Synthesis of magnetic activated carbon from passion fruit seeds and its application in the adsorption of methylene blue dye / Síntese de carvão ativado magnético de sementes de maracujá e a sua aplicação na adsorção do corante azul de metileno

Abstract

Brazil is a large producer of passion fruit, approximately 600 thousand tons of this fruit were produced in the year 2019, but about 70% of the mass is considered waste. The waste from this production could be aggregated in the synthesis of activated carbons for the treatment of textile effluents. However, the activated carbons need to be changed periodically and these can be removed from the inserted medium using magnetic fields by incrementing particles sensitive to this field on the surface of the adsorbent reducing the cost of the operation. The present work presents a comparison of 3 different carbons: (i) conventionally activated carbon with NaOH as an activating agent from dried passion fruit seeds at 500 °C, (ii) magnetic field sensitive activated carbon synthesized in the laboratory using FeCl3 6H2O as magnetizing agent at 700 °C and (iii) commercial activated carbon. The three types of carbons were characterized using the analyses of: FTIR, PZC, TGA and nitrogen physisorption. The adsorption tests were performed on the adsorption of methylene in batch. Four kinetic models were evaluated to predict the adsorption kinetics: pseudo-first order, pseudo-second order, Elovich and Weber-Moris and four adsorption isotherm models: Langmuir, Freundlich, Temkin and Redlich-Peterson. The characterization of the conventional activated carbon presented an adsorbent with a degradation curve by TGA that followed the dry seeds, presenting low concentration of non-volatile material, the FTIR showed in its surface ketone groups and CH2, with a pHPZC on the surface of approximately 5.46, with a microporous surface with 690 m² g-1 of type I for microporous surfaces and shows . The magnetization of this carbon significantly changed the properties on the surface, cation ionic surface; the TGA showed more non-volatile compounds, sensitivity of magnetic field, showing more acidic components on its surface such as hydroxyls and carboxyls and the surface pHPZC is 4.14 and a microporous surface with 501 m² g-1 specific surface area. The kinetic and isothermal tests showed promising, the conventional activated carbon showed a higher adsorption capacity than the commercial one and the magnetic activated carbon showed a similar adsorption capacity as the commercially available. All the adsorbents presented very similar behaviors among them, presenting a kinetics with characteristics of physisorption of a pseudo-first order adsorption and the adsorption isotherms presented a favorable behavior, endothermic and with Langmuir adsorption. Thus, the present work infers in the potential use of magnetic activated carbons in relation to the ones available in the market, with similar performance.