Enhanced Cd²⁺ Adsorption Using Engineered Hydroxyapatite Nanoparticles

The hydroxyapatite (HAp) nanoparticles were effectively engineered through a combination of ultrasonication and microwave techniques. This process significantly enhances the adsorbent’s specific surface area and particle size compared to microwave and other combinational techniques. The particle size of the adsorbent was effectively reduced to 30 ± 3 nm (length) and 10 ± 3 nm (width), with a crystallite size of 10 nm, an enhanced specific surface area of 105 m²/g. These modifications led to a significant acceleration in Cd²⁺ adsorption capacity, 195 mg/g at pH 7 in 20 min. The pseudo-second-order kinetic and Langmuir isotherm fitting confirm that Cd²⁺ adsorption occurs through chemisorption and that the adsorption is monolayer, respectively. The positive value of ΔH and the negative value of ΔG indicate that the adsorption of Cd²⁺ ions was endothermic and spontaneous, respectively. Very high and sustained regeneration efficiency was observed for HAp-UM (ultrasound and microwave treated HAp), 95% after seven regeneration 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.