Behavior of a Mixture of Metals for Competiting Adsorption Sites of Untreated and Alkali-Treated Rice Husk

Elements are released into water bodies, affecting the environment and human health. To address this problem, the adsorption-desorption capacity of untreated rice husk (URH) and rice husk treated with alkali (RHTA) for Cu(II), Pb(II), Fe(II), Cd(II), and Zn(II) was investigated. Analyses during the process were performed by X-ray diffraction (XRD), scanning electron microscopy, and elemental analysis by energy dispersive X-ray spectroscopy (SEM-EDS), as well as Fourier transform infrared spectroscopy (FTIR). Adsorption-desorption kinetics and isotherms were carried out. The FTIR analysis of RHTA revealed a lowering of intensity of the bonding signals (OH, C-O, CH₂, CH₃, SiO₂, Si-OH) and loss of some signals due to the adsorbent-elements interaction. The adsorption on RHTA presented higher adsorption of Fe, followed by Pb, Zn, Cu, and Cd. Meanwhile, in URH, the adsorption was Fe > Pb > Cu > Zn and Cd. On the other hand, the desorption values for RHTA were Zn > Cd > Pb > Cu > and Fe and Zn > Cd > Cu > Pb and Fe for URH. The adsorption kinetics showed that data fit (R² ≥ 0.89) to pseudo-second-order kinetics and Freundlich isotherms (R² ≥ 0.84) for both ATRH and URH, indicating that the process occurs in the multilayer form and is controlled by chemisorption, where the adsorption depends on the active sites of the adsorbent surface.