Application of Hydroxyapatite Functionalized Magnetic Rice Husk Biochar for the Adsorption Removal of Cu(II) Ions from Aqueous Solutions

Biochar with abundant functional groups, porous structure, and high specific surface area has been widely used as an adsorbent for the removal of metal ions, and its low production costs make it economically suitable for large-scale application in various industries. Hydroxyapatite nanoparticles were used to functionalize magnetic rice husk biochar (MBC) to produce hydroxyapatite functionalized magnetic rice husk biochar (HAP@MBC) in this study. The functional groups, magnetism performance, surface morphology, and aperture structure of HAP@MBC were characterized to explore the feasibility of its application in the treatment of wastewater containing Cu(II) ions. It is found that the maximum adsorption removal rate reaches 96.02% at 35°C, pH=5.0, with an adsorbent dosage of 0.65 g/L and an initial Cu(II) ion concentration of 50 mg/L. Additionally, adsorption kinetics, isotherms, and thermodynamics were analyzed to probe into the adsorption mechanism of Cu(II) ions by HAP@MBC. The results show that the adsorption process is very close to the pseudo-second-order model, indicating that the adsorption process is mainly controlled by chemisorption. The adsorption of Cu(II) ions can also be fitted by the intraparticle diffusion model, revealing that the adsorption rate is limited by intraparticle diffusion as well as other factors. Moreover, the adsorption process is consistent with the Langmuir isotherm, and the saturated adsorption capacity is calculated to be 81.59 mg/g. Thermodynamic analysis shows that the value of adsorption enthalpy change is 48.35 kJ/mol, suggesting the adsorption process is chemisorption. More precisely, it is a spontaneous entropy-increasing reaction, and an increase in the temperature is conducive to the adsorption. To sum up, HAP@MBC exhibits perfect adsorption properties and has the potential to be a novel and promising adsorbent for the removal of Cu(II) ions.