Experimental factors at different levels used for the ultrasound-assisted leaching experiment at 35 °C for 2 h.

Factor	Code	Unit	Low axial (−2)	Low factorial (−1)	Centre point (0)	High factorial (+1)	High axial (+2)
S/L ratio	A	%	0.01	0.5	1.0	1.5	2
Ultrasound power	B	W	50	88	125	163	200
Lemon juice conc.	C	%	0	20	40	60	80
H2O2 conc.	D	%	0	5	10	15	20