Accelerating Cd2+ adsorption using hydroxyapatite nanoparticles engineered by combination of ultrasound and microwave treatment

Abstract

<title>Abstract</title> The hydroxyapatite (HAp) nanoparticle was effectively engineered through a combination of ultrasonication followed by microwave techniques. This modification significantly enhances the adsorbent's characteristics compared to microwave and other combinational techniques. The particle size of the adsorbent was effectively reduced to 30×10 ± 3 nm, with a crystallite size of 10 nm, and an enhanced specific surface area of 105 m²/g. These modifications led to a significant acceleration in Cd²⁺ adsorption capacity, reaching 185 mg/g at pH 7 within 20 min. Both pseudo-second-order kinetic and Langmuir isotherm fittings confirmed monolayered Cd²⁺ adsorption on homogeneous sites via chemisorption. Thermodynamic studies indicated an exothermic and spontaneous behavior of the Cd²⁺ ion adsorption process. Very high and sustained regeneration efficiency was observed for HAp-UM, reaching 95% after seven cycles. The simple and rapid synthesis of HAp-UM demonstrates a drastic enhancement in Cd²⁺ ion removal capacity, making it a promising option for wastewater treatment.