Adsorption of cationic and anionic azo dyes on sepiolite clay: Equilibrium and kinetic studies in batch mode

In the present work, sepiolite clay adsorptive properties were investigated for textile azo dyes in aqueous solution. The clay (78 wt.% sepiolite) was characterized in terms of physical, textural, chemical and mineralogical properties. Basic Red 46 and Direct Blue 85 azo dyes were selected as adsorbates. Adsorption equilibrium data were successfully fitted to Freundlich and Langmuir equations. For Basic Red, the maximum adsorption capacities, predicted by the Langmuir model, were 110 mg/g (at 25 °C) and 310 mg/g (at 35 °C). The value obtained at 25 °C, matched perfectly the cation exchange capacity of the clay. For Direct Blue, the adsorption capacity was 332 mg/g. Adsorption equilibrium was also evaluated for Direct Blue dye in a synthetic effluent, containing salt and auxiliary dyeing chemicals. In this mixture, the amount of dye adsorbed by sepiolite decreased, but very considerable values were still reached (106 mg/g). Adsorption kinetics was studied and modeled. Homogeneous solid diffusion coefficient and effective pore diffusivities were calculated for both dyes. Sepiolite clay showed to be an advantageous adsorbent in terms of price and versatility, being effective under a wide pH range, for both anionic and cationic dyes. Its use as adsorbent does not require further purification or chemical modifications, which is beneficial for the environment and for the economy of the treatment.