Kinetic and Thermodynamic Studies of the Biosorption of Ni(II) by Modified Rape Straw

Biosorption equilibrium, kinetics and thermodynamics of Ni(II) onto the modified rape straw (MRS) by ZnCl2 were studied in a batch system with respect to pH, temperature and initial metal ion concentration. The Ni(II) removal rate increased with rising pH values, reaching about 99.7% at pH 6.5. The biosorption efficiency of Ni(II) to the biomass decreased as the initial concentration of metal ions was increased. But it is less likely to be affected by temperature from 303K to 323K. The adsorption data fit Langmuir adsorption isotherm at 303K and 313K. According to Langmuir isotherm, the theoretical maximum adsorption capacity of MRS was 9.17mg·g-1 and 10.45mg·g-1 at 303K, 313K, respectively. The pseudo-first-order and pseudo-second-order kinetic models were applied to test the experimental data for initial Ni(II). The pseudo-second-order kinetic model provided the best correlation of the used experimental data. Thermodynamic parameters can be calculated by Gibbs equation, ΔG is negative, but ΔH is positive, it can be concluded that the process of removing Ni(II) by MRS are spontaneous and endothermic. The XPS spectrum of nickel-treated biosorbent reveals that Ni(II) is really sorbed onto biosorbent.