Removal of Pb (II) from aqueous solutions using functionalized walnut shell

The excessive discharge of Pb(II) into the environment has increasingly aroused great concern [1]. In this work, the process of removal of Pb(II) by walnut shell modified by carboxylic groups was evaluated. The modified walnut shell was characterized by the FT-IR and SEM techniques. The adsorptive properties of the modified walnut shell were investigated as a function of pH (2-6), contact time (1-20 min), temperature (15–55 ◦C), and initial Pb(II) concentrations (50-200 mg/L). The percent adsorption increases in the pH range of 3-5, and the adsorption of Pb(II) is maximum and constant at pH 5 up to 6. The equilibrium time is attained after 10 minutes.The highest percentage removal (98.5%) was obtained at pH 5.5, the adsorbent dose of 0.8 g/L, and the initial metal concentration of 100 mg/L. The kinetic investigations of Pb(II) removal suggested that the adsorption processes followed a pseudo-second order kinetics model. Also according to the Langmuir isotherm, the maximum adsorption capacity of 196.1 mg/g was acquired for Pb(II), which is better or comparable with some previously reported adsorbents. It should be noted that based upon the results obtained for temperature, the adsorption process is spontaneous at room temperature, and it was endothermic in nature. The adsorbent was reused without significant deterioration after elution with a suitable eluent.