Nano-hydroxyapatite modified biochar : insights into the dynamic adsorption and performance of lead (II) removal from aqueous solution

Adsorption of lead as Pb(II) using biochar is an environmentally sustainable approach to remediate this kind of pollution affecting wastewater. In this study, rice straw biochar (BC) was modified by combination with nano-hydroxy-apatite (HAP), resulting in a material designated as BC@nHAP, with enhanced adsorption performance. Based on Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) analyses, it was evidenced that, after modification, HAP greatly enhanced surface functional groups (i.e., –COOH and/or –OH) of raw biochar's surface. Batch tests showed that the maximum sorption capacity of BC (63.03 mg g⁻¹) was improved due to the modification, reaching 335.88 mg g⁻¹ in BC@nHAP. Pseudo-second order (PSO) kinetics fitted well the adsorption data (R² = 0.99), as well as the Langmuir isotherm model (showing an adsorption value of 335.88 mg g⁻¹ for q_e). The results of thermodynamic calculations showed that the adsorption was primarily governed by chemisorption process. FTIR spectroscopy and XPS spectrum after adsorption further confirmed that the adsorption mechanisms were ion exchange with Pb²⁺ and surface complexation by –OH and –COOH. In addition, BC@nHAP revealed a brilliant regeneration capability. The maximum adsorption capacity by BC@nHAP was higher than that of raw biochar or other previously reported adsorbents. Therefore, BC@nHAP could be seen as a new sorbent material with high potential for real-scale heavy metal removal from wastewater, and specifically as a capable candidate new sorbent for Pb(II) removal from wastewater, which has clear implications as regard preservation of environmental quality and public health.