primary treatment (Sample C). However, the relative decrease of the total AOX observed after the mixture of the acidic D0 effluent with the alkaline effluent AEPM (Figure 1), coming from the primary treatment plant (Sample D), as well as that promoted by the biological treatment (Samples E and F) are much lower than the corresponding decreases observed for the compounds 1-5 (Figure 5). This is probably due to the resistance to degradation of high-molecular-weight chlorinated compounds (1). The contribution of carbohydrate-derived chlorinated compounds to AOX of effluents range from 15 to 20% in the acidic and alkaline bleaching effluents to about only 1% in the final effluents leaving the secondary treatment. The AOX of this effluent is in the range of 0.20-0.25 kg/tAD, a figure that is in the lower range of AOX levels typically observed for ECF bleached hardwood kraft pulp mills (18).
Acknowledgments Thanks to FCT and ESF for the award of a Ph.D. grant to C. Freire, to the Forest and Paper Research Institute RAIZ, for sampling and supplying the effluent samples and also for performing the AOX analyses. We thank specially Luis Machado (RAIZ) for the kind collaboration in sampling and discussion of results.
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(16) Barton, D. A.; Payne, T. W. Water Sci. Technol. 1999, 40 (11-12), 297. (17) Severtson, S. J.; Barnejee, S. International Environmental Conference Proceedings; TAPPI Press: Atlanta, GA, 1995; Book 1, pp 263-267. (18) NCASI, Technical Bulletin no. 667; National Council for Air and Stream Improvement: Research Triangle Park, NC, 1994.
Received for review April 25, 2002. Revised manuscript received October 2, 2002. Accepted December 6, 2002. ES0200847
