To determine the time required for equilibrium adsorption, we
varied the contact time between adsorbent and adsorbed in the
range [15–1800] min at pH 4, the initial concentrations of
Cd(II) and Cr(III) have been fixed at 200 mg L1, and the concentration
of clay in the suspension at 1 g L1. The quantities
of adsorbed cadmium and chromium per gram of clay were
calculated by the method of residues using Eq. (2). These
quantities are plotted as a function of contact time in Fig. 2.
The adsorption of Cd II on JCKp and JCKb reached equilibrium
around 120 min. and showed a slight decrease in the
case of JCKp and a decrease for the largest raw clay JCKb that
could be attributed to desorption of cadmium. Also, some
other studies that analyzed the kinetics of adsorption of Cd
II on kaolinite noted a decrease after 12 h (Srivastava et al.,
2005). Similarly, the adsorption of cadmium on vermiculite
exhibits this decrease of retention after 65 h (da Fonseca
et al., 2006). However, the adsorption of Cr(III) on JCKp is
almost instantaneous, since after 15 min, a plateau is reached.
The adsorption of Cr(III) on JCKb is slower since it is completed
after 30 min.