3.2.1. Effect of contact time
The isotherm of Cd2+ ions (pH = 5.6) adsorption by nHAp and
nHApCs is displayed in Fig. 5A. As obviously seen, the Cd2+ ions
removal was very fast at the beginning (within 20 min), and a
slower kinetics continued next until equilibrium. Generally, we
noticed that a contact time of 90 min was sufficient to ensure
saturation in the Cd2+ ions sorption capacity by these sorbents. In
order to investigate the rate of Cd2+ sorption, the linear form of
pseudo-second-order kinetic model was employed, where t/qt was
plotted versus t (see Fig. 5B). The second-order constants (k, qe, and
h) for nHAp and nHApCs were evaluated from Eq. (2) and listed in
Table 1. Interestingly, the pseudo-second-order kinetic model was
perfect in fitting the experimental results, where a close matching
was attained between the experimental and calculated equilibrium
sorption. Interestingly, the uptake capacity we obtained at
nHAp in this investigation was much (4 times) higher than those
reported earlier [30]. The equilibrium sorption capacity of the
nHApCs composite was ca. 1.33 times higher than that of nHAp.
This is likely because the availability of a large number of –NH2
groups capable of coordinately binding with Cd2+ ions.