3.2. MO removal3.2.1. Adsorption ki

3.2. MO removal
3.2.1. Adsorption kinetics Fig. 3a shows the adsorption kinetics of MO on PAC-HNO3 and xFe3O4/PAC-HNO3. Clearly, for all the sorbents, MO uptake was fast during the first 60 min and pseudo-equilibrium was reached in 180min. After the equilibrium period, the amount of adsorbed MO did not significantly change with time. The quick adsorption may be due to the smaller particle size of PAC-HNO3 and xFe3O4/ PAC-HNO3 sorbents, leading to presenting a more accessible surface as well as a high availability of active sites on sorbent surfaces at the beginning of the experiments, whereas the observed plateau corresponds to a slow rate of adsorption may be due to accumulation of dye particles on the active sites. In order to understand the mechanism of MO adsorption onto the surface of the sorbents, the Lagergren first-order and the pseudo-second order kinetic models were used to analyze the kinetic experimental data. The fittings of the Lagergren first-order and pseudo-second order kinetic models to the kinetic experimental data for the MO adsorption onto the adsorbents at pH 5.0 are shown in Fig. 3a. Table 2 summarizes the adsorption kinetic model parameters obtained from these models. Clearly, the R2 values for the pseudo-second-order model are close to unity and the calculated qe values agree well with the experimental values, demonstrating that the pseudo-second-order model can be applied to