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.