The linear regression data of different models.

Adsorption model	Parameters	Cd–SIP	Pb–SIP	Cu–SIP	Zn–SIP	NIP
		Cd(ii)	Pb(ii)	Cu(ii)	Zn(ii)	Cd(ii)	Pb(ii)	Cu(ii)	Zn(ii)
Lagergren-first-order	Q e,cal (mmol g−1)	2.27	2.20	2.16	2.12	0.50	0.47	0.44	0.43
	k 1 (min−1)	0.12	0.11	0.11	0.11	0.11	0.11	0.11	0.12
	R 2	0.9991	0.9992	0.9991	0.9992	0.9991	0.9984	0.9979	0.9986
Pseudo-second-order	Q e,cal (mmol g−1)	2.76	2.63	2.54	2.45	0.62	0.57	0.53	0.49
	k 2 (g mmol−1 min−1)	0.047	0.046	0.046	0.044	0.194	0.189	0.186	0.181
	R 2	0.9974	0.9962	0.9954	0.9967	0.9948	0.9936	0.9954	0.9948
Elovich equation	α (mmol g−1 min−1)	0.77	0.70	0.66	0.61	0.15	0.12	0.16	0.17
	β (g mmol−1)	1.66	1.69	1.69	1.69	7.30	7.40	7.60	9.16
	R 2	0.9995	0.9993	0.9994	0.9992	0.9991	0.9994	0.9991	0.9992