Aluminium migration through a geosynthetic clay liner

Water treatment plants produce significant amounts of aluminium-rich residual solids (i.e. sludge) as a result of various processes to treat raw source water. Monofilling of these alum-based residual solids or other alum-rich sludge or leachates can be performed cost-effectively with geosynthetic clay liners (GCLs). However, there is currently a paucity of literature related to aluminium migration through GCLs and the influence of this aluminium migration on the engineering behaviour of GCLs. This paper presents results of GCL hydraulic conductivity, diffusion, and batch testing performed with aluminium sulfate solutions. Hydraulic conductivity test results show that modest increases in hydraulic conductivity with the aluminium solutions are observed after hydration with distilled water (k < 5 × 10⁻¹¹ m/s). Diffusion testing with the same GCL has established an aluminium diffusion coefficient, D_t, of 1.5 × 10⁻¹⁰ m²/s and a linear distribution coefficient, K_d, of 30 ml/g. Batch testing performed confirms an initial high rate of aluminium uptake to the sodium bentonite, as well as non-linear behaviour. Adsorbed cation distributions on the bentonite and differential thermal analysis after 22 pore volumes of permeation with the aluminium solutions suggest that cation exchange and precipitation are mechanisms responsible for the attenuation observed as well as changes in hydraulic conductivity. An illustrative example is provided in the text to provide some perspective on the possible use of results presented herein for alum residual monofills.