2.3 Column experiments
To ensure complete saturation of the porous medium, 20 mL of deionized water (pH=7) was first added to the cylindrical chamber, and the outlet was adjusted to ensure no bubbles escaped from the bottom. The quartz sand was packed in layers, divided into 10 parts, according to the total amount; each layer was approximately 90 mm high. After each quartz sand layer was placed, the sand was stirred using a glass rod and compacted to a set thickness with a compacting hammer. Then, the next quartz sand layer was added, and each time, the deionized water level was kept 2 cm above the top of the sand column. The final average porosity of the saturated sand column was n =41.67%.
Prior to the experiment, the sand column was washed by injecting deionized water at a steady water flow velocity that was higher (e.g.,v =0.348 cm/s) than that in the subsequent test (e.g.,v =0.260 cm/s) until the effluent turbidity was nearly zero. The laboratory experiments were conducted to investigate the transport of either Pb2+ or SPs and the cotransport of Pb2+ with SPs. The injection concentration of SPs was set to C inj=0.5, 2 and 4 mg/mL. According to the situations likely to be encountered in geoenvironmental engineering (Agbozu and Bassey, 2016; Tansel and Rafiuddin, 2016), the Pb2+ solution concentration was set toC M=100, 200 and 400 ng/mL. The Darcy velocity was set to v =0.087 and 0.260 cm/s. The temperature was controlled atT =22±0.2 °C.
Because of the high coordination capacity of ethylenediaminetetraacetic acid disodium (EDTA), it readily forms stable chelates with Pb2+ adsorbed on the surface of SPs. In this experiment, 15 mL of a 0.05 mol/L EDTA disodium salt solution was used as the extraction reagent, which resulted in a high extraction efficacy (> 90%) (Fangueiro et al., 2002; Qiao et al., 2017). The leachate was shaken for 4 h in a constant-temperature oscillator (HY-4, Changzhou, China; vibration amplitude: 0−20 mm; vibration frequency: 0−300 rpm). Subsequently, Pb2+ and SPs were separated using a desktop low-speed centrifuge at a rotation speed of 4500 rpm (L3-6K, Hunan Co., China). The supernatant was collected to measure the Pb2+absorbance.
After completion of the transport tests, samples were collected from the different sections, and the sampling thickness of each section was approximately within 1 cm. Then, the concentrations of the deposited Pb2+ and SPs were measured, and the PSD of the SPs was obtained by laser diffraction (the wet method). In addition, a 5-megapixel metallographic microscope (CX40M, Ningbo Sunny Instrument Co., China) was used to examine the deposition state of SPs, which could easily distinguish the difference between the colored SPs and white quartz sand, as evident in the microstructure photos.