Grantee Research Project Results
Final Report: The Effects of Aging and Sorbent Decomposition on the Bioavailability of Toluene and Xylene in Solid Waste
EPA Grant Number: R827131Title: The Effects of Aging and Sorbent Decomposition on the Bioavailability of Toluene and Xylene in Solid Waste
Investigators: Barlaz, Morton A. , Knappe, Detlef R.U.
Institution: North Carolina State University
EPA Project Officer: Aja, Hayley
Project Period: October 1, 1998 through September 30, 2001 (Extended to March 31, 2002)
Project Amount: $425,000
RFA: EPA/DOE/NSF/ONR Joint Program on Bioremediation (1998) RFA Text | Recipients Lists
Research Category: Land and Waste Management , Hazardous Waste/Remediation
Objective:
The central hypothesis of this research project was that kinetically limited sorption/desorption processes and humification control the bioavailability and leaching of organic contaminants in landfills, and that humification may be an endpoint in the fate of toluene and o-xylene. The overall objective of this research project was to develop an understanding of the factors that control the bioavailability and fate of organic contaminants sorbed to components of municipal solid waste (MSW). Specific objectives were to: (1) estimate the expected distribution of sorbed contaminants in MSW by measuring the sorptive capacity of the dominant organic refuse components in fresh and biodegraded form; (2) determine whether the sorptive uptake of organic contaminants in MSW is reduced in the presence of leachate as a result of organic contaminant binding by dissolved organic macromolecules; (3) determine the impacts of aging and leachate composition on contaminant desorption rates; (4) compare rates of desorption and biodegradation to determine whether desorption limits bioavailability; (5) determine the effects of refuse decomposition on bioavailability; and (6) identify whether humification is an important sequestration mechanism for aromatic xenobiotics in solid waste.
Summary/Accomplishments (Outputs/Outcomes):
We conducted experiments with individual components of MSW, including polyvinyl chloride (PVC), high-density polyethylene (HDPE), newsprint, office paper, and model food and yard waste (rabbit food). Each of the biopolymer composites (office paper, newsprint, and rabbit food) were tested in both fresh and anaerobically decomposed form. To characterize biopolymer-containing sorbent organic matter, fresh and anaerobically decomposed materials were characterized by chemolytic and spectroscopic techniques. For biodegradable MSW components, biodegradation resulted in the preferential removal of cellulose and hemicellulose, and the enrichment of lignin. Furthermore, protein accumulated during degradation, suggesting that microbial cell components constituted a portion of the degraded materials. Relative to cellulose and hemicellulose, losses of lipophilic extractives were smaller, a result that is explained by microbial resynthesis of lipids (cell walls, lipid bilayers of cell membranes) and the poor biodegradability of such lipids. Ash contents increased upon biodegradation, a result that is explained in part by the conversion of organic matter to CO2 and CH4, and by the addition of sodium carbonate for pH neutralization, especially during the preparation of degraded rabbit food. Anaerobic degradation decreased the organic carbon contents of office paper and rabbit food by about 9.5 percent, while that of newsprint remained approximately constant.
We collected batch isotherm data in phosphate-buffered organic-free water, as well as in acidogenic and methanogenic leachate. We used flame-sealed glass ampules to minimize volatilization losses, and we added sodium azide to the liquid phase to prevent aerobic biological activity. Acidogenic leachate was prepared by recirculating water through fresh residential MSW, and methanogenic leachate was generated by recirculating water through decomposed refuse. The isotherm data showed that alkylbenzene sorption to plastics was greater than to biopolymer composites, because of differences in sorbate/sorbent solubility parameter compatibility. Alkylbenzene sorption to biopolymer composites yielded linear isotherms, and normalized sorption capacities [log (Koc/Kow)] decreased linearly with increasing sorbent polarity as expressed by the O-alkyl/alkyl ratio. Compared to other measures of sorbent polarity, such as the polarity index [(O+N)/C], an improved correlation resulted with the O-alkyl to alkyl ratio because it also recognized the importance of lipophilic extractives on HOC sorption. Leachate composition had little effect on alkylbenzene sorption with one exception; volatile fatty acids in acidogenic leachate converted PVC from a glassy to a rubbery polymer. The results of the isotherm study showed that sorbent organic matter affinity for hydrophobic organic contaminants (HOCs) increases with the extent of MSW decomposition, because of the recalcitrance of plastics and the preferential degradation of polar biopolymers. Furthermore, the plasticizing effect of volatile fatty acids in acidogenic leachate may enhance the bioavailability of HOCs sorbed to glassy organic matter in MSW, or in soils contaminated with acidogenic leachate.
To determine the effects of aging on alkylbenzene desorption rates, we performed abiotic batch desorption tests in both acidogenic and methanogenic leachates. Prior to desorption testing, sorbents were exposed to 14C-labeled toluene for 30 and 250 days in flame-sealed ampules. To assure sterility during aging, flame-sealed ampules were γ-irradiated. Selected samples containing 14C-labeled o-xylene also were analyzed after 250 days. Desorption tests showed that alkylbenzene desorption rates varied greatly among MSW components (PVC was slowest, while fresh rabbit food and newsprint were fastest). Furthermore, desorption rates decreased as aging time increased, suggesting that the bioavailability of HOCs in landfills decreases over time as HOCs are physically sequestered or undergo humification. At the completion of desorption tests, any remaining 14C that was sorbed (extractable with benzyl alcohol), associated with humic matter (extractable with NaOH), and associated with nonextractable organic matter, such as humins (recoverable as 14CO2 upon combustion of extracted solids), was quantified. We obtained mass balances of 99-102 percent, and about 4 to 6 percent of the initially added 14C was not extractable from degraded biopolymer-composites after 250 days of aging.
A single-parameter polymer diffusion model successfully described PVC and HDPE desorption data, but it failed to simulate desorption rate data for biopolymer composites such as rabbit food, newsprint, and office paper. To describe desorption rate data for biopolymer composites, a three-parameter biphasic polymer diffusion model was employed, which successfully simulated both the initial rapid and the subsequent slow desorption of toluene and o-xylene. Toluene desorption rates from MSW mixtures were predicted for typical MSW compositions for the years 1960 and 1997. For the older MSW mixture, which had a low plastics content, the model predicted that 50 percent of the initially sorbed toluene desorbed over a period of 5.8 days. In contrast, the model predicted that 50 percent of the initially sorbed toluene desorbed over a period of 4 years for the newer MSW mixture. These results suggest that toluene desorption rates from old MSW mixtures exceed methanogenic toluene degradation rates (toluene half-lives of ~ 30 to 100 days have been reported for methanogenic systems), and imply that biodegradation kinetics control the rate at which sorbed toluene is mineralized in old landfills. This finding is consistent with the high frequency with which toluene is found in the leachate from older landfills.
In addition, the disposal of organic solvents in landfills was relatively common in the 1960s, but illegal after the mid-1970s. For newer MSW mixtures with a larger plastics content, toluene desorption rates are substantially slower; therefore, toluene desorption kinetics most likely control the rate at which sorbed toluene can be mineralized in new landfills.
For bioavailability tests, 14C-toluene aged under abiotic conditions in the presence of individual waste components for periods ranging from 4 to 180 days. After aging, Pseudomonas putida F1, a well-characterized toluene degrader, was added to the aged toluene-sorbent mixture and toluene bioavailability was determined by monitoring 14CO2 production over time. We also monitored 14CO2 production in sorbent-free positive controls, and all tests were conducted in triplicate. For all sorbents, there was a phase of rapid toluene biodegradation, after which the 14CO2 production rate decreased and ultimately stopped, despite the presence of residual toluene sequestered in the solid phase. The extent of biodegradation varied markedly among sorbents, and was correlated with the sorbents' chemical characteristics. 14CO2 recovery ranged from 68.8 percent for toluene aged on degraded newsprint for 4 days to 45.8 percent for toluene aged on fresh office paper for 4 days. In general, toluene bioavailability was reduced by aging (e.g., 14CO2 recovery decreased from 68.8 to 47.6 percent for toluene aged on degraded newsprint for 4 and 180 days, respectively). Solid phase analysis at the termination of bioavailability tests revealed that 14C was present in all three major forms of humic substances; humic acid, fulvic acid, and humin. High-performance size exclusion chromatography showed that 14C in humic and fulvic acid fractions was present in molecular size fractions greater than that of free toluene. Further research is required to determine whether the 14C in these fractions was physically sequestered or covalently bound. We measured the overall 14C mass balances of 95-101 percent. After the project end date, we observed that the γ-irradiation process converted a fraction of the initially added toluene to one or more nonvolatile compounds. We will conduct tests that will identify these compounds and their effect on the obtained results.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 7 publications | 1 publications in selected types | All 1 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Wu BY, Taylor CM, Knappe DRU, Nanny MA, Barlaz MA. Factors controlling alkylbenzene sorption to municipal solid waste. Environmental Science & Technology 2001;35(22):4569-4576. |
R827131 (2001) R827131 (Final) |
not available |
Supplemental Keywords:
bioavailability, cellulose, desorption, hemicellulose, humic substances, landfills, lignin, solid waste, sorption, toluene, o-xylene., RFA, Scientific Discipline, Toxics, Waste, Ecosystem Protection/Environmental Exposure & Risk, Bioavailability, Environmental Chemistry, HAPS, chemical mixtures, Bioremediation, 33/50, fate and transport, Toluene, landfills , Xylenes, kinetic studies, aging, contaminant release, desorption rates, solid waste, humification, Xylene, municipal solid waste, Xylenes (isomers and mixture), leachateProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.