Dye adsorption by modified cotton. Steric and energetic interpretations of model parameter behaviours

The variations of the three parameters, viz. anchorage number, receptor site density and half-saturation concentration, as determined theoretically (Khalfaoui et al, 2002) and related to the adsorption energy, exhibited different behaviours when examined in terms of the adsorption of various dyes on to modified cotton. When plotted against the percentage nitrogen content of these cottons, variations in the effective receptor site density and the inverse of the adsorbed molecules anchorage number allowed the adsorption process to be described topographically in terms of the parallel or perpendicular adsorption of the dye molecule on to the adsorbent surface. The presence of ionic or van der Waals forces in such adsorption was also considered. The values of the model parameters were found to be related to the magnitude of the steric hindrance arising from the anchorage geometry of the dye molecule. Where the site density was high, a form of dimerization of the dye molecules was favoured. The best conditions for obtaining a high adsorption capacity, necessary for textile dyeing and wastewater depollution, were considered. Through the use of the half-saturation capacity, c(1/2), it was possible to determine the change in the adsorption energy when the receptor site spacing was decreased and to relate such variation to the rate of steric hindrance, tau.