Hydraulic conductivity of bentonite permeated with acid mine drainage

Abstract

Bentonite, especially sodium bentonite, is used extensively either by itself or as a component of soil mixtures in many geoenvironmental engineering applications, including the construction of waste-containment barriers. An important aspect of the design of such barriers is the compatibility assessment of the soil barrier with the intended waste. A series of laboratory hydraulic conductivity (K) experiments were conducted to assess the compatibility of a sodium bentonite paste with acid mine drainage (AMD). The K of the water-permeated bentonite was observed to be of the order of 2 × 10 –9 cm/s. Permeation of the bentonite with AMD increased K slightly by almost one order of magnitude to an average value of 1 × 10 –8 cm/s as a result of the contraction of electrical double layers surrounding clay particles. The application of a confining stress (30–50 kPa) to the samples minimized the increase in K. Confining stress likely compressed the samples and reduced the size of the voids created by double-layer contraction, resulting in lower K. Effluent chemical profiles obtained during the K tests indicated the appearance of some cations and elements contained in the AMD permeant, at its original concentration (influent concentration) after a few pore volumes of AMD permeation. Iron and aluminum likely precipitated as oxyhydroxide and oxysulphate minerals. The pH of the effluent decreased from 7.6 during water permeation to 3.0 after only three pore volumes of AMD permeation, reflecting the limited capacity of the bentonite to neutralize acidic solutions. Zinc was slightly attenuated by adsorption (3.15 mequiv./100 g of dry bentonite) but was reported in the effluent, after approximately nine pore volumes. The results are relevant to the design and construction of bentonite slurry walls around acid-generating tailings and acid water containment ponds at mine sites.Key words: hydraulic conductivity, swelling, waste containment, confining stress, heavy metals, ionic strength, saturation index.