Evaluation of materials derived from rice husk as adsorbents of cationic and anionic dyes

Abstract

ABSTRACT The rice husk was chemically treated with acetic acid and calcined at 600 °C, in an uncontrolled atmosphere to obtain the ash (RHA), and in an oxygen-limited atmosphere to produce the biochar (BC). The influence of the thermal treatment on the adsorbent properties for removing cationic (methylene blue) and anionic (methyl orange) dyes was investigated. Both materials showed amorphous structures rich in silica, with acicular morphology and a high roughness. High surface areas (157.2 and 215.5 m²g-1) and pH at the point of zero charge close to neutrality (6.73 and 6.58) for RHA and BC were found, respectively. The anionic dye was adsorbed only by BC due to its contribution to the π- π stacking interaction between the C=C bonds of the adsorbent and the benzene rings of the dye. In contrast, the cationic dye was adsorbed by electrostatic interaction between the Si-O-Si and Si-OH groups of the adsorbents, with the positive charge of the dye. All adsorption isotherms adjusted better to the SIPS model, suggesting adsorption in homogeneous and heterogeneous sites. The observed results suggest good adsorption capacities and the interactions between surface groups on the adsorbents and functional groups of the contaminants are the most important mechanism in the adsorption of dyes of different ionic natures on surfaces derived from rice husk.

Keywords

• agro-industrial waste
• rice husk ash
• biochar