RESUMO
Caffeine is the most widespread active pharmaceutical compound in the world, generally studied as a tracer of human pollution, since caffeine levels in surface water correlate with the anthropogenic load of domestic wastewater. This work investigated the use of different steel wastes named as SW-I, SW-II, SW-II, SW-IV, SW-V, and SW-VI in the adsorption of caffeine. These materials were pretreated and characterized by scanning electron microscopy, Fourier transform infrared spectroscopy, X-ray diffraction, and point of zero charge. The samples are mainly composed of iron (hematite and magnetite). The caffeine adsorption test indicated that SW-VI (steel slag dust) is the most efficient and promising (removal around 51.68%) in relation to the other residues, which it was selected for further studies. Equilibrium time was reached within 96 h of contact between the adsorbent and the adsorbate, with removal of 84.00%, 81.09%, and 73.19% for the initial concentrations of 10 mg L-1, 20 mg L-1, and 30 mg L-1 of caffeine. The pseudo-first-order, pseudo-second-order, and Elovich models presented a good fit to the experimental data. However, the pseudo-first order model described better the experimental behavior. Adsorption isotherms were performed at three temperatures (298, 308, and 318 K). The maximum adsorption capacity was 17.46 ± 2.27 mg g-1, and experimental data were better fitted by the Sips isotherm. Values of ΔG° and parameters equilibrium of the models of Langmuir, Sips, and Temkin were calculated from the standard enthalpies and standard entropies estimated. The values of ΔG° were negative for the temperatures studied indicating that the adsorption process is viable and spontaneous. Negative values for ΔH° were also found, indicating that the process of caffeine adsorption by SW-VI is an exothermic process (0 to -40 kJ mol-1). Thus, the adsorption of caffeine by SW-VI is a physical process. The SW-VI material showed economic viability and promising for the adsorption of caffeine in aqueous media.
Assuntos
Cafeína , Poluentes Químicos da Água , Humanos , Adsorção , Águas Residuárias , Aço , Poluentes Químicos da Água/análise , Óxido Ferroso-Férrico , Cinética , Concentração de Íons de Hidrogênio , Termodinâmica , Espectroscopia de Infravermelho com Transformada de Fourier , Ferro , Água , Poeira , Preparações FarmacêuticasRESUMO
RESUMO Este estudo investigou a eficiência da remoção de nitrato em meio aquoso usando adsorção em argilas montmorilonitas comerciais, comparativamente a uma resina aniônica comumente usada nessa operação. Algumas dessas argilas foram intercaladas como o íon hexadeciltrimetilamônio para avaliação. Áreas superficiais específicas, capacidades de troca catiônica e espectros de infravermelho médio foram obtidos para caracterização das argilas. Testes preliminares indicaram a nanoargila funcionalizada com aminopropiltrietoxisilano e octadecilamina como a mais eficiente na adsorção de nitrato, conquanto não tenha superado em eficiência a resina aniônica comercial também testada. Curvas cinéticas e isotermas de equilíbrio de adsorção de nitrato em fase aquosa na nanoargila e na resina comercial foram determinadas e modeladas para comparação, como também para seleção de modelos representativos e para estimativa de propriedades termodinâmicas dos sistemas estudados. Todos os ensaios foram realizados a 25°C e com pH neutro. Isotermas de equilíbrio também foram determinadas a 35 e 45°C. Os resultados obtidos mostram que o processo adsortivo estudado é físico, espontâneo e favorável tanto para a resina como para a nanoargila, as quais possuem muito boa afinidade com o nitrato. O modelo mais representativo para a cinética foi o de pseudossegunda ordem, enquanto para o equilíbrio foi a isoterma de Freundlich.
ABSTRACT This study investigated the efficiency of nitrate removal in aqueous medium using adsorption on commercial montmorillonites compared to an anionic resin widely used in this operation. Some of these clays were intercalated with hexadecyltrimethylammonium ion for this investigation. Specific surface areas, cation exchange capacities and mid-infrared spectra were determined for clays characterization. Preliminary tests indicated that the nanoclay functionalized with aminopropyltriethoxysilane and octadecylamine was the most efficient in nitrate adsorption, although it did not exceed anionic resin in efficiency. Adsorption kinetic and equilibrium of nitrate from aqueous phase on the nanoclay and anionic resin were determined and modeled for comparison, as well as for the selection of representative models and estimation of thermodynamic properties of the studied systems. All experiments were carried out at 25°C and at pH 7. Equilibrium isotherms were also determined at 35 and 45°C. The obtained results show that the adsorption processes studied are physical, spontaneous and favorable for both the resin and the nanoclay, which have very good affinity with nitrate. The most representative models for the systems were the pseudo-second order kinetic model and the Freundlich equilibrium isotherm.