Characterization of an absorbent based on Raphanus sativus (L. var.), a solid residue from biodiesel production, for the removal of methylene blue

Abstract

In this study, an adsorbent produced from Raphanus sativus (L. var.), a residue from the biodiesel industry, was investigated for the removal of methylene blue (MB) from textile effluents aiming to: (i) obtain physical and textural characteristics of the given carbon, and (ii) understand the MB adsorption mechanism onto such carbonaceous structures. The following methods were employed: SEM-EDS microscopy, gas adsorption and Raman spectroscopy. Physical and textural characteristics of the carbon obtained by classical methodologies such as BET, BJH and SEM analyses were corroborated by the Raman analysis. For the experimental conditions used on the preparation of the activated carbon of Raphanus sativus, the main physical parameters measured were specific surface area (236.3 m2.g-1), total pore volume (0.06 cm-3.g-1) and average pore size (1.4 nm). SEM and Raman analysis revealed that a microporous structure of carbon was obtained as the ratio between the disorder band and the graphite band was higher than unity, thus indicating that MB adsorption may occur predominantly at the surface of the adsorbent, given that the diameter of MB molecule was estimated to be nearly 1.5 nm. Raman analysis of solid MB revealed that MB molecules are predominantly found as dimmers in the solid however they are adsorbed as monomer in the Raphanus sativus carbon. Results evidence that MB adsorption probably occurs by bonding of monomer MB through the central nitrogen atom of its molecule; MB bonding through sulfur atom may also occur but at lesser intensity while MB bonding through amino nitrogen atom does not seem probable. © 2010 WIT Press.