Challenges and opportunities of Napier grass-derived biochar via liquefied gas for humic acid adsorption and DFT analysis

The purpose of this research was to evaluate the efficiency of humic acid (HA) removal by the Napier grass-derived biochar. Napier grass was pyrolyzed in a pilot furnace at 600 °C (CNP_600). The adsorption kinetics followed the pseudo-second-order model, while the adsorption isotherm was consistent with the Freundlich isotherm. In practical application, the CNP_600 biochar showed the reasonable removal of disinfection byproduct precursors in terms of dissolved organic carbon (DOC) and UV-254 absorbance of 49.0% and 70.3%, respectively. The specific UV absorbance (SUVA) decreased from 2.0 to 1.2 L/mg·m. This result clarified that the CNP_600 biochar effectively adsorbed aromatic hydrocarbons, which were key precursors to disinfection byproduct (DBP). The adsorption mechanism was identified as chemisorption, a result that aligns well with the Density Functional Theory (DFT) analysis. According to the DFT calculations, the oxygen atoms associated with the –COOH, –OH, and –O– functional groups exhibit regions of negative potential, suggesting their capability to donate electrons and function as nucleophilic sites. In contrast, the hydrogen atoms present on the CNP surface display positive potential zones, indicating their vulnerability to electrophilic interactions with HA molecules. In conclusion, CNP_600 biochar potential as an adsorbent for DOM removal under the tested experimental conditions.