After the column was completely filled with

After the column was completely filled with water, the water was gravity-drained from the column and collected. The collected water (mp) was then weighed using a balance. The mp was measured repeatedly, and the mean of the measurements was used to calculate the column pore volume (Vp), and the porosity (η) of the quartz sand filled column.equation(a)η=mpρwater×Vp×100%.
The delayed breakthrough of the arsenic in the column relative to the inert fluorescein sodium tracer was represented by the dimensionless retardation factor (R) ( van Halem et al., 2010), which can be calculated as follows:equation(b)RAs=VViAsCC0=0.5VVitracerCC0=0.5where C AY-NH2 the measured value; C0 is the initial value of the injection solution, and the inert tracer is fluorescein sodium. The V/Vi value equals the water volume (V) divided by the column pore volume (Vi).
3. Results and discussion
3.1. Optimal conditions for iron coating
3.1.1. Optimal loading time
The brown color of the coating materials on the surface of quartz sand in Column I gradually darkened over time (Fig. S1). The coloration was relatively uniform in the column at different times, indicating that the iron coating homogeneously formed during injection. The color of the coating materials showed no change, and the Fe content in the outflow water did not decrease significantly after 96 h of loading. In addition, no significant changes in flow rate and injection pressure were observed during the reagent injection process, indicating that no clogging occurred in the column during injection. Consequently, 96 h was set as the optimal loading time for chorionic villi sampling (CVS) system.