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Table 1

Experimental range and levels of independent variables

Range and levels
VariablesLow level (−1)Center level (0)High level (+1)ΔXia
Initial pH of solution (pH), X1 3 ( − 1) 5 (0) 7 (1) 
Initial dye concentration (C0), X2 50 ( − 1) 175 (0) 300 (1) 125 
Adsorbent dosage (d), X3 0.2 ( − 1) 1.1 (0) 2 (1) 0.9 
Temperature (T), X4 23 ( − 1) 36.5 (0) 50 (1) 13.5 
Range and levels
VariablesLow level (−1)Center level (0)High level (+1)ΔXia
Initial pH of solution (pH), X1 3 ( − 1) 5 (0) 7 (1) 
Initial dye concentration (C0), X2 50 ( − 1) 175 (0) 300 (1) 125 
Adsorbent dosage (d), X3 0.2 ( − 1) 1.1 (0) 2 (1) 0.9 
Temperature (T), X4 23 ( − 1) 36.5 (0) 50 (1) 13.5 

aStep change values.

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