THERMODYNAMIC EVALUATION OF THE NI AND V ADSORPTION PROCESS BY PRECIPITATED LIGNINS FROM KRAFT BLACK LIQUOR.

Abstract

In the present study an analysis of the adsorption of Ni and V on lignin from national black liquor was carried out using thermodynamic principles. A comparison was established between both the precipitated lignins and one commercial analytical grade lignin. The adsorptive capacity of both materials was evaluated through the construction of isotherms whose data was fitted to the following models: Freundlich, Langmuir and Redlich - Peterson. For all these models, the best fit for the obtained data was the one of Langmuir’s.  For the adsorption process, the thermodynamic parameters DGº, DHº y DSº, were also calculated. The adsorption enthalpy yielded values of less than 40 KJ / mol, which can not clarify what type of adsorption (physical or chemical) is ocurring, since both types of adsorption may be present. With respect to the Gibbs free energy, negative values were obtained for the adsorption of Ni, suggesting a spontaneous process, while for V values were positive, suggesting a not spontaneous process. In the case of the calculated entropy, a negative value was obtained for the adsorption of nickel, which is indicative of the adsorption process. On the other hand, for V this value was small and positive, which combined with the low adsorption obtained, does not permit to accurately interpret the results.