Adsorption of methyl orange and methylene blue on activated biocarbon derived from birchwood pellets

This study explored the adsorption capacity of a physically activated biocarbon derived from the pyrolysis of birchwood pellets (ABPB) towards two dyes - methyl orange (MO, anionic) and methylene blue (MB, cationic), at pH 2, 7 and 11. Compared to mineral commercial activated carbon (CAC mineral ), ABPB exhibited lower SSA, pores volume and surface; higher external surface and average pore diameter, similar ash content and aromaticity, and stronger hydrophilicity and polarity. The maximum adsorption capacity on ABPB was equal to 220 mg/g for MO and 91 mg/g for MB after 17 h. Batch tests with variable adsorbent amount (0.5–2.5 g/L) showed for both ABPB and CAC mineral better results for lower adsorbent dose. Under all tested conditions, ABPB showed higher or analogous adsorption capacity compared to CAC mineral . Based on the pK a of MB (3.80) and MO (3.46) and on the pH PZC of ABPB (5.3), the adsorption was favored at pH 2 for MO and at pH 11 for MB. Kinetics analysis and isotherm modelling revealed that, although many different physicochemical interactions occurred between ABPB and the dyes molecules, chemisorption is the rate-controlling step and prevalent mechanism. In conclusion, this study may provide support to further research aimed at exploring the effect of ABPB's physicochemical properties on the efficiency and mechanisms of dyes adsorption. • The adsorption capacity of a physically activated biochar from birchwood pellets has been investigated. • Methyl orange (anionic) and methylene blue (cationic) have been used as model adsorbates. • Activated birchwood pellets biochar showed higher or analogous adsorption capacity compared to commercial activated carbon. • Chemisorption is the rate-controlling step and prevalent adsorption mechanism.