2.1 Experimental apparatus
The test apparatus used in the experiments is made of Plexiglas (Fig. 1(a)) with a long seepage distance. The inner diameter of the hollow cylindrical chamber is D =8 cm, and the length is L =90 cm. The length-to-diameter ratio is L /D =11.25, which can be regarded as a completely one-dimensional sand column test. To facilitate sand sample filling and sampling for measuring the deposition concentration after testing, the cylindrical chamber is divided into three equal sections, each of which is 300 mm long. The three sections are connected by bolts with rubber rings to ensure good sealing (Fig. 1(b)).
The suspension (Pb2+, SPs or Pb2+mixed with SPs) in the water tank was pumped into the cylindrical chamber at a constant velocity using a peristaltic pump (BT600-2J, LongerPump Co., Baoding, China; control range: 120−1200 mL/min). The flow velocity was measured with a digital flow meter installed at the column inlet. The mixed solutions were maintained in suspension with the help of a motorized stirrer in the water tank. The water flowed vertically from the top to the bottom of the chamber, and the gravity force slightly accelerated the transport and deposition of SPs due to a coincident direction with the hydrodynamic forces.
Mesh screens 0.5 mm in diameter were placed at both the inlet and outlet of the chamber to prevent the loss of quartz sand particles. Leachate was collected using 30-mL plastic tubes at regular time intervals using a fraction collector (Huxi CBS-A 100, China) for measuring the concentrations of Pb2+ and SPs. The concentration of Pb2+ was determined using the graphite furnace method (TAS-990G; Persee General Instrument Co., Beijing, China) based on the concentration−absorbance relationship. The turbidity of SPs was measured using a turbidity meter (Type 2100N, HACH Co., USA), and the concentration of SPs was then calculated according to the concentration-turbidity relationship determined a priori.
2. 2