Adsorption of Zn(II) IONS by Ziziphus joazeiro barks in aqueous solutions

In this study, five types of "Ju & aacute;" bark (Ziziphus joazeiro) were investigated for the removal of Zn(II) ions from aqueous solutions. The tested samples were natural barks, natural powder, washed with water (WW), washed with ethanol at 80 % (EE), and washed with 0.5 N NaOH. The adsorptive tests investigated were kinetics, equilibrium isotherms, thermodynamics, and desorption. The WW adsorbent presented better adsorption with the metal confirmed by scanning electronic microscopy with electronic dispersive spectroscopy (SEM-EDS). After reaching the equilibrium time, all adsorbents, except NaOH, presented a reduction in the adsorptive capacity, indicating the behavior of disassociation of Zn(II) ions from the material when exposed to times greater than 60 min. Physical adsorption was confirmed for the WW adsorbent by associating the FTIR spectrum, as well as the Delta H degrees being less than 80 kJ.mol(-1) and the easy recoverable due to desorption being 100 % of the metal after 10 min of contact with the solution acid. These conditions suggest an adsorption mechanism through hydrogen bonds between the water molecules solvated with zinc and the carboxylic groups of the adsorbent. The isotherm at 25 degrees C showed the formation of two distinct layers where the best fit for BET liquid phase adsorption isotherm, and a maximum removal of 71.9 % for the first stage. At first equilibrium, the best-fit isotherm (Sips) presented characteristics of homogeneous systems (monolayer), and the thermodynamic conditions demonstrate that adsorption is endothermic and viable adsorption mainly from 25 degrees C.