Impregnation of zinc oxide modified clay over alginate beads: a novel material for the effective removal of congo red from wastewater

Abstract

Alginate hydrogel beads were impregnated with zinc oxide (ZnO) modified sepiolite clay and applied for the removal of congo red (CR) dye from its aqueous solution in batch and packed column systems. After the synthesis, the beads were characterised by FTIR, XRD, and SEM which revealed the surface structures as well as forces of interaction between the beads and the dye leading to the adsorption process. Batch equilibrium data were subjected to Freundlich, Langmuir, Temkin and Dubinin–Radushkevich (D–R) isotherm models. The experimental data were best fitted to the Temkin isotherm with R² > 0.995, a maximum adsorption capacity (Q_max) of 546.89 mg g⁻¹ was obtained from the Langmuir model and an average R_L value of 0.58 suggested a favourable adsorption. Kinetic data were analysed with pseudo first-order, pseudo second-order, and Elovich kinetic models. The pseudo first-order model fitted best the data with a coefficient of determination, R² of 0.997 and statistical errors (% SSE) of less than 1%. An intraparticle diffusion model showed that the adsorption process develops in stages, as rapid, moderate and slow phases. Data from packed column adsorption were subjected to Clark, Thomas, Adams–Bohart and Yoon–Nelson models. The continuous flow process showed a flow rate, column height and concentration dependent breakthrough curves with the Yoon–Nelson model as the best fit of all the models giving R² of 0.999 and % SSE of less than 0.1%. The synthesized material was confirmed by characterization to have surfaces and functional groups suitable for adsorption, the adsorption capacity is comparably high with other synthetic materials.