Characterization of coconut shell-based activated carbon and its application in the removal of Zn(II) from its aqueous solution by adsorption

Adsorption kinetics and loading capacity of Zn(II) onto a coconut shell-based activated carbon (AC) was investigated at different solution concentrations, pH values, and temperatures. Characterization of the AC was performed by using scanning electron microscopy, N-2 adsorption and desorption isotherms, pore size distribution, surface area determination, determination of point of zero charge (pH(PZC)), and Boehm titration. The equilibrium studies showed that the Zn(II) adsorption onto the AC increases from 1.98 to 4.24 mg/g by increasing the initial Zn(II) concentration from 23 to 148 mg/L. The kinetic experiments revealed that the initial adsorption rate of Zn(II) is rapid and decreased afterwards. The adsorption rate significantly increased with raising the temperature from 25 to 45 degrees C. The activation energy for the adsorption of Zn(II) ions onto the AC was 54.23 kJ/mol. Both of the adsorption kinetics and loading capacity decreased with increasing the acidity of solution. The results of Boehm titrations, determination of pH(PZC), and the time course of the system pH during the sorption process showed that the adsorption of Zn(II) ions onto the AC takes place through cation-exchange mechanism with acidic surface functional groups. The results of linear and nonlinear regression methods showed that the equilibrium data among different two- and three-parameter isotherm models closely obey from a three-parameter isotherm model, namely Koble-Corrigan. The results of linear regression method revealed that the kinetic data among different kinetic models closely obey from Elovich model. The rate-limiting step in the adsorption of Zn(II) onto the AC was assessed through the Webber and Morris plots.