## 3 Results and Discussion
### Batch Tests
#### 3.2.1 Adsorption Kinetics
The change was due to the increase in TNT adsorption [34], and the high h values of 2.762 mg/(kg\(\cdot\)min) and 3.676 mg/(kg\(\cdot\)min) for TNT concentration at 50.0 mg/L and 100.0 mg/L demonstrated that TNT adsorption on loess might be via surface interchange mechanisms until all the surface-active groups were saturated [35]. The R\({}^{2}\) value of PSOK is more than 0.9, indicating that PSOK is also suitable for describing the adsorption process of TNT on the loess, and the process is largely due to physical adsorption, while film diffusion is a rate-limiting factor [34]. The regression was conducted by WMIDM and, as expected, its R\({}^{2}\) values were close to 0.9 (Table S1), implying that the TNT adsorption process is attributed to particle diffusion through the solution to the external surface of the loess or the boundary layer of the solute molecule, where intraparticle diffusion is a rate-limited step [35]. This further proves that not only film diffusion but also intraparticle and boundary layer diffusion are the adsorption-controlling factors [31]. However, their parameters, including k\({}_{3}\) and c, are rather small and almost unchanged, while only decreasing a little with increasing TNT concentration, from 10.0 mg/L to 100.0 mg/L, showing that the control potential is rather small and can even be ignored. Thus, the results demonstrate that TNT adsorption on the loess is largely due to surface physical adsorption and then chemical adsorption. Compared with PSOK, the R\({}^{2}\) values of EE are small, close to 0.8, confirming that TNT adsorption is not on the whole due to chemical adsorption [18]. Like the TNT adsorption on the loess, aniline adsorption has also two stages (fast and slow) and then reaches equilibrium (Figure S2b). Regardless of whether the initial concentration of aniline is 10.0, 25.0 or 75.0 mg/L, the experimental results fitted well with four studied models (Table S1), implying that the adsorption of aniline and TNT on the loess has similar mechanisms. However, compared with TNT adsorption, the fast and slow stages of aniline adsorption were short, 40 min and 80 min, and the equilibrium time was 120 min, which is significantly shorter than in the report by [PERSON] et al. (2004) [8]. The adsorption capacities of aniline were 77.92, 159.38 and 286.67 mg/kg at initial concentrations of 10.0, 25.0 and 75.0 mg/L, larger than those of TNT under the same conditions [32]. The parameters of aniline adsorption kinetics are also listed in Table S4. Obviously, the values of R\({}^{2}\) are in an order of PSOK \(>\) PFOK \(>\) WMIDM \(>\) EE. The results show the four models are also suitable to describe the adsorption process of aniline, suggesting that aniline and TNT share similar adsorption mechanisms. Furthermore, the FTIR of the loess before and after the adsorption of TNT and aniline presented few differences (Figure S3), indicating that the basic physical and chemical functional group structures of the loess had not been altered after the adsorption of TNT or aniline, further proving the similarity of TNT and aniline adsorption mechanisms. Still, there are also obvious differences in their corresponding parameters, such as the rate constants of k\({}_{1}\), k\({}_{2}\), k\({}_{3}\) and h for aniline being much greater than those for TNT, indicating that aniline is more easily adsorbed by the loess than TNT. In particular, the value of k\({}_{3}\) for aniline is more than one hundred times that for TNT, illustrating that aniline has a stronger intra-particle diffusion driving force on the loess than that of TNT. Moreover, at the same experimental conditions, the adsorption capacity and rate of TNT were higher than those of aniline, but its equilibrium time was longer than that of aniline. The h value of PSOK for TNT adsorption was smaller than that for aniline adsorption (Table S1). The adsorption process for TNT is mainly controlled by surface physical adsorption at the first stage, while aniline is controlled by surface electrostatic adsorption.