Brazil nut shells as a new biosorbent to remove methylene blue and indigo carmine from aqueous solutions

The adsorption of methylene blue and indigo carmine, respectively a basic and an acid dye, was studied on raw Brazil nut shells. The dye removal from solution by BNS was governed by: (i) polarization effects between the colored ions and the surface sites, leading to physisorbed species due to weak electrostatic forces and (ii) diffusion limitations affecting the kinetic parameters. Thermodynamic studies showed that the adsorption of methylene blue and of indigo carmine was spontaneous and exothermic occurring with entropy decrease. H0 values confirmed the physical nature of the adsorption processes. The adsorption followed the Langmuir model and pseudo-second order kinetics over the entire range of tested concentrations but the process was controlled by intraparticle diffusion. The maximal uptakes were 7.81 mg g−1, for methylene blue, and 1.09 mg g−1 for indigo carmine, at room temperature. These results indicate that Brazil nut shells may be useful as adsorbent either for basic or acid dyes.