Atomic layer deposition surface functionalized biochar for adsorption of organic pollutants: improved hydrophilia and adsorption capacity

Atomic layer deposition (ALD) thin film coating was applied to improve the hydrophilia of biochar derived from black willow. 2 (2Al, 0.82 wt% Al₂O₃), 5 (5Al, 1.40 wt% Al₂O₃), and 10 (10Al, 2.36 wt% Al₂O₃) cycles of alumina ALD were applied. The biochars were characterized by inductively coupled plasma–atomic emission spectroscopy, nitrogen adsorption and desorption, scanning electron microscopy, and Fourier transform infrared spectroscopy. The adsorbents were utilized for the removal of methylene blue (MB) from an aqueous solution to evaluate their adsorption capacities. The 5Al biochar showed the highest adsorption capacity, compared to the uncoated biochar and other Al₂O₃ coated biochars, due to its improved hydrophilia. The amount of MB adsorbed onto the 5Al biochar was almost three times that adsorbed onto the uncoated biochar during the first hour of adsorption experiments. Adsorption isotherms were modeled with the Langmuir and Freundlich isotherms. The data fit well with the Langmuir isotherm, and the maximum adsorption capacities were found to be 26.8 and 35.0 mg/g at 25 °C for the uncoated biochar and 5Al biochar, respectively. The adsorbed MB amount per square meter achieved 1.3 mg/m² onto the 5Al biochar, and it was twice the amount on the uncoated biochar. The experimental data were analyzed by pseudo-first-order and pseudo-second-order kinetics models of adsorption. The pseudo-second-order model better describes adsorption kinetic data for the uncoated biochar and 5Al biochar than the pseudo-first-order model does.