Adsorption of microcystin contaminants by biochars derived from contrasting pyrolytic conditions: Characteristics, affecting factors, and mechanisms

The growing incidence of microcystins (MCs) in the environment has become an issue of global concern for the high ecological and human health risks. Herein, a comparative adsorption of three MCs (MC-LR, MC-YR and MC-RR) by spent mushroom substrate (SMS)-derived biochars from contrasting pyrolytic conditions (temperature: 600/300 °C; and gas steam: CO2/N2) was surveyed to better understand the mechanisms and factors affecting the adsorption performance. For biochar preparation, 600 °C and CO2 led to greater levels of aromaticity, ash, SBET, and porosity, while 300 °C and N2 created more surface functional groups. The adsorption of MCs by biochars was a pH-dependent and endothermic physisorption process, following the pseudo-second-order kinetics (R² = 0.99) and linear isotherm model (R² > 0.88). The distribution coefficients K_d (0.98–19.2 L/kg) varied greatly among MCs (MC-YR > MC-RR > MC-LR) and biochars (BC600 > BN600 > BC300 > BN300), which depends on the combined effects of hydrophobicity, electrostatic attraction, H-bonding, cation bridging, and the amounts of adsorption sites on biochars. Higher ash, SBET, and total pore volume of BC600 facilitated the adsorption capacity for MCs relative to other biochars. Furthermore, the co-adsorption efficacy for MCs (K_d = 1.09–8.86 L/kg) was far below those for the single adsorption, indicating strong conflicts among competing MCs. This study sheds light on the roles of pyrolytic temperature and gas steam in biochar properties, and elucidates the mechanisms and factors affecting the adsorption performance of different MCs, which lays a foundation for MCs removal from water. Copyright © 2020 Elsevier B.V. All rights reserved.