## 2 Materials and Methods
### Up-Flow Percolation Column Test  
In this study, up-flow column percolation tests were performed following the procedure described in USEPA 1314 [23], with slight modifications. Briefly, each column was packed with the sample material (i.e., contaminated soil or one of five different stabilized soils). Each sample material included five subsamples with the column divided into five continuous layers of material. The packing of the column started from the top (outflow) and ended at the bottom (inflow), and the column was inverted prior to the experiment. Clean sand (20-30 mesh; Hi-Tech, Seongnam-Si, Korea) was compressed in both ends of the columns to provide an even flow pattern. In addition, filter papers (Whatman filter paper Grade 2) and plastic plates (thickness, 3 mm) containing several small holes were also placed at both ends (plates on the bottom and top positions) to prevent losing any packing materials from the column. We recorded the mass of the empty column and the end caps, tubing materials, packed materials (soils, clean sand, filter paper, and plastic \begin{table}
\begin{tabular}{l c c} \hline \hline
**Parameter** & **Unit** & **Value** \\ \hline Soil texture: Loamy sand & & \\ Sand (0.020–2.000 mm) & \% & 80.0 \\ Silt (0.002–0.020 mm) & \% & 14.0 \\ Clay (\(<\)0.002 mm) & \% & 6.0 \\ pH (H\({}_{2}\)O, 1:5) & & 7.6 \\ Soil-moisture content & \% & 8.5 \\ Electric conductivity (EC) & mS/cm & 0.034 \\ Oxidation-reduction potential (ORP) & mV & 80.5 \\ Redox potential (Eh) & mV & 280.5 \\ Organic matter (OM), loss on ignition (LOI) & \% & 4.07 \(\pm\) 0.07 \\ Arsenic & mg/kg & 82.5 \(\pm\) 7.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Major physicochemical properties of the As-contaminated soil. plates), and the total mass of completely packed columns. The experimental conditions, including the column dimensions and sample sizes are listed in Table 2.  
The inert tubes comprising polytetrafluoroethylene (PTFE) with 2 mm ID were connected to both end caps of the column. The tubes were connected with shut-off valves and tubes from the bottom end were connected to a peristaltic pump (Shenchen LabV1, China). The inlet tubes from the peristaltic pump were dipped into a 2 L glass beaker containing an eluent solution (1 mM CaCl\({}_{2}\)). The top outlet tubes were connected to an eluate collection bottle. The eluent solution for the column percolation test was prepared following the USEPA Method 1314 [23]. Briefly, calcium chloride (1.0 mM CaCl\({}_{2}\)) solution was prepared by dissolving 0.11 g of solid calcium chloride in 1 L of deionized water. The pH of the eluent was 5.8. The solution was freshly prepared before initiating the percolation tests. A peristaltic pump with five channels was used to supply the eluent throughout the columns. Nitrogen gas (an inert gas) was bubbled through the eluent (which was contained in a beaker) prior to release into the columns. A schematic diagram of the column percolation test is provided in Figure 1A (see Figure S1A for column setup). After careful preparation of the experiment, the pump was turned on and columns filled with the eluent, wetting the packed materials as the eluent reached the top of the columns. Once the column packing was completely wet, the pump was stopped, and the columns were allowed to equilibrate. The original procedure in USEPA Method 1314 required the column to be equilibrated for 21 \(\pm\) 3 h. However, this study used stabilizing agents mixed with the soil. Therefore, to cure the soil-stabilizer mixture, we followed the equilibration time (21 \(\pm\) 3 h) with a further 28 days of resting to ensure equilibrium was reached. After 28 days of curing, the pump was turned on and the experiment began. The eluent flow rate was controlled at approximately 0.75 \(\pm\) 0.25 L/S per day [23]. The eluate fractions were collected at different L/S ratios (Table 2). All experiments were conducted at ambient room temperature (of approximately 25 \({}^{\circ}\)C).