A new adsorbent modified from walnut shell for the adsorption of Ni(II) from aqueous solution

A new adsorbent modified from walnut shell (MWNS) was prepared by grafting aspartic acid onto walnut shell (WNS). Two samples (MWNS1, MWNS2) were characterized through SEM, elemental analysis, XPS, point of zero charge analysis and N-2-BET analysis. The adsorption behaviors of Ni(II) onto WNS, MWNS1 and MWNS2 were studied. The results showed that the adsorption kinetics of Ni(II) onto MWNS1 and MWNS2 followed the pseudo-second-order model, while the ones onto WNS followed the pseudo-first-order model. The adsorption mechanism was investigated using the intraparticle diffusion model. The equilibrium data of Ni(II) adsorption onto MWNS2 were analyzed using the Langmuir, Freundlich and Dubinin-Redushkevich (D-R) models. Meanwhile, the mean feature concentration (C-e') correlated with temperature was determined. The values of the mean free energy (E) obtained from the D-R model indicated the adsorption process was conducted by monolayer chemisorption for C-e<C-e' and followed by physical adsorption for C-e>C-e'. The thermodynamic parameters (Delta G, Delta H and Delta S) showed that the Ni(II) adsorption onto MWNS2 was spontaneous, endothermic and entropy driven. Recycling property of MWNS2 was studied.