Avaliação das propriedades adsortivas de carvões ativados produzidos via tratamento termoquímico convencional e não convencional com vistas à remoção de cádmio em meio aquoso

Abstract

The conventional (muffle oven) and non-conventional method (microwave oven) were evaluated as heat sources for the production of activated carbon in order to adsorb cadmium in aqueous solution. For this, the Taguchi method was used to evaluate the influence factors in the production and optimization of three types of carbons; MF, carbonized and activated in muffle in a single step; MW, carbonized and activated in single microwave stage; and MFW, carbonized in muffle and activated in microwave oven in two steps. The activating agent for all carbons was HNO3, and the precursor used was coffee husks. Several physicochemical characteristics were evaluated in optimized carbons through Infrared Spectroscopy (FTIR), Thermogravimetric analysis (TGA), Elemental analysis (CHN), X-ray diffraction (XRD), Boehm titration Method, pHpzc and specific surface area estimation by methylene blue adsorption study and iodine number. Furthermore, it also evaluated the adsorption characteristics of the activated carbons optimized by pH influence and adsorbent dosage in the cadmium adsorption, adsorption kinetics, equilibrium and thermodynamics, in addition to preliminary adsorption tests of other metals and competition. The results showed that the increase in temperature favored the cadmium adsorption, and that adsorption process is chemical in nature for all activated carbons. The kinetic model that presented the best fitting was the pseudo-second order model, and the equilibrium model that presented the best fitting was Langmuir-Freundlich. In general, the carbon prepared in muffle oven (MF) had better adsorption characteristics at room temperature. The carbon prepared by microwave heating (MW) presented better structural characteristics, such as high number of acid functional groups, more porous and large surface area. In addition, the activated carbon carbonized in muffle oven and activated in microwave heating (MFW) had better functional characteristics, as large acid functional groups in comparison to the other activated carbons. Therefore, it is feasible to use muffle oven and microwave heating to prepare adsorbents using coffee husks as precursor and HNO3 as activating agent. However, the thermochemical treatment has high influence on physicochemical characteristics of the activated carbon and, therefore, its adsorption capacity.