Uptake of cationic dyes by cement kiln dust: Sorption mechanism and equilibrium isotherm

The mechanistic aspects of the sorption of Basic Blue-3 and Basic Red-22 on to cement kiln dust (CKD) have been investigated. The by-pass kiln dust before and after washing with water was characterized by X-ray diffraction methods, IR spectroscopy and elemental analysis. Two forms of by-pass kiln dust were used in this study without any treatment, i.e. in powder and pelletized form (the latter being achieved using water or sodium silicate). Equilibrium isotherms to assess the maximum capacity of the two basic dyes on the two forms of CKD were evaluated using a computer program. This allowed two-, three- and four-parameter adsorption models to be studied; it was found that the piecewise Freundlich isotherm yielded an excellent overall fit. Washing with water led to the loss of some active sites responsible for the sorptive capacity. The increase in capacity observed after pelletization with water rather than with sodium silicate could be explained by an increase in hydroxy moieties that undergo exchange reactions. In both cases of pelletization, the adsorption capacity was less than for the unwashed powdered form. The uptake order was found to be: unwashed powder > pellets with water > pellets with sodium silicate.