## 3 Results and Discussion  
### Silica Gel and Activated Carbon Adsorption Experiment  
From the adsorption curves of the high-concentration TCE (Figure 2) and PCE (Figure 3) by the silica gel, it can be seen that the adsorption of the high-concentration TCE and PCE by the silica gel takes approximately 60 min to reach adsorption equilibrium. The equilibrium adsorption capacities of silica gel for the TCE and PCE are 4.221-6.893 mg L\({}^{-1}\) and 4.927-8.095 mg L\({}^{-1}\), respectively (Table 7). In addition, the removal percentages (R%) of silica gel for the TCE and PCE are approximately 67.00-72.90% and 69.94-76.16%, respectively, indicating that the removal efficiency of silica gel for TCE and PCE is not much different. Figure 3: Adsorption experiment of different initial concentrations of PCE by silica gel. Figure 2: Adsorption experiment of different initial concentrations of TCE by silica gel. From the adsorption curves of the low-concentration TCE (Figure 4) and PCE (Figure 5) by the activated carbon, it can be seen that the adsorption of the low-concentration TCE and PCE by the activated carbon can reach equilibrium in approximately 180 s. The equilibrium adsorption capacity of the activated carbon at different concentrations of the TCE and PCE is 0.520-0.909 mg L\({}^{-1}\) and 0.604-1.048 mg L\({}^{-1}\), respectively (Table 8). In addition, the removal percentages (R%) of activated carbon for the low-concentration TCE and PCE are approximately 88.72-89.50% and 91.54-92.92%, respectively, indicating that the removal efficiency of activated carbon for PCE is better than that for TCE. Regardless of the high concentration or low concentration of the adsorbate, with the change in initial concentration, the adsorbate removal percentage has no obvious change trend, indicating that the initial solute concentration has no significant effect on the adsorbate removal percentage. Although the removal rate of activated carbon for TCE and PCE is higher than that of silica gel, activated carbon is expensive and difficult to recycle, and the adsorption rates of TCE and PCE are too fast, which makes the saturated adsorption capacity of the adsorbent lower [42]. Therefore, if the pollutant has a high concentration, activated carbon is added for adsorption, and a large amount of activated carbon is needed to adsorb the pollutant to meet the maximum contaminant level (MCL), which leads to excessively high adsorbent usage and higher adsorption costs. Although silica gel is inexpensive and easy to obtain, the TCE and PCE removal percentages are lower than those of activated carbon. Therefore, if the pollutants from a high concentration relative to the MCL are all adsorbed by silica gel, a large amount of adsorbent needs to be added, which leads to excessively high adsorption costs. In addition to the considerations of the above experimental results, coupled with the research results of [PERSON] et al. [43], activated carbon is an adsorbent with an irregular surface, suitable for the Freundlich model, which is prone to error at high concentrations. Therefore, in this study, the adsorption of TCE and PCE was carried out by using a relatively high equilibrium adsorption capacity and inexpensive silica gel at a high concentration, and, when the pollutant was adsorbed to a low concentration, the adsorbent was changed to activated carbon for adsorption to facilitate a better adsorbent addition method and lower adsorption cost. Figure 6: Experimental data of (**a**) activated carbon and (**b**) silica gel adsorption onto TCE and PCE, fitted to linearized forms of Langmuir isotherms.