3.2.5. Competitive adsorption behavior of Cd2+ and Zn2+
The relationship between the adsorption capacity and the initial
concentration of Cd2+ and Zn2+ with the total concentration
changeless was shown in Fig. 8a. It suggested that the equilibrium
adsorption capacity of Cd2+ and Zn2+ ranged from 0.446 to 1.473
and 0.172 to 1.661mmolg−1 in the binary component systems,
respectively, which were less than those in the single component
solutions 1.664mmolg−1 for Cd2+ and 1.838mmolg−1 for Zn2+. It
indicated that one type of the metal ion present interfered with the
uptake of the other in the binary component system. The adsorption
maximumof total adsorptionwasapproximately constant. This fact
indicates that Cd2+ and Zn2+ were adsorbed in the same sites.
The equilibrium adsorption capacity of Cd2+ or Zn2+ with the
presence of Zn2+ or Cd2+ was shown in Fig. 8b and c, respectively.
It was shown that, when both Cd2+ and Zn2+ were present in the
solution together, some reduction of the Zn2+ or Cd2+ adsorbed
could be observed. In Fig. 8b, the equilibrium adsorption capacity
of Cd2+decreased from 1.607 to 0.848mmolg−1 with the initial
concentration of Zn2+ varying from 10−4 to 10−2 mol L−1, However,
in Fig. 8c, when the initial concentration of Cd2+ varied from
10−4 to 10−2 mol L−1, the equilibrium adsorption capacity of Zn2+
decreased from 1.662 to 0.154mmolg−1. Comparing with the equilibrium
adsorption capacity in the single system,we could conclude
that the interference of Cd2+ with the Zn2+ uptake was much more
pronounced, since a distinct reduction of the Zn2+ adsorbed was
observed even at a relatively low Cd2+ concentration. It confirmed
that the MNHAP adsorbents had better affinity for cadmium ion
than for zinc ion. The reason was due to the greater Langmuir
constant of Cd2+ than that of Zn2+ in the adsorption process. Similar
results were obtained by Corami et al. [39] when Cd2+ was
removed from multi-metal (Cd + Pb + Zn + Cu) solutions by sorption
on hydroxyapatite.