Fractal characteristics of adsorption of direct dye compounds onto clay particles
粘土颗粒吸附直接染料的分形特征

The adsorption processes of direct fast light black G and direct scarlet 4BE onto illite and montmorillonite clay particles were investigated. The adsorption could be divided into two parts, one is high-speed boundary coverage, and the other is slow-speed interlayer adsorption. Not only is a pseudo-second-order reaction kinetic equation applicable for all the entire processes, but also fractal-like properties were found in the main interlayer adsorption stage, except for direct fast light black G adsorption onto illite. Classical and fractal forms of the Langmuir adsorption model were used to analyse the non-linear adsorption isotherms of direct dye compounds adsorption onto the clay particles, and good simulation effects were found for them, except for the fractal model of direct scarlet 4BE onto montmorillonite particles. For direct scarlet 4BE absorbing onto both clay particles at an initial concentration of 150 mg·L-1, a highest amount adsorbed were given in each adsorption isotherm, and the amount adsorbed by boundary covering is higher than that by interlayer adsorption. This may be ascribed to different amounts adsorbing onto clay particle surface through boundary coverage and interlayer adsorption mechanisms at different initial concentration of direct scarlet 4BE. Furthermore, the incorporation of dye molecules between the layers might lead to expansion of the interlayer distance and geometric structure alterations. The surface of the dye-adsorbed clay particles show smooth edges, loose structure and few layers, which is most marked on the surface of adsorbed montmorillonite particle or clay surfaces adsorbing direct fast light black G. After adsorption of dye compounds, the volume of N2 absorbed onto montmorillonite decreased less than onto illite, so the corresponding specific surface area and pore volume show the same trend. The average pore diameter of illite after dyes adsorption decreased more obvious than that montmorillonite absorbed dyes. The surface fractal dimensions Ds for clay particles with dyes absorbed are higher than for the clays without dyes absorbed because the interlayer expansion and pore opening counteract the decrease in Ds through surface roughness screening and pore blocking mechanisms. Therefore the surface roughness of the clay particles is increased after adsorption of the dyes.