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Independent variables and their corresponding levels.
| Independent variables | Unit | Levels (α = 2) | ||||
|---|---|---|---|---|---|---|
| − α | Low (− 1) | Center (0) | High (+ 1) | +α | ||
| (X1) MG concentration | mg L−1 | 6.0 | 12 | 18 | 24 | 30 |
| (X2) pH | – | 3.0 | 5.0 | 7.0 | 9.0 | 11 |
| (X3) Nanohybrid adsorbent | Mg | 6.0 | 12 | 18 | 24 | 30 |
| (X4) Temperature | °C | 10 | 20 | 30 | 40 | 50 |
| (X5) Contact time | min | 3.0 | 6.0 | 9.0 | 12 | 15 |
