Environ. Sci. Technol. 2004, 38, 3542-3550
Chemical Characterization and Sorption Capacity Measurements of Degraded Newsprint from a Landfill L I X I A C H E N , † M A R K A . N A N N Y , * ,†,‡ D E T L E F R . U . K N A P P E , * ,§ TRAVIS B. WAGNER,§ AND NOPAWAN RATASUK† School of Civil Engineering and Environmental Science, University of Oklahoma, Carson Engineering Center, 202 West Boyd Street, Room 334, and Sarkeys Energy Center, University of Oklahoma, 100 East Boyd Street, Room 510, Norman, Oklahoma 73019, and Department of Civil, Construction, and Environmental Engineering, North Carolina State University, Campus Box 7908, Raleigh, North Carolina 27695-7908
Newsprint samples collected from 12-16 ft (top layer (TNP)), 20-24 ft (middle layer (MNP)), and 32-36 ft (bottom layer (BNP)) below the surface of the Norman Landfill (NLF) were characterized by infrared (IR) spectroscopy, cross-polarization, magic-angle spinning 13C nuclear magnetic resonance (CP-MAS 13C NMR) spectroscopy, and tetramethylammonium hydroxide (TMAH) thermochemolysis gas chromatography/mass spectrometry (GC/MS). The extent of NLF newsprint degradation was evaluated by comparing the chemical composition of NLF newsprint to that of fresh newsprint (FNP) and newsprint degraded in the laboratory under methanogenic conditions (DNP). The O-alkyl/alkyl, cellulose/lignin, and lignin/resin acid ratios showed that BNP was the most degraded, and that all three NLF newsprint samples were more degraded than DNP. 13C NMR and TMAH thermochemolysis data demonstrated selective enrichment of lignin over cellulose, and TMAH thermochemolysis further exhibited selective enrichment of resin acids over lignin. In addition, the crystallinity of cellulose in NLF newsprint samples was significantly lower relative to that of FNP and DNP as shown by 13C NMR spectra. The yield of lignin monomers from TMAH thermochemolysis suggested that hydroxyl groups were removed from the propyl side chain of lignin during the anaerobic decomposition of newsprint in the NLF. Moreover, the vanillyl acid/aldehyde ratio, which successfully describes aerobic lignin degradation, was not a good indicator of the anaerobic degradation of lignin on the basis of the TMAH data. The toluene sorption capacity increased as the degree of newsprint degradation increased or as the O-alkyl/alkyl ratio of newsprint decreased. The results of this study further verified that the sorbent O-alkyl/ alkyl ratio is useful for predicting sorption capacities of natural organic materials for hydrophobic organic contaminants. * Corresponding authors. Phone: (405)325-4234; fax: (405)3254217; e-mail:
[email protected] (M.A.N.). Phone: (919)515-8791; fax: (919)515-7908; e-mail:
[email protected] (D.R.U.K.). † School of Civil Engineering and Environmental Science, University of Oklahoma. ‡ Sarkeys Energy Center, University of Oklahoma. § North Carolina State University. 3542
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Introduction Many older municipal solid waste (MSW) landfills are a source of groundwater contamination because they (i) lack an engineered liner and (ii) accepted hazardous wastes prior to promulgation of the Resource Conservation and Recovery Act (RCRA) in 1976. To date, factors controlling the fate of co-disposed organic contaminants in MSW landfills are poorly understood. MSW landfills differ from soil and sediment environments because of their high organic carbon content and extraordinary degree of heterogeneity. The change of content, composition, and structure of organic materials during landfill stabilization is a crucial factor, not only to the pollution potential assessment, but also to the reclamation of the landfill. According to the U.S. EPA, paper products, including office paper, newspaper, and cardboard, make up the largest component of MSW in the United States, 38.1% of generated wastes and about 33.2% of wastes after recycling (1). Newsprint, the paper material studied in this research, mainly consists of cellulose (∼48%), hemicellulose (9-18%), lignin (22-25%), and some minor components, such as lipids, protein, and ash (2-5). Several factors contribute to the slow rate of newsprint degradation in landfills, such as moisture limitation, poor shredding, low cellulose/lignin ratio (high lignin content), the manner in which lignin is associated with cellulose/hemicellulose, the lack of inoculum, and the crystallinity of cellulose (2, 6-9). Of these factors, the association between lignin and cellulose/hemicellulose is especially important because either physical blockage or the presence of resistant chemical bonds can hinder the microbial degradation of cellulose and hemicellulose in lignocellulosic materials (2, 3, 10). Although soluble lignin monomers, dimers, and oligomers are readily degraded or mineralized in anaerobic environments (11-17), the degradation of natural lignin polymers does not occur as quickly (15, 18, 19). For example, Dittmar and Lara (20) estimated the halflife of lignin to be ∼150 yr in sulfate-reducing mangrove sediments. Thus the molecular weight of lignin fragments is one important factor that controls the extent and rate of anaerobic degradation (12, 14, 21, 22). Both the content and characteristics of organic carbon in MSW change as waste decomposes in landfills. On one hand, the sorbent organic matter content in waste decreases due to degradation and mineralization. At the same time, preferential decomposition of polar components such as cellulose and hemicellulose over relatively nonpolar components such as lignin and lipids increases the hydrophobic characteristics of the sorbent organic matter during waste decomposition (4, 5). These changes in composition and structural characteristics affect the sorption capacity of MSW as an organic sorbent and as a result influence the bioavailability and transport of hydrophobic organic contaminants (HOCs) in MSW landfills. The objectives of this study were (i) to characterize the structural and compositional change of newsprint samples collected at the Norman Landfill (NLF) as a function of landfill depth, (ii) to measure the toluene sorption capacity of these newsprint samples, and (iii) to relate changes in toluene sorption capacity to a structural parameter that is a measure of the newsprint degradation state. Underlying this research is the hypothesis that the anaerobic degradation of lignocellulosic organic matter (OM), as represented by newsprint, in a landfill results in an increasingly hydrophobic residue and that the changes in OM characteristics affect the fate and transport of HOCs in a landfill. Earlier results (5) have 10.1021/es0305914 CCC: $27.50
2004 American Chemical Society Published on Web 05/25/2004
shown that this hypothesis is valid for model MSW constituents that were anaerobically degraded in laboratory reactors; however, long-term mechanisms of lignocellulose degradation under field conditions and the effects of the resulting changes in OM characteristics on HOC sorption have not been identified to date. Results obtained with NLF samples were compared with fresh newsprint and a degraded newsprint sample that was prepared in a laboratory reactor under methanogenic conditions (5). Toluene, which has been detected at concentrations of up to 3 µg/L in NLF leachate (23), was selected for the sorption study as a representative HOC.
Materials and Methods Site and Sampling. The NLF is located on the alluvial plain of the Canadian River, 2 miles south of Norman, OK. The landfill accepted MSW from 1922 to 1985, after which the landfill was capped with local clay. The waste in the landfill is predominantly residential and commercial solid waste, with some suspected hazardous waste disposal. The landfill never utilized liners or leachate collection systems, and a leachate plume has developed, extending at least 225 m from the landfill edge toward the Canadian River (24-27). NLF newsprint samples were obtained from three cores collected in April and May 2001. As determined by aerial photos, the area sampled accepted municipal solid waste from 1970 to 1985 (27). A geoprobe direct push machine equipped with a macrocore piston rod sampler (Geoprobe System) was used to retrieve landfill core samples. A probe rod with diameter of 1.5 in. was used to obtain samples at 4 ft intervals to the base of the landfill, which was approximately 40 ft deep. Three sections of the cores were analyzed: 12-16, 20-24, and 32-36 ft from the top of the landfill to represent different ages of wastes (i.e., top layer, middle layer, and bottom layer, respectively). A start depth of 12 ft from the top was selected to avoid influences from the capping soil. Core samples were stored on ice until they were received at the laboratory, where they were kept at 4 °C. Physically distinguishable newsprint samples separated from each layer were soaked in Nanopure water for 24 h. The newsprint samples were freeze-dried and ground with a mortar and pestle prior to analysis. Fresh and Laboratory-Degraded Newsprint Samples. Fresh newsprint (FNP) was collected from The News & Observer Recycling Division (News & Observer Publishing Co., Garner, NC). Anaerobically degraded newsprint (DNP) was prepared from FNP in laboratory reactors as described (4, 5). Elemental Analysis. The carbon content of NLF newsprint samples was measured by the Service Central d’Analyse du CNRS (Vernaison, France) using a microanalysis protocol requiring only a few milligrams of sample. For FNP and DNP, the carbon content was measured using a CHN analyzer (Perkin-Elmer PE2400 CHN elemental analyzer, Perkin-Elmer Corp., Norwalk, CT) as described (5). Infrared (IR) Spectroscopy. IR spectra were obtained in a frequency range from 4000 to 200 cm-1 using a 283B scanning infrared spectrophotometer (Perkin-Elmer) at a scan rate of 250 cm-1/min. The ground sample was mixed with potassium bromide (infrared grade, Aldrich Chemical Co.) in a 1:100 ratio and pressed into a pellet. Solid-State 13C Nuclear Magnetic Resonance (13C NMR) Spectroscopy. Cross-polarization, magic-angle spinning (CPMAS) 13C NMR spectra were obtained with a Chemagnetics CMX-II solid-state NMR spectrometer operating at 75.694 MHz and using a Chemagnetics 5 mm double-resonance magic-angle spinning probe. A quasi-adiabatic sequence (28) using two-pulse phase modulation (TPPM) decoupling (29) at 75 kHz was employed. The 13C CP contact pulse of 1 ms length was divided into 11 steps of equal length with
ascending radio frequency field strength, while the 1H contact pulse had a constant radio frequency field strength of 301.00 MHz. At least 720 scans were acquired with a delay of 15.0 s. The sample spinning frequency was 6.0 kHz, maintained to within a range of (5 Hz or less with a Chemagnetics speed controller. Tetramethylammonium Hydroxide (TMAH) Thermochemolysis. TMAH thermochemolysis was performed according to Nanny and Ratasuk (26). Briefly, approximately 3.0 mg of sample (weighed to the nearest tenth of a milligram) was weighed into an ampule with 100 µL of TMAH (25% in methanol) (Aldrich). The ampule was evacuated and flamesealed on a vacuum line before being baked at 250 °C for 30 min. The products were extracted from the ampules with methylene chloride (CH2Cl2), and transferred to an amber vial with a Teflon septum. After the extract was blown to dryness under a gentle stream of N2 gas, the vial was reconstituted with 100 µL CH2Cl2 containing 32 ng/µL n-eicosane prior to gas chromatography/mass spectrometry (GC/MS) analysis (an HP 6980 gas chromatograph interfaced to an HP-5973N mass spectrometer). An MDN-5S (SigmaAldrich) fused silica capillary column (30 m × 0.25 mm i.d. × 0.25 µm film thickness) was used for the separation. The GC oven was temperature-programmed from 60 to 150 °C at a rate of 15 °C/min and then from 150 to 240 °C at 4 °C/min. The injector and interface were kept at 250 and 280 °C, respectively. 4-Hydroxy-3-methoxybenzaldehyde (vanillin; Fisher Scientific) and 1,2,3,4,4a,4b,5,6,10,10a-decahydro-1,4a-dimethyl-7-(1-methylethyl)-1-phenanthre necarboxylic acid (abietic acid; Aldrich) were used as external standards to determine the molar response factors for lignin and resin acids, respectively. The two standards were mixed quantitatively (1-2 mg each) in an ampule, and the same TMAH thermochemolysis procedure was performed as described for the newsprint samples. On the basis of the peak area and molar response factor, the content of each product from TMAH thermochemolysis was calculated. Isotherm Experiments. Toluene sorption to NLF newsprint samples was measured in flame-sealed glass ampules (1.5 mL). Ampules were baked in a muffle furnace at 550 °C for 4 h prior to use. Ultrapure water amended with a phosphate buffer (1 mM) and sodium azide (200 mg/L), to prevent aerobic sorbent and sorbate degradation, served as the liquid phase. Initial sorbate concentrations ranged from 10 to 100 µg/L, and liquid/solid mass ratios ranged from 10/1 to 17/1. Using a gastight syringe, [14C]toluene was spiked into the suspension at the ampule bottom immediately prior to ampule sealing. Methanol contributions from the toluene stock solution were kept below 0.1% v/v in isotherm tests to avoid cosolvent effects. During the 5 d equilibration period (5), ampules were tumbled end-over-end to ensure effective mixing. Once equilibrated, ampules were centrifuged at 2000 rpm for 20 min, after which a 1 mL sample was removed for scintillation counting. With each isotherm experiment, triplicate blanks containing toluene but no solids were included. Results from these blanks showed that no toluene losses occurred during the equilibration period. Solid-phase toluene concentrations were calculated using a mass balance approach. Toluene isotherm experiments with FNP and DNP were conducted using a similar procedure (5).
Results and Discussion IR Spectroscopy. The IR spectra of FNP, DNP, and newsprint samples from the top (TNP), middle (MNP), and bottom (BNP) layers of the NLF are shown in Figure 1. All newsprint samples exhibited significant IR absorption at the O-H stretching band at 3100-3500 cm-1, the aliphatic C-H stretching band at 2850-3000 cm-1, and the C-O stretching band at 1030-1200 cm-1. These sorption signals are mainly VOL. 38, NO. 13, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Distribution of Functional Groups (%) in Newsprint Samples Based on 13C NMR Integration Results functional group
FNP
DNP
TNP
MNP
BNP
carbonyl (160-200 ppm) aromatic (110-160 ppm) anomeric (90-110 ppm) cellulose alcohol (60-90 ppm) methoxy (50-60 ppm) alkyl (0-50 ppm) C/La O-A/Ab
2.1 12.2 13.7 62.9
2.8 17.1 11.6 55.5
0.6 18.9 10.4 49.6
1.7 17.8 11.3 51.5
1.3 23.4 8.9 42.6
5.8 3.3 4.3 25.0
7.7 5.4 2.7 13.9
10.1 10.5 2.1 6.7
9.8 7.9 2.3 9.2
9.8 14.0 1.6 4.4
a Cellulose/lignin ratio equal to (anomeric + cellulose alcohol)/ (aromatic + methoxy). b O-Alkyl/alkyl ratio. The region for the O-alkyl group is 50-110 ppm.
FIGURE 1. IR spectra (absorbance vs frequency (cm-1)) of newsprint samples: FNP, DNP, TNP, MNP, and BNP.
FIGURE 2. CP-MAS solid-state 13C NMR spectra of newsprint samples: FNP, DNP, TNP, MNP, and BNP. from the vibration or stretching of cellulose, a major component of newspaper (30, 31). The IR spectra of FNP and DNP are very similar, indicating no significant difference in IR-sensitive groups between the two samples. However, NLF newsprint samples, especially BNP, have a sharp response and high resolution in the region of 1000-1200 cm-1. This increase in intensity and resolution is possibly from clay, which may be attached to the NLF newsprints. Most clays have strong absorption bands in the regions of 3600-3670, 3400-3450, 1050-1075, 905-945, and 500-550 cm-1 (30), all of which are observed in the TNP and BNP IR spectra. CP-MAS 13C Nuclear Magnetic Resonance Spectroscopy. CP-MAS 13C NMR spectra of the five newsprint samples are shown in Figure 2, and 13C NMR integration results are presented in Table 1 as the percentage distribution of functional groups according to their chemical shift regions. Spectral results show that the samples consist primarily of cellulose (major signals 45-60, 60-70, 70-95, and 100-110 ppm) with varying amounts of aromatic carbon. Compared 3544
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to FNP, DNP exhibits a decreased 13C NMR response in both the anomeric and cellulose alcohol regions with an increased response in the aromatic, methoxy, and alkyl regions (Table 1). This is in agreement with the loss of cellulose and hemicellulose due to degradation and the selective enrichment of lignin (major signals 10-45, 50-60, and 110-155 ppm) and terpenoids (major signals 10-45 ppm). The decrease in cellulose groups and the increase in lignin and alkyl terpenoid groups are more pronounced in NLF samples than in DNP, suggesting that the cellulose in these samples is further decomposed and the lignin and alkyl terpenoids are subsequently enriched relative to the sample degraded under laboratory conditions. The O-alkyl/alkyl (O-A/A) ratio was calculated (Table 1) as a measure of the degradation state of newsprint, in which the O-alkyl carbon is expressed by the region of 50-110 ppm. This ratio is useful for assessing the decomposition of natural organic materials (32) and correlating the sorption capacity of fresh and laboratory-degraded MSW constituents (5). In addition, because both methoxy and aromatic carbons originate mainly from lignin, the (anomeric + cellulose alcohol)/(methoxy + aromatic) ratio was calculated (Table 1) as an estimate for the cellulose/lignin (C/L) ratio, which is widely used to evaluate the extent of MSW degradation (33, 34). As shown in Table 1, both O-A/A and C/L ratios decreased in the order FNP > DNP > MNP > TNP > BNP, suggesting that the extent of degradation is in the order BNP > TNP > MNP > DNP > FNP. The result for the NLF samples was somewhat unexpected because one could argue that the extent of degradation should increase with depth (i.e., with residence time). However, residence time is not the only factor that affects the extent of degradation of MSW components. Microbial activity in landfills varies spatially and is affected by such factors as pH and moisture content. The observation that TNP was degraded to a greater extent than MNP suggests that conditions for microbial growth were more favorable in the region from which the TNP sample was collected. For example, water percolating through the cover soil may absorb primarily in the upper reaches of the landfill such that, during certain periods (e.g., rain events), the moisture content, and thus the microbial activity, could be greater in the region from which the TNP sample was collected than in the region from which the MNP sample was collected. Compared to the C/L ratio that ranged from 1.6 to 4.3, the O-A/A ratio exhibited a much broader range of 4.4-25.0, making it a more sensitive measure of anaerobic degradation of paper-based materials. Moreover, the O-A/A ratio represents the change of the hydrophilicity in MSW (5). In this study, the decrease in the O-A/A ratio of degraded newsprints relative to that of FNP reflects the loss of polarity upon decomposition, which is consistent with the preferential degradation of cellulose over lignin and alkyl terpenoids. In
FIGURE 3. TIC of TMAH thermochemolysis products from newsprint samples: FNP, DNP, TNP, MNP, and BNP. The labeled peaks are identified in Table 2. “L” and “R” refer to peaks originating from lignin-derived and resin acid-derived compounds, respectively. addition, on the basis of the chemolytically measured lignin content of fresh and degraded MSW components and the corresponding C/L ratio calculated for these samples from the 13C NMR data given in Wu et al. (5), the estimated lignin contents in TNP, MNP, and BNP are 31%, 31%, and 33%, respectively, which is greater than the value of 22-25% for fresh newsprint (2-5). This increase in lignin content strongly suggests that lignin in NLF newsprint samples is being selectively preserved relative to cellulose. In the cellulose alcohol region (60-90 ppm) of the 13C NMR spectra (Figure 2), the couplets at 62 and 65 ppm and 82 and 89 ppm are, respectively, from amorphous and crystalline C-6 and C-4 carbons in cellulose. The relative intensity of crystalline to amorphous peaks for C-4 and C-6 is typically used to determine cellulose crystallinity (35, 36). FNP and DNP contained significant crystalline cellulose shown by more intense crystalline C-6 and C-4 peaks at 65 and 89 ppm than amorphous signals at 62 and 82 ppm, respectively. In contrast, NLF samples have more intense responses from amorphous carbon than from crystalline C-4 and C-6 carbons, except for the C-6 in BNP. This suggests that the cellulose in the newsprint recovered from the NLF landfill was generally low in crystallinity. Although many studies have demonstrated preferential degradation of amorphous over crystalline cellulose (37-39), Pichler et al. (40, 41) observed that crystalline cellulose in MSW was efficiently removed after 46 weeks of nonaerated windrow composting following 9 weeks of aerated composting. TMAH Thermochemolysis. TMAH thermochemolysis is used to characterize lignin structure because it specifically
cleaves the β-O-4 bonds of lignin that have a hydroxyl group in an adjacent position (i.e., R- or γ-carbon or both) on the propyl side chain (42, 43). This method is effective in the study of lignin degradation because its products are highly sensitive to changes on the lignin propyl side chain (42-44). The GC/MS total ion current (TIC) spectra for products from TMAH thermochemolysis are displayed in Figure 3. Peak identifications are listed in Table 2. The spectra consist predominantly of lignin monomers and resin acids. Lignin monomers are mostly methylated derivatives of the vanillyl units, suggesting that newsprints were made primarily from softwoods (45). Resin acids, refractory components in softwoods, were detected in the form of methyl esters resulting from TMAH thermochemolysis. Resin acids are usually used as internal sizing agents in papermaking (46) and are also intrinsically present in low-quality paper, especially newspaper (47). For quantitative assessment of newsprint degradation, the average TMAH yields of lignin and resin acid derivatives (Figure 4) from duplicate samples were normalized to the organic carbon (OC) content obtained from elemental analysis measurements (foc, Table 3). The relatively low lignin yields from FNP and DNP (21.1 and 25.9 mg/g OC, respectively) compared to the relatively higher lignin yields from TNP and MNP (40.5 and 33.8 mg/g OC, respectively) suggest that the lignin content of newsprint samples increased with increasing extent of degradation. These results are in agreement with the increased lignin content and decreased C/L ratio values of degraded newsprint samples as indicated by the 13C NMR results (Figure 2 and Table 1). However, the VOL. 38, NO. 13, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Identified Products from TMAH Thermochemolysis for Newsprint Samples product
FNP
DNP
TNP
MNP
BNP
X X X
X X
X X X
X X
X X
X X X X X X X
X X X X X X
X X X X
X X X X X X X X X X X X X X X X X X
X X X X
Lignin L1 L2 L3 L4 L5 L6 L7 L8 L9 L10 L11 L12 L13 L14 L15 L16 L17 L18,19 L20 L21 L22 L23,24 L25 L26
phenol, 3,4-dimethoxybenzene, 4-ethenyl-1,2-dimethoxy1,2,4-trimethoxybenzene benzoic acid, 4-methoxy-, methyl ester benzene, 1,2,3-trimethoxy-5-methyl3,4-dimethoxyphenethyl alcohol benzaldehyde, 3,4-dimethoxybenzene, 1,2-dimethoxy-4-(1-propenyl)ethanone, 1-(3,4-dimethoxyphenyl)3,4-dimethoxyphenylacetone benzoic acid, 3,4-dimethoxy-, methyl ester benzaldehyde, 3,4,5-trimethoxy1-(3,4-dimethoxyphenyl)-2-methoxy propane cis/trans-2-(3,4-dimethoxyphenyl)-1-methoxyethylene 1-propanone, 1-(3,4-dimethoxyphenyl)cis/trans-1-(3,4-dimethoxyphenyl)-1-methoxy-1-propene 2-propenoic acid, 3-(4-hydroxy-3-methoxyphenyl)-, methyl ester benzene, 1,2,4-trimethoxy-5-(1-propenyl)- (E, Z) 1-(3,4-dimethoxyphenyl)-2-methoxyethane benzenebutanoic acid, 3,4-dimethoxy3-(3,4-dimethoxyphenyl)propanoic acid, methyl ester threo/erythro-1-(3,4-dimethoxyphenyl)-1,2,3-trimethoxypropane cis/trans-1-(3,4-dimethoxyphenyl)-1,3-dimethoxy-1-propene 2-propenoic acid, 3-(3,4-dimethoxyphenyl)-, methyl ester Resin Acids 1-phenanthrenecarboxylic acid, 7-ethenyl-1,2,3,4,4a,5,6,7,8,9,10,10adodecahydro-1,4a,7-trimethyl-, methyl ester pimaric acid, 1-phenanthrenecarboxylic acid, 7-ethenyl-1,2,3,4,4a,4b,5,6,7,9,10,10adodecahydro-1,4a,7-trimethyl-, methyl ester isopimaric acid, 1-phenanthrenecarboxylic acid, 7-ethenyl-1,2,3,4,4a,4b,5,6,7,8,10,10adodecahydro-1,4a,7-trimethyl-, methyl ester 1-phenanthrenecarboxylic acid, 7-ethenyltetradecahydro-1,4a,7-trimethyl-, methyl ester 1-phenanthrenecarboxylic acid, 1,2,3,4,4a,5,6,7,8,9,10,10a-dodecahydro1,4a-dimethyl-7-(1-methylethyl)-, methyl ester 1-phenanthrenecarboxylic acid, 1,2,3,4,4a,4b,5,6,8a,9,10,10a-dodecahydro-1,4a-dimethyl-7(1-methylethyl)-, methyl ester 1-phenanthrenecarboxylic acid, 1,2,3,4,4a,6,7,8,10,10a-decahydro-1,4a-dimethyl-7(1-methylethyl)-, methyl ester abietic acid, 1-phenanthrenecarboxylic acid, 1,2,3,4,4a,4b,5,6,10,10adecahydro-1,4a-dimethyl-7-(1-methylethyl)-, methyl ester dehydroabietic acid, 1-phenanthrenecarboxylic acid, 1,2,3,4,4a,9,10,10aoctahydro-1,4a-dimethyl-7-(1-methylethyl)-, methyl ester
R1 R2 R3 R4 R5 R6 R7 R8 R9
BNP sample, the most degraded newsprint sample on the basis of its low O-A/A and C/L ratio values and the fact that it contained the highest lignin concentration (Table 1 and Figure 2), produced an unexpectedly low yield of lignin (33.1 mg/g OC), a level similar to that produced by MNP (Figure 4). Previous research showed that the presence of β-O-4 bonds with adjacent hydroxyl groups is the limiting factor in the decomposition of lignin by TMAH thermochemolysis; β-O-4 bonds without adjacent hydroxyl groups cannot be cleaved by TMAH (42, 43). As a result, the low lignin monomer yield from BNP is hypothesized to result from a substantial loss of hydroxyl groups on R- and/or γ-carbons of the lignin propyl side chain. This suggests that, besides enrichment, lignin in BNP may be anaerobically degraded via propyl side chain alteration after a long residence time in a landfill. The ratio of 3,4-dimethoxybenzoic acid methyl ester (methylated vanillic acid) and 3,4-dimethoxybenzaldehyde (methylated vanillin), i.e., the vanillyl acid/aldehyde (Ad/Al) ratio, is an indicator of the relative state of aerobic lignin degradation (43, 44). The Ad/Al ratio increases with degradation since vanillic acid is generated upon oxidative cleavage of the CR-Cβ bond in the lignin propyl side chain during aerobic degradation (42-44, 48, 49). Vanillyl Ad/Al ratios of newsprint samples shown in Figure 5 vary between 0.45 and 0.98, which is in the range of 0.15-1.3 for fresh wood samples 3546
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X X X X X X X X X X
X X X X X X X X X X
X X X X X X X X X X X X X X
X X X X X X X X X X X
X
X
X
X
X
X
X
X
X
X
X
X
X
X X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
(43, 50, 51). It is difficult to state that the significantly (p < 0.002) lower values of the Ad/Al ratio for NLF samples relative to FNP and DNP result from anaerobic lignin degradation because no solid evidence exists in the literature demonstrating generation of vanillin by anaerobic lignin degradation. Chen et al. (11) did find vanillin as a metabolite in anaerobic degradation of a lignin model compound; however, it was accompanied by a greater production of vanillic acid. Thus, the Ad/Al ratio values for the degraded newsprint samples indicate that no significant side chain oxidation occurred with respect to the R-carbon of the propyl side chain of lignin in both an anaerobic laboratory reactor and a landfill environment. This suggests an anaerobic lignin degradation pathway different from that for aerobic degradation. A similar result was observed by Dittmar and Lara (20) for lignin-containing materials sampled from different depths of sulfate-reducing mangrove sediments. Their CuO oxidation results revealed that the vanillyl Ad/Al ratio varied from 0.3 to 1.0 and was scattered with depth. They found that even highly degraded lignin did not show any oxidation of the propyl side chain as evident from stable Ad/Al ratios. This suggests that the Ad/Al ratio, which is successfully used to describe the degree of aerobic lignin degradation, may be a poor indicator for anaerobic lignin degradation.
FIGURE 4. Yields (mg/g OC) and ratios of lignin and resin acids released by TMAH thermochemolysis of newsprint samples: FNP, DNP, TNP, MNP, and BNP.
TABLE 3. Organic Carbon Fraction of Newsprint Samples and Partition Coefficients Describing Toluene Sorption
FNP DNP TNP MNP BNP a
foc
Kp (L/kg)
Koca (L/kg OC)
0.451b 0.455b 0.371 0.407 0.270
13.0 ((0.2)b 25.8 ((0.4)b 24.7 ((3.0)c 20.8 ((2.4)c 37.6 ((2.8)c
28.8b 56.7b 66.6 51.1 139.4
Koc ) Kp/foc. b From ref 5. c Based on 10-12 isotherm data points.
FIGURE 5. Vanillyl Ad/Al ratios of newsprint samples: FNP, DNP, TNP, MNP, and BNP. Resin acids were substantially enriched in NLF newsprint samples (Figure 4). Previous studies have illustrated that resin acids are very recalcitrant in anaerobic environments and are quite toxic to anaerobic bacteria, particularly to methanogens (52, 53). The chemical structures of the resin acid compounds identified in this study are shown in Figure 6, and the names are listed in Table 2. A total of nine resin acids were identified as either pimaric acid type (R1-R4) or abietic acid type (R5-R9), and three of them (R1, R2, and R4) were
FIGURE 6. Structures of resin acids. The name of each compound is referenced in Table 2. found as two isomers with different elution times (Figure 3). Of all resin acids identified, dehydroabietic acid (R9) existed in all samples at the highest concentration. Previous research on anaerobic sludge blankets suggests that this compound may be one of the more refractory resin acids in anaerobic environments (54). The observation of the dihydro derivative of pimaric acid, 1-phenanthrenecarboxylic acid, 7-ethenyltetradecahydro-1,4a,7-trimethyl-, methyl ester (R4), in TNP and BNP is interesting because of the complete loss of double bonds in the resin ring, suggesting reduction and hydrogenation of pimaric acid. To our knowledge, this compound has only been reported once in anaerobic river sediment downstream from a Kraft pulp mill (55). The fully reduced form of pimaric acid, 7-ethyltetradecahydro-1,4a,7-trimethyl-1-phenanthrenecarboxylic acid (tetrahydropimaric acid), which, in addition to ring reduction, has the ethylene group reduced to an ethyl group, was not detected in the NLF newsprint samples, but has been observed in fresh softwoods (56, 57) and in anaerobic sediments downstream from Kraft pulp mills (58-60). Thus, R4 may be an anaerobic metabolite of pimaric acid, which suggests that resin acids are reductively degraded by hydrogenation of the ring double bonds. Although no literature evidence exists for this assumption regarding R4, previous researchers found a similar process for abietic acid (R8), which was degraded to tetradecahydro-1,4a-dimethyl7-isopropyl-1-phenanthrenecarboxylic acid (tetrahydroabietic acid) by double bond saturation in anaerobic environments (54, 61). The ratio of TMAH analyzable lignin monomers to resin acids (lignin/resin acids), presented in Figure 4, exhibits the relative degradability of these two components. The decreased lignin/resin acids ratio in DNP and NLF samples relative to FNP suggests a significant loss of original lignin relative to resin acids during the anaerobic degradation of newsprint. It is interesting to note that the lignin/resin acids ratio decreased in the order FNP > DNP > MNP > TNP > BNP, following the same trend as C/L and O-A/A ratios from 13C NMR data (Table 1). These results demonstrate not only that consistent estimates regarding the degradation degree of different newsprint samples can be obtained from TMAH thermochemolysis and 13C NMR data, but also that the VOL. 38, NO. 13, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Toluene isotherms for fresh and degraded newsprint samples. The inset shows the low liquid-phase concentration region; solid lines represent best fits of the linear partitioning model. anaerobic degradability of newsprint components follows the order cellulose > lignin > resin acids. Sorption Capacity for Hydrophobic Organic Contaminants. To relate changes in sorbent organic matter characteristics that result from anaerobic sorbent degradation to HOC sorption capacity, toluene isotherm experiments were conducted with NLF newsprint samples as well as with FNP and DNP. As shown in Figure 7, toluene isotherms on lignocellulosic materials were well described by a linear partitioning model (q ) KpC, where q is the equilibrium solidphase concentration, C is the equilibrium liquid-phase concentration, and Kp is the partition coefficient). To compare the affinity of the organic carbon in newsprint samples for toluene, Kp values were divided by the sorbent organic carbon content (foc) to yield the carbon-normalized partition coefficient (Koc ) Kp/foc). Table 3 summarizes Kp and Koc values describing toluene sorption to fresh and degraded newsprint samples. Both the Kp and Koc values in Table 3 illustrate that BNP, the most degraded newsprint sample, had the largest toluene sorption capacity while FNP had the smallest. This result was expected on the basis of the 13C NMR and TMAH thermochemolysis data that showed a selective preservation of more hydrophobic regions (lignin, resin acids) over more hydrophilic regions (cellulose, hemicellulose) as the extent of newsprint degradation increased. To relate HOC sorption capacity to a structural parameter describing the sorbent organic matter, the normalized sorption parameter [log(Koc/Kow)] describing toluene sorption to newsprint samples was plotted as a function of the sorbent O-alkyl/alkyl ratio determined by 13C NMR (Table 1). Figure 8 illustrates that the toluene sorption capacity of the three NLF newsprint samples increased with decreasing sorbent hydrophilicity as expressed by the O-alkyl/alkyl ratio. As shown by Wu et al. (5), the ratio of O-alkyl to alkyl groups appears to be a suitable sorbent organic matter descriptor because it recognizes the importance of both sorbent polarity and aliphatic group content on HOC sorption. O-Alkyl groups are attributed to hydrophilic regions of the sorbent, and numerous studies have shown that HOC sorption capacity decreases with increasing sorbent hydrophilicity (e.g., see refs 5 and 62-65). The importance of aliphatic regions of the sorbent, i.e., regions containing poly(methylene) groups, on HOC sorption was recognized more recently (5, 66-69), and results suggest that HOC sorption capacity increases approximately linearly with increasing alkyl group content (67, 68). Figure 8 further shows that toluene sorption capacities of the TNP and MNP samples were predicted reasonably well by the trend line established for three fresh and three 3548
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FIGURE 8. Dependence of the normalized sorption parameter on the sorbent O-alkyl/alkyl ratio. The straight line represents the line of best fit obtained with fresh and laboratory-degraded MSW components (see ref 5).
FIGURE 9. Dependence of the normalized sorption parameter on the sorbent O-alkyl/alkyl ratio for NLF newsprint samples, fresh and laboratory-degraded MSW components (5), and natural organic materials (67). laboratory-degraded MSW components by Wu et al. (5). In contrast, the sorption capacity of BNP, the most hydrophobic newsprint sample, was not well predicted, suggesting that the trend line may not be valid for more hydrophobic sorbents. To extend the range of sorbent O-alkyl/alkyl ratios, log(Koc/Kow) values describing toluene sorption to NLF newsprint samples were also plotted in conjunction with log(Koc/Kow) values describing toluene and o-xylene sorption to fresh and laboratory-degraded MSW components (5) and phenanthrene sorption to natural organic materials (67) as shown in Figure 9. It should be noted that the abscissa in Figure 9 is presented on a logarithmic scale to illustrate more clearly the manner in which HOC sorption capacity increased for sorbents with O-alkyl/alkyl ratios of less than about 5, at which point log(Koc/Kow) values increased exponentially with decreasing O-alkyl/alkyl ratio. Figure 9 suggests that the O-alkyl/alkyl ratio is a suitable predictor of HOC sorption capacity for sorbents with an O-alkyl/alkyl ratio value greater than about 1. It is unclear at this point whether HOC sorption capacities are not affected by the O-alkyl/alkyl ratio at values less than about 1, as suggested by the data of Salloum et al. (67), or whether improved quantitative 13C NMR data are needed to accurately determine the O-alkyl/alkyl ratio for relatively hydrophobic sorbents (e.g., the kerogens studied by Salloum et al.; 67). Environmental Significance. Substantial structural and compositional changes occurred during the degradation of newsprint in the Norman Landfill. 13C NMR and TMAH
thermochemolysis data illustrated that (i) the loss of cellulose/ hemicellulose and aliphatic hydroxyl groups in lignin and (ii) the preservation of resin acids and the remaining lignin contributed to a increase in hydrophobicity of degraded newsprint samples. One implication of the increased hydrophobicity of degraded newsprint samples is an increased sorption capacity for HOCs. For example, the Koc value describing toluene sorption to BNP, the most degraded newsprint sample evaluated in this study, was almost 5 times larger than that describing toluene sorption to FNP. The results of this study suggest, therefore, that the HOC sorption capacity of newsprint and related lignocellulosic materials should increase with time because (i) the loss of sorbent organic matter as a result of biogasification processes is primarily related to hydrophilic cellulose/hemicellulose with negligible sorption capacity and (ii) the more hydrophobic sorbent organic matter (lignin, resin acids), which likely explains the majority of HOC sorption to fresh lignocellulosic materials, is essentially preserved and may become more hydrophobic over time. With respect to HOC sorption, the results of this study also illustrated that the O-alkyl/alkyl ratio of sorbent organic matter is a structural parameter that is useful for predicting the HOC sorption capacity of fresh and degraded MSW components and a variety of natural organic materials.
Acknowledgments We thank Mr. Scott Christensen from the U.S. Geological Survey (Oklahoma City) for collecting the core samples and Dr. Margaret Eastman from Oklahoma State University for assistance with the 13C NMR analysis. This research was funded by the National Science Foundation (Grants BES9732969 and BES-0001700).
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Received for review August 11, 2003. Revised manuscript received February 17, 2004. Accepted April 19, 2004. ES0305914
