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| Factor | Factor level | Unit | |||
|---|---|---|---|---|---|
| −1 | 0 | +1 | |||
| X 1 | Adsorbent dose | 6 | 12 | 18 | mg |
| X 2 | Contact time | 1 | 3 | 5 | h |
| X 3 | Initial concentration | 2 | 4 | 6 | mg L−1 |
| X 4 | Temperature | 20 | 30 | 40 | °C |
| X 5 | pH | 5 | 7 | 9 | — |
| X 6 | Ionic strength | 0 | 5 | 10 | mg L−1 |
| X 7 | HMDI/HP | 10 | 25 | 40 | Ratio |
| X 8 | MN/HP | 0.5 | 1 | 1.5 | Ratio |
| X 9 | Stirrer speed | 50 | 100 | 150 | rpm |
