Modified natural zeolite with crown ethers such as 12crown4, 15crown5, 6crown18 and benzo15crown5 have been characterized using Fourier transform infra-red, scanning electron microscope and Brunauer–Emmett–Teller instruments. Concentration of 0.01 mg/L of complexing agent and modification time of 240 h were obtained as optimum parameters. Effective parameters such as pH, adsorbent dose, contact time, type and concentration of crown ethers, ion strength, and initial concentration of lithium ion and temperature of complex formation were also investigated and optimized in this adsorption process. Under the optimized experimental conditions pH equal to 7, one gram of adsorbent in 2 mL solution of 0.01 mg/L and temperature of 40°C, the most lithium adsorption (87%) was acquired. The effect of interference ions on the adsorption process was also considered. The results showed that Freundlich model is one of the best descriptive models for this process. The adsorption kinetics of lithium on modified zeolite follows the pseudo second-order model. Thermodynamic parameters were calculated and shown that adsorption process is an endothermic and spontaneous process. Statistical analysis of experimental results was done by fuzzy logic tool of MATLAB software and three-dimensional diagrams of different parameters effect on the adsorption percentage were also presented.
