1. Kinetics of Cadmium ions Adsorption on Tree Fern from Aqueous Solutions Chung-Chi Wang* and Yuh-Shan Ho # School of Public Health, Taipei Medical University Figure 1. Pseudo-second order adsorption kinetics of cadmium onto tree fern at various initial concentrations Introduction In recent years many cheap, widely-available materials have been identified as suitable adsorbents for the removal of metal ions from wastewaters. Traditionally, the kinetics has been described by the pseudo-first order equation typical of that derived by Lagergren. In recent years, pseudo-second order rate law has been considerable as one of the most appropriate models. In the present research the mechanism of cadmium sorption on tree fern has been studied. The factors studied include the influence of tree fern particle size, dose and initial cadmium ions concentration on the diffusion processes and the sorption kinetics of the system. Materials and Methods The raw tree fern was dried in an oven at 105°C for 24 h. The adsorbent was screened to get geometrical size such as 44-53, 61-74, 74-88, 88-104 and 104-124 µm. The stock cadmium solutions were prepared with distilled water and Cd(NO 3 ) 2 ·4H 2 O. All working solutions were prepared by diluting the stock solution with distilled water. A range of cadmium concentrations from 65 to 280 mg/dm 3 were used as simulated wastewater. 6.8 g sample of tree fern was added to each 1.7 dm 3 of cadmium solution to a 2 dm 3 adsorber vessel. They were agitated with the speed of 200, 250, 300 and 350 rpm and a constant water bath at 25°C for 120 min. 2.5 mL of the solution were withdrawn at suitable time intervals and filtered through a 0.45  m membrane filter. It was analysed by an inductively coupled plasma atomic emission spectroscopy analyser (ICP-AES) for the concentration of cadmium. Results and Discussion The results show that the adsorption capacity of cadmium increases from 10.9 to 15.2 mg/g with the increases of the tree fern dose and the sorption capacity of cadmium increases from 9.6 to 15.3 mg/g with the increases of the cadmium concentration. Kinetics Model Lagergren pseudo-first order model Ho pseudo-second order model Figure 2. Effect of particle size on the adsorption of cadmium onto tree fern Conclusion  The equilibrium adsorption capacity is effected by changing initial cadmium concentrations, tree fern particle size and dose.  The process of adsorption of cadmium ions on tree fern based on the assumption of the pseudo-second order mechanism.  The rate constant of adsorption, the equilibrium capacity and the initial adsorption rate predict from the pseudo second-order model. Table 1. Parameters for the effect of initial cadmium concentration Table 2. Parameters for the effect of tree fern dose Table 3. Parameters for the effect of initial cadmium concentration C 0 : initial cadmium concentration (mg/dm 3 ); h, initial adsorption rate (mg/g min); q e, equilibrium capacity (mg/g); k, pseudo-second order rate constant (g/mg min); r, correlation coefficient