Unraveling the mechanism of efficient adsorption of riboflavin onto activated biochar derived from algal blooms.

Riboflavin is commercially produced primarily by bio-fermentation. Nonetheless, purification and separation are particularly complex and costly. Adsorption from the fermentation liquor is an alternative riboflavin separation technology during which a cost-efficient adsorbent is highly desired. In this study, a low-cost activated algal biomass-derived biochar (AABB) was applied as an adsorbent to efficiently adsorb riboflavin from an aqueous solution. The adsorption capacity of riboflavin on AABB increased with the increase in pyrolysis temperature and initial riboflavin concentration. The adsorption isotherms were well described by the Freundlich and Langmuir models. The AABB displayed excellent adsorption performance and its maximum adsorption capacity was 476.9 mg/g, which was 6.8, 6.8, and 5.2 times higher than that of laboratory-prepared activated rape straw biochar, activated broadbean shell biochar and commercial activated carbon, respectively, which was mainly ascribed to its larger specific surface area and abundant functional groups. The mass transfer model results showed that mass transfer resistance was dependent on both the film mass transfer and porous diffusion. Raman and Fourier transform-infrared spectra confirmed the presence of π-π interactions and hydrogen bonding between riboflavin and the AABB. The adsorption of riboflavin onto AABB was a spontaneous process, which was dominated by van der Waals forces. These results will be beneficial for developing effective riboflavin recovery technologies and simultaneously utilizing waste algal blooms.