Synthesis of Clinoptilolite-Polyacrylonitrile nano Composite for removal of zirconium from aqueous solution: Investigation and evaluation of effective parameters on sorption process, kinetic, thermodynamic and adsorption isotherms parameters

In the present research, composite adsorbents consisting of nano clinoptilolite and polyacrylonitrile were prepared. The synthesized composites were characterized by XRD, XRF, FT-IR, DTG and SEM analysis techniques, and finally the adsorption behavior of the composites toward zirconium was investigated. In this nano adsorbent, nano-zeolite clinoptilolite acts as an active component for the absorption of zirconium ions, and polyacrylonitrile plays a bining role. In addition, by changing the size of zeolite particles from micrometer to nano scale, the adsorption capacity and the kinetic of the adsorption process was increased significantly. The absorption rate by nano composites was very rapid and more than 75% of the maximum absorption capacity for zirconium was obtained in the first 5 hours. The SEM image showed that zeolite particles are bonded to each other by a polyacrylonitrile polymer. The porous structure of the nano composite allowed permeation of the ions from solution into nano composite beads and reaching the ion exchange sites. The effect of pH, initial ion concentration, contact time, and temperature were examined. The optimum contact time and pH were 24 h and 2, respectively. The maximum adsorption capacity of the composite was 18.65 mg.g^-1 and the composite was able to remove 80% of Zr⁺⁴ from 0.01 meq.mL^-1 aqueous solutions. The kinetic and thermodynamic parameters were extracted. The experimental data were well fitted with a pseud-second order kinetic model with good correlation coefficients. In addition, the theoretical values obtained from the equation showed a good agreement with experimental values. Therefore, the pseudo-second order kinetic equation can be considered as a suitable model for interpreting experimental data. The agreement of the experimental data with the Pseudo-second order kinetic model showed that overall rate constant controlled by chemical sorption. In addition, the constant rate of absorption by nano composites was higher than that of a clinoptilolite-polyacrylonitrile-macrometric composite. In other words, the absorption of zirconium ion on nano composite was significantly higher than that of a zeolite composite with macro dimensions. Positive ΔH° and negative ΔG° were indicative of the endothermic and spontaneous nature of process. The equilibrium data were analyzed by the Langmuir, Freundlich, and Dubinin–Radushkviech isotherm models. D-R isotherm model indicated that ions were uptake through an ion exchange process. The obtained R_L values range between 0 to 1, indicating Zr adsorption was favorable. Comparison of Q₀ values for adsorbents showed that the nano composite has the highest absorption capacity for zirconium ion. This high Q₀ value can be explained by the nano scale size of the composite.