| Factors | Coding | level | ||
| −1 | 0 | 1 | ||
| Initial pH | X1 | 3.0 | 7.0 | 11.0 |
| TC concentration (mg·L−1) | X2 | 50 | 100 | 150 |
| Activated carbon dosage (g·L−1) | X3 | 0.5 | 1.0 | 1.5 |
| Adsorption time (h) | X4 | 3.0 | 6.0 | 9.0 |