Article pubs.acs.org/est
Kinetics of PCDD/Fs Formation from Non-Wood Pulp Bleaching with Chlorine Xueli Wang, Haijun Zhang, Yuwen Ni, Qinqin Du, Xueping Zhang, and Jiping Chen* Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China S Supporting Information *
ABSTRACT: Chlorine bleaching is still practiced by most of nonwood pulp and paper mills, resulting in a considerable amount of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) formation and emission. In this study, the effects of primary chlorination conditions on the formation of PCDD/Fs from nonwood pulp bleaching with elemental chlorine were investigated. It was found that lowchlorinated PCDD/Fs were usually formed and then underwent further chlorination to form highly chlorinated PCDD/ Fs with increasing chlorination time. Higher available chlorine dosages and lower system pH values greatly accelerated dioxin formation, and pH 3 was the threshold for the formation of tetra- to octa-CDD/Fs. Higher temperatures promoted the formation of lower-chlorinated PCDD/Fs, while caused significant reduction of tetra- to hepta-CDDs and penta- to octa-OCDFs. PCDFs were formed much faster than PCDDs. A first-order kinetic model showed a good fit to the data for tetra- to oct-CDFs formation under different chlorination conditions, indicating that chlorine substitution was the rate determining step for their formation. Finally, the optimum chlorination conditions for minimizing and eliminating the formation of 2,3,7,8-TCDD/TCDF in nonwood pulp bleaching with elemental chlorine were established.
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INTRODUCTION Pulp and paper mills are important emission sources of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). These are mainly formed during bleaching with chlorine, and are usually 2,3,7,8-TCDF and 2,3,7,8-TCDD.1−4 In the last two decades, elemental chlorine free and total chlorine free bleaching technologies have been widely used in North America and Europe, and emissions of PCDD/Fs from pulp bleaching have consequently been greatly reduced.5−7 However, chlorine bleaching technology is still adopted by most of nonwood pulp mills in China, India, and Southeast Asia.8−10 In 2007, China developed its National Implementation Plan for the POPs Convention, in which pulp and paper industries that used chlorine bleaching were identified as important emission sources of dioxins, and needed to be controlled.11 Pulp bleaching is a chemical process for increasing the pulp brightness by delignification and lignin removal. Most PCDD/ Fs are generated from dibenzo-p-dioxin and dibenzofuran (DBD/F) precursors or the lignin matrix at the chlorine bleaching stage;12,13 and the key formation reactions include chlorine substitution and coupling of phenol and quinine structures. The chlorine substitution reaction requires positively charged chlorine species, and the reaction rate depends on the concentration of electrophilic chlorine. The factors affecting chlorine bleaching efficiency, such as the available chlorine © 2014 American Chemical Society
dosage, lignin content, temperature, pH, and chlorination time, clearly influence the electrophilic chlorine content, resulting in variations in the formed PCDD/Fs.12,14 Several previous studies have reported the formation of dioxins in wood pulp bleaching with mixtures of molecular chlorine and chlorine dioxide.12−15 It was found that the amounts of 2,3,7,8-TCDD and 2,3,7,8-TCDF formed increased as an S-shaped curve with increasing molecular chlorine use.13,14 The increase of kappa number, an indicator of the residual lignin content or pulp bleachability, was found to favor formation of organic chlorine compounds, whereas no dependence of dioxin formation on the kappa number was observed.12,14,16 Until now, the influences of the pH and temperature of the pulp bleaching system on dioxin formation have been unknown, although it was found that increasing the system pH and temperature caused a substantial decrease in the total amount of organic chlorine compounds formed at the chlorination stage.17,18 In addition, a study of the formation kinetics of PCDD/Fs from DBD/F by direct chlorination and DBD/F spiked wood pulp chlorination indicated that the formation of mono- to tri-CDD/Fs decreased, whereas the Received: Revised: Accepted: Published: 4361
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pH-dependence experiments (pH 0.5, 1, 2, 3, 4, 5, and 6) were performed at a constant temperature of 20 °C with available chlorine content in the pulp of 5%. Chlorine- dosagedependence experiments were conducted at 20 °C at pH 1. The final available chlorine contents in the pulps were adjusted to 5%, 8%, 10%, 12%, 14%, and 16%. In all the experiments, pulp chlorination was terminated at 2 h, and the experimental operation was similar to that used for the formation kinetics experiments. Extraction and Clean-Up. After chlorination, the pulp was passed through a coarse Buchner funnel to separate the solid and aqueous phases. The freeze-dried pulp was spiked with 13 C12 labeled 2,3,7,8-substituted PCDD/Fs internal standards. Extraction was carried out for 16 h in a Soxhlet apparatus using 500 mL of toluene. The filtrate was transferred to a 1 L separating funnel and extracted three times with 50 mL of methylene dichloride. Finally, the pulp and aqueous extracts were mixed and concentrated to approximately 1 mL using a rotary evaporator. Sample cleanup was achieved by a multilayer silica gel column and an alumina column. The final clean extract was transferred stepwise to a microvial and evaporated to dryness with a stream of nitrogen. Ten microliters of the recovery standard (13C12-1,2,3,4-TCDD and 13C12-1,2,3,7,8,9HeCDD, 100 ng mL−1) were added to the microvial for highresolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS) analysis. Instrumental Analysis and Quality Control. The purified extract was analyzed using an Autospec Ultima high resolution mass spectrometer (Micromass, UK) interfaced with a Hewlett−Packard (Palo Alto, CA) 6890 Plus gas chromatograph (HRGC/HRMS). Details of the instrumental analysis conditions were given in our previous report.4 2,8-di-CDD/F, 1,3,7-tri-CDF, and 2,3,8-tri-CDD standards were used for qualitative analysis of the mono- to tri-CDD/F congeners. The concentrations of the mono- to tri-CDD/Fs were estimated from the relative responses of di- and tri-CDD/F to 13C121,2,3,4-TCDD. The determined values of the sample detection limit ranged from 0.033 pg g−1 pulp to 0.45 pg g−1 pulp for 2,3,7,8-substituted PCDD/Fs. The recoveries of internal standards were all within the range 40−130%. Thirteen key experimental points (chlorination time: 5 min, 20 min, 70 min, 180 min; pH value: 0.5, 2, 4; available chlorine dosage: 8%, 12%, 16%; chlorination temperature: 20 °C, 40 °C, 60 °C) were conducted in duplicate. The average relative deviations of 2,3,7,8-chlorine substituted congeners ranged from 7.7% to 18.0%, and the relative deviations of summed tetra- to octaCDD/Fs for different treatments were all less than 16.1%.
formation of tetra-CDD/F increased with increasing chlorination time.19 Kinetic studies can be used to correlate the dioxin formation rate with reagents and bleaching conditions, enabling determination of the basic principles for reducing the formation and emission of dioxins from pulp bleaching with chlorine. As mentioned above, only a few studies have focused on the formation kinetics of dioxins from chlorine bleaching of wood pulp. To our knowledge, there have as yet been no studies exploring the kinetics of dioxin formation from nonwood pulp bleaching with chlorine, although several studies have reported the levels of PCDD/Fs in nonwood pulps.4,20−23 Nonwood pulp usually contains less lignin, more silica, and more coloring matter than wood pulp does; therefore, the formation kinetics of PCDD/Fs from chlorine bleaching of nonwood pulp may be somewhat different from that from wood pulp. In the present work, chlorine bleaching experiments were conducted using different chlorination times, pH values, temperatures, and available chlorine dosages, with the specific aim of developing a kinetic model of PCDD/Fs formation. The obtained results are helpful in determining and applying the best available technology and best environmental practice (BAT/BEP) in the nonwood pulp and papermaking industries, in order to reduce PCDD/Fs emission from nonwood pulp and paper mills. Furthermore, this study was also expected to give an insight into the possibility and mechanism of dioxin formation from other low-temperature (or ambient temperature) chlorination processes.
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EXPERIMENTAL SECTION Chlorination Experiments. Unbleached pulp, which was made by alkaline digestion of wheat straw with NaOH, was obtained from a nonwood pulp and paper mill in China. Prior to the experiments, the pulp was mixed homogeneously, without washing. The kappa number of the test pulp was determined using KMnO4 solution, according to the ISO 302: 2004 method.24 The kappa number of the test pulp was 16.5. Chlorine−water was prepared by mixing hypochlorite and hydrochloric acid. The available chlorine content of the chlorine−water was determined titrimetrically,25 by measuring the amount of thiosulfate consumed in the induced iodine− azide reaction. A series of experiments were carried out to study the formation of PCDD/Fs under different chlorination conditions. In the formation kinetics experiments, 506 mL aliquots of reagent water were added to eight flasks each containing 20 g of dried pulp. The pH of the pulp slurry was adjusted to 1 using concentrated hydrochloric acid . The pulp slurry was stirred continuously at 20 °C in a water bath for 2 h, and then 160 mL of chlorine−water were added, except in the case of the process blank. The flask was immediately capped and the pulp slurry was stirred continuously after the addition of chlorine−water. The final solid content in the pulp slurry was 3% by weight, and the available chlorine content was 5% chlorine based on the pulp. The flasks containing the pulp samples were taken away from the water bath at 5 min, 10 min, 20 min, 40 min, 70 min, 120 min, and 180 min, respectively, and aqueous sodium thiosulfate was immediately added to quench the residual chlorine in the pulp samples. The process blank was removed after 3 h. Temperature-dependence experiments were performed over the temperature range 20−70 °C; the pH of the pulp slurry was 1 and the available chlorine content in the pulp was 5%. The
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RESULTS AND DISCUSSION Effects of Available Chlorine Dosage. The available chlorine is an indicator of the total chlorine species of all oxidation states, including Cl2, ClO−, ClO2−, ClO2, ClO3−, and ClO5−.26 Among these, molecular chlorine (Cl2), which can undergo heterolytic fission to form intermediate positive ion chlorine (Cl+), plays an important role in the formation of organic chlorine compounds from pulp bleaching.27,28 Theoretically, the content of Cl2 should positively correlate with the available chlorine dosage when the pH and temperature of the pulp are fixed. The addition of chlorine−water obviously promoted PCDD/ Fs formation (Supporting Information (SI) Table S3). When the pH was adjusted to 1 and the temperature was fixed at 20 °C, the summed concentrations of tetra- to octa-CDFs (R2 = 4362
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0.66, p < 0.05) and I-TEQ values (R2 = 0.72, p < 0.05) both linearly increased with increasing available chlorine content from 5% to 16% (Figure 1a). However, a quadratic polynomial
Figure 2. Ratios of summed tetra- to octa-CDFs to summed tetra- to octa-CDDs under different pulp bleaching conditions.
mainly come from further chlorination of mono- and dichlorinated phenols and quinones. These lower-chlorinated phenols or quinones are mostly released from the lignin matrix by side-chain displacement reactions caused by chlorination.30−32 Compared with non-2,3,7,8-chlorine substituted dioxin isomers (less toxic), more 2,3,7,8-chlorine substituted PCDD/Fs were formed from pulp bleaching with chlorine, especially 2,3,7,8-TCDD and 2,3,7,8-TCDF. A linear increase in the 2,3,7,8-TCDF content with increasing available chlorine dosage was found (R2 = 0.81, p < 0.05) (SI Figure S2). Effects of pH. The solubility of chlorine in water and the speciation of aqueous free chlorine are pH dependent.33,34 The speciation of aqueous free chlorine is a well-understood equilibrium, described by the equation Cl2 + H2O ⇌ H+ + HOCl + Cl−, and the equilibrium constant (K) can be expressed as
Figure 1. Effects of available chlorine dosage (a), pH value (b), and temperature (c) on the formation of tetra- to octa-CDD/Fs from nonwood pulp bleaching with chlorine.
K=
[H+][Cl−][H O C l] [Cl 2] +
(1)
−
where [H ], [Cl ], [HOCl], and [Cl2] are the concentrations of H+, Cl−, HOCl, and Cl2 in aqueous solution, respectively. According to eq 1, a lower pH leads to a higher concentration of Cl2 and a lower concentration of HOCl.34 The influence of pH on the formation of PCDD/Fs from pulp bleaching was investigated, with the available chlorine dosage adjusted to 5% and the reaction temperature fixed at 20 °C. It was found that summed concentrations of tetra- to octaCDFs (R2 = 0.98, p < 0.01), summed concentrations of tetra- to octa-CDDs (R2 = 0.45, p < 0.05) and I-TEQ values (R2 = 0.93, p < 0.01) all decreased exponentially with increasing pH from 0.5 to 6 (Figure 1b). For pH ≥ 3, the total concentrations and I-TEQ values of tetra- to octa-CDD/Fs formed from the pulp bleaching were both comparable to those in the raw pulp without bleaching (blank) (SI Table S3). This suggests that a pH value of 3 is a threshold for the formation of tetra- to octaCDD/Fs from pulp bleaching with chlorine. The concentrations of di-CDD (R2 = 0.88, p < 0.05) and diCDF (R2 = 0.88, p < 0.05) increased linearly with increasing pH from 0.5 to 4 (SI Figure S3), although the Cl 2 concentration gradually decreased.34 The rapid formation of di-CDD/Fs should result from a progressive increase in the HOCl concentration and significant occurrence of OCl− with increasing pH. OCl− tends to attack quinone oxygens in lignin through nucleophilic addition to form phenoxyl radicals.35 Debenzo-p-dioxin and furan structures (DBD/F) can be
relationship (R2 = 0.50, p < 0.05) (Figure 1a) was observed between the summed concentration of tetra- to octa-CDDs and available chlorine dosage, and these total PCDDs yields peaked at available chlorine contents of 8−12%. In addition, it was found that the ratio of the total tetra- to octa-CDFs to the total tetra- to octa-CDDs exponentially increased with increasing available chlorine dosage (R2 = 0.85, p < 0.01) (SI Figure S1), and the tetra- to octa-CDFs were formed much more than tetra- to octa-CDDs (Figure 2). This implies that a higher available chlorine dosage favors the formation of tetra- to octaCDFs. Normalization of individual homologue groups from di- to octa-CDD/Fs to the total concentrations of PCDD/Fs indicated that the formation of lower-chlorinated PCDD/Fs and OCDD was accelerated by adding chlorine−water, and the proportion of tri- and tetra-chlorinated PCDFs increased with increasing available chlorine dosage (SI Table S4). These lower-chlorinated PCDD/Fs partly originate from the chlorination of dibenzo-p-dioxin and dibenzofuran (DBD/F) precursors during oxidative coupling of lignin precursors.14,29 Moreover, the condensation reactions of lower-chlorinated phenols or quinones also make important contributions to the formation of lower-chlorinated PCDD/Fs. The most likely mechanism for OCDD formation is the condensation reactions of higher-chlorinated phenols or quinones, which should 4363
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formed from the condensation of these phenoxyl radicals,36 and then they can be released as di-CDD/Fs from the lignin matrix by side-chain displacement reactions caused by chlorination.13 Small amounts of di-CDD/Fs were formed at pH values above 4, as a result of lack of molecular Cl2.34 Unlike di-CDD/Fs, the amount of tri-CDD/Fs homologues decreased with increasing pH (SI Table S6). Effects of Temperature. The influence of temperature on PCDD/Fs formation can be attributed to two contradictory factors. On the one hand, the reaction rate constant for PCDD/ Fs formation increases with increasing temperature, according to the Arrhenius equation; on the other hand, the solubility of chlorine decreases with increasing temperature.33 As shown in Figure 1c, when the pH was adjusted to 1 and the available chlorine dosage was 5%, the summed concentrations of tetrato octa-CDFs (R2 = 0.99, p < 0.01) and I-TEQ values (R2 = 0.99, p < 0.01) both decreased exponentially with increasing temperature from 20 to 70 °C. However, the summed concentrations of tetra- to octa-CDDs peaked at 30 °C, and then rapidly decreased with increasing temperature. These results suggest that a higher chlorination temperature can inhibit the formation of tetra- to octa-CDD/Fs. The distribution pattern of di- to octa- CDD/Fs homologue groups varied significantly depending on the temperature. Higher temperatures (30−70 °C) promoted the formation of lower-chlorinated PCDD/Fs. Compared with those at 20 °C, the summed concentrations of di- and tri-CDDs and those of di- and tri-CDFs formed in temperature range 30−70 °C increased 0.7−1.3 fold and 41.2−58.9 fold, respectively. A large amount of OCDD was formed at 30−40 °C. The concentrations of OCDD at 30 and 40 °C were 4.1 fold and 1.0 fold higher, respectively, than that at 20 °C. However, the formation of tetra- to hepta-CDDs and penta- to octa-OCDFs decreased significantly in the temperature range 30−70 °C (SI Table S8). PCDD/Fs Formation as a Function of Chlorination Time. Under the conditions temperature 20 °C, pH 1, and available chlorine dosage 5%, the summed concentrations of tetra- to octa-CDFs (R2 = 0.96, p < 0.01) and I-TEQ values (R2 = 0.94, p < 0.01) both exponentially increased with increasing chlorination time to 180 min (Figure 3); however, the summed concentrations of tetra- to octa-CDDs increased linearly with increasing chlorination time (R2 = 0.89, p < 0.01) (Figure 3). The apparent formation rate of tetra- to octa-CDFs was much higher than that of tetra- to octa-CDDs, especially for long
chlorination times. The ratio of the total tetra- to octa-CDFs to the total tetra- to octa-CDDs reached 2.2−3.2 in the time range 20−180 min, and the largest ratio was obtained at 20 min (SI Table S3 and Figure 2). The di- to octa-CDD/Fs homologue profiles varied significantly, depending on the chlorination time. Di- and triCDD/Fs probably formed rapidly at shorter chlorination times (5−40 min), and then the amounts gradually decreased with increasing chlorination time from 70 to 180 min. The highest summed concentrations of di- and tri-CDFs and of di- and triCDDs were found at 40 and 20 min, respectively. In contract, larger amount of highly chlorinated dioxins, that is, tetra- to hepta-CDDs and tetra- to octa-CDFs, were formed for chlorination times in the range 70−180 min (SI Table S10). Compared with the case of pulp without bleaching, the total concentration of these highly chlorinated dioxins formed at chlorination times of 70−180 min increased about 1.2−7.6 fold. These results show that low−chlorinated PCDD/Fs are probably formed first and then undergo continuous chlorination to form highly chlorinated PCDD/Fs during the chlorine bleaching process. Kinetic Modeling. During pulp bleaching with chlorine, most PCDD/Fs are formed from precursors via the reactions of chlorine substitution and oxidative coupling. We supposed that chlorine substitution was the rate-determining step for PCDD/ Fs formation. The overall formation of PCDD/Fs can therefore be schematically represented by the following irreversible reaction: precursors + Cl → PCDD/Fs + other products
(2)
According to the proposed reaction scheme, the formation rate of PCDD/Fs (r) can be written as follows: r = d[PCDD/Fs]/dt = k[precursors]np [Cl]nCl
(3)
The superscripts np and nCl indicate the reaction orders for precursors and Cl, respectively, and k is the reaction rate constant. According to the Arrhenius equation, k = Aexp(−(Ea/RT)), where A is the pre-exponential factor and E is the activation energy. [Cl] represents the total concentration of chlorine species of chlorination states, that is, the concentration of water-soluble elemental chlorine. Because dioxin precursors, such as phenols, quinines, DBD/F structures and the lignin matrix, excessively exist in the pulp during bleaching, the precursor concentration term can be assumed to be constant in each experiment and the PCDD/Fs formation reaction can be defined as a first-order reaction; eq 3 can therefore be written as ⎛ Ea ⎞ ⎟[Cl] d[PCDD/Fs]/dt = A 0exp⎜ − ⎝ RT ⎠
(4)
np
where A0 = A[precursors] . At a certain temperature and a certain pH value, [Cl] in the reaction system will be proportional to the available chlorine dosage [ClA], and eq 4 can therefore be given by d[PCDD/Fs]/dt = k1[ClA]
(5)
When the available chlorine dosage and chlorination temperature are fixed, [Cl] in the reaction system will be a function of pH value. According to eq 1, [Cl] can be expressed as K−1exp {−B(pH)} (B is a constant), and thus eq 4 can be modified to Figure 3. Effect of chlorination time on the formation of tetra- to octaCDD/Fs from nonwood pulp bleaching with chlorine.
d[PCDD/Fs]/dt = k 2exp{−B(pH)} 4364
(6)
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Figure 4. Experimental and calculated formation rate of tetra- to octa-CDFs from nonwood pulp bleaching with chlorine.
When pH value of reaction system is fixed at 1 and available chlorine dosage is a constant, [Cl] in the reaction system can be expressed to be a function of chlorination temperature. Alkan et al. found that the variation of elemental chlorine solubility in chlorine-water system with temperature conformed to an exponential function: [Cl] = 4.46 × 10−8 × A1exp(A2/T) (A1 and A2 are constants).33 Therefore, eq 4 can be written as d[PCDD/Fs]/dt = A3exp{(A 2 R − E a)/(RT )}
(chlorination time, chlorination temperature, system pH value and available chlorine dosage) on the degradation rate of dioxins are unknown. Therefore, the parameters of degradation kinetics were not introduced in the established formation kinetic model. Practical Significance for Pulp and Paper Industries. 2,3,7,8-TCDD/TCDF turned out to be the most abundant toxic congeners in the pulp chlorination process.37−39 In 2008, the United Nations Environment Program (UNEP) published BAT/BEP guidelines for minimizing or eliminating the formation of 2,3,7,8-TCDD/TCDF in wood and nonwood bleaching processes, in which the substitution of ClO2 or chlorine-free chemicals for molecular chlorine together with the prevention of precursor addition, were recommended.40 According to the results obtained in our study, the optimization of the bleaching conditions can be used as a supplementary BAT approach to reduce the formation of 2,3,7,8-TCDD/ TCDF from nonwood pulp bleaching with chlorine. Our results indicate that the pulp bleaching conditions, such as chlorination time, initial pH, temperature, and available chlorine dosage, greatly affect dioxin formation. As shown in Figure 5, the formation of 2,3,7,8-TCDD/TCDF increased with increasing chlorination time and increasing available chlorine dosage; but decreased with increasing pH and temperature. During real nonwood pulp bleaching with elemental chlorine, the chlorination time is usually scheduled for 1 h and the available chlorine dosage is usually fixed at 5%; however, the system pH and temperature may vary moderately. Based on our experimental results, the recommended BAT approaches for reducing the formation of 2,3,7,8-TCDD/ TCDF from nonwood pulp bleaching with elemental chlorine are as follows: an optimum system pH near 3 and optimum chlorination temperature of about 30 °C. Compared with the amounts formed at pH 2, the formation of 2,3,7,8-TCDD and 2,3,7,8-TCDF at pH 3decreased 4 fold and 17 fold, respectively. The formation of 2,3,7,8-TCDD and 2,3,7,8-TCDF at 30 °C decreased by 4 fold and 5 fold, respectively, compared with the those formed at 20 °C. When the system pH is above 3,
(7)
where A3 is a constant, and A3 = 4.46 × 10−8 × A0 × A1. Equations 4−7 were used to model the experimentally determined values of [PCDD/Fs] (nmol kg−1) versus chlorination time, available chlorine dosage, pH and temperature, respectively. The goodness of fit was estimated on the basis of one-way ANOVA regression results (SI Tables S13) and the determination coefficient (R2). As shown in Figure 4, four kinetic equations all fitted the data on summed tetra- to octa-CDFs very well. This indicated that chlorine substitution was the rate−determining step for the formation of tetra- to octa-CDFs. However, except eq 6, the other three kinetic equations did not show a good fit to the data for the apparent formation rate of summed tetra- to octa-CDDs. The lower formation rate and relatively higher oxidative degradation of summed tetra- to octa-CDDs should be responsible for the poorer fit. In addition, the established kinetic equations also did not fit the data on di- and tri-CDD/Fs. The most possible reason is that the coupling of phenol and quinine structures should be a more important process during the formation of these lower chlorinated PCDD/Fs. The oxidative degradation of formed dioxins can inevitably happened during the pulp chlorine bleaching process, due to the massive existence of oxidation species (HOCl and OCl−). For example, higher chlorination temperatures caused significant reduction of formed tetra- to hepta-CDDs and penta- to octa-OCDFs (SI Table S8). However, the degradation mechanisms of dioxins during the pulp bleaching may be more complex and up to now no documents reported the degradation kinetics. The effects of chlorination conditions 4365
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(5) Renberg, L.; Johansson, N. G.; Blom, C. Destruction of PCDD and PCDF in bleached pulp by chlorine dioxide treatment. Chemosphere 1995, 30 (9), 1805−1811, DOI: 10.1016/00456535(95)00068-J. (6) Toyota, K.; Kaneko, R.; Jikibara, T.; Kawasaki, K.; Maezawa, K.; Terada, K.; Yano, K.; Tanaka, K.; Shigemoto, T. Effect of dioxins reduction with ECF conversion in kraft pulp bleaching mills in Japan. Organhalogen Compd. 2007, 69 (237), 962−965. (7) Torres, A. L.; Roncero, M. B.; Colom, J. F.; Pastor, F. I. J.; Blanco, A.; Vidal, T. Effect of a novel enzyme on fibre morphology during ECF bleaching of oxygen delignified Eucalyptus kraft pulps. Bioresour. Technol. 2000, 74 (2), 135−140, DOI: 10.1016/S09608524(99)00178-9. (8) Thacker, N. P.; Nitnaware, V. C.; Das, S. K.; Devotta, S. Dioxin formation in pulp and paper mills of India. Environ. Sci. Pollut. Res. 2007, 14 (4), 225−226, DOI: 10.1065/espr2007.02.386. (9) Rolf, B.; Christina, J.; Lars-Åke, L.; Yngve, L. Non wood pulping technology present status and future. IPPTA J. 2009, 21 (1), 115−120. (10) Oanh, N. T. K. A comparative study of effluent toxicity for three chlorine-bleached pulp and paper mills in Southeast Asia. Resour. Conserv. Recy. 1996, 18, 87−105, DOI: 10.1016/S0921-3449(96) 01171-8. (11) The People’s Republic of China-National Implementation Plan for the Stockholm Convention on Persistent Organic Pollutants. In Ministry of Environmental Protection of the People’s Republic of China 2007. http://www.pops.int/documents/implementation/nips/ submissions/China_NIP_En.pdf. (12) Dallons, V. J.; Whittemore, R. C.; LaFleur, L. E.; Brunk, R. Study of 2,3,7,8-TCDD and 2,3,7,8-TCDF formation at 23 bleach plants. TAPPI Pulping Conf., [Proc.] 1990, 153−159. (13) Hrutfiord, B. F.; Negri, A. R. Dioxin sources and mechanisms during pulp bleaching. Chemosphere 1992, 25 (1−2), 53−56, DOI: 10.1016/0045-6535(92)90478-A. (14) Berry, R. M.; Luthe, C. E.; Voss, R. H.; Wrist, P. E.; Axegard, P.; Gellerstedt, G.; Lindblad, P.-O.; Pöpke, I. The effects of recent changes in bleached softwood kraft mill technology on organochlorine emissions an international perspective. Pulp Pap. Can. 1991, 92 (6), T155−T165. (15) Yetis, U.; Selcuk, A.; Gokcay, C. F. Reducing chlorinated organics, AOX, in the bleachery effluents of a Turkish pulp and paper plant. Water Sci. Technol. 1996, 34 (10), 97−104. (16) Axegard, P.; Renberg, L. The influence of bleaching chemicals and lignin content on the formation of polychlorinated dioxins and dibenzofurans. Chemosphere 1989, 19 (1−6), 661−668, DOI: 10.1016/0045-6535(89)90387-1. (17) Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp; U.S. Congress, Offoce of Technology Assessment: Washington, DC, 1989. (18) Smeds, A.; Holmbom, B.; Pettersson, C. Chemical-stability of chlorinated components in pulp bleaching liquors. Chemosphere 1994, 28 (5), 881−895, DOI: 10.1016/0045-6535(94)90005-1. (19) Dimmel, D. R.; Riggs, B. K.; Pltts, G.; White, J.; Lucas, S. Formation mechanisms of polychlorinated dibenzo-p-dioxins and dibenzofurans during pulp chlorination. Environ. Sci. Technol. 1993, 27, 2553−2558, DOI: 10.1021/es00048a037. (20) Zheng, M. H.; Bao, Z. C.; Wang, K. O.; Xu, X. B. Levels of PCDDs and PCDFs in the bleached pulp from Chinese pulp and paper industry. Bull. Environ. Contam. Toxicol. 1997, 59 (1), 90−93. (21) Zhang, Q. H.; Xu, Y.; Wu, W. Z.; Xiao, R. M.; Feng, L.; Schramm, K.-W.; Kettrup, A. PCDDs and PCDFs in the wastewater from Chinese pulp and paper industry. Bull. Environ. Contam. Toxicol. 2000, 64 (3), 368−371. (22) Zheng, M. H.; Bao, Z. C.; Zhang, B.; Xu, X. B. Polychlorinated dibenzo-p-dioxins and dibenzofurans in paper making from a pulp mill in China. Chemosphere 2001, 44 (6), 1335−1337, DOI: 10.1016/ S0045-6535(00)00488-4. (23) Fang, L. P.; Zheng, M. H.; Liu, W. B.; Hui, Y. M.; Guo, L. Profile of PCDD in effluents from non-wood pulp and paper mills. Organohalogen Compd. 2009, 71, 3116−3118.
Figure 5. The effects of different chlorination conditions on the formation of 2,3,7,8-TCDD and 2,3,7,8-TCDF from nonwood pulp bleaching with chlorine.
hypochloric acid becomes the dominant compound in chlorine−water system, which causes severe degradation of carbohydrate and thus degrades the paper quality. Moreover, cellulose oxidation intensifies with increasing chlorination temperature, and this also degrades the paper quality. In future studies, a series of feasibility tests for the recommended BAT approaches for minimizing the formation of dioxins from Chinese nonwood pulp and paper mills should be conducted.
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ASSOCIATED CONTENT
S Supporting Information *
Tables on concentrations of di- up to octa- CDD/Fs homologue groups and seventeen 2,3,7,8-chlrine substituted PCDD/Fs in pulp during different chlorination conditions, and ANOVA results for the data fitting. Figures on the variations of 2,3,7,8-TCDF concentrations and the ratios of PCDFs to PCDDs with available chlorine dosage, on the variation of diCDD/F concentration with pH value. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone/fax: +86 411 84379562; e-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No.21037003). REFERENCES
(1) Standardized Toolkit for Identification and Quantification of Dioxin and Furan Releases, 2.1 ed.; UNEP Chemicals Geneva: Switzerland, 2005. (2) Swanson, S. E.; Rappe, C.; Malmstrom, J.; Kringstad, K. P. Emissions of PCDDs and PCDFs from the pulp industry. Chemosphere 1988, 17 (4), 681−691, DOI: 10.1016/0045-6535(88)90248-2. (3) Amendola, G.; Barna, D.; Blosser, R.; Lafleur, L.; Mcbride, A.; Thomas, F.; Tiernan, T.; Whittemore, R. The occurrence and fate of PCDDs and PCDFs in five bleached kraft pulp and paper-mills. Chemosphere 1989, 18 (1−6), 1181−1188, DOI: 10.1016/00456535(89)90253-1. (4) Wang, X. L.; Ni, Y. W.; Zhang, H. J.; Zhang, X. P.; Chen, J. P. Formation and emission of PCDD/Fs in chinese non-wood pulp and paper mills. Environ. Sci. Technol. 2012, 46 (21), 12234−12240, DOI: 10.1021/es303373b. 4366
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(24) Pulps-Determination of Kappa Number ISO 302:2004; International Organization for Standardization, 2004. (25) Kurzawa, J.; Kurzawa, Z.; Janowicz, K. Determination of microgram and nanogram amounts of active chlorine in water by iodine azide reaction induced by thiosulfate or thioammeline. Anal. Chim. Acta 1991, 252 (1−2), 127−132, DOI: 10.1016/0003-2670(91) 87206-M. (26) Kubala, S. W.; Tilotta, D. C.; Busch, M. A.; Busch, K. W. Determination of chloride and available chlorine in aqueous samples by flame infrared-emission. Anal. Chem. 1989, 61 (24), 2785−2791, DOI: 10.1021/ac00199a020. (27) Kotiaho, T.; Shay, B. J.; Cooks, R. G.; Eberlin, M. N. Electrophilic aromatic Cl+ addition and Co.+ substitution in the gasphase. J. Am. Chem. Soc. 1993, 115 (3), 1004−1014, DOI: 10.1021/ ja00056a027. (28) Sarkanen, K. V. The chemistry of delignification in pulp bleaching. Pure. Appl. Chem. 1962, 5, 219−232. (29) Hrutfiord, B. F.; Negri, A. R. Chlorinated dibenzofurans and dibenzodioxins from lignin models. Tappi J. 1992, 75, 129−134. (30) Luthe, C. E.; Berry, R. M.; Voss, R. H. Chlorinated dioxins in the production of bleached kraft pulp resulting from the use of sawmill wood chips contaminated with polychlorinated phenols. TAPPI Eng., Environ. Conf. 1992, 859−867. (31) Luthe, C. E.; Berry, R. M.; Voss, R. H. Formation of chlorinated dioxins during production of bleached kraft pulp from sawmill chips contaminated with polychlorinated phenols. Tappi J. 1993, 76 (3), 63−68. (32) Luthe, C. E.; Dlvd, J. s. Octachlorinated dioxin in pulps and effluents: Where does it come from? Chemosphere 1996, 32 (12), 2409−2425, DOI: 10.1016/0045-6535(96)00138-5. (33) Alkan, M.; Oktay, M.; Kocakerim, M. M.; Copur, M. Solubility of chlorine in aqueous hydrochloric acid solutions. J Hazard Mater. 2005, 119 (1−3), 13−18, DOI: 10.1016/j. (34) Cherney, D. P.; Duirk, S. E.; Tarr, J. C.; Collette, T. W. Monitoring the speciation of aqueous free chlorine from pH 1 to 12 with Raman spectroscopy to determine the identity of the potent lowpH oxidant. Appl. Spectrosc. 2006, 60 (7), 764−772, DOI: 10.1366/ 000370206777887062. (35) Zhan, H. Y.; Liu, Q. J.; Chen, J. C.; N., Y. R.; Han, Q.; Zhai, H. M. Pulping Principle and Engineering, 3rd ed.; China Light Industry Press: China, Beijing, 2011 (in Chinese). (36) Xu, F.; Yu, W.; Gao, R.; Zhou, Q.; Zhang, Q.; Wang, W. Dioxin formations from the radical/radical cross-condensation of phenoxy radicals with 2-chlorophenoxy radicals and 2,4,6-trichlorophenoxy radicals. Environ. Sci. Technol. 2010, 44, 6745−6751, DOI: 10.1021/ es101794v. (37) Wagman, N. Trace analysis of PCDDs and PCDFs in unbleached and bleached pulp samples. Organohalogen Compd. 1995, 23, 337−382. (38) Kitunen, V. H.; Salkinoja-Salonen, M. S. Occurrence of PCDDs and PCDFs in pulp and board products. Chemosphere 1989, 19, 721− 726, DOI: 10.1016/0045-6535(89)90397-4. (39) Beck, H.; Drop, A.; Eckart, K.; Mathar, W.; Wittkowski, R. PCDDs, PCDFs and related compounds in paper products. Chemosphere 1989, 19, 655−660, DOI: 10.1016/0045-6535(89)90386-X. (40) UNEP. Guidelines on Best Available Techniques and Provisional Guidance on Best Environmental Practices, Relevant to Article 5 and Annex C of the Stockholm Convention on Persistent Organic Pollutants, Production of Pulp Using Elemental Chlorine or Chemical Generating Elemental Chlorine; Secretariat of the Stockholm Convention on Persistent Organic Pollutants, UNEP: Switzerland, 2008.
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Kinetics of PCDD/Fs Formation from Non-Wood Pulp Bleaching with Chlorine Xueli Wang, Haijun Zhang, Yuwen Ni, Qinqin Du, Xueping Zhang, and Jiping Chen* Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China S Supporting Information *
ABSTRACT: Chlorine bleaching is still practiced by most of nonwood pulp and paper mills, resulting in a considerable amount of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) formation and emission. In this study, the effects of primary chlorination conditions on the formation of PCDD/Fs from nonwood pulp bleaching with elemental chlorine were investigated. It was found that lowchlorinated PCDD/Fs were usually formed and then underwent further chlorination to form highly chlorinated PCDD/ Fs with increasing chlorination time. Higher available chlorine dosages and lower system pH values greatly accelerated dioxin formation, and pH 3 was the threshold for the formation of tetra- to octa-CDD/Fs. Higher temperatures promoted the formation of lower-chlorinated PCDD/Fs, while caused significant reduction of tetra- to hepta-CDDs and penta- to octa-OCDFs. PCDFs were formed much faster than PCDDs. A first-order kinetic model showed a good fit to the data for tetra- to oct-CDFs formation under different chlorination conditions, indicating that chlorine substitution was the rate determining step for their formation. Finally, the optimum chlorination conditions for minimizing and eliminating the formation of 2,3,7,8-TCDD/TCDF in nonwood pulp bleaching with elemental chlorine were established.
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INTRODUCTION Pulp and paper mills are important emission sources of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). These are mainly formed during bleaching with chlorine, and are usually 2,3,7,8-TCDF and 2,3,7,8-TCDD.1−4 In the last two decades, elemental chlorine free and total chlorine free bleaching technologies have been widely used in North America and Europe, and emissions of PCDD/Fs from pulp bleaching have consequently been greatly reduced.5−7 However, chlorine bleaching technology is still adopted by most of nonwood pulp mills in China, India, and Southeast Asia.8−10 In 2007, China developed its National Implementation Plan for the POPs Convention, in which pulp and paper industries that used chlorine bleaching were identified as important emission sources of dioxins, and needed to be controlled.11 Pulp bleaching is a chemical process for increasing the pulp brightness by delignification and lignin removal. Most PCDD/ Fs are generated from dibenzo-p-dioxin and dibenzofuran (DBD/F) precursors or the lignin matrix at the chlorine bleaching stage;12,13 and the key formation reactions include chlorine substitution and coupling of phenol and quinine structures. The chlorine substitution reaction requires positively charged chlorine species, and the reaction rate depends on the concentration of electrophilic chlorine. The factors affecting chlorine bleaching efficiency, such as the available chlorine © 2014 American Chemical Society
dosage, lignin content, temperature, pH, and chlorination time, clearly influence the electrophilic chlorine content, resulting in variations in the formed PCDD/Fs.12,14 Several previous studies have reported the formation of dioxins in wood pulp bleaching with mixtures of molecular chlorine and chlorine dioxide.12−15 It was found that the amounts of 2,3,7,8-TCDD and 2,3,7,8-TCDF formed increased as an S-shaped curve with increasing molecular chlorine use.13,14 The increase of kappa number, an indicator of the residual lignin content or pulp bleachability, was found to favor formation of organic chlorine compounds, whereas no dependence of dioxin formation on the kappa number was observed.12,14,16 Until now, the influences of the pH and temperature of the pulp bleaching system on dioxin formation have been unknown, although it was found that increasing the system pH and temperature caused a substantial decrease in the total amount of organic chlorine compounds formed at the chlorination stage.17,18 In addition, a study of the formation kinetics of PCDD/Fs from DBD/F by direct chlorination and DBD/F spiked wood pulp chlorination indicated that the formation of mono- to tri-CDD/Fs decreased, whereas the Received: Revised: Accepted: Published: 4361
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pH-dependence experiments (pH 0.5, 1, 2, 3, 4, 5, and 6) were performed at a constant temperature of 20 °C with available chlorine content in the pulp of 5%. Chlorine- dosagedependence experiments were conducted at 20 °C at pH 1. The final available chlorine contents in the pulps were adjusted to 5%, 8%, 10%, 12%, 14%, and 16%. In all the experiments, pulp chlorination was terminated at 2 h, and the experimental operation was similar to that used for the formation kinetics experiments. Extraction and Clean-Up. After chlorination, the pulp was passed through a coarse Buchner funnel to separate the solid and aqueous phases. The freeze-dried pulp was spiked with 13 C12 labeled 2,3,7,8-substituted PCDD/Fs internal standards. Extraction was carried out for 16 h in a Soxhlet apparatus using 500 mL of toluene. The filtrate was transferred to a 1 L separating funnel and extracted three times with 50 mL of methylene dichloride. Finally, the pulp and aqueous extracts were mixed and concentrated to approximately 1 mL using a rotary evaporator. Sample cleanup was achieved by a multilayer silica gel column and an alumina column. The final clean extract was transferred stepwise to a microvial and evaporated to dryness with a stream of nitrogen. Ten microliters of the recovery standard (13C12-1,2,3,4-TCDD and 13C12-1,2,3,7,8,9HeCDD, 100 ng mL−1) were added to the microvial for highresolution gas chromatography/high-resolution mass spectrometry (HRGC/HRMS) analysis. Instrumental Analysis and Quality Control. The purified extract was analyzed using an Autospec Ultima high resolution mass spectrometer (Micromass, UK) interfaced with a Hewlett−Packard (Palo Alto, CA) 6890 Plus gas chromatograph (HRGC/HRMS). Details of the instrumental analysis conditions were given in our previous report.4 2,8-di-CDD/F, 1,3,7-tri-CDF, and 2,3,8-tri-CDD standards were used for qualitative analysis of the mono- to tri-CDD/F congeners. The concentrations of the mono- to tri-CDD/Fs were estimated from the relative responses of di- and tri-CDD/F to 13C121,2,3,4-TCDD. The determined values of the sample detection limit ranged from 0.033 pg g−1 pulp to 0.45 pg g−1 pulp for 2,3,7,8-substituted PCDD/Fs. The recoveries of internal standards were all within the range 40−130%. Thirteen key experimental points (chlorination time: 5 min, 20 min, 70 min, 180 min; pH value: 0.5, 2, 4; available chlorine dosage: 8%, 12%, 16%; chlorination temperature: 20 °C, 40 °C, 60 °C) were conducted in duplicate. The average relative deviations of 2,3,7,8-chlorine substituted congeners ranged from 7.7% to 18.0%, and the relative deviations of summed tetra- to octaCDD/Fs for different treatments were all less than 16.1%.
formation of tetra-CDD/F increased with increasing chlorination time.19 Kinetic studies can be used to correlate the dioxin formation rate with reagents and bleaching conditions, enabling determination of the basic principles for reducing the formation and emission of dioxins from pulp bleaching with chlorine. As mentioned above, only a few studies have focused on the formation kinetics of dioxins from chlorine bleaching of wood pulp. To our knowledge, there have as yet been no studies exploring the kinetics of dioxin formation from nonwood pulp bleaching with chlorine, although several studies have reported the levels of PCDD/Fs in nonwood pulps.4,20−23 Nonwood pulp usually contains less lignin, more silica, and more coloring matter than wood pulp does; therefore, the formation kinetics of PCDD/Fs from chlorine bleaching of nonwood pulp may be somewhat different from that from wood pulp. In the present work, chlorine bleaching experiments were conducted using different chlorination times, pH values, temperatures, and available chlorine dosages, with the specific aim of developing a kinetic model of PCDD/Fs formation. The obtained results are helpful in determining and applying the best available technology and best environmental practice (BAT/BEP) in the nonwood pulp and papermaking industries, in order to reduce PCDD/Fs emission from nonwood pulp and paper mills. Furthermore, this study was also expected to give an insight into the possibility and mechanism of dioxin formation from other low-temperature (or ambient temperature) chlorination processes.
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EXPERIMENTAL SECTION Chlorination Experiments. Unbleached pulp, which was made by alkaline digestion of wheat straw with NaOH, was obtained from a nonwood pulp and paper mill in China. Prior to the experiments, the pulp was mixed homogeneously, without washing. The kappa number of the test pulp was determined using KMnO4 solution, according to the ISO 302: 2004 method.24 The kappa number of the test pulp was 16.5. Chlorine−water was prepared by mixing hypochlorite and hydrochloric acid. The available chlorine content of the chlorine−water was determined titrimetrically,25 by measuring the amount of thiosulfate consumed in the induced iodine− azide reaction. A series of experiments were carried out to study the formation of PCDD/Fs under different chlorination conditions. In the formation kinetics experiments, 506 mL aliquots of reagent water were added to eight flasks each containing 20 g of dried pulp. The pH of the pulp slurry was adjusted to 1 using concentrated hydrochloric acid . The pulp slurry was stirred continuously at 20 °C in a water bath for 2 h, and then 160 mL of chlorine−water were added, except in the case of the process blank. The flask was immediately capped and the pulp slurry was stirred continuously after the addition of chlorine−water. The final solid content in the pulp slurry was 3% by weight, and the available chlorine content was 5% chlorine based on the pulp. The flasks containing the pulp samples were taken away from the water bath at 5 min, 10 min, 20 min, 40 min, 70 min, 120 min, and 180 min, respectively, and aqueous sodium thiosulfate was immediately added to quench the residual chlorine in the pulp samples. The process blank was removed after 3 h. Temperature-dependence experiments were performed over the temperature range 20−70 °C; the pH of the pulp slurry was 1 and the available chlorine content in the pulp was 5%. The
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RESULTS AND DISCUSSION Effects of Available Chlorine Dosage. The available chlorine is an indicator of the total chlorine species of all oxidation states, including Cl2, ClO−, ClO2−, ClO2, ClO3−, and ClO5−.26 Among these, molecular chlorine (Cl2), which can undergo heterolytic fission to form intermediate positive ion chlorine (Cl+), plays an important role in the formation of organic chlorine compounds from pulp bleaching.27,28 Theoretically, the content of Cl2 should positively correlate with the available chlorine dosage when the pH and temperature of the pulp are fixed. The addition of chlorine−water obviously promoted PCDD/ Fs formation (Supporting Information (SI) Table S3). When the pH was adjusted to 1 and the temperature was fixed at 20 °C, the summed concentrations of tetra- to octa-CDFs (R2 = 4362
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0.66, p < 0.05) and I-TEQ values (R2 = 0.72, p < 0.05) both linearly increased with increasing available chlorine content from 5% to 16% (Figure 1a). However, a quadratic polynomial
Figure 2. Ratios of summed tetra- to octa-CDFs to summed tetra- to octa-CDDs under different pulp bleaching conditions.
mainly come from further chlorination of mono- and dichlorinated phenols and quinones. These lower-chlorinated phenols or quinones are mostly released from the lignin matrix by side-chain displacement reactions caused by chlorination.30−32 Compared with non-2,3,7,8-chlorine substituted dioxin isomers (less toxic), more 2,3,7,8-chlorine substituted PCDD/Fs were formed from pulp bleaching with chlorine, especially 2,3,7,8-TCDD and 2,3,7,8-TCDF. A linear increase in the 2,3,7,8-TCDF content with increasing available chlorine dosage was found (R2 = 0.81, p < 0.05) (SI Figure S2). Effects of pH. The solubility of chlorine in water and the speciation of aqueous free chlorine are pH dependent.33,34 The speciation of aqueous free chlorine is a well-understood equilibrium, described by the equation Cl2 + H2O ⇌ H+ + HOCl + Cl−, and the equilibrium constant (K) can be expressed as
Figure 1. Effects of available chlorine dosage (a), pH value (b), and temperature (c) on the formation of tetra- to octa-CDD/Fs from nonwood pulp bleaching with chlorine.
K=
[H+][Cl−][H O C l] [Cl 2] +
(1)
−
where [H ], [Cl ], [HOCl], and [Cl2] are the concentrations of H+, Cl−, HOCl, and Cl2 in aqueous solution, respectively. According to eq 1, a lower pH leads to a higher concentration of Cl2 and a lower concentration of HOCl.34 The influence of pH on the formation of PCDD/Fs from pulp bleaching was investigated, with the available chlorine dosage adjusted to 5% and the reaction temperature fixed at 20 °C. It was found that summed concentrations of tetra- to octaCDFs (R2 = 0.98, p < 0.01), summed concentrations of tetra- to octa-CDDs (R2 = 0.45, p < 0.05) and I-TEQ values (R2 = 0.93, p < 0.01) all decreased exponentially with increasing pH from 0.5 to 6 (Figure 1b). For pH ≥ 3, the total concentrations and I-TEQ values of tetra- to octa-CDD/Fs formed from the pulp bleaching were both comparable to those in the raw pulp without bleaching (blank) (SI Table S3). This suggests that a pH value of 3 is a threshold for the formation of tetra- to octaCDD/Fs from pulp bleaching with chlorine. The concentrations of di-CDD (R2 = 0.88, p < 0.05) and diCDF (R2 = 0.88, p < 0.05) increased linearly with increasing pH from 0.5 to 4 (SI Figure S3), although the Cl 2 concentration gradually decreased.34 The rapid formation of di-CDD/Fs should result from a progressive increase in the HOCl concentration and significant occurrence of OCl− with increasing pH. OCl− tends to attack quinone oxygens in lignin through nucleophilic addition to form phenoxyl radicals.35 Debenzo-p-dioxin and furan structures (DBD/F) can be
relationship (R2 = 0.50, p < 0.05) (Figure 1a) was observed between the summed concentration of tetra- to octa-CDDs and available chlorine dosage, and these total PCDDs yields peaked at available chlorine contents of 8−12%. In addition, it was found that the ratio of the total tetra- to octa-CDFs to the total tetra- to octa-CDDs exponentially increased with increasing available chlorine dosage (R2 = 0.85, p < 0.01) (SI Figure S1), and the tetra- to octa-CDFs were formed much more than tetra- to octa-CDDs (Figure 2). This implies that a higher available chlorine dosage favors the formation of tetra- to octaCDFs. Normalization of individual homologue groups from di- to octa-CDD/Fs to the total concentrations of PCDD/Fs indicated that the formation of lower-chlorinated PCDD/Fs and OCDD was accelerated by adding chlorine−water, and the proportion of tri- and tetra-chlorinated PCDFs increased with increasing available chlorine dosage (SI Table S4). These lower-chlorinated PCDD/Fs partly originate from the chlorination of dibenzo-p-dioxin and dibenzofuran (DBD/F) precursors during oxidative coupling of lignin precursors.14,29 Moreover, the condensation reactions of lower-chlorinated phenols or quinones also make important contributions to the formation of lower-chlorinated PCDD/Fs. The most likely mechanism for OCDD formation is the condensation reactions of higher-chlorinated phenols or quinones, which should 4363
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formed from the condensation of these phenoxyl radicals,36 and then they can be released as di-CDD/Fs from the lignin matrix by side-chain displacement reactions caused by chlorination.13 Small amounts of di-CDD/Fs were formed at pH values above 4, as a result of lack of molecular Cl2.34 Unlike di-CDD/Fs, the amount of tri-CDD/Fs homologues decreased with increasing pH (SI Table S6). Effects of Temperature. The influence of temperature on PCDD/Fs formation can be attributed to two contradictory factors. On the one hand, the reaction rate constant for PCDD/ Fs formation increases with increasing temperature, according to the Arrhenius equation; on the other hand, the solubility of chlorine decreases with increasing temperature.33 As shown in Figure 1c, when the pH was adjusted to 1 and the available chlorine dosage was 5%, the summed concentrations of tetrato octa-CDFs (R2 = 0.99, p < 0.01) and I-TEQ values (R2 = 0.99, p < 0.01) both decreased exponentially with increasing temperature from 20 to 70 °C. However, the summed concentrations of tetra- to octa-CDDs peaked at 30 °C, and then rapidly decreased with increasing temperature. These results suggest that a higher chlorination temperature can inhibit the formation of tetra- to octa-CDD/Fs. The distribution pattern of di- to octa- CDD/Fs homologue groups varied significantly depending on the temperature. Higher temperatures (30−70 °C) promoted the formation of lower-chlorinated PCDD/Fs. Compared with those at 20 °C, the summed concentrations of di- and tri-CDDs and those of di- and tri-CDFs formed in temperature range 30−70 °C increased 0.7−1.3 fold and 41.2−58.9 fold, respectively. A large amount of OCDD was formed at 30−40 °C. The concentrations of OCDD at 30 and 40 °C were 4.1 fold and 1.0 fold higher, respectively, than that at 20 °C. However, the formation of tetra- to hepta-CDDs and penta- to octa-OCDFs decreased significantly in the temperature range 30−70 °C (SI Table S8). PCDD/Fs Formation as a Function of Chlorination Time. Under the conditions temperature 20 °C, pH 1, and available chlorine dosage 5%, the summed concentrations of tetra- to octa-CDFs (R2 = 0.96, p < 0.01) and I-TEQ values (R2 = 0.94, p < 0.01) both exponentially increased with increasing chlorination time to 180 min (Figure 3); however, the summed concentrations of tetra- to octa-CDDs increased linearly with increasing chlorination time (R2 = 0.89, p < 0.01) (Figure 3). The apparent formation rate of tetra- to octa-CDFs was much higher than that of tetra- to octa-CDDs, especially for long
chlorination times. The ratio of the total tetra- to octa-CDFs to the total tetra- to octa-CDDs reached 2.2−3.2 in the time range 20−180 min, and the largest ratio was obtained at 20 min (SI Table S3 and Figure 2). The di- to octa-CDD/Fs homologue profiles varied significantly, depending on the chlorination time. Di- and triCDD/Fs probably formed rapidly at shorter chlorination times (5−40 min), and then the amounts gradually decreased with increasing chlorination time from 70 to 180 min. The highest summed concentrations of di- and tri-CDFs and of di- and triCDDs were found at 40 and 20 min, respectively. In contract, larger amount of highly chlorinated dioxins, that is, tetra- to hepta-CDDs and tetra- to octa-CDFs, were formed for chlorination times in the range 70−180 min (SI Table S10). Compared with the case of pulp without bleaching, the total concentration of these highly chlorinated dioxins formed at chlorination times of 70−180 min increased about 1.2−7.6 fold. These results show that low−chlorinated PCDD/Fs are probably formed first and then undergo continuous chlorination to form highly chlorinated PCDD/Fs during the chlorine bleaching process. Kinetic Modeling. During pulp bleaching with chlorine, most PCDD/Fs are formed from precursors via the reactions of chlorine substitution and oxidative coupling. We supposed that chlorine substitution was the rate-determining step for PCDD/ Fs formation. The overall formation of PCDD/Fs can therefore be schematically represented by the following irreversible reaction: precursors + Cl → PCDD/Fs + other products
(2)
According to the proposed reaction scheme, the formation rate of PCDD/Fs (r) can be written as follows: r = d[PCDD/Fs]/dt = k[precursors]np [Cl]nCl
(3)
The superscripts np and nCl indicate the reaction orders for precursors and Cl, respectively, and k is the reaction rate constant. According to the Arrhenius equation, k = Aexp(−(Ea/RT)), where A is the pre-exponential factor and E is the activation energy. [Cl] represents the total concentration of chlorine species of chlorination states, that is, the concentration of water-soluble elemental chlorine. Because dioxin precursors, such as phenols, quinines, DBD/F structures and the lignin matrix, excessively exist in the pulp during bleaching, the precursor concentration term can be assumed to be constant in each experiment and the PCDD/Fs formation reaction can be defined as a first-order reaction; eq 3 can therefore be written as ⎛ Ea ⎞ ⎟[Cl] d[PCDD/Fs]/dt = A 0exp⎜ − ⎝ RT ⎠
(4)
np
where A0 = A[precursors] . At a certain temperature and a certain pH value, [Cl] in the reaction system will be proportional to the available chlorine dosage [ClA], and eq 4 can therefore be given by d[PCDD/Fs]/dt = k1[ClA]
(5)
When the available chlorine dosage and chlorination temperature are fixed, [Cl] in the reaction system will be a function of pH value. According to eq 1, [Cl] can be expressed as K−1exp {−B(pH)} (B is a constant), and thus eq 4 can be modified to Figure 3. Effect of chlorination time on the formation of tetra- to octaCDD/Fs from nonwood pulp bleaching with chlorine.
d[PCDD/Fs]/dt = k 2exp{−B(pH)} 4364
(6)
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Figure 4. Experimental and calculated formation rate of tetra- to octa-CDFs from nonwood pulp bleaching with chlorine.
When pH value of reaction system is fixed at 1 and available chlorine dosage is a constant, [Cl] in the reaction system can be expressed to be a function of chlorination temperature. Alkan et al. found that the variation of elemental chlorine solubility in chlorine-water system with temperature conformed to an exponential function: [Cl] = 4.46 × 10−8 × A1exp(A2/T) (A1 and A2 are constants).33 Therefore, eq 4 can be written as d[PCDD/Fs]/dt = A3exp{(A 2 R − E a)/(RT )}
(chlorination time, chlorination temperature, system pH value and available chlorine dosage) on the degradation rate of dioxins are unknown. Therefore, the parameters of degradation kinetics were not introduced in the established formation kinetic model. Practical Significance for Pulp and Paper Industries. 2,3,7,8-TCDD/TCDF turned out to be the most abundant toxic congeners in the pulp chlorination process.37−39 In 2008, the United Nations Environment Program (UNEP) published BAT/BEP guidelines for minimizing or eliminating the formation of 2,3,7,8-TCDD/TCDF in wood and nonwood bleaching processes, in which the substitution of ClO2 or chlorine-free chemicals for molecular chlorine together with the prevention of precursor addition, were recommended.40 According to the results obtained in our study, the optimization of the bleaching conditions can be used as a supplementary BAT approach to reduce the formation of 2,3,7,8-TCDD/ TCDF from nonwood pulp bleaching with chlorine. Our results indicate that the pulp bleaching conditions, such as chlorination time, initial pH, temperature, and available chlorine dosage, greatly affect dioxin formation. As shown in Figure 5, the formation of 2,3,7,8-TCDD/TCDF increased with increasing chlorination time and increasing available chlorine dosage; but decreased with increasing pH and temperature. During real nonwood pulp bleaching with elemental chlorine, the chlorination time is usually scheduled for 1 h and the available chlorine dosage is usually fixed at 5%; however, the system pH and temperature may vary moderately. Based on our experimental results, the recommended BAT approaches for reducing the formation of 2,3,7,8-TCDD/ TCDF from nonwood pulp bleaching with elemental chlorine are as follows: an optimum system pH near 3 and optimum chlorination temperature of about 30 °C. Compared with the amounts formed at pH 2, the formation of 2,3,7,8-TCDD and 2,3,7,8-TCDF at pH 3decreased 4 fold and 17 fold, respectively. The formation of 2,3,7,8-TCDD and 2,3,7,8-TCDF at 30 °C decreased by 4 fold and 5 fold, respectively, compared with the those formed at 20 °C. When the system pH is above 3,
(7)
where A3 is a constant, and A3 = 4.46 × 10−8 × A0 × A1. Equations 4−7 were used to model the experimentally determined values of [PCDD/Fs] (nmol kg−1) versus chlorination time, available chlorine dosage, pH and temperature, respectively. The goodness of fit was estimated on the basis of one-way ANOVA regression results (SI Tables S13) and the determination coefficient (R2). As shown in Figure 4, four kinetic equations all fitted the data on summed tetra- to octa-CDFs very well. This indicated that chlorine substitution was the rate−determining step for the formation of tetra- to octa-CDFs. However, except eq 6, the other three kinetic equations did not show a good fit to the data for the apparent formation rate of summed tetra- to octa-CDDs. The lower formation rate and relatively higher oxidative degradation of summed tetra- to octa-CDDs should be responsible for the poorer fit. In addition, the established kinetic equations also did not fit the data on di- and tri-CDD/Fs. The most possible reason is that the coupling of phenol and quinine structures should be a more important process during the formation of these lower chlorinated PCDD/Fs. The oxidative degradation of formed dioxins can inevitably happened during the pulp chlorine bleaching process, due to the massive existence of oxidation species (HOCl and OCl−). For example, higher chlorination temperatures caused significant reduction of formed tetra- to hepta-CDDs and penta- to octa-OCDFs (SI Table S8). However, the degradation mechanisms of dioxins during the pulp bleaching may be more complex and up to now no documents reported the degradation kinetics. The effects of chlorination conditions 4365
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(5) Renberg, L.; Johansson, N. G.; Blom, C. Destruction of PCDD and PCDF in bleached pulp by chlorine dioxide treatment. Chemosphere 1995, 30 (9), 1805−1811, DOI: 10.1016/00456535(95)00068-J. (6) Toyota, K.; Kaneko, R.; Jikibara, T.; Kawasaki, K.; Maezawa, K.; Terada, K.; Yano, K.; Tanaka, K.; Shigemoto, T. Effect of dioxins reduction with ECF conversion in kraft pulp bleaching mills in Japan. Organhalogen Compd. 2007, 69 (237), 962−965. (7) Torres, A. L.; Roncero, M. B.; Colom, J. F.; Pastor, F. I. J.; Blanco, A.; Vidal, T. Effect of a novel enzyme on fibre morphology during ECF bleaching of oxygen delignified Eucalyptus kraft pulps. Bioresour. Technol. 2000, 74 (2), 135−140, DOI: 10.1016/S09608524(99)00178-9. (8) Thacker, N. P.; Nitnaware, V. C.; Das, S. K.; Devotta, S. Dioxin formation in pulp and paper mills of India. Environ. Sci. Pollut. Res. 2007, 14 (4), 225−226, DOI: 10.1065/espr2007.02.386. (9) Rolf, B.; Christina, J.; Lars-Åke, L.; Yngve, L. Non wood pulping technology present status and future. IPPTA J. 2009, 21 (1), 115−120. (10) Oanh, N. T. K. A comparative study of effluent toxicity for three chlorine-bleached pulp and paper mills in Southeast Asia. Resour. Conserv. Recy. 1996, 18, 87−105, DOI: 10.1016/S0921-3449(96) 01171-8. (11) The People’s Republic of China-National Implementation Plan for the Stockholm Convention on Persistent Organic Pollutants. In Ministry of Environmental Protection of the People’s Republic of China 2007. http://www.pops.int/documents/implementation/nips/ submissions/China_NIP_En.pdf. (12) Dallons, V. J.; Whittemore, R. C.; LaFleur, L. E.; Brunk, R. Study of 2,3,7,8-TCDD and 2,3,7,8-TCDF formation at 23 bleach plants. TAPPI Pulping Conf., [Proc.] 1990, 153−159. (13) Hrutfiord, B. F.; Negri, A. R. Dioxin sources and mechanisms during pulp bleaching. Chemosphere 1992, 25 (1−2), 53−56, DOI: 10.1016/0045-6535(92)90478-A. (14) Berry, R. M.; Luthe, C. E.; Voss, R. H.; Wrist, P. E.; Axegard, P.; Gellerstedt, G.; Lindblad, P.-O.; Pöpke, I. The effects of recent changes in bleached softwood kraft mill technology on organochlorine emissions an international perspective. Pulp Pap. Can. 1991, 92 (6), T155−T165. (15) Yetis, U.; Selcuk, A.; Gokcay, C. F. Reducing chlorinated organics, AOX, in the bleachery effluents of a Turkish pulp and paper plant. Water Sci. Technol. 1996, 34 (10), 97−104. (16) Axegard, P.; Renberg, L. The influence of bleaching chemicals and lignin content on the formation of polychlorinated dioxins and dibenzofurans. Chemosphere 1989, 19 (1−6), 661−668, DOI: 10.1016/0045-6535(89)90387-1. (17) Technologies for Reducing Dioxin in the Manufacture of Bleached Wood Pulp; U.S. Congress, Offoce of Technology Assessment: Washington, DC, 1989. (18) Smeds, A.; Holmbom, B.; Pettersson, C. Chemical-stability of chlorinated components in pulp bleaching liquors. Chemosphere 1994, 28 (5), 881−895, DOI: 10.1016/0045-6535(94)90005-1. (19) Dimmel, D. R.; Riggs, B. K.; Pltts, G.; White, J.; Lucas, S. Formation mechanisms of polychlorinated dibenzo-p-dioxins and dibenzofurans during pulp chlorination. Environ. Sci. Technol. 1993, 27, 2553−2558, DOI: 10.1021/es00048a037. (20) Zheng, M. H.; Bao, Z. C.; Wang, K. O.; Xu, X. B. Levels of PCDDs and PCDFs in the bleached pulp from Chinese pulp and paper industry. Bull. Environ. Contam. Toxicol. 1997, 59 (1), 90−93. (21) Zhang, Q. H.; Xu, Y.; Wu, W. Z.; Xiao, R. M.; Feng, L.; Schramm, K.-W.; Kettrup, A. PCDDs and PCDFs in the wastewater from Chinese pulp and paper industry. Bull. Environ. Contam. Toxicol. 2000, 64 (3), 368−371. (22) Zheng, M. H.; Bao, Z. C.; Zhang, B.; Xu, X. B. Polychlorinated dibenzo-p-dioxins and dibenzofurans in paper making from a pulp mill in China. Chemosphere 2001, 44 (6), 1335−1337, DOI: 10.1016/ S0045-6535(00)00488-4. (23) Fang, L. P.; Zheng, M. H.; Liu, W. B.; Hui, Y. M.; Guo, L. Profile of PCDD in effluents from non-wood pulp and paper mills. Organohalogen Compd. 2009, 71, 3116−3118.
Figure 5. The effects of different chlorination conditions on the formation of 2,3,7,8-TCDD and 2,3,7,8-TCDF from nonwood pulp bleaching with chlorine.
hypochloric acid becomes the dominant compound in chlorine−water system, which causes severe degradation of carbohydrate and thus degrades the paper quality. Moreover, cellulose oxidation intensifies with increasing chlorination temperature, and this also degrades the paper quality. In future studies, a series of feasibility tests for the recommended BAT approaches for minimizing the formation of dioxins from Chinese nonwood pulp and paper mills should be conducted.
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ASSOCIATED CONTENT
S Supporting Information *
Tables on concentrations of di- up to octa- CDD/Fs homologue groups and seventeen 2,3,7,8-chlrine substituted PCDD/Fs in pulp during different chlorination conditions, and ANOVA results for the data fitting. Figures on the variations of 2,3,7,8-TCDF concentrations and the ratios of PCDFs to PCDDs with available chlorine dosage, on the variation of diCDD/F concentration with pH value. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Phone/fax: +86 411 84379562; e-mail: [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the National Natural Science Foundation of China (Grant No.21037003). REFERENCES
(1) Standardized Toolkit for Identification and Quantification of Dioxin and Furan Releases, 2.1 ed.; UNEP Chemicals Geneva: Switzerland, 2005. (2) Swanson, S. E.; Rappe, C.; Malmstrom, J.; Kringstad, K. P. Emissions of PCDDs and PCDFs from the pulp industry. Chemosphere 1988, 17 (4), 681−691, DOI: 10.1016/0045-6535(88)90248-2. (3) Amendola, G.; Barna, D.; Blosser, R.; Lafleur, L.; Mcbride, A.; Thomas, F.; Tiernan, T.; Whittemore, R. The occurrence and fate of PCDDs and PCDFs in five bleached kraft pulp and paper-mills. Chemosphere 1989, 18 (1−6), 1181−1188, DOI: 10.1016/00456535(89)90253-1. (4) Wang, X. L.; Ni, Y. W.; Zhang, H. J.; Zhang, X. P.; Chen, J. P. Formation and emission of PCDD/Fs in chinese non-wood pulp and paper mills. Environ. Sci. Technol. 2012, 46 (21), 12234−12240, DOI: 10.1021/es303373b. 4366
dx.doi.org/10.1021/es404347h | Environ. Sci. Technol. 2014, 48, 4361−4367
Environmental Science & Technology
Article
(24) Pulps-Determination of Kappa Number ISO 302:2004; International Organization for Standardization, 2004. (25) Kurzawa, J.; Kurzawa, Z.; Janowicz, K. Determination of microgram and nanogram amounts of active chlorine in water by iodine azide reaction induced by thiosulfate or thioammeline. Anal. Chim. Acta 1991, 252 (1−2), 127−132, DOI: 10.1016/0003-2670(91) 87206-M. (26) Kubala, S. W.; Tilotta, D. C.; Busch, M. A.; Busch, K. W. Determination of chloride and available chlorine in aqueous samples by flame infrared-emission. Anal. Chem. 1989, 61 (24), 2785−2791, DOI: 10.1021/ac00199a020. (27) Kotiaho, T.; Shay, B. J.; Cooks, R. G.; Eberlin, M. N. Electrophilic aromatic Cl+ addition and Co.+ substitution in the gasphase. J. Am. Chem. Soc. 1993, 115 (3), 1004−1014, DOI: 10.1021/ ja00056a027. (28) Sarkanen, K. V. The chemistry of delignification in pulp bleaching. Pure. Appl. Chem. 1962, 5, 219−232. (29) Hrutfiord, B. F.; Negri, A. R. Chlorinated dibenzofurans and dibenzodioxins from lignin models. Tappi J. 1992, 75, 129−134. (30) Luthe, C. E.; Berry, R. M.; Voss, R. H. Chlorinated dioxins in the production of bleached kraft pulp resulting from the use of sawmill wood chips contaminated with polychlorinated phenols. TAPPI Eng., Environ. Conf. 1992, 859−867. (31) Luthe, C. E.; Berry, R. M.; Voss, R. H. Formation of chlorinated dioxins during production of bleached kraft pulp from sawmill chips contaminated with polychlorinated phenols. Tappi J. 1993, 76 (3), 63−68. (32) Luthe, C. E.; Dlvd, J. s. Octachlorinated dioxin in pulps and effluents: Where does it come from? Chemosphere 1996, 32 (12), 2409−2425, DOI: 10.1016/0045-6535(96)00138-5. (33) Alkan, M.; Oktay, M.; Kocakerim, M. M.; Copur, M. Solubility of chlorine in aqueous hydrochloric acid solutions. J Hazard Mater. 2005, 119 (1−3), 13−18, DOI: 10.1016/j. (34) Cherney, D. P.; Duirk, S. E.; Tarr, J. C.; Collette, T. W. Monitoring the speciation of aqueous free chlorine from pH 1 to 12 with Raman spectroscopy to determine the identity of the potent lowpH oxidant. Appl. Spectrosc. 2006, 60 (7), 764−772, DOI: 10.1366/ 000370206777887062. (35) Zhan, H. Y.; Liu, Q. J.; Chen, J. C.; N., Y. R.; Han, Q.; Zhai, H. M. Pulping Principle and Engineering, 3rd ed.; China Light Industry Press: China, Beijing, 2011 (in Chinese). (36) Xu, F.; Yu, W.; Gao, R.; Zhou, Q.; Zhang, Q.; Wang, W. Dioxin formations from the radical/radical cross-condensation of phenoxy radicals with 2-chlorophenoxy radicals and 2,4,6-trichlorophenoxy radicals. Environ. Sci. Technol. 2010, 44, 6745−6751, DOI: 10.1021/ es101794v. (37) Wagman, N. Trace analysis of PCDDs and PCDFs in unbleached and bleached pulp samples. Organohalogen Compd. 1995, 23, 337−382. (38) Kitunen, V. H.; Salkinoja-Salonen, M. S. Occurrence of PCDDs and PCDFs in pulp and board products. Chemosphere 1989, 19, 721− 726, DOI: 10.1016/0045-6535(89)90397-4. (39) Beck, H.; Drop, A.; Eckart, K.; Mathar, W.; Wittkowski, R. PCDDs, PCDFs and related compounds in paper products. Chemosphere 1989, 19, 655−660, DOI: 10.1016/0045-6535(89)90386-X. (40) UNEP. Guidelines on Best Available Techniques and Provisional Guidance on Best Environmental Practices, Relevant to Article 5 and Annex C of the Stockholm Convention on Persistent Organic Pollutants, Production of Pulp Using Elemental Chlorine or Chemical Generating Elemental Chlorine; Secretariat of the Stockholm Convention on Persistent Organic Pollutants, UNEP: Switzerland, 2008.
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