Table 1. Box–Behnken Design Variables for the Study.

	Levels
Factors	Low (−1)	Medium (0)	High (+1)
Fe(VI) concentration (ppm), X1	400	500	600
PTA concentration (mg/50 mL model fuel), X2	100	200	300
Mixing speed (rpm), X3	4,400	7,600	10,800
Mixing temperature (°C), X4	40	50	60