Factors Controlling Desorption Rates of Hydrophobic Organic Contaminants from Municipal Solid Waste

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Title: Factors Controlling Desorption Rates of Hydrophobic Organic Contaminants from Municipal Solid Waste
Author: Mitchell, Lisa A.
Advisors: Detlef R. U. Knappe, Committee Chair
Morton A. Barlaz, Committee Member
Joel J. Ducoste, Committee Member
Abstract: The overall goal of this research was to determine desorption rates of hydrophobic organic contaminants (HOCs) from municipal solid waste (MSW). Toluene and o-xylene, two alkylbenzenes that frequently occur in landfill leachates, served as model HOCs. HOC desorption rates were measured for individual MSW components expected to contribute to HOC sorption [high density polyethylene (HDPE), and poly(vinyl chloride) (PVC), newsprint, office paper, and degraded rabbit food as a model food and yard waste]. To quantify effects of MSW decomposition on HOC desorption rates, newsprint and office paper were tested in both fresh and anaerobically degraded forms. Effects of aqueous phase composition were evaluated by comparing HOC desorption rates in both acidogenic and methanogenic leachates to those obtained in ultrapure water. To determine the effects of aging (i.e., contaminant-sorbent contact time), desorption tests in ultrapure water were performed after aging times of 1 day, 1 week, 1 month, 6 months, and 9 months. Toluene desorption rates in acidogenic and methanogenic leachates and o-xylene desorption rates in ultrapure water were measured after aging times of 1 week and 6 months. To compare HOC desorption rates in a quantitative manner, experimental data were described by polymer diffusion models. A single-parameter polymer diffusion model was used to describe HOC desorption rates from PVC and HDPE, while a three-parameter biphasic polymer diffusion model was implemented to describe HOC desorption rates from biopolymer composites. Desorption tests showed that HOC desorption rates varied greatly with sorbent characteristics. HOC desorption rates from plastics were rapid for rubbery polymers such as HDPE and slow for glassy polymers such as PVC. For biopolymer composites, an initial phase of rapid HOC desorption was followed by an extended period of slow HOC desorption. In general, HOC desorption rates in acidogenic or methanogenic leachates were similar to those observed in ultrapure water. Exceptions were experiments conducted with PVC and fresh office paper in acidogenic leachate, in which cases enhanced HOC desorption rates were obtained. Volatile fatty acids such as propionic or butyric acids, which are important constituents of acidogenic leachate organic matter, may have plasticized PVC and thus enhanced HOC desorption rates. For fresh office paper, prior work by Wu (2002) showed that the lower pH of acidogenic leachate was primarily responsible for enhanced HOC desorption rates. Regarding sorbate characteristics, desorption rates for toluene were generally faster than those of o-xylene, a larger more hydrophobic molecule. For biopolymer composites, the HOC fraction associated with the slowly releasing sorbent organic matter fraction increased with increasing aging time, and, compared to toluene, this trend was more pronounced for o-xylene. Model predictions for simulated MSW mixtures showed that both waste composition and HOC characteristics greatly affect HOC desorption rates. Predicted half-lives for toluene and o-xylene were <1 and ~7 days, respectively, for a MSW mixture typical of the year 1960 and ~600 and 1525 days, respectively, for a MSW mixture typical of the year 2000. The principal changes in MSW composition that occurred between 1960 and 2000 was a decrease in food and yard waste and an increase in plastics. In particular, the greater presence of glassy polymers, such as PVC, in the newer MSW mixture led to slower alkylbenzene desorption rates. These predictions suggest that the mineralization rate of alkylbenzenes in old landfills may be controlled by biological processes while desorption processes may control alkylbenzene biodegradation rates in newer landfills.
Date: 2005-05-31
Degree: MS
Discipline: Civil Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/1965


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