Many hydrophobic compounds can diffuse through the hydrophobic high-density polyethylene (HDPE) geomembranes used to contain contaminated material in waste disposal and environmental engineering applications. Scavenger materials layered with or added to HDPE can intercept diffusing molecules via transformation or sorption, slowing the perceived rate of diffusion across the barrier.
Diffusion of a variety of chlorinated organic contaminants across HDPE, HDPE sandwich membranes, and HDPE/poly(vinyl alcohol) (PVA) composites containing zero-valent iron and powdered activated carbon (PAC) was measured. Reaction of carbon tetrachloride with zero-valent iron and sorption of trichloroethylene to PAC in sandwich membranes were fast, effectively delaying breakthrough until scavenger activity was exhausted. The diffusion of 1,2,4-trichlorobenzene and 2,3',4',5-tetrachlorobiphenyl was measured for HDPE, PAC-containing HDPE, and an HDPE/PVA composite using PAC as a scavenger in the PVA layer. Because diffusion of these bulky aromatic compounds was slow, sorption rates within the HDPE matrix were limited by diffusion from the bulk membrane to the PAC particle surface. The reduced sorption rate allowed some contaminant molecules to leak across the barriers, but the PAC captured enough to substantially slow the overall contaminant flux. In both studies, scavengers were more effective (more of the scavenger was available for reaction or sorption) when placed in a hydrophilic matrix layered with the HDPE.
Because PAC successfully intercepted such of variety of contaminants in our initial studies, factors controlling the sorption rate in active membranes were further explored via comparison of PAC- and single-walled carbon nanotube (SWCNT)-containing PVA films. This study highlights the role of macropore/micropore diffusion on the kinetics of uptake by membrane-bound sorbent particles. While SWCNTs exhibit similar sorption capacities as PAC in aqueous solution, access of 1,2,4-trichlorobenzene to the sorption sites on the SWCNTs was severely limited in the PVA matrix.
This work provides experimental evidence for improved containment and a theoretical framework useful for the design of prototype and field-scale active membrane barriers. These findings also identify some limitations associated with the membrane encapsulation of scavenger materials and highlight the need for additional laboratory studies.
University of Minnesota Ph.D. dissertation. December 2009. Major: Civil Engineering. Advisor: Dr. William A. Arnold. 1 computer file (PDF); xix, 143 pages, appendices A-D.Ill. (some col.)
Surdo, Erin Mehleis.
Active membranes for containing and treating environmental contamination..
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