ERVirOR.
sci. TeChROl. 1993,27, 1334-1339
Reactions of Dibenzo-pdioxin and Dibenzofuran with Electrophilic Reagents Lucinda 6. Sonnenberg' and Donald R. Dimmei
Institute of Paper Science and Technology, 500 10th Street NW, Atlanta, Georgia 30318 The levels of polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) formed during chlorine bleaching of pulp may be reduced if precursors such as dibenzop-dioxin (DBD) and dibenzofuran (DBF) are destroyed prior to chlorination. Research was undertaken to find reagents that react effectively with DBD and DBF and to better understand the chemistry of dioxin precursors. The reactivities of DBD and DBF toward ozone, nitrogen dioxide, hydrogen peroxide, oxygen under ultraviolet light, and nitric acid were examined. The precursors reacted with radical, charged, and neutral electrophiles. In many cases, the degradability of the precursors depended on whether they were dissolvedin water or adsorbed to ligninfree fibers. Ozone was the most effective reagent tested for destroying DBD and DBF; nitrogen dioxide was the next most effective. Introduction The formation of polychlorinated dibenzo-p-dioxins (PCDD) and polychlorinated dibenzofurans (PCDF) during chlorine bleaching of pulp has been the target of extensive research by the paper and pulp industry. Several means of reducing their occurrence have been investigated, including the use of alternative bleaching techniques (1, 2) and nonchlorine bleaching reagents (3, 4). Another potential method of minimizing PCDD/F formation is by pretreatment of the pulp to remove PCDD/F precursors prior to chlorination. Dibenzo-p-dioxin (DBD) and dibenzofuran (DBF) are present in pulp (5). There is evidence that chlorination of these compounds leads to a significant portion of the observed PCDD/F in effluent and pulp (6). The major PCDD/F congeners that are formed are shown in reactions 1 and 2. Molar yields of approximately 6% 2,3,7,8tetrachlorodibenzo-p-dioxin(TCDD) and 1% 2,3,7,8tetrachlorodibenzofuran (TCDF) have been reported when DBD- and DBF-spiked pulp is chlorinated; the extent of the disappearance of DBD and DBF was not addressed (3).
react with DBD and DBF in the presence of lignin so that subsequent chlorine bleaching will not produce PCDD/F. There are two potential sites of reaction in DBD and DBF: the aromatic ring and the electron-rich ether oxygens. The ring may become oxidized (7,8),cleaved (91, or substituted (IO) when exposed to electrophilic reagents such as chlorine, ozone, and nitrogen dioxide. Although the reactivity of phenolic and alkoxy1 oxygen toward many electrophilic oxidants has been well-investigated (7,1O,II), there is less information about chemical activity of the oxygen in diary1 ethers. The goal of this research was to find an effective reagent for destroying DBD and DBF and to better understand the chemistry of these PCDD/F precursors. Several compounds were examined, including radical, charged, and neutral electrophiles. Specifically,ozone, nitrogen dioxide, hydrogen peroxide, oxygen under ultraviolet light, and nitric acid were evaluated. The reactions were first conducted in an aqueous solution to determine appropriate reaction conditions. The precursors were then adsorbed to cellulose fibers (cotton linters) and treated. In this paper, the term adsorption includes any loose, reversible association of the precursors with the linters. The cotton linters provided the heterogeneous conditions of pulp bleaching without competing lignin reactions. Experimental Section
The chlorination of parts-per-billion levels of DBD/F in the presence of percentage levels of lignin (a phenolic, macromolecular constituent of pulp) suggests that DBD/F are highly reactive toward chlorine. It is possible that electrophilic reagents other than chlorine will selectively
Solution Reactions. Aqueous reactions were conducted in 50-mL serum bottles containing 30 mL of solution to which approximately 100 pg each of DBD and DBF were added. After the reaction, approximately 100 pg of the internal standard, 2-methoxybiphenyl, was mixed with the solution. In some cases the internal standard was added later in the reaction workup in order to minimize its degradation; in these cases the DBD and DBF values were not recovery corrected and are noted. The solution was extracted with 2 mL of hexane. Adsorbed-Phase Reactions. The cotton linters, were homogenized in a mixer, sequentially rinsed with water, methanol, acetone, and methylene chloride, and then Soxhlet-extracted overnight in hexane. The linters were soaked for approximately 1 h in a methylene chloride solution of DBD and DBF at levels approximately two times the desired spike level. The methylene chloride was removed by rotoevaporation. Approximately half of the DBD and DBF remained on the linters, as determined by control samples conducted in each study. Control samples consisted of spiked linters that underwent all of the procedural steps except exposure to the reagent. The control values were used to determine initial spike values. Blank samples were also conducted for each reagent studied. Duplicates or triplicates were conducted for each reagent. Ten-gram samples of linters were used for each experiment. After the reaction, the linters were filtered (when appropriate) and usually washed with distilled deionized water. The filtrate was spiked with about 50 or 100 pg of internal standard, mixed, and then extracted three times
1334 Environ. Scl. Technol., Voi. 27, No. 7, 1993
0013-936X/93/0927-1334$04.00/0
DBD
2,3,7,8TCDD
-2 DBF
2,3,7,S TCDF
1,2,7,8TCDF
@ 1993 American Chemlcal Society
with hexane. The linters were spiked with internal standard, wetted with acetone, and Soxhlet-extracted overnight with hexane. The extracts were rotoevaporated to approximately 1mL. For many extracts, approximately 100 pg of recovery standard, diphenyl ether, was added to the concentrated extracts to monitor the recovery of the internal standard. Analyses. Quantitation of the precursors was carried out by gas chromatography with flame ionization detection using a 30-m DB-5 column. Typical operating conditions were as follows: splitless injection of 1 pL; injector temperature, 245 "C; detector temperature, 300 "C; oven temperature, 90 OC (1min), then 10 OC/min to 295 "C; and helium carrier gas. Detection limits were in the partsper-billion range. Recoveries of the internal standard varied from 75 to 99 % for the linters and from 67 to 98% for solutions. Recoveries from combined extracts of nitric acid samples varied from 80 to 109% . Reagents. (a) Ozone. A 500 pM ozone stock solution was prepared by bubbling an 02/03 stream from a Welsbach T 816 ozonator (at 2.5 L/min 02/08 flow rate, 115 V) into cold, unbuffered water, which was diluted to prepare solutions for aqueous reactions. After 15-min reaction times, the solutions were purged with N2 to remove residual ozone. In the cotton linters experiments, 500mL of a 500pM stock solution was added to the spiked linters; reaction times were 15 min. In the pH studies, five solutions with pHs varying from 2 to 12 were prepared. The room temperature solutions were bubbled with an 02/03 stream (4 L/min 02/03 flow rate). Final pHs were taken after the residual ozone was purged. The ozone concentrations were determined by standard iodometric titration with thiosulfate. (b) Nitrogen Dioxide. Gaseous nitrogen dioxide was injected through septa into DBD/F solutions in sealed bottles using a gas-tight syringe. The bottles were shaken during 30-min reaction times. The fleeting,light-blue color development upon injection suggests that there was or other immediate transformation of the NO2 to "02 nitrogen oxides. The mixtures were quenched with 2 g of NaHS03. Dry, spiked linters were placed in a flask and brought under vacuum; approximately 200 mg of NO2 was then added. After 5 min of treatment, the linters were exposed to air so that any NO present would be converted to NO2. After 10 rnin of reaction time a t room temperature, the linters were stripped of any remaining NO2 by repeated application of vacuum and N2 purging. A single replicate was treated for 40 min. Dry linters were used so that maximum concentrations of NO2 could be obtained and without unquantified transformation of NO2 to "02 "03.
(c) Hydrogen Peroxide. Two attempts were made to determine the effect of pH on the degradation of aqueousphase DBD and DBF by hydrogen peroxide. In the first, 3 M peroxide solutions were pH-adjusted with 1 M HC1 and 0.3 M NaOH and after 1h quenched with a minimum of 3 g of NaHS03. In the second experiment, solutions at various pHs without peroxide were prepared in addition to the aqueous hydrogen peroxide samples, to determine the effects of pH alone. Quenching took 30 to 45 min. The role of reactive intermediate radicals formed during quenching to the peroxide reaction was investigated; however, the results were inconclusive due to high vari-
ability. The participation of reactive intermediates from the quenching agents in the peroxide chemistry cannot be ruled out. In the procedures utilizing Fenton's reagent, 80 mL of a solution of 0.23 M H2S04 and 0.26 M FeSOd was spiked with approximately 200 pg of each precursor; 22 mL of a 0.9 M H202 solution was quickly added. One control consisted of sulfuric acid, ferrous sulfate, and water, and a second control consisted of sulfuric acid and hydrogen peroxide. After 15 rnin the solutions were extracted with hexane. Internal standard was added to the hexane extracts; therefore, the results were not recovery corrected. Fifty milliliters of 5.9 M H202 was added to the linters and allowed to react for 1h and then the slurry was filtered. Only the aqueous filtrate was quenched with sodium bisulfite because severe interferences occurred when the linter slurry was quenched. Consequently, after treatment with peroxide, the spiked linters were washed three times with distilled, deionized water and quickly extracted; the wash water was added to the aqueous filtrate prior to extraction. (d) Oxygen/Ultraviolet Light. A Rayonet photochemical reactor equipped with a merry-go-round was used in the photochemical studies. Preliminary studies indicated that more degradation occurred under conditions of pH = 7 (versuspH = 111,oxygensaturation (versusanoxic), and in the presence of UV light (versus the dark). Lamps with 3500-A wavelength maximum were used in these preliminary studies. The degradation of DBD and DBF with time was monitored in neutral pH, 02-saturated solutions; UV light lamps. One bottle was was provided by 30004 ,A, maintained in the darkness while the remaining bottles were placed in the photoreactor. Bottles were removed at 10 min, 1h, 2 h, and 3 h and placed in the dark until all of the samples were ready for extraction. In the sensitizer experiments, fluorescein diacetate, anthracene, and rose bengal were added to oxygenated solutions of 3 ppm of each precursor and irradiated for 3 = 3000 A. Three of the solutions h with UV lamps with A, contained 1 X 103 M of each photosensitizer (however, solubilities of the fluorescein diacetate and anthracene in water are limited to about 7 and 0.4 pM (121,respectively). In the OdUV linters experiments, 200 mL of oxygensaturated water (at 20-25 "C) was added to 10 g of linters (i.e., 5% consistency), and the slurry was mixed as it was irradiated for 3 h. Temperature was not controlled; the flask became warm. (e)Nitric Acid. In the cotton linters experiments, 100 mL of a 4.06 M "03 solution was added to 10g of spiked cotton linters. One-hour reaction times were used for aqueous and cotton linters conditions. The filtrate and linters extracts were combined, concentrated, spiked with recovery standard, and analyzed as described above. The concentrations of the acids used were the maximum concentrations the cotton linters could withstand without linter degradation.
Results and Discussion
Ozone. The precursors degraded at ozone concentrations considerably lower than the concentrations required by the other reagents (micromolar versus molar concentrations). Some DBD and DBF remained after treatment with 500 p M ozone in the aqueous experiments Environ. Sci. Technol., Vol. 27, No. 7, 1993 1535
90
70 100 UM
240 uM
%DBD/F 6o Remaining 5o
0
50
150
250
350
2
550
450
Ozone Concentration (pM)
Table I. Reactions of DBD and DBF in the Presence of Linters pmol after reaction
av % remaining
reagent
DBD
DBF
DBD
DBF
DBD
DBF
03
0.57 0.57 3.15 3.15 1.05 1.05 1.37
0.49 0.50 2.17 2.17
0.05 0.05 0.02 NDc 1.02 1.04 NDc
0.06 0.06
18"
112"
1.44 1.46
