Remoção de hidrocarbonetos policíclicos aromáticos de efluentes líquidos por adsorção em carvões ativados

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are a class of substances that have two or more aromatic rings in their molecules. PAHs can be transported over long distances in air and water and some are able to interact with DNA to promote mutagenic and carcinogenic responses. Because of their high stability and difficult degradation by microorganisms, it is difficult to remove PAHs using only biological processes and conventional physicochemical methods. In this way activated carbon is the most used porous solid for the removal of PAHs in solution through the adsorption process. In the present work, the removal of model PAHs (naphthalene, phenanthrene, fluorene and fluoranthene) from aqueous solutions by adsorption in different activated carbons was studied. Commercial activated carbons with different physicochemical characteristics were selected and as innovation for this field of research was used an activated carbon produced from residual biomass (cane straw). For the adsorption kinetics modeling, three kinetic models were tested to fit the experimental data: pseudo-first order, pseudo-second order, and Weber and Morris. The adsorption equilibrium was studied by comparing the performance of the activated carbons in the removal of the four PAHs over a wide concentration range (10-300 mg L-1). The adsorption equilibrium data were determined according to the isotherms of Langmuir, Freundlich and other models poorly explored in the literature, such as Langmuir-Freundlich, Sips, Radke-Plausnitz, Tóth, Redlich-Peterson, Fritz-Schlunder I and Fritz- Schlunder II. The parameter estimation procedure employed the isotherms in their original nonlinear form, avoiding problems associated with linearization. In addition to the interpretation of the results, the adsorbents were characterized by X-ray diffraction, N2 physisorption, temperature programmed desorption of CO and CO2, Boehm titration, thermogravimetric analysis and scanning electron microscopy. By PCA statistical analysis, performed with the quantitative data obtained in the characterization verified the presence of three groups of activated carbons with significant differences and, for this reason, the kinetic and equilibrium modeling was based only on these activated carbons, CASA- 2, CAA-5 and CAP-7. The kinetic data obtained a better fit for the pseudo-second order model and the Langmuir and Sips isotherms models adjusted better the equilibrium data for all PAHs studied and for all the activated carbons tested. Both, kinetic and the equilibrium data modeling indicated that the adsorbed amount of PAHs follows the order naphthalene > phenanthrene  fluorene > fluoranthene for all the activated carbons, showing a dependence on the molecular size of PAHs. Regarding the performance of the selected activated carbons in the PCA, the following order of the values of maximum quantity adsorbed (Qm) was found: CAP-7> CAA-5> CASA-2