Sustainable synthesis of carbide slag waste derived hydroxyapatite and its application for cationic dye adsorption

Hydroxyapatite (HAp) and its composites have gained significant recognition as an effective biocompatible adsorbent for a variety of pollutants, attributed to its extensive surface area, robust chemical and thermal stability, and non-toxic nature. Yet, its industrial-scale application is obstructed by the cost of synthesis. This study investigates the sustainable and cost-effective route of HAp synthesis using carbide slag waste. The carbide slag-derived hydroxyapatite (CS-HAp) is synthesized via a two-step neutralization method at ambient conditions by employing carbide slag waste as an eco-friendly calcium source. The thorough characterization of CS-HAp revealed a favorable Ca/P ratio of 1.86 with adequate structural and thermal properties. The prepared CS-HAp is proposed as an economical adsorbent by employing it for the removal of model pollutant methylene blue (MB) dye from an aqueous solution. Batch adsorption experiments assessed the influence of operational parameters such as initial dye concentration, initial pH, contact time, and adsorbent dose on MB removal. The adsorption kinetics were best described by the pseudo-second-order model with an equilibrium adsorption capacity of 36 mg/g close enough to the actual equilibrium adsorption capacity of 32 mg/g. The Freundlich isotherm model adequately represented the adsorption isotherms with R² of 0.9712, indicating a multilayer adsorption process.