An investigation of toluene and TCE diffusion through EVOH in aqueous solutions

Barrier systems are designed to control contaminant migration. Geomembranes are commonly used as part of a composite liner in landfill cover, base liner systems and sometimes vapour barriers. In landfill applications, the primary function of the geomembrane liner is to control contaminant transport from the leachate (dissolved phase) and landfill gas (vapour phase) out of the landfill thus minimizing environmental impact. In vapour barrier applications, membranes are used to mitigate intrusion of toxic volatile organic compounds (VOCs) into buildings originating from legacy contaminated sites, for example: petroleum stations, dry cleaners, industrial manufacturing sites and waste facilities (especially old dumps). Traditional geomembranes used in well designed and constructed barrier systems have been very effective at minimizing contaminant escape but it is also known that they are not impermeable (Rowe 2005, 2009). Contaminants can migrate through geomembranes by the process of either advection or diffusion. Advection involves movement of contaminants in a fluid due to a pressure differential or hydraulic gradient (e.g. landfill gas or leachate through flaws, holes or tears in a geomembrane (Rowe 2012)). Diffusion is the transport of molecules, by random molecular motion, from areas of high to low concentrations (Crank 1968). For a well designed and constructed barrier system, diffusion becomes the dominant form of contaminant migration (Rowe et al. 2004, Rowe 1998, 2005). The most common geomembranes used in barrier systems are made of high density or linear low density polyethylene (HPDE, LLDPE). The diffusive characteristics of HDPE liners are well established (Parks and Nibras 1993, Sangam and Rowe 2001; Joo et al. 2004, 2005) and although HDPE is substantially better than LLDPE, neither offer substantial diffusive resistance to many VOCs (McWatters 2010). Ethylene vinyl alcohol (EVOH) has the potential to be a much better diffusive barrier to VOCs than PVC, LLDPE or HDPE. To illustrate this, the diffusion of toluene and trichloroethylene (TCE) in aqueous solution through a 0.015 mm thick, 32 mol% EVOH thin-film is examined in this paper. McWatters and Rowe (2010) presented preliminary results (obtained over a period of two years), which showed that co-extruded LLDPE/EVOH/LLDPE geomembrane provided a much improved diffusive barrier to benzene, toluene, ethylbenzene and xylenes (BTEXs) than LLDPE alone. These coextruded LLDPE geomembranes with inner barrier materials of EVOH are designed to merge the functionality and durability of LLDPE with a very low-permeability inner barrier. This paper presents updated results for toluene after 6.5 years of testing. Temperature plays a critical role in the diffusive process (Crank 1975). Geomembrane applications extend to barrier systems built for elevated temperature environments (Rowe 2005, 2012), however most ABSTRACT: Novel geomembranes that include a thin-film of ethylene vinyl alcohol (EVOH) have the potential to minimize the migration of toxic volatile organic compounds (VOCs) from landfills or from contaminated (brownfield) sites into buildings and subsurface structures. The diffusion of toluene in dilute aqueous solution through an EVOH thin-film and a co-extruded LLDPE/EVOH/LLDPE geomembrane was investigated. The thin-film was 0.015 mm-thickness 32 mol% EVOH. The toluene permeation coefficients, Pg, through this thin film ranged from 2.5 x10 -13 m 2 s -1 at 23 o C to 48 x10 -13 m 2 s -1 at 50 o C. The Pg value of toluene through the co-extruded geomembrane was 5 x10 -15 m 2 s -1 . The diffusion of a chlorinated hydrocarbon, trichloroethylene (TCE), was also investigated for the thin-film. The TCE permeation coefficients, Pg, through this thin-film ranged from 3.5 x10 -13 m 2 s -1 at 23 o C to 65 x10 -13 m 2 s -1 at 50 o C.