Dye Adsorption from Aqueous Solutions by Using Hydrogel‐Rice Husk Biochar Composites: Isothermal, Kinetic, and Thermodynamic Aspects

Abstract The rapid growth of global industrialization generates more organic pollutants, jeopardizing ecological safety and harming human health. The aim of this study was to examine the adsorption of methyl green (MG), methylene blue (MB), and cresyl violet (CV) onto the composite made from polyacrylamide and rice husk biochar. The incorporation of rice husk biochar, derived from agricultural waste, introduced a highly porous structure and abundant surface functional groups into the hydrogel matrix, significantly improving its adsorption performance. The synthesized composite underwent characterization through SEM, FTIR, TGA, and DCS. The performance of the synthesized composite in dye removal was investigated by analyzing the impact of the dye's initial concentration, contact duration, and solution pH. The hydrogel‐biochar composite (B4) exhibited maximum adsorption capacities of 21.972, 23.53, and 22.11 mg/g for MG, MB, and CV, respectively, under optimized experimental conditions. Different adsorption kinetic and isotherm models were evaluated to determine the adsorption mechanism. The kinetic data were best described by the pseudo‐first‐order model, while the equilibrium data were most accurately fitted by the Temkin isotherm model. The calculated equilibrium adsorption capacities (q e ) from the experiment for MG, MB, and CV were 21.97, 23.53, and 22.11 mg/g, respectively. The intra‐particle diffusion model was found to be the rate‐limiting mechanism of dye adsorption. Thermodynamic analysis verified that MG, MB, and CV adsorption onto hydrogel‐rice husk biochar composite was spontaneous and endothermic. The synthesized composite exhibited a good regeneration capacity. Furthermore, the reusability of hydrogel for up to four cycles enhances its ability to adsorb dyes from wastewater.