Enhanced Removal of Cu(II) Ions from Aqueous Solution by Poorly Crystalline Hydroxyapatite Nanoparticles

Poorly crystalline and well-dispersed hydroxyapatite (HAP) nanoparticles were synthesized and used as novel adsorbents for the removal of Cu(II) from aqueous solution. Various factors affecting the adsorption such as adsorbent crystallinity, pH, adsorbent dosage, contact time, temperature, competing cations, and the presence of humic acid were investigated in detail. Results showed that the HAP calcined at lower temperature was poorly crystalline and had better adsorption capacity for Cu(II) than those calcined at higher temperature. Cu(II) removal was increased with increases of pH, adsorbent dosage, temperature, and the presence of humic acid, but decreased as the existence of competing divalent cations. Kinetic studies showed that pseudo-second-order kinetic model better described the adsorption process. Equilibrium data were best described by Langmuir model, and the estimated maximum adsorption capacity of poorly crystalline HAP was 41.80 mg/g at 313 K, displaying higher efficiency for Cu(II) removal than many previously reported adsorbents. Thermodynamics studied revealed that the adsorption of Cu(II) by poorly crystalline HAP was spontaneous, endothermic, and entropy-increasing in nature. This study showed that poorly crystalline HAP could be used as an efficient adsorbent material for the removal of Cu(II) from aqueous solution.