Effect of acidic mine drainage on the polymer properties of an HDPE geomembrane

A laboratory exposure programme was conducted to assess the effects of acidic mine drainage (AMD) from metallic mine wastes on the bulk polymer properties of a 1.5-mm-thick HDPE geomembrane. Coupons of the geomembrane were immersed in tanks containing a synthetic AMD maintained at 20, 40 and 60°C. Two other solutions (acidic water and deionised water at 20, 40, and 60°C) were also used. Specimens were periodically removed from the tanks and tested to determine whether changes were occurring in the melt flow index (MFI), polymer structure (as determined by Fourier transform infrared spectroscopy, or FTIR), and oxidation induction time. Only modest effects of AMD exposure were observed in the MFI and FTIR tests over the 22-month exposure period. The OIT changed appreciably during exposure, and followed a first-order (exponential in time) degradation pattern. Greater depletion rates occurred at higher temperatures and in the chemical solutions. An Arrhenius model was used to estimate the antioxidant depletion time for HDPE geomembranes immersed in AMD. Conservative estimates of the depletion time range from 46 years to 426 years depending on field temperature, initial OIT, and exposure condition.