Mechanistic and theoretical investigation of acetaminophen adsorption on titanium-coated biochar: Statistical physics models and DFT insights

Abstract

This study investigated acetaminophen removal using biochar from Pinus elliottii modified by titanium deposition under different conditions. The raw biochar was initially obtained, and then the magnetron sputtering technique was applied to deposit thin films of titanium on the biochar surface under different conditions. The effect of titanium deposition was evaluated by an integrated approach that included physicochemical characterization of the adsorbent materials, statistical physics modeling, and theoretical analysis by Density Functional Theory (DFT), allowing an in-depth understanding of the adsorption mechanism at the macroscopic and molecular levels. Models based on statistical physics indicated that the removal process occurs predominantly by physical adsorption (adsorption energy between 19.71 and 24.75 kJ mol⁻¹), with the formation of double layers and variations in the adsorption energy according to the metal deposition conditions. The steric parameters (n > 0.5) revealed changes in the adsorption geometry, and the number of molecules adsorbed per site correlated with the titanium deposition conditions. DFT analysis showed electronegative regions in the O11 and O16 atoms with Condensed Double Descriptors (CDD = –0.0637 and –0.0511), associated with forming hydrogen bonds with the biochar. The distribution of the frontier orbitals showed a HOMO-LUMO gap of 5.24 eV, indicating the high electronic stability of the molecule. The results demonstrate that titanium deposition selectively modifies the adsorptive properties of biochar, interaction energies, and adsorption geometry.