A novel approach to modifying eggshell-based adsorbent for the removal of acid red 1 and crystal violet dyes: kinetics, isotherm, and thermodynamics study

The pollution of synthetic dyes from textile wastewater has raised environmental and health concerns, prompting the search for economical, sustainable waste-derived adsorbents. This study investigates the ability of modified eggshell powder to adsorb two deleterious dyes, the anionic Acid Red 1 (AR1) and the cationic Crystal Violet (CV), from aqueous solutions. Eggshells, a prevalent biowaste of calcium carbonate, were treated with ferrous sulfate using co-precipitation to generate modified eggshells that enhance their adsorptive characteristics. Different analyses were performed on the adsorbent, including XRD, zeta potential, and XRF. The adsorption experiments were conducted to examine the effects of varying solution pH from 2 to 9, contact duration ranging from 5 to 60 min, initial dye concentration ranging from 10 to 100 mg/L, and adsorption temperature from 20 to 60 °C. It indicates that the efficiency of dye removal was considerably influenced by pH, as the maximum adsorption for CV and AR1 dyes was found at pH 9 and pH 2, respectively. The modified eggshells exhibited a maximal adsorption capacity of 138 mg.g− 1 for CV and 124 mg.g− 1 for AR1, which increased in tandem with the dye concentration. The kinetic analysis demonstrated that CV adsorption conforms to a pseudo-second-order model, signifying the influence of pore diffusion and surface interactions, whereas AR 1 adherence aligns with a pseudo-first-order model. The Freundlich and Langmuir models were effectively aligned with isotherm modeling to validate the mechanisms of physical and chemical adsorption. As the negative entropy changes and the Gibbs free energy values indicate a non-spontaneous phenomenon, the adsorption process is exothermic and becomes less favorable at elevated temperatures. Modified eggshells treated dyes wastewater better as a low-cost adsorbent, making them a viable and environmentally beneficial alternative.