DISCUSSION
As illustrated in Fig. 2, initially the amount of Cd(II) ions adsorbed increase rapidly, but then the process slows down and subsequently attains a constant value after 240 min i.e., when adsorption equilibrium is established. This phenomenon was due to a large number of vacant surface site for metal ions adsorption during the initial stage. Near the equilibrium the remaining vacant surface sites were difficult to be occupied due to repulsive forces between the solute molecules on the solid and bulk phase. The adsorption of Cd(II) by composite materials increases as the initial Cd(II) concentration increased. An increase in the initial Cd(II) concentration leads to an increase in adsorption capacity of the Cd(II) onto the adsorbent. This is due to the increase in the concentration gradient between the bulk and the surface of the prepared adsorbent with the increase in initial Cd(II) concentration. Base on the correlation coefficients, the adsorption of Cd(II) is best described by the second-order equation. In many cases the first order equation does not fit well to the whole range of contact time and is generally applicably over the initial stage of the adsorption processes.