Efficiency of dry drinking water treatment sludge as adsorbent for methylene blue and acid red 97 removal from aqueous solutions

In this study, drinking water treatment sludge (DWTS) was applied after drying as adsorbent for the removal of methylene blue (MB) and acid red 97 (AR97) from aqueous solutions. DWTS powder was characterized by X-ray fluorescence, Fourier transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) methods. The thermal behavior of DWTS was examined and leaching tests were carried out to study its chemical stability. DWTS is mainly composed of aluminum (24.65%) and silicium (6.157%) and it has a mesoporous structure with a surface area of 92 m2.g-1. Batch adsorption experiments of MB and AR97 were achieved taking into account the effect of pH, initial dye concentration, adsorbent mass, contact time and temperature. The pseudo-second-order kinetic model provided the best correlation for both dyes, and the intra-particle diffusion is not the only rate-limiting step during the adsorption. Furthermore, the adsorption of MB and AR97 followed the Langmuir isotherm model with a maximum adsorption capacity equal to 46.30mg.g-1 and 135.13mg.g-1, respectively. Thermodynamically, the negative values   of ΔG° for both the dyes assumes that the adsorption is favored and spontaneous and that the reaction is endothermic (ΔH°>0) and exothermic (ΔH°<0) for MB and AR97, respectively. The entropy ΔS° is positive and proves that there is randomness at the liquid/solid interface during the adsorption process. The results also exhibited that the adsorption of both dyes is a physical process. Finally, DWTS can be considered an effective and low-cost adsorbent due to its simple physical conditioning requiring only dewatering and grinding.