Methylene blue adsorption from glycerol solution onto the acicular habit of α-goethite

Abstract

The mechanism of methylene blue adsorption onto the surface of synthetic acicular habit of α-goethite from glycerol solution has been studied through batch experiment at 25, 30 and 35 ⁰C in a glass cell of minimal dead volume. To describe the adsorption results, an attempt was made to fit the data to the Langmuir, Freundlich, Temkin and Dubinin equations. It was evident that adsorption capacity increased rapidly from 4.7 to 27.2 x 10^-5 M for the equilibrium concentration of 0.13 to 1.90 x 10^-5 M at 25 ^oC to a maximum of 33.1 x 10^-5 M for 30 ^oC for equilibrium concentration of 2.74 M. Drop in the height of the plateau was observed and remained unchanged for 35 ^oC, and a maximum appeared in each isotherm before plateau. The adsorption process was found to be exothermic with an estimated mean entropy change of 23.84 kJ/mol. Also, an initial estimation of the surface acidity was made and it has been suggested that the surface of this mineral is heterogeneous if the location of these sites are presumed to be the adsorption sites identified as Lewis and Bronsted acid types. Furthermore, thermodynamic parameters ∆H, ∆G and ∆S for the adsorption were determined from the data and some features of the adsorption mechanism were enumerated on the basis of reaction scheme that incorporates simple adsorption phase exchange at the glycerol/α-FeOOH interface.

KEY WORDS: Surface interface, Crystal habit, Heterogeneity, Micropore volume 

Bull. Chem. Soc. Ethiop. 2011, 25(1), 25-35.