Equilibrium and kinetic studies of caffeine adsorption from aqueous solutions on thermally modified Verde-lodo bentonite

Abstract

The removal of pharmaceutical compounds from water and wastewater is a subject of interest to the scientific community, since these substances have been related to several environmental and health problems. Usual techniques are not effective to remove pharmaceuticals and adsorption is an alternative technology with high potential to treat contaminated water. This work aimed to investigate the use of thermally modified bentonite Verde-lodo for batch adsorption of caffeine from aqueous solution. A kinetic study occurred at atmospheric pressure and 25 °C. The adsorption equilibrium time was 40 h, and the models adjusted to experimental data were pseudo-first order, pseudo-second order, Boyd and intraparticle diffusion and the pseudo-second order model showed the best fit. The resistance to external film was the limiting step on mass transfer. The equilibrium study was performed at temperatures of 15, 25, 40 and 60 °C. The Langmuir model was adjusted to data and the highest adsorption capacity was obtained at 60 °C (0.73 mmol/g). The Freundlich and Dubinin-Radushkevich models were also fitted to the experimental curves. The first one showed the best fit for temperatures of 40 and 60 °C and the second was the best for temperatures of 15 and 25 °C. The characterization techniques of scanning electron microscopy, mercury porosimetry, helium pycnometry and nitrogen physisorption indicated changes on bentonite's surface, such as increase on the bulk density, reduction on skeletal density and reduction in the volume of micropores and mesopores.