Adsorption of Zinc ions by native and modified acacia bark (Acacia auriculiformis)

Abstract

Modified sorption materials were produced by treating earleaf acacia (Acacia auriculiformis) bark with low-concentration (1-3%wt) sulfuric acid solutions. The sorption properties of native and modified acacia bark for zinc (II) ions were studied. Based on the values of the initial and equilibrium concentrations of zinc ions in the solution, the materials' sorption capacity (A, mmol/g) was found and adsorption isotherms were plotted. It was found that the isotherms are type I isotherms according to the IUPAC classification and describe the process of monomolecular adsorption. It was found that acacia bark treatment with H2SO4 solutions resulted in an increase in the sorption capacity of the latter with respect to zinc (II) ions due to an increase in the area of the sorption material due to the extraction of part of the biopolymers in the matrix of the sorption material into solution. This circumstance leads to an increase in the pore space of modified samples of sorption material, which is proved by a decrease in the degree of crystallinity. The thermodynamic process parameters were found using the resulting adsorption process constants of the Langmuir and Dubinin-Radushkevich models: sorption energy (E, kJ/mol) and Gibbs energy (ΔG, kJ/mol), which values indicate that the processes of zinc (II) ions adsorption by native and modified acacia bark are physical adsorption processes. The kinetics has been studied and kinetic dependences of the processes of Zn2+ ions adsorption by native and modified acacia bark have been plotted. The resulting kinetic dependences were analyzed based on the Boyd diffusion model and it was determined that in all four cases mixed adsorption occurred, that is, both external and internal diffusion are limiting.