Mass transfer and equilibrium studies for the sorption of chromium ions onto chitin

Abstract

The kinetics and equilibrium of chromium sorption onto chitin were studied with respect to pH, particle size, initial metal ion concentration, sorbent concentration and stirring rate. In order to select the main rate-limiting step in the overall uptake mechanism, a single external mass transfer diffusion model and intraparticular mass transfer diffusion models were used. External film mass transfer coefficients gave a mean value of 1.16×10 −3 cm s −1. Chromium sorption onto chitin was mainly located on the surface. However, the stirring rate had no noticeable effect on the overall uptake rate of chromium. This observation led to the conclusion that external mass transfer resistance was not the major limiting phenomenon. Intraparticle diffusion coefficients gave a mean value of 2.78×10 −3 mmol g −1 s −0.5 (0.17 mmol g −1 h −0.5), indicating a poor intraparticular diffusion into the chitin or into the pores. The equilibrium isotherms were analysed using the monolayer and multilayer sorption models. Finally, the chromium binding capacity of chitin was compared with that of Rhizopus arrhizus, which possesses a high chitin content in the cell wall.