Avaliação de estruturas derivadas da casca de arroz como adsorventes de corantes catiônico e aniônico

Abstract

The presence of dyes in aqueous media generates environmental and human health impacts. They can be removed of these systems by adsorption and an alternative to reduce costs of this method is the use of agro-industrial residues as adsorbents, such as rice husk, as raw material in the development of new adsorbents. Its thermal treatment by different routes can generate ash (RHA) or biochar (BC), depending on the conditions of the heating atmosphere, determining different adsorbent characteristics for these materials. Thus, the objective of this work is to produce RHA and BC from rice husk and evaluate them as adsorbents in the removal of cationic (methylene blue) and anionic (methyl orange) dyes from aqueous media by the relationship between the removal capacity and the properties of each material. The rice husk was chemically treated with acetic acid 1,7 M, and then calcined at 600 °C, in an uncontrolled atmosphere, to obtain the RHA, and in an oxygen-limited atmosphere, to obtain the BC. The samples were chemically and structurally characterized by XRF, XRD and FTIR-ATR. Surface area determination was done by BET analysis and microestructural evaluation by SEM-EDS. The zero-charge point (ZPC) was determined for each material and adsorption studies were carried out in batches, evaluating the influence of pH, contact time and initial concentration of the dye solution. RHA and BC showed amorphous structures rich in silica, with acicular morphology and high roughness. High surface areas (157.2 and 215.5 m²g-1, respectively) and ZPC close to neutrality (6.73 and 6.58, respectively) for RHA and BC were found. The anionic dye, methyl orange, was adsorbed only by BC, with greater removal efficiency at pH 2 and 4 and kinetic curves better fitted by the pseudo-first order model. This pH value contributed to the π- π stacking interaction between C=C bonds of the adsorbent and the benzene rings of the dye. The cationic dye, methylene blue, was better adsorbed at pH 10 and with a better fit to the pseudo-second order model. Its adsorption occurred mainly by electrostatic interaction between the siloxane and silanol groups of the adsorbents, negatively charged, with the positive charge of the dye. All adsorption isotherms, obtained under the best adsorption conditions, adjusted better to the SIPS model, suggesting adsorption in homogeneous and heterogenous sites, with calculated qmáx of 53.19, 45.03 and 602.60 mg g-1 for methyl orange in BC and methylene blue in BC and RHA, respectively. The observed results suggest good adsorption capacities, and the interactions between surface groups are the most important mechanism in the adsorption of dyes of different ionic natures on surfaces derived from rice husk.