Developing low-cost, environmentally friendly adsorbents is crucial for sustainable wastewater treatment and remediation of global wastewater pollution. CaO is a good adsorbent due to its high reactivity with CO₂, low cost, and high adsorption capacity. However, its practical implications are constrained by rapid deactivation from particle sintering and poor reversibility, limiting its application. In this study, we used a straightforward & sustainable procedure to create calcium oxide nanoparticles (CaO NPs) by thermally decomposing CaCO₃ derived from waste eggshells. FESEM was used to analyze the nanoparticles' size & chemical composition before they were used to adsorb Congo Red (CR) dye from aqueous solutions. Results indicated that their nano-sized structure, strong reactivity, and surface hydroxyl groups were responsible for their 99.40% removal efficiency and max 94.7 mg/g adsorption capacity in just one hour. Adsorption study was made by the Langmuir isotherm model & the Freundlich isotherm model of adsorption (R² = 0.96, & R² = Developing low-cost & environment-friendly adsorbents is crucial for sustainable wastewater treatment and also remediation of global wastewater pollution. CaO is a good adsorbent due to its high reactivity with CO₂, low cost, and high adsorption capacity. However, its practical implications are constrained by rapid deactivation from particle sintering and poor reversibility, limiting its application. In this study, we used a straightforward & sustainable procedure to create calcium oxide nanoparticles (CaO NPs) by thermally decomposing CaCO₃ derived from waste eggshells. FESEM was used to analyze the nanoparticles' size & chemical composition before they were used to adsorb Congo Red (CR) dye from aqueous solutions. Results indicated that their nano-sized structure, strong reactivity, and surface hydroxyl groups were responsible for their 99.40% removal efficiency and max 94.7 mg/g adsorption capacity in just one hour. Adsorption study was made by the Langmuir isotherm model & the Freundlich isotherm model of adsorption (R² = 0.96, & R² =