Sorption studies of acid dyes on chitosan

Abstract

The feasibility and capability of chitosan as an adsorbent for the removal of acid dyestuffs, namely, Acid Green 25, Acid Orange 10, Acid Orange 12, Acid Red 18 and Acid Red 73 from aqueous solution have been studied. The experimental data were analysed using Langmuir, Freundlich, Redlich-Peter sonequations for each individual dye. The Langmuir isotherm equation was found to provide the best prediction for the sorption of all five acid dyes for the entire concentration ranges. Based on the Langmuir isotherm analysis, the monolayer adsorption capacities were determined to be equal to 645.1, 922.9, 1006.3, 693.2 and 728.2 mg per g chitosan for Acid Green 25, Acid Orange 10, Acid Orange 12, Acid Red 18 and Acid Red 73, respectively. The difference in capacities may due to the effect of molecular size and the number of sulfonate groups of each dye. The results demonstrated that monovalent and/or smaller dye molecules have superior adsorption capacities due to an increase in dye/chitosan ratio in the system, enabling a deeper penetration of dye molecules to the internal pore structure of chitosan. Due to the inherent bias in using the correlation coefficient resulting from linearization, alternative single component parameters determined by non-linear regression were employed in this study. Five error functions were used, namely, the sum of the squares of the error (SSE); a hybrid fractional error function (HYBRID); Marquardt's percent standard deviation (MPSD); the average relative error (ARE) and the sum of the absolute errors (EABS). It was found that the Redlich-Peterson isotherm had the lowest values and provided the best fit to the experimental data. The monolayer adsorption capacities of chitosan increased with increasing temperature from 25C to 60C and showed no further increment at 80C due to the increased the mobility of the large dye ions and the maximum swelling within the internal structure of the chitosan at 60C. Furthermore, the amount of dye adsorbed increased with decreasing particle size due to the inability of the large dye molecule to completely penetrate into the internal pore structure of chitosan and the increase in active surface area exposure for the adsorption of dyes onto the chitosan. The reformation of the crystalline region within the chitosan occurs due to an increase in homogeneity of the internal structure of chitosan with higher DD% leading to a decrease in the overall monolayer equilibrium capacities. The kinetics of acid dye removal were investigated by substituting experiment data into those kinetic models, i.e. the pseudo-first order, the pseudo-second order, the modified second order and the Elovich equations. The sorption kinetics of Acid Green 25, Acid Orange 10, Acid Orange 12, Acid Red 18 and Acid Red 73 onto chitosan can be fully correlated by the Elovich equation. The kinetic model was determined in accordance with the minimum error function and the agreement between the rate equations and the differentiation of kinetic equations.