Adsorption potential of biochar obtained from pyrolysis of raw and torrefied Acacia nilotica towards removal of methylene blue dye from synthetic wastewater

Abstract

Torrefaction, as a pretreatment of biomass coupled with pyrolysis process, has gathered significant attention in recent years to obtain higher quality bio-oil. Being an energy-intensive process, the application of byproducts such as biochar from the integrated process might offset the extra energy provided during the torrefaction process. With this hypothesis, present work aimed to investigate the performance of biochar (BC) obtained from pyrolysis of native (BC-raw) and torrefied biomass (BC-torrefied) towards the synthetic wastewater treatment containing methylene blue (MB) dye. Both biochars were characterized by their physicochemical properties. The impact of time, dose of adsorbent, pH, concentration of MB, and temperature were examined during batch adsorption process. Results showed that BC-torrefied (103.47 m2/g) has higher Brunauer–Emmett–Teller (BET) surface area than BC-raw (80.40 m2/g). Both biochars behave in similar fashion toward MB removal; however, BC-torrefied had greater adsorption capacity (158.13 mg/g from Sips isotherm at 50 mg/L MB concentration) because of higher BET surface area, pore volume, and complexation on surface as compared to BC-raw (85.68 mg/g from Sips isotherm at 50 mg/L MB concentration). The experimental data were in good agreement with Sips isotherm and pseudo-second-order kinetic model for both biochars. The MB adsorption was unprompted and endothermic. Further, it was observed that hydrogen bonding, electrostatic force of attraction, ion exchange, surface complexation, and π-π interaction between MB dye and adsorbent were primarily accountable for adsorption process. Thus, BC-torrefied could be novel adsorbent for water treatment, and its application will facilitate the integrated torrefaction-pyrolysis process. © 2021, The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.