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

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.