Comparison of porous and non-porous biochar for trimethoprim removal: Quantifying uncertainty in sorption and thermodynamic parameters over different pH and temperature

Abstract

This study conducted a comparative analysis of non-porous (CN400) and porous (CN600) biochar for trimethoprim (TMP) removal under varying pH (3, 7, and 10) and temperature conditions (20, 26, and 32 °C). Bayesian nonlinear regression was used to quantify uncertainty in both adsorption isotherms and derived thermodynamic parameters. The biochar was characterized by yield, point of zero charge, scanning electron microscopy with energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, X-ray diffraction, and N₂ adsorption-desorption isotherms. The experimental data were statistically interpreted using the pseudo-second-order, modified intraparticle diffusion, and Langmuir isotherm model. The results showed that the non-porous structure of CN400 had a smaller surface area than that of CN600 (4 vs. 372 m² g⁻¹, respectively). However, CN400 exhibited a higher sorption capacity (9.44 vs. 3.48 mmol kg⁻¹) at pH 7 and 20 °C, and significantly faster sorption kinetics than CN600 (2 vs. 87 min, respectively). Based on frontier molecular orbital theory calculations, TMP sorption onto CN400 was driven by three synergistic mechanisms including electrostatic attraction, hydrogen bonding, and π–π electron donor–acceptor interactions. Interestingly, raising the temperature from 20 to 32 °C resulted in a 1.8-fold increase in Q_max for CN400 and up to a 6.9-fold increase for CN600, attributed to enhanced pore diffusion kinetics at high solution temperatures. Bayesian inference-based thermodynamic analysis confirmed that the TMP sorption process was both spontaneous and exothermic. Our statistical findings suggest that tailoring CN400/CN600 biochar could offer a synergistic strategy to accelerate TMP adsorption and maximize sorption capacity in water treatment systems.