Adsorption equilibrium, thermodynamics, kinetics, mechanism and process design of zinc(II) ions onto cashew nut shell

Cashew nut shell (CNS) is an agricultural waste was investigated as a new adsorbent for the removal of zinc(II) from aqueous environment. Effects of solution pH, CNS dose, contact time, initial zinc(II) concentration and temperature on removal efficiency were tested and optimum conditions were evaluated. The equilibrium data were fitted well with Langmuir isotherm model and pseudo-second-order kinetic model. Langmuir monolayer adsorption capacity of CNS was examined as 24.98 mg/g. Changes in standard Gibbs free energy (▵G°), standard enthalpy (▵H°) and standard entropy (▵S°) showed that the sorption of zinc(II) ions onto CNS are spontaneous and exothermic at 303–333 K. Sorption process was found to be controlled by both surface and pore diffusion. A batch adsorber was designed for different CNS dose to effluent volume ratios using Langmuir equation. Effective diffusivity values were found to be 1.927 × 10−11 (10 mg/L), 2.135 × 10−11 (20 mg/L), 2.267 × 10−11 (30 mg/L), 2.305 × 10−11 (40 mg/L) and 2.362 × 10−11 (50 mg/L) m2/s. © 2011 Canadian Society for Chemical Engineering