Interpretations on the mechanism of In(III) adsorption onto chitosan and chitin: A mass transfer model approach

The mass transport mechanism of the In(III) adsorption on chitin and chitosan was elucidated in this research. The chitosan and chitin presented specific surface area values of 3.60 and 4.30 m(2) g(-1), with average pore radius of 14.30 and 33.10 angstrom, respectively. It was identified in chitosan and chitin, typical groups, such as N-H and C-O. It was found that both materials present a rigid surface without the presence of visible pores and cavities, with a geometry that resembles plates. Further analysis indicates the presence of the In(III) on the surface of both materials. The isotherm experiments indicated that the temperature caused an increase in the adsorption capacity for both materials. The Langmuir model was chosen for describing the equilibrium in both cases. The External Mass Transfer Model (EMTM) and Pore Volume and Surface Diffusion Model (PVSDM) were used for describing the indium concentration decay curves for the chitosan and chitin. Nevertheless, the modified Biot number (between 8.82 x 10(-4) and 2.71) indicated that, in this case, the external mass transfer was the dominant mechanism. In addition, chitin and chitosan were efficient adsorbents for In(III), since that only 0.35 mg L-1 of In(III) remained in solution after adsorption. Chitosan provided faster adsorption kinetics, and this fact was attributed to its higher external surface area. The findings revealed that the overall mass transport mechanismis as follows: the indium species in the bulk solution are transferred to the adsorbent surface and then, adsorbed by surface precipitation and/or coordination bonds. (C) 2020 Elsevier B.V. All rights reserved.