Porous medium, Pb2+ and SPs
Natural quartz sand (mainly composed of the mineral constituent SiO2; ρ s=2.65 g/cm3) prepared by mechanical crushing was used as the porous medium. It had a white, lustrous surface with a high acid and alkali resistance. Its particle size uniformly ranged from 1−2 mm, and the mean value wasd g=1.5 mm. The impurities and chemical substances on the quartz sand surface, such as iron and aluminum oxides, were removed by rinsing with 0.01 mol/L NaOH and HNO3solutions for 10 min prior to the experiment, and the quartz sand was then repeatedly washed with deionized water until the water pH remained constant.
Heavy metal ions of Pb2+ with an atomic weight of 207.2 were used. Due to the low solubility of Pb2+ and its high adsorption ability, SPs (or SiO2 particles) can more notably influence Pb2+ transport. Pb2+ solutions with different concentrations were prepared by diluting a 100 µg/mL Pb(NO3)2 solution with deionized water. Given that Pb2+ ions in solution commonly result in a slightly acidic state, the Pb2+ solution was adjusted to pH=5.5 by adding an appropriate amount of 0.01 mol/L NaOH or HNO3 solution.
Two commercial spherical SPs were prepared by dry grinding of natural high-quality color quartzite (Fuhong Mineral Products Co., Shanxi, China) insoluble in acidic and alkaline solutions. The median diameters of the two SPs were D 50=13.4 and 24.7 μm. Fig. 2 shows the measured PSDs by laser diffraction (LA-950 Mode, HORIBA, Japan; wet method). The two particle-size ranges were d =1−60 and 1−120 µm. The injected SP actually had a very wide PSD, which reflects the situation in practical engineering whereby the mobilized solid particles may be very inhomogeneous. The two SPs were red and black, and the density of both SPs was ρ sp=2.2 g/cm3.