Adsorção de zinco a partir de cascas de arroz funcionalizadas: estudo em batelada e em coluna de leito fixo

Abstract

Water contamination by heavy metals poses a critical threat to aquatic ecosystems and public health. Among these elements, zinc (Zn2+) stands out due to its high toxicity and persistence in the environment, compromising biodiversity and water potability. Conventional wastewater treatment processes have limitations in removing residual heavy metals, which underscores the need for alternative, higher-performance technologies. In this context, adsorption using agro-industrial residues, such as rice husks, emerges as a promising strategy for removing zinc at low concentrations, combining effectiveness with the promotion of the circular economy. Accordingly, this study aimed to evaluate the efficiency of functionalized rice husks for zinc adsorption in both batch and fixed-bed systems, given the lack of comparative data between these models. To this end, raw rice husks, whole and crushed (0,30-1,40 mm), were subjected to acid treatment (TA) with H3PO4, basic treatment (TB) with NaOH, and combined treatment (TC) with sequential application of both reagents at different concentrations. In batch experiments, preliminary tests assessed the effect of pH (4 and 5) and husk particle size on process performance, establishing the optimal condition as the use of crushed material at pH 5, a value supported by the electrochemical characterization of the solids and the zinc specification diagram. Under these conditions, the effect of chemical modification on metal removal was evaluated, identifying the TC 1,0-1,0 adsorbent as the most effective. This selection was corroborated by surface analysis, revealing functional groups with affinity for Zn2+ cations, such as SiO- and R-COO-Na+. The material was then subjected to kinetic studies, with data best fitting the pseudo-second order (PSO) model (qe: 0,88 meq g-1, k2: 1,55 g meq-1 min-1). Mass transfer dynamics were analyzed using the intraparticle diffusion model (IPDM), indicating external and internal resistances. In equilibrium tests, the Sips isotherm provided the best fit to experimental data (ns: 0,64; ks: 10,3 L meq-1, qmax: 1,18 meq g-1). Regeneration of the TC 1,0-1,0 adsorbent over three adsorption/desorption cycles with HCl and NaOH showed greater effectiveness under basic conditions, maintaining zinc removal capacity for up to 4 reuse cycles. In continuous operation, packed-column design and kinetic modeling were performed for TC 1,0-1,0 beds with different heights (5, 10, and 15 cm, at 5 meq L-1) and initial zinc concentrations (3, 4, and 5 meq L-1, at 15 cm). Higher metal contents reduced the column’s breakthrough time (tt,u), while increased bed height extended it. Maximum efficiencies were observed for the 15 cm bed at 3 meq L-1 (69,44%) and the 10 cm bed at 5 meq L-1 (68,44%). Breakthrough curve kinetics were best described by the modified dose-response (MDR) model (qMDR: 0,78-1,22 meq g-1, αMDR: 6,58-10,65), except for the 15 cm bed at 3 meq L-1, which was better represented by the Clark model (Ac: 44,13, rc: 0,11 min-1, nF: 1,03). Although the highest adsorption capacity occurred in fixed-bed operation, the TC 1,0-1,0 adsorbent showed significant zinc removal in both systems, corroborating its applicability in advanced water and wastewater treatment.