A constructed fixed bed of light expanded clay aggregate coated with nano-titania particles has been applied as an effective adsorbent fixed bed to remove an anionic azo dye, direct red 16, from aqueous solution. This process was carried out using a recycled batch system under environmental temperature. Initial pH of the adsorbate solution imposed a severe effect on the adsorption process so that the best and worst performances of the prepared beds belonged to the acidic and basic pHs of 3 and 10, respectively. Various equilibrium and kinetic models were applied for analyzing experimental data related to several initial concentrations of DR16. Langmuir-Freundlich isotherm and mixed surface reaction and diffusioncontrolled kinetic model satisfactorily described equilibrium and kinetic adsorption data, respectively. The models suggested that there is heterogeneity in this system and diffusion step at the same time with surface reaction determines the overall rate of the adsorption process. Appropriate and considerably stable performance of the used nano-titania/light expanded clay aggregate fixed beds after regeneration is one of the outstanding characters of the prepared beds. It is noteworthy that based on tests, the removal efficiency of the pollutant with the initial concentration of 25 mg/L was about 95% after 25 min (under pH 3 and T ¼ 25 C). It is while the adsorbent beds still worked steadily after about 65 times of reuses which can keep approximately all the adsorption capacity remaining. Therefore, features such as the high and relatively fast removal of the adsorbate, the facility and high rate in the regeneration can candidate the prepared fixed beds as an effective adsorbent for wastewater treatment.
