Nonpurgeable Total Organic Halide Analysis and the Characterization of River Water Quality Adjacent to the Discharge from a Kraft Mill Geoffrey B. Watts’lt and Bruce R. Locke
Department of Chemical Engineering, Florida A&M University and Florida State University, Tallahassee, Florida 32316-2 175 Examination of river water quality downstream of the outfall from a kraft mill has shown that the mill effluent water (bleach liquor) contains significant amounts of chlorinated organic compounds. These compounds, which appear to be polymeric in nature, may be indirectly detected in the river water using the nonpurgeable total organic halide (NPTOX) test. Approximately 30% of the chlorinated organic content of the river is contained in an acid insoluble material (Fenextract), which was separated from the other components in a river-water sample by pH adjustment. Fenextract appears to be a macromolecular chlorinated thiolignin that is formed in the kraft bleaching process when chlorine dioxide is the primary oxidizing agent. Chemical and spectroscopic characterization of Fenextract has allowed some additional insight to be gained into the reaction mechanism occurring during the chlorine dioxide bleaching process. The other 70% of the organic chlorine content in the river, which cannot be precipitated from the river by pH adjustment, was also attributed to high molecular weight chlorinated lignin substances.
Two types of cellulose are manufactured at the kraft mill in this study, “dissolving pulp” and “southern bleach kraft”, each requiring a different bleaching treatment. Dissolving pulp uses chlorine, hypochlorite, and chlorine dioxide. The southern bleach kraft uses only chlorine dioxide. Both products are extracted with sodium hydroxide after bleaching. The combined bleach and alkaline extraction liquors represent, after treatment in an aeration lagoon, the major portion of the effluent discharged from the kraft mill to the Fenholloway River. To determine whether the Fenholloway River was the source of the apparent residential well pollution, the chemical quality of the river water had to be evaluated as completely as possible. During the course of the evaluation, a brown-colored organic solid was isolated from the river water and partially characterized. The solid, which was termed “Fenextract”, appears to be a high molecular weight chlorinated thiolignin that is derived from the kraft mill bleaching process. To our knowledge,such a substance has never been previously described in the scientific literature. Materials and Methods
Introduction
The disposal of spent liquors from kraft mills into rivers, lakes, and oceans continues to be a worldwide environmental problem. Available information on the chemical composition of spent bleach liquors has increased in recent years (1-6).Previous work has documented the effects of kraft mill discharges on water quality and aquatic biota in the receiving lakes and rivers. The motivation for the present study was the need to identify the sources of contamination of residential drinking water wells in the vicinity of the Fenholloway River in Taylor County, Florida. The Fenholloway River is a low-flow stream, in which a major portion of water downstream of a kraft mill comes from the output of the kraft mill. Prior to establishing any relationship between the well contamination and the river, it was necessary to characterize the river water quality and to find unique chemical signatures in the water. These are the goals of the present study. A forthcoming study will address the issues of groundwater characterization.
* Author to whom all correspondence should be addressed.
Present address: GeoSolutions Inc., P.O. Box 7638, Tallahassee,
FL 32314. 0013-936X/93/0927-2311$04.00/0
0 1993 American Chemical Society
The measurements in the present study were obtained at two sampling stations on the Fenholloway River; (1)5 mi upstream of the kraft mill and (2) about 1000 f t downstream from the kraft mill effluent outfall. Water Quality Analyses. Total organic carbon (TOC) was measured using an organic carbon analyzer (7). Purgeable organic compounds and extractable organic compounds were measured using EPA methods 624 and 625, respectively (8). A Hewlett-Packard particle beamreverse phase-liquid chromatography/mass spectrometer was used to analyze a sample of river water from station 2. The nonpurgeable total organic halide (NPTOX) analysis used m this study is a modification of EPA method 9020 (9) for total organic halogen (TOX) in which the water sample was first purged for 30 min with ultrapure helium or carbon dioxide gas to remove volatile chlorinated organics (e.g., chloroform). This analysis has a method detection limit (MDL) of 10 pg/L NPTOX expressed as chlorine. Color measurements were made on a river-water sample and a sample of the supernatant liquid after acidification and isolation of the lignin component. A visual comparison method (EPA method 110.2, 10) was used. Envlron. Sci. Technol., Vol. 27,
No. 12, 1993
2811
Preparation of Fenextract. A sample of Fenholloway River water taken at station 2 was treated according to the following scheme. The sample was filtered through a glass fiber filter to remove river debris, and while stirring, the pH was adjusted to approximately 2 with concentrated sulfuric acid. After the mixture was heated to a temperature of about 90 "C, stirring was stopped and the solution cooled, thus allowing the lignin precipitate to settle out. The supernatant liquid was decanted and the residue transferred to a centrifuge tube in which it was centrifuged at 3600 rpm for about 1h. The supernatant liquid from this process was removed by decanting and pipeting, after which the precipitate was suspended in 0.001 M sulffiric acid, stirred, and recentrifuged. This precipitate was then redissolved in 0.01 M sodium hydroxide solution, and the above acidification procedure was repeated to obtain reprecipitated lignin product. After the acid wash and centrifuging steps were repeated three more times, the dark brown residue was given a final wash with deionized water followed by centrifuging to effect the separation of the wash water from the lignin product. The amorphous brown lignin residue was dried overnight in an overn at about 60 "C. A sample of the supernatant was then subjected to ultrafiltration using the vacuum technique described by Sjostrom (12). One liter of the supernatant was concentrated through an immersible-CX ultrafilter (Millipore Corp., Bedford, MA) having a 30 000-Da nominal molecular weight limit (NMWL). The filtration, which was done in a vigorously stirred vessel to reduce membrane clogging, was continued until the retentate volume reached about 20 mL. Then, 100 mL of deionized water was added to wash the filter, and the filtration was continued until the retentate volume was again reduced to 20 mL. This step was repeated once more, and then the retentate plus any washings were transferred to a sample bottle for NPTOX analysis. The filtrate from the above step was then filtered using the same procedure; in this case, though, a 10 000 NMWL ultrafilter was used. Analysis of Fenextract. A sodium hydroxide-sodium carbonate solvent was used for light scattering molecular weight determination using the Zimm plot analysis (22). This solution was prepared by dissolving 0.35 g of NaOH and 1.75 g of NazC03 into 500 mL of HPLC-grade water. The solvent was then filtered through a 0.02-~m filter prior to use. For Zimm plot analysis, a stock solution of Fenextract was prepared with the above solvent at a concentration of 1.00 x 10-4 g/mL. The stock solution was filtered through a 0.45-mm filter, and three dilutions were made from it. Four concentrations were measured for the Zimm plots: 10, 7.57,4.99, and 2.50 X lo4 g/mL. The dn/dc determination was made on a Wyatt/Optilab 903 interferometric refractometer using a 0.2-mm cell. A stock solution of 2.02 X 10-3 g/mL was prepared from the above hydroxide-carbonate solution, and measurements were made in duplicate at seven concentrations in the range 0.25-1.75 X 10-3 g/mL. Zimm plots were generated on a Dawn Model F laser photometer. Because of the low solubility of Fenextract in the majority of solvents tested, an infrared spectrum of the solid was obtained using a horizontal attenuated total reflectance (ATR) accessory in conjunction with a Nicolet Model 205 Fourier transform infrared (FT-IR) spectrometer. A Fenextract sample was prepared by grinding it to a fine powder, and then spreading it evenly in the well of 2312
Environ. Scl. Technol., Vol. 27, No. 12, 1993
Table I. Inorganic Parameters parameter temperature ("C) conductance (pWcm) PH chloride (mg/L) sulfate (mg/L) sodium (mg/L) iron (mg/L) manganese (mg/L)
upstream station 1 26.2 72 6.4 10
35 2.6 0.59 0.008
downstream station 2 25.2 1780 7.1 590
260 490 0.53 0.19
the ATR powder device such that the crystal surface was completely covered. The sample was held in place against the crystal surface using the Clamper (Spectra-Tech), which allows an even pressure to be applied to the crystal. A background spectrum was first recorded by collecting 100 scans of the ATR accessory without any sample. The sample was then loaded into the cell: and a horizontal ATR spectrum of it was obtained using the same experimental conditions. The fluorescence emission spectrum of Fenextract was measured using a Hitachi MPF-PA fluorescence spectrophotometer. An emission spectrum was generated by irradiating the sample with light at a fixed wavelength (usually ,A, for adsorption) and scanning the emission (fluorescence) spectrum. In this case six different excitation wavelengths (Aex) were utilized, including 240,250, 260,270,280, and 290 nm. The sample of Fenextract, in an aqueous solution at a neutral pH, was irradiated at room temperature in a standard 1-cm quartz cell. The visible and ultraviolet spectrum of Fenextract over a range of 200-700 nm was measured on a Perkin Elmer Lambda 5 UV/vis spectrophotometer. Limited solubility of Fenextract in dimethyl sulfoxided6 and acetone-& solvents prevented generation of a useful liquid-phase '3C-NMR spectrum of the solid. However, a Fenextract solid-state 13C-NMRspectrum was obtained using the cross polarization-magic angle spinning (CPMAS) technique. The spectrum was obtained on a Bruker SY-2OOWP NMR spectrometer at 50 MHz. Results River-WaterCharacterization. Analytical results for water samples from monitoring stations 1and 2 are given in Table I for field parameters (temperature, pH, and specific conductance) and inorganic moities including chloride, sulfate, sodium, iron, and manganese. The field parameters indicate a large conductance increase; however, the major focus of this study is on the organic chemical quality of the river water in order to determine a unique chemical tracer to identify the kraft mill effluent. TOC is a more convenient and direct expression of total organic content than BOD or COD because organic carbon that may be present in the plant wastewater may not respond either to biological or chemical degradation. Samples taken from the river at monitoring stations 1and 2 gave a TOC of 73 mg of C/L and 140 mg of C/L, respectively. The elevated level of TOC measured upstream of the plant is due to naturally occurring organic humic substances in the water. According to Malcom (231, the dissolved organic carbon content of organically colored stream waters is extremely variable, ranging from 5 mg of C/L to greater than 50 mg of C/L. The TOC value recorded at monitoring station 2 can be considered representative
Table 11. Extractable Organic Componentss
extractable organic
concentration (pg/L)
2,4-dichlorophenol 8 2,4,64richlorophenol 24 2,3,4,6-tetrachlorophenol 6 bis(2-ethyl hexy1)phthalate 26 sulfonyl bismethaneb 30 40 dimethylcyclohexeneb dimethyl trisulfideb 5 dimethylhexadieneb 8 trichlorodifluoroethaneb 80 methoxyphenylpropanoneb 10 4-hydroxy-3-methoxyben~aldehyde~ 10 tetrachloromethoxyphenolb 5 hexadecanoic acidb 10 unidentified components (no.) 300 (7) a Water sample taken from monitoring station 2. b Tentatively identified compound.
of the wastewater organic carbon content since almost all riverflow at this point is attributable to the effluent. No unidentified purgeable organic compounds were reported in the sample from station 2. The only volatile organic component detected was chloroform at a concentration of 39pg/L. This result is consistent with the vigorous aeration that the effluent undergoes prior to discharge. The data in Table I1indicate that few extractable organic compounds were detected in a river-water sample from monitoring station 2; the total of both identified and tentatively identified compounds amounts to significantly less than 0.5% of the TOC measured in the river. No discernible peaks other than two internal standards were detected by PB-LC/MS after a direct aqueous injection of river water (from monitoring station 2) after filtration through a 0.2-pm filter. Extraction of the riverwater sample with methylene chloride after pH adjustment produced acid and base-neutral fractions that were analyzed by PB-LC/MS using different gradient elution programs. While several small peaks were observed in both fractions, none of the peaks could be identified. Repeated attempts to improve the data, including positive ion chemical ionization, did not further assist the identification. Nonpurgeable Total Organic Halogen (NPTOX) Analysis. It was apparent from the preceding testing that the compound-specific analytical techniques were not adequate to address this particular effluent. Thus, a sample of river water was analyzed using a “global” parameter, nonpurgeable total organic halogen (NPTOX). Rather than identifying individual halogen-containing compounds, this method measured the total organically bound halogen present. The results for the Fenholloway River samples give NPTOX of 90 pg/L for the upstream sample and 15 000 and 16 100 mg/L for the downstream sample. Fenextract Measurement. Having established that the river downstream of the kraft mill discharge contains elevated amounts of nonvolatile organic chlorine components, which are probably of relatively high molecular weight since the components are not eluted through a chromatographic column, further attempts were made to chemically characterize this material. The task was made somewhat easier by an observation during field preservation (adjustment of the pH with nitric acid to a value of 2) of a river-water sample that was intended for “metals” analysis. A color change was noted, which on closer
Table 111. NPTOX/Molecular Weight Distribution
sample Fenholloway River water supernatant Fenextract (by difference) supernatant ultrafiltration rententate > 30 000 NMWL 30 000 > rententate > 10 000 NMWL filtrate < 10 000 NMWL
NPTOX (mg/L) 16.1 11.1
5.0
0.43 0.37 7.2
examination proved to be due to the precipitation of a dark brown solid from the water. This solid was either finely dispersed or colloidal in nature because it was difficult to filter. This solid was assumed to be a lignin derivative. About 70 kg of organic material for each 1000 kg of pulp is dissolved from the pulp into the bleaching liquor, and of this amount approximately 70% is lignin and 27% is polysaccharide (8). A method described by Marton (14) for separating kraft lignin from black liquor was utilized to isolate the lignin derivative from the river water. The yield of lignin extracted from the Fenholloway River sample, hereinafter referred to as “Fenextract”, was approximately 110mg/L of river water. It, thus represents a major organic component in the river water. Because it was apparent that a substantial portion of the NPTOX in the river was associated with high molecular weight substances, an attempt was made to measure the molecular mass distribution of the NPTOX in the river. A sample of river water was obtained at monitoring station 2 and was treated according to the procedure described in the Materials and Methods section to remove the precipitable lignin component. After the supernatant liquid was filtered through a 0.45-mm filter, both the original river sample and the supernatant were analyzed for NPTOX (Table 111). The organically bound chlorine associated with Fenextract (5 mg/L) is the difference between the NPTOX value for the river water (16.1 mg/ L) and the supernatant (11.1 mg/L). Hence, Fenextract accounts for about 30% of the total NPTOX. Both the filtrate and the retentate from this experiment were analyzed for NPTOX. The sum of the retentate and the filtrate NPTOX values indicates that only about 70 % of the original NPTOX was recovered. This result is due presumably to irreversible sorption of NPTOX on the filter. Sjostrom (11)has noted that recovery was inversely related to molecular mass (i.e., the lower the molecular mass, the higher the recovery). Although, due to sorption problems, it was not possible to obtain an accurate molecular weight distribution for NPTOX, it is clear that the NPTOX is not evenly dispersed throughout the molecular weight range. About 30% of the total NPTOX in the river-water sample is incorporated into the high molecular weight fraction [e.g., in Fenextract] while approximately 45% of the NPTOX resides in the low molecular weight fraction (1000-10 000 Da). Presumably, the intermediate and lower molecular weight NPTOX fractions are also associated with ligninrelated substances since only aromatically bound chlorine normally survives the alkaline extraction process. The other major organic component in the bleach liquor (e.g., hemicellulose) is not susceptible to chlorination, It was apparent that Fenextract is a major contributor to the color measured in the river. A river-water sample Environ. Sci. Technol., Voi. 27, No. 12, 1993 2313
taken at monitoring station 2 had a color of 2200 PCU, while the same sample after removal of the Fenextract component had a color measurement of 750 PCU. The difference, 1450PCU or 66 % of the total color in the river, was attributable to the Fenextract lignin. The hemicellulosic component of the river water was not isolated or characterized because (1)the polysaccharides would be difficult to separate from the river water due to their high solubility and relatively low molecular weight; (2) there are no sensitive analytical methods or indicator parameters available to detect the presence of hemicellulosic materials in water; and (3) perhaps most importantly, these substances are readily biodegradable, ultimately to carbon dioxide. Hemicellulose-type compounds will be decomposed under the aerobic/anaerobic conditions that exist in the river. Characterization of Fenextract. Fenextract, like kraft lignin obtained from black liquor, appears to be highly soluble in alkaline solution. At high enough concentrations (e.g., such as in the Fenholloway River), Fenextract is readily precipitated from neutral or basic solution by acidification. When present in low concentrations in basic or neutral solution, Fenextract cannot be precipitated by pH adjustment. In this case, the only visual change that occurs upon acidification is a decrease in intensity of the solution's yellow color. Fenextract is soluble in N-methylpyrolidone but is insoluble or only slightly soluble in ether, alcohol, hydrocarbon, and chlorinated hydrocarbon solvents. The solubility behavior of Fenextract differs from that of kraft lignin in that kraft lignin is soluble in dioxane and pyridine and partially soluble in methyl alcohol (15). An elemental analysis was conducted on Fenextract to determine the distribution of the major elements (e.g., carbon, hydrogen, and oxygen) in the material. Analytical results were obtained by combusting the sample followed by quantification with an element-specific detector. The elemental composition of Fenextract consisted of 54.27 % carbon, 5.20% hydrogen, 29.12 % oxygen, 1.32 % nitrogen, 3.41% sulfur, and 3.50% chlorine. Note that the Fenextract sample contains approximately equivalent amounts of chlorine and sulfur, thus indicating that Fenextract is a chlorothiolignin. The six elements that were reported in this analysis account for 96.8 % of the mass of the sample. The measured weight-average molecular weight by light scattering was 1.44 (& 0.03) X 106 Da. The RMS radius was 50 f 3 nm. The radius of gyration obtained for Fenextract is acceptable for a molecule of 1.44 X lo6 molecular weight with an extended conformation (e.g., rod or helix). For a spherical molecule of molecular weight 1.44 X 106, the radius of gyration would be much larger. Since the second virial coefficient (1.11f 0.14 X is positive, no aggregation of the sample is suggested. Figure 1 shows the FT-IR spectrum for powdered Fenextract after background subtraction. The infrared spectrum of Fenextract consists of several broad and unresolved bands for which little structural information can be derived. Tentative band assignments for the major absorption bands in Figure 1though have been made in accordancewith Hergert's (16)empirical scheme for mildly prepared wood lignins. A broad absorption band at approximately 3500 cm-1 is attributed to the 0-H stretching vibration. This band presumably includes contributions from carboxylic, phenolic, and aliphatic OH groups in the chlorothiolignin molecule. The absorption at 1650 2314 Envlron. Scl. Technol., Vol. 27, No. 12, 1993
98
96
t
t
Narinumbor (em-')
Figure 1. Fenextract ATR Infrared spectrum.
A
250
am
154
c
e"ci ;:::
Im
50
L b
Pam Per Million (ppm)
Figure 2. Fenextract solld-state l9C CP/MAS NMR.
cm-l is assigned to a carbonyl (C=O) stretching in parasubstituted aryl ketones while a shoulder on this band at approximately 1700cm-l is also a carbonyl stretching band for unconjugated ketone and carboxyl groups. The peak at 1200 cm-' is attributed to a C-0stretching vibration, and the band at 1055 cm-l is due to a C-0deformation band of secondary alcohols and aliphatic ethers. Previous workers (17-19) have obtained liquid-phase carbon-13 nuclear magnetic resonance (13C-NMR)spectra of lignins and related compounds in deuterated solvents such as dimethyl sulfoxide-d6and acetoned6 or dioxaneds mixed with heavy water. Fenextract was found to be even less soluble in dioxane-dg and dimethyl sulfoxide-de than in the corresponding protonated solvents. Consequently, no useful solution W-NMR spectra of Fenextract could be generated. An attempt was made to circumvent these solubility limitations by measuring a solid-state l3CNMR spectrum of the chlorothiolignin sample using the cross polarization-magic angle spinning (CP-MAS) NMR technique. Figure 2 shows the solid-state l3C CP-MAS NMR spectrum of Fenextract. Band assignments in Figure 2 are mainly given in accordance with Kringstad's (27) scheme. This author has used the observed 13C-NMR chemical shifts of about 60 lignin model compounds to make a detailed analysis of the liquid-phase 13C-NMR spectra of kraft lignins. Fenextract contains significant amounts of aliphatic carbon (15-100 ppm) and aromatic carbon (110-160ppm). Major contributors to the incompletely resolved aromatic resonance bonds in the chlorothiolignin sample are the ring carbons of guaiacol to-methoxyphenol). A strong
Table IV, Carbon Distribution in Fenextract and Indulin AT by W-NMR. carbon type
Fenextract (% )
Indulin AT (% )
aliphatic (15-100 ppm) aromatic (110-160 ppm) carboxyl (172 ppm) carbonyl (196-206 ppm) methoxyl(56-58 ppm)
55 34 5 6 NQ
42 56 2 ND -9
4
ND = not detected; NQ = not quantified.
signal is observed for carboxyl carbon (COOH, 172 ppm) which, based on the chemical shift value, appears to be mainly aliphatic in nature. The methoxyl carbon (OCH3, 56 ppm) band dominates the unresolved methine, methylene, and methyl carbon resonances in the aliphatic carbon region. A broad signal at 190-210 ppm is tentatively assigned to carbonyl carbon (C=O) from aldehyde and ketone groups, and a resonance band at 70-73 ppm is attributed to the a-carbon in 8-aryl ether structures. In comparison to Fenextract, the 13C-NMRspectra of both spruce milled wood lignin and kraft lignins (I7) show aliphatic carbon signals that are apparently much less intense compared to the aromatic carbon signal. Signal integration was not performed by these authors, hence no quantitative estimates of the relative proportions of aliphatic and aromatic carbon in kraft lignin were provided. To confirm this observation, the 13C-NMRspectrum was obtained for a powdered sample of Indulin AT (purified kraft pine lignin, Westvaco Corp., North Charleston, SC) under the same conditions used to generate the Fenextract W N M R spectrum. The spectrum is similar to the liquidphase 1%-NMR spectrum of kraft lignin (17)except for the lower signal resolution. Table IV summarizes the approximate contributions of the different types of carbon atoms in Fenextract and Indulin AT samples. Fenextract contains about 55% aliphatic carbon and 34% aromatic carbon while Indulin AT has 42% and 56% aliphatic and aromatic carbon, respectively. Carboxylcarbon represents about 5 % of the total carbon in Fenextract and 2 % of the carbon in kraft lignin. Carbonyl carbon, which accounts for about 6 % of the carbon in Fenextract, was not detected in the Indulin AT sample. Neither was a carbonyl carbon signal observed in the liquid-phase 13C-NMRspectrum of kraft lignin, a fact that was attributed to the long relaxation times of the carbonyl group (17). Approximately 9% of the carbon in the Indulin AT sample is methoxyl carbon. No estimate was made of the methoxyl carbon content of Fenextract because the NMR signal was not sufficiently resolved. However, a methoxyl carbon content of 2.2% was measured in Fenextract using a different chemical assay technique, i.e., the Zeisel method (20). Figure 3 depicts the UV/visible spectrum of Fenextract that was measured in alkaline solution (pH 10)and in acid solution (pH 3). In comparison to the UV spectra of many lignin preparations (21) there is no maximum at 280 nm; rather, a distinct shoulder extending from about 260 to 285 nm is apparent. In alkaline solution, a small shift to longer wavelengths is observed. This pH dependence, which is also observed for lignin preparations (14, is attributed to both the ionizable hydroxyl and carboxyl groups present in Fenextract. Figure 4 shows the fluorescence spectra of Fenextract that were obtained a t six different excitation wavelengths. A broad, unresolved emission band (with a maximum at
I
e
d
Wavelength (nm)
Flguro 3. Fenextract ultraviolet absorption spectrum.
5.425 4.650
3.875
3.lW 1.325 1.56
0.775
0.W
303
145
390
435
.lBo
525
575
615
660
705
754
Wwelingth
Figure 4. Fenextract fluorescence spectrum.
approximately430 nm) was obtained. The band's intensity increases with increasing excitation wavelength. The quantum yield of the chlorothiolignin sample is low, as evidenced by the appearance of Raman bands at higher Aex.
Discussion There is considerable environmental interest in ascertaining the organic chemical composition of spent chlorination and alkali extraction liquors discharged from pulp mills worldwide (1-5). To date, nearly 300 individual low molecular weight compounds have been identified in spent chlorination/alkali extraction liquor ( I , 5). Ultrafiltration experiments suggest that low molecular mass (MW < 1OOO) materials represent about 25 % of the organically bound chlorine in the combined spent chlorination and alkali extraction liquor (1,5). The results of this study indicate that only a small number of low molecular weight components occur at trace concentrations in the effluent from the subject kraft mill, a facility that uses chlorine dioxide as a primary bleaching agent. Furthermore, from the data in Tables I1 and 111, less than 0.15% of the organically bound chlorine in the effluent is present as low relative molecular mass material. Clearly, the type of oxidizingagent used in the bleaching process affects the chemical composition of the kraft mill effluent. Voss et al. (2) reported that total replacement of chlorine by chlorine dioxide significantly reduces the number of low molecular weight chlorinated phenolic Environ. Scl. Technol., Vol. 27. No. 12. 1993 2315
compounds found in bleach liquor. Substitution of chlorine dioxide also influences the elemental composition of the relatively low molecular mass components by reducing the content of the organically bound chlorine (2). The structures of the high molecular mass substances in spent chlorination liquor are poorly defined mainly due to the difficulties in their isolation from bleach liquor and the complexities of analyzing polymeric materials (1).The finding of the present study that a solid high molecular mass chlorothiolignin (Fenextract) can be isolated from the receiving river water by pH adjustment has, to our knowledge, never been previously described in the scientific literature. Therefore, Fenextract may be a unique byproduct of the manufacture of southern bleach kraft, a process that uses chlorine dioxide bleaching exclusively. The partial chemical characterization of Fenextract in this study allows additional insight into the mechanism of chlorine dioxide bleaching of lignin. Gierer (22) has suggested a mechanism for the kraft pulping process in which the main reaction involves cleavage of the various ether bonds in lignin leading to substantial depolymerization of the lignin. The lower molecular weight lignin degradation products dissolve in the alkaline pulping liquor. Fricke (15)reported the molecular weight of the kraft lignin degradation products to be in the range 14 00050 000 Da depending on the cooking conditions. A different series of reactions takes place when the kraft pulp is bleached. For example, during pulp bleaching with chlorine, chlorine reacts in molecular form with residual lignin by an electrophilic substitution mechanism (23). Oxidation by chlorine and the addition of hydrogen chloride leading to depolymerization of the lignin are the other important chemical reactions that occur (1). Chlorine dioxide oxidation of some lignin model compounds has been studied by Sarkenan et al. (24)and Ishikawa et al. (25). The authors determined that guaiacol derivatives were transformed into hydrogen methyl muconate and quinones. These products were obtained in small yields; the major fractions of the product mixtures consisted of darkly colored unstable materials that were considered to be formed from quinoid products by polymerization. Lindgren (26)has proposed a mechanism for the chlorine dioxide oxidation of some lignin-related phenol model compounds. For kinetic reasons, a radical mechanism was proposed, commencing with the creation of a phenoxy radical (11)(Figure 5) by the chlorine dioxide oxidation of a lignin guaiacol structure (I). Attack of chlorine dioxide occurs at the position (containing a methoxyl group) ortho to the phenolic hydroxyl group, thus forming an intermediate chlorite ester of o-quinol(II1). The chlorite esters are then transformed either into a quinoid product (IV) or a methyl muconate (VI). Formation of methyl muconate-type structures has the overall effect of increasing the relative proportion of aliphatic carbon in the reaction product at the expense of the aromatic carbon. Fenextract's significantly lower ratio of aromatic to aliphatic carbon (as measured by ISC-NMR) compared to its precursor (kraft lignin) plus the chlorothioliginin compound's increased carboxyl and carbonyl group content (Table IV) suggest that muconic acid formation may be the preferred reaction route in the chlorine dioxide oxidation of kraft lignin. The polymeric product mixtures reported by Sarkenan et al. (24)and Ishikawa et al. (25) may, therefore, have been derived from muconic acidtype structures rather than from quinoid products. In2918
Envlron. Scl. Technol., Vol. 27, No. 12, 1993
L
Polymer
0
Q
0
6
L.
COOCHJ
COOH (VI)
L=Lignin
Flgure 5. Chlorine dioxide oxidation of lignin.
creasing the proportion of carboxyl groups in the residual lignin also allows the lignin to dissolve in the chlorine dioxide bleach liquor. Creation of a phenoxy radical by chlorine dioxide oxidation of the phenolic groups in lignin also could explain the large molecular weight of the chlorothiolignin product. These free radicals are highly reactive and can readily dimerize and subsequently polymerize. Conventional environmental analytical techniques, e.g., gas chromatography/mass spectrometry (GC/MS) are of little utility for tracing water contamination resulting from the discharge of effluent from the subject kraft mill. Few low molecular weight organic compounds amenable to analysis by GCIMS were found in the effluent. Those compounds that were detected are present in low concentrations. The NPTOX analysis, a nonspecific test that measures all nonvolatile organically bound chlorine compounds in water, is an inexpensive and sensitive indicator test for tracking the high molecular weight chlorinated lignins discharged from kraft mills into the environment. An NPTOX value of 15-16 mg/L was measured for the wastewater discharged from the subject kraft mill. Given that the NPTOX analysis has a MDL of 10 pg/L, dilution of the kraft mill effluent by a factor of approximately 1500 is allowable while still achieving detection. The observation of low levels of NPTOX in the upstream river-water sample was unexpected. This sampling station is approximately 5 mi upstream of the kraft mill plant, and the water at this point constitutes surface drainage from pristine areas of Taylor County. This NPTOX is probably of natural origin. Grimvall et al. (27) have reported the presence of AOX or adsorbable organic halogen (Le., the sum of purgeable and nonpurgeable halogenated substances) in several unpolluted rivers and peat bogs in Sweden. The authors attribute the formation of AOX to the enzyme-induced chlorination of humic substances in the presence of low concentrations of chloride ion. Asplund et al. (28) have measured the AOX/TOC
ratio in many surface water bodies in areas of Sweden unaffected by industrial discharges. The NPTOX/TOC ratio for the upstream Fenholloway sample (1200 pg/g) is consistent with the range of values (730-8600 pg/g) reported by Asplund et al. (28). An NPTOX/TOC ratio of 112 000 pg/g was calculated for the downstream sample, which is about 100 times larger than the upstream value. The high levels of NPTOX measured in the downstream Fenholloway sample are, therefore, directly related to the wastewater discharge from the kraft mill. Thus, measurement of the NPTOX/TOC ratio in a river-water sample allows organochlorine compounds derived from kraft mills to be distinguished from naturally occurring organochlorine compounds. Conclusions
The chemical characterization of the water quality in the outfall from a kraft mill has lead to a new measurement, termed NPTOX, of the nonpurgeable total organic halide, that can be used to trace the water contamination due to the kraft mill. In addition, an acid-insoluble precipitate, Fenextract, has been isolated that consists of large molecular weight chlorothiolignin derivation. The structure of this molecule gives some indication that the chlorine dioxide oxidation reactions in pulp bleaching lead to aromatic ring cleavage and polymerization of smaller lignin fragments. Acknowledgments
Support from the Florida Department of Environmental Regulation WQ005 Water Quality Assurance Trust Fund for this work is gratefully appreciated. Literature Cited Kringstad, K. P.; Lindstrom, K. Spent Liquors from Pulp Bleaching. Environ. Sci. Technol. 1984, 18, 236.4. Voss, R. H.; Wearing, J. T.; Wong, A. In Advances in the Identificationand Analysis of OrganicPollutantsin Water; Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981. Lindstrom, K.; Nordin, J.; Osterbergin, F. In Advances in the Identification and Analysis of Organic Pollutants in Water; Keith, L. H., Ed.; Ann Arbor Science: Ann Arbor, MI, 1981. Van Loon, W. M.; Boon, J. J.; DeJong, R. J.; deGroot, B. Isolation of MacromoleculesChlorolignosulfonicAcids and LignosulfonicAcids from Pulp Mill Effluents and the River Rhine Using XAD-8 MacroporousResin and Ultrafiltration. Environ. Sci. Technol. 1993, 27, 332. Kringstad, K. P. Environmental Aspects on the Future Developments of Pulp Bleaching. In Wood Processing and Utilization;Kennedy, J. F., Phillips, G. O., Williams, P. A., Eds.; John Wiley and Sons: New York, 1989. National Council of the Paper Industry for Air and Stream Improvement Inc. Pulping Effluents in the Aquatic Environment - Part I: A Review of the Published Literature. NCASI Technical Bulletin No. 572, Oct 1989. Standard Methods. Standard Methods for the Examination of Water and Wastewater,15thed.; AmericanPublic Health Association: Washington, DC, 1980. U.S. EPA. Methods for Organic Chemical Analysis of Municipal and Industrial Wastewaters;EPA 60014-82-057;
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Received for review October 8, 1992. Revised manuscript received March 29, 1993. Accepted June 22,1993." Abstract published in Advance ACS Abstracts,August 15,1993.
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