Influence of structural and surface properties of Nb₂O₅ pellets on methylene blue adsorption and adsorbent reuse capacity

Abstract

This study aimed to describe the preparation of Nb₂O₅ pellets by uniaxial pressing, to investigate the influence of heat treatment at different temperatures (800, 900, and 1000 °C) on the physical, surface, microstructural, and adsorptive properties of the materials, and to assess their potential as reusable adsorbents for methylene blue removal. Specimens treated at 800 °C contained a mixture of monoclinic and hexagonal phases, with a predominance of monoclinic structures. By contrast, heat treatment at 900 °C and above resulted in complete conversion to the monoclinic phase. The surface of Nb₂O₅ particles contained basic groups, possibly oxygen ions (O^-). Experimental investigations showed that methylene blue adsorption followed second-order kinetics, suggesting chemical diffusion as the dominant process. Maximum adsorption occurred at pH 6.0. Pellets remained intact after the adsorption test. A study was carried out to determine the reusability of pellets over five adsorption cycles in methylene blue solutions at different concentrations. The maximum adsorbed concentration was 10.4 mg.L^-1. The adsorption capacity of Nb₂O₅ specimens gradually decreased from 56% in the first cycle to 25% in the fifth cycle. Despite this reduction in removal efficiency, Nb₂O₅ pellets showed promise as reusable adsorbents in heterogeneous processes because of their high potential adsorption capacity and easy separation from the reaction medium.

Keywords:
adsorption; methylene blue; niobium pentoxide; reuse