Adsorption of methylene blue onto carbons made of residues from the biodiesel industry

Abstract

The performance of carbons produced from the cold-pressed cake of Raphanus sativus (L. var.), a residue from the biodiesel industry, was evaluated for the adsorption of methylene blue (MB) from synthetic aqueous solutions. The study has focused on the following three topics: (i) physical and textural characterization of the carbon, (ii) batch investigation of equilibrium and kinetics of MB adsorption at room temperature, and (iii) analysis of MB adsorption mechanism onto such carbonaceous structures. Regarding the fi rst topic, carbon was characterized using several methods such as Brunauer–Emmett–Teller (BET), Barrett–Joyner–Halenda (BJH), scanning electron microscopy (SEM), and Raman spectroscopy. A microporous carbon was found thus indicating that MB adsorption may occur predominantly at the surface of the carbon. Regarding the second topic, equilibrium tests revealed that Langmuir-based maximum MB uptake capacity was 19.8 mg g –1 and that MB kinetics onto carbon was best described by a pseudo second-order model. Data fi tting analysis confi rmed that MB adsorption onto such carbons was predominantly superfi cial. And fi nally, regarding the third topic, MB adsorption may probably occur by bonding of monomer MB through the central nitrogen atom of its molecule and through sulfur atom while MB bonding through amino nitrogen atom does not seem probable. The study reveals that carbons made of R. sativus are suitable materials for the removal of dyes from aqueous solutions, mainly if fifraction of carbons is used, because MB adsorption was found to be predominantly superfi cial.