(a).

Kinetic model	Linear equation	Initial Cr(VI) concentration, mg/L	Reaction constant	R 2
Pseudo-first-order	$\log (q_e-q_t)=\log q_e-\frac{k_1}{2.303}t$	40	k 1 = 0.0230 min−1	0.842
		60	k 1 = 0.0115 min−1	0.882
		100	k 1 = 0.0115 min−1	0.906
		120	k 1 = 0.0046 min−1	0.870
Elovich	$q_t=\frac{1}{\beta }\ln \left(\alpha \beta \right)+\frac{1}{\beta }\ln t$	20	α = 0.0081 mg·g−1 ·min−1   β = 11.11 g/mg	0.907
		40	α = 0.0227 mg·g−1 ·min−1 β = 2.3923 g/mg	0.965
		60	α = 0.0518 mg·g−1 ·min−1 β = 2.6316 g/mg	0.861
		100	α = 0.5897 mg·g−1 ·min−1 β = 2.4390 g/mg	0.902
		120	α = 0.3663 mg·g−1 ·min−1 β = 1.8315 g/mg	0.956