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Coded units used for the variable parameters for the statistical analysis
| Variable | Unit | Coded values | ||||
|---|---|---|---|---|---|---|
| − α | − 1 | 0 | + 1 | + α | ||
| pH solution (X1) | 4 | 5 | 6 | 7 | 8 | |
| Contact time (X2) | min | 60 | 90 | 120 | 150 | 180 |
| Adsorption dosage (X3) | g/L | 1 | 1.5 | 2 | 2.5 | 3 |
| Initial concentration of CV (X4) | mg/L | 400 | 500 | 600 | 700 | 800 |
