Infiltration of Pb(II) solution in compacted bentonite/sand mixture under unconfined conditions

Compacted mixture of bentonite and sand has been proposed for using as buffer/backfill material in the disposal of high-level radioactive waste in many countries. The hydraulic behaviors and deformation properties of this material can be influenced by the chemical composition of groundwater. In the present work, the infiltration test of Pb(II) solution was conducted on compacted Gaomiaozi (GMZ) bentonite/sand mixture under unconfined conditions. The moisture migration characteristics and swelling properties were analyzed. Results show that the moisture migrates more quickly in the GMZ bentonite/sand mixture infiltrated with Pb(II) solutions than that in the pure GMZ bentonite hydrated with pure water for a same bentonite dry density. This difference could be explained by the chemical influence of Pb(II) on the thickness of diffuse double layer. For the bentonite/sand mixture with high bentonite content, its hydraulic conductivity depends on the bentonite dry density, regardless of the influence of the sand content. The hydration rate of the specimen is a function of the infiltration time and the distance from the wetting end. The largest swelling deformation was observed at the wetting end. At a given location, the vertical displacement was larger than any unilateral one of both sides. After the infiltration test, the inductively coupled plasma emission spectrometer, mercury intrusion porosimetry, and environmental scanning electron microscope tests were conducted for investigation of the Pb(II) distribution and microstructural characteristics of the tested specimen. Results show that, as the distance from the wetting end increases, the Pb(II) content nonlinearly decreases from 4.48 to 0.07 mg/g while the micro-pore increases and the macro-pore decreases.