Thermodynamic Investigation on Chrome and Aluminum Tanning

Abstract

Chromium(III) and aluminum(III) have some history of use inthe leather industry, and the leathers tanned by them are greatlydifferent in hydrothermal stability. However, the direct measureof thermodynamic characteristics of the tanning process isdifficult due to the binary fibers/solution system. In this paper,the thermodynamics of chrome/aluminum tanning reaction wasobtained by means of adsorption thermodynamics. The hidepowder was used to investigate the adsorption behavior of bothCr(III) and Al(III), and the surface electrochemical property ofhide powder was characterized by Zeta potential analyzer. Theresults showed that the adsorption isotherm of both Cr(III) andAl(III) could be well described by the Langmuir equation, andthe quantity of Cr(III) absorbed at pH 4 is similar to that ofAl(III) at pH 3.5 using the dimension of mol/g collagen fibers.Then the dimensionless equilibrium constants of such binaryfibers/solution systems were obtained. The data of Gibbs freeenergy change (?G) indicate that the interaction between Cr(III)and collagen is slightly stronger (about 1.8 kJ·mol-1) than Al(III),which is insufficient to assume the large difference in stability ofchrome-tanned and alum-tanned leathers. However, ?S of theadsorption process is positive for Cr(III) and negative for Al(III),and the addition of Cr(III) results in bigger conformationalchange of collagen than Al(III) indicated by both the CD spectraand SEM images, which suggest that the big differences inconformation of collagen induced by Cr(III) and Al(III) may bethe foundation of difference in stability.