3.2.1. Adsorption-Desorption Test

The sorption of drugs by soils was investigated using the oscillation balance method following GB/T 21851-2008 [46]. The optimal soil/solution ratios and adsorption-desorption equilibrium time used in the formal test were obtained by a pre-experiment (Table 13).


**Table 13.** Optimal soil/solution ratios and equilibrium time in sorption tests.

Soil samples (0.5–5 g) were weighed and placed in 50 mL centrifuge tubes covered with tin foil. Appropriate amounts of aqueous solution of di fferent drug concentrations (0, 1, 2, 3, 4, and 5 mg/L) were added to the tubes to reach the optimum soil/solution ratios. The tubes were shaken at 25 ± 2 ◦C at 180 r/min. Three replicates were used for each concentration group.

After the adsorption reached equilibrium, drugs in solution and soils of all groups were extracted and detected using methods described in 3.2.3 and 3.2.4. Adsorption rates were calculated and the Freundlich model (lg Cs = lg Kd + 1/n lg Ce) was applied to fit sorption isotherms.

#### 3.2.2. Soil Mobility Test

Mobility was investigated by soil thin-layer chromatography following GB/T 31270-2014 [30]. 10 μg of each drug was spotted 2.0 cm from the lower end of the coated soil sheet plates (containing 10 g of soil, 0.5 mm soil thickness, air-dried). Blank controls and three replicates were used. After the

solvent had evaporated, the glass plate was placed at an angle of 30◦ in a chromatographic bath, and was then unfolded by distilled water at room temperature until the water surface reached the leading edge of the glass plate. The soil layer was divided into 6 × 3 cm segments. Each soil segmen<sup>t</sup> was scraped into a 10 mL centrifuge tube to detect drug content (3.2.3 and 3.2.4). The Rf values of drugs were calculated.

## 3.2.3. Soil Drug Extraction

Drugs were extracted from soils according to [48]. Briefly, 10 mL of methanol was added to the soil in centrifuge tubes. The tubes were then vortexed for 1 min, sonicated at 25 ◦C for 10 min, and centrifuged at 8000 r/min for 5 min. The supernatant was passed through a 0.45 μm filter to obtain the pre-treated sample solution. Extraction accuracy was checked by a recovery test (Table 14). Addition concentrations were 1, 2, 4 mg/L, and five replicates for each concentration were tested in each soil.


**Table 14.** Extracted drug recovery in test soils (%).

Drug extraction from water: an appropriate volume of testing water was passed through a 0.22 μm filter to obtain the pre-treated sample solution.
