Simultaneous removal of Cu2+, Ni2+ and Zn2+ ions using leftover Azadirachta indica twig ash

A novel adsorbent namely, Azadirachta indica twig ash, was identified and evaluated for adsorption of Cu2+, Ni2+ and Zn2+ ions. The properties of novel adsorbent were studied by various characterization techniques such as Scanning Electron Microscopy, Fourier Transformation Infrared analysis, X-ray diffraction, proximate and ultimate analysis. Furthermore, an optimization study was conducted to determine the optimal value of process parameters. Various dimensionless numbers have been derived and used to elucidate the adsorption dynamics in the present study. Application of Artificial Neural Network in data modeling together with comparative analysis of novel adsorbents with other adsorbents has been done. Elemental analysis revealed 38.47% Carbon, 1.89% Hydrogen, 49.21% Oxygen and 10.43% Nitrogen in the A. indica twig ash. Outcome of X-ray diffraction showed amorphous characteristic (58.4%) of an adsorbent. BET surface area and bulk density was found to be 71.35 m(2)/g and 0.47 g/cm(3) for A. indica twig ash. Furthermore, the correlation plot showed a deviation of 0.16% for Cu2+, 0.09% for Ni2+ and 0.08% for Zn2+ between experimental and Artificial Neural Network predicted values which implied that the chosen algorithm and methodology were appropriate for prediction. The obtained values of film and pore diffusivity were 2.13 x 10(-12), 5.17 x 10(-12), 3.43 x 10(-11) cm(2) sec(-1) and 3.70 x 10(-13), 4.44 x 10(-13), 5.55 x 10(-13) cm(2) sec(-1) for Cu2+, Ni2+ and Zn2+ ions, respectively. These values revealed that the adsorption mechanism depends on the pore diffusion. The values of dimensionless numbers N-k and indicated that the adsorption of Cu2+, Ni2+ and Zn2+ ions on A. indica twig ash surface was predominantly controlled by diffusion. Moreover, isotherm and kinetic modeling demonstrated superior values of the regression coefficient for the Langmuir isotherm and pseudo-second order kinetic model. This reflected that the adsorption occurred in monolayer fashion. Simultaneous removal of metal ions followed an order based on electronegativity: Cu2+ > Ni2+ > Zn2+. Positive Delta H (126,579.20 kJ/mole, 96,527.95 kJ/mole and 73,076.65 kJ/mole) and Delta S (437.65 J/mol K, 331.15, J/mol K and 248.71 J/mol K) for Cu2+, Ni2+ and Zn2+ ions, respectively showed that binding of metal ions on the surface of A. indica twig ash was endothermic with increased turbulence at the solid-aqueous interface.