Diffusion of Chloride and Dichloromethane Through an HDPE Geomembrane

A methodology for determining diffusion coefficients through geomembranes is presented and used to estimate the diffusion coefficient for dichloromethane and chloride from dilute solutions through a 2 mm thick high density polyethylene geomembrane. The likely diffusion coefficient for dichloromethane at 23°C was found to be 2×10⁻¹² m²/s (6×10⁻⁵ m²/yr). The chloride tests were run for more than 900 days. Over this period only negligible diffusion of chloride was observed. Based on these results, a likely upper bound for the diffusion of chloride is 6×10⁻¹⁵ m²/s (2×10⁻⁷ m²/yr). An examination of different cell materials for use in diffusion tests showed that both stainless steel and polytetrafluoroethylene (PTFE) are problematic with respect to dichloromethane. In both cases there was significant loss of mass from the control cells. For stainless steel this loss was attributed to abiotic degradation of the dichloromethane. For PTFE it was attributed to diffusion through the PTFE. Glass cells with a surface area to volume ratio of less than 1 cm⁻¹ were found to be suitable for the diffusion test, with negligible mass loss being observed.