Comparative study for adsorption of methylene blue dye on biochar derived from orange peel and banana biomass in aqueous solutions.

Biochar pyrolyzed at 800 °C from banana (B) and orange peels (OP) was applied for sorption of methylene blue (MB) dye in a batch system. OP showed better affinity for MB dye than B with rapid increase in sorption capacity and percent removal for both biochars attaining equilibrium at 30 min. Chemisorption was suggested as the rate limiting step based on the best fitting of the pseudo-second-order reaction kinetics to the batch adsorption data. Linear increase in sorption capacity was seen as the initial MB dye concentration increased from 50 to 300 mg g with a 40 % decrease in removal efficiency. An increase of 90 mg g in sorption capacity for both biochars with a 15 and 30 % increase in removal efficiency for OP and B, respectively, was observed after increasing the solution pH from 2 to 6 or 8. An increase in sorption capacity of about 150 mg g was seen by increasing the biochar dose from 0.1 to 0.5 g. Langmuir isotherm model represented the adsorption data well as reflected by the high values of R (0.99) when using both biochar, while least representation of adsorption data was seen in H-J isotherm as estimated from very low R (0.6-0.66) for both types of biochar. An endothermic nature of MB dye sorption was suggested based on the linear increase in sorption capacity with an increase in solution temperature from 30 to 60 °C. This was further confirmed by the observed positive changes in standard entropy and standard enthalpy while negative values of Gibbs-free energies proposed the non-spontaneous natures of MB dye sorption on to both biochars. The effective sorption of MB dye demonstrated the potential of plant-based biochar as economically viable adsorbents for MB dye.