9811 Pseudo-second-order constants T (℃) k2/(g/(mg·min) qe(mg/g) h/(g/(mg·min) R2 20.
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[3]
2 Adsorption isotherm To optimize the design of an adsorption system for the adsorption of Cr(VI) onto GPC, it is important to establish the most appropriate correlation for the equilibrium curves. In the present study, the equilibrium data of the adsorption of Cr(VI) onto the GPC at pH 4. 0, 6. 0 and 8. 0 were fitted with the Langmuir equation and the Freundlich equation. Langmuir isotherm. The Langmuir model is applicable to homogeneous adsorption, where the adsorption of each molecule onto the surface has equal adsorption activation energy. Langmuir isotherm is represented by the following equation [12]: Ce/qe=l/(qm·KL)+Ce/qm (4) where Ce is the equilibrium concentration of Cr(VI) solution (mg/L), qe is the amount of adsorbed at equilibrium (mg/g), the constant qm and KL are Langmuir constants related to the adsorption capacity (mg/g) and the free energy of adsorption (L/mg), and the linear plot of Ce/qe versus Ce shows that adsorption follows Langmuir isotherm (Fig. 6). Values of qm and KL were calculated from the slope and intercept of the linear plot and were presented in Table 2. Table 2 showed that the Langmuir isotherm could yield relatively poor fits for Cr(VI) adsorption onto the grapefruit peel sorbent, with R2 values of 0. 9640, 0. 9346 and 0. 8791 for the adsorption at pH 4. 0, 6. 0 and 8. 0, respectively. Langmuir qm values for Cr(VI) were similar at the different pH in study. Conformation of the experimental data into Langmuir isotherm model with high R2 value indicates the homogeneous nature of the adsorbent surface. The essential characteristics of Langmuir isotherm can be expressed in term of a dimensionless parameter RL, also known as the separation factor, which is defined by [13]: RL=1/(1+ KL C0) (5) where KL is the Langmuir constant and Co is the initial Cr(VI) concentration (mg/L). The RL values indicates that the type of isotherms will be unfavorable (RL>1), linear (RL=1), favorable (0<RL<1) or irreversible (RL<0). The RL values were found to be between 0. 1684 and 0. 5934 at pH of 4, 6 and 8, which indicates favorable adsorption [17]. Freundlich isotherm. The Freundlich isotherm is an empirical model based on adsorption on heterogonous surface. It is expressed by the following equation [2]: lnqe=lnKF + 1/n lnCe (6) where qe is the adsorption capacity (mg/g), Ce is the equilibrium concentration of Cr(VI) in solution (mg/L), KF (mg1-1/nL1/ng-1) is the Freundlich constants or adsorption capacity, and 1/n stands for adsorption intensity. The values of KF and 1/n were calculated from intercept and slope of the plot between ln Qe vs ln Ce (Fig. 7, Table 2). The correlation coefficients (R2) were found to be 0. 9871, 0. 9658 and 0. 9198 for the adsorption at pH 4. 0, 6. 0 and 8. 0, respectively. 1/n is the heterogeneity factor and can be used as a measure of the deviation from linearity of the adsorption []Crini, G., Kinetic, equilibrium studies on the removal of cationic dyes from aqueous solution by adsorption onto a cyclodextrin polymer. Dye Pigment. 77(2) (2008).
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8791 Freundlich constants pH KF (mg1-1/nL1/ng-1) 1/n R2 4.
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9198 Summary In this study, grapefruit peel was converted into activated carbon (GPC) by chemical activation with ZnCl2 and used for the removal of Cr(VI) from aqueous solutions. The adsorption was found to be dependent on pH. At low pH a higher removal rate of Cr(VI) was obtained. An increase in temperature from 20 to 60℃ enhanced the adsorption capacity. The dynamical data fit very well with the pseudo-second-order kinetic model and the calculated adsorption capacities of 23. 98, 24. 33 and 24. 81 mg/g were equal to the actual values of the experiments at the initial Cr(VI) of 100 mg/L, at the temperature 20, 30 and 40℃. Freundlich model has better correlation coefficient than the Langmuir model in the concentration studied at different pH. The calculated isotherm parameters confirmed the favorable adsorption of Cr(VI) on the GPC. GPC is an effective adsorbent for the removal of Cr(VI) from aqueous solutions. Acknowledgements The authors especially thank the Guangxi Key Laboratory of Environmental Pollution Control Theory and Technology for the research assistance and the financial support from the Scientific Research and the Technical Development Plan Projects of Guangxi, China (GuiKeGong 10124002-2). References.
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