Three-dimensional hierarchical porous P,O co-doped pinecone biochar for tetracycline removal: behavior and mechanism

Abstract

Tetracycline (TC) presents significant risks to the human health and has the potential to cause substantial harm to ecological systems due to its excessive utilization. However, carbon-based materials have attracted considerable interest for their efficacy in removing TC from aquatic environments. In this study, phosphorus and oxygen co-doped pinecone biochar (po-PBC) with a three-dimensional (3D) hierarchical porous architecture was synthesized and employed for the adsorption of TC. To elucidate the adsorption characteristics of po-PBC for TC, four adsorption kinetic models and four isotherm models were utilized. The adsorption data were best described by the pseudo-second-order kinetics model, as evidenced by an R² value of 0.9975. Furthermore, the removal efficiency of TC using po-PBC increased to 81.67% at 313 K for an initial concentration of 150 mg L⁻¹ in comparison to unmodified PBC. The observed increase can be attributed to the co-doping of phosphorus and oxygen heteroatoms, along with the presence of a three-dimensional hierarchical porous structure. The Langmuir isotherm model, exhibiting R² values between 0.9830 and 0.9926, accurately characterized various equilibrium isotherms. The maximum adsorption capacities for TC at temperatures of 298 K, 308 K, and 318 K were determined to be 1137.09 mg g⁻¹, 1379.23 mg g⁻¹, and 1401.77 mg g⁻¹, respectively. The relatively low average free energies, ranging from 0.11 to 0.63 kJ mol⁻¹, corroborated the physical nature of the adsorption process. Thermodynamic parameters (ΔH > 0, ΔG < 0, and ΔS > 0) further indicated that the adsorption process was spontaneous and endothermic. Moreover, the adsorption capacity of TC was retained at 88.12% after five successive adsorption-regeneration cycles. Therefore, po-PBC demonstrates significant potential for the removal of TC from wastewater. Additionally, it offers novel insights into the reduction, stabilization, and resource utilization of pinecones.