Adsorptive properties of biochar derived from biorefinery industry for basic dye removal

The aim of this work was to investigate the use of biorefinery industry by-products such as humins as adsorbents for methylene blue (MB). After thermal treatment, these by-products are called biochars. Three biochars were used as adsorbents for MB removal: (i) a biochar obtained after thermal treatment of humins (called raw biochar), (ii) a biochar obtained after NaOH treatment of the biochar from (i) (called biochar-OH), (iii) a biochar obtained after mixing humins with lignin, this mixture being then submitted to the same thermal treatment than (i) (called humins-lignin biochar). Structural characterization was done using ATR-FTIR, BET surface area analysis, and scanning electron microscopy (SEM). An elemental analysis was conducted to determine C, H, N, S, and O content in samples. Adsorption experiments were carried out as a function of time, pH, different background electrolytes (NaNO3 and CaCl2), and initial MB concentration. Kinetics data showed a good compatibility of all adsorbents with the pseudo-second-order model. The affinity of MB for biochar increased noticeably with the increase in pH, particularly for biochar-OH and humins-lignin biochar. Both the NaNO3 and CaCl2 background electrolytes had a negative effect on adsorption processes, and Ca2+ and Na+ acted as competitor ions with MB on the surface. The Langmuir model was more suitable for biochar-OH and humins-lignin biochar, whereas neither the Langmuir nor the Freundlich isotherms fitted experimental results for raw biochar due to its very low adsorption capacity. The maximum adsorption capacity predicted by the Langmuir model allowed us to establish the following classification of biochars: biochar-OH > humins-lignin biochar > > > raw biochar. Modifications of the surface had a positive impact on the adsorption capacity due to the additional available functional groups incorporated into the surface through lignin enrichment and the clean-up of blocked pores through NaOH treatment. Furthermore, formation of macropores on biochar-OH through the NaOH treatment created additional adsorption sites and probably promoted the adsorption of MB via a pore-filling mechanism.