Influence of surface hydrophobicity/hydrophilicity of biochar on the removal of emerging contaminants

This study investigated the removal of Acetaminophen (ACT) using biochars having different physicochemical characteristics. Biochars subjected to post-pyrolysis heat-treatment at 300 °C for different treatment times (0, 3.5, 8 and 24 h) were used. The resulting biochars were characterized using FTIR and X-ray diffraction spectroscopy. Experiments for ACT adsorption with different biochars loads (0.0, 0.05, 1, and 2 g L⁻¹) were performed. Using the best performing material, ACT adsorption was investigated for additional biochar loads (4.0, and 6.0 g L⁻¹) and experiments to test the effect of ionic strength were undertaken for different ions (chloride, carbonate, and nitrate) at three different concentrations (0.0, 1.0, 5.0 mM). The results showed that the changes to the surface of the thermally treated biochars increased the adsorption of ACT. The changes in the amount of oxygen-containing functional groups on the surface of the modified biochars (e.g., C[dbnd]O from 47.8 a.u. to 152 a.u. in the untreated and thermally treated biochars, respectively), as well as modifications to their crystalline structure are considered to be the reason for the observed improvement. Adsorption isotherms and kinetic models suggest the generation of an adsorbate monolayer and chemisorption as the rate-limiting step. The different anions tested were found to have a significant influence on ACT adsorption, related to their electronegativity and steric effect, as confirmed by the multivariate analysis.