Thermal and thermal-acid treated sewage sludge for the removal of dye reactive Red 120: Characteristics, kinetics, isotherms, thermodynamics and response surface methodology design

Abstract

Sludge from wastewater treatment plant was used as low-cost adsorbent to remove the dye Reactive Red 120 (RR 120) from an aqueous solution. Adsorbents were prepared through thermal (pyrolysis) and chemical treatment (functionalization with HNO3) of sewage sludge. Adsorbents were characterized through physical-chemical and textural analyses. The mesoporous nature of pyrolyzed sludge (PS) and functionalized slugde (FS) influenced the adsorption of RR 120. Moreover, adsorbents surface was rich mainly in carboxylic groups. Batch kinetic experiments were conducted on the statistical design elaborated with central composite rotational design under different pH (3.5–11.5) and temperature (30 °C–60 °C) to achieve the best operational conditions. The response surface methodology (RSM) indicated optimal dye removal efficiency above 94% and 98% at pH and temperature pairs of 3.5/60 °C and 5.4/60 °C for PS and FS, respectively. Kinetics data revealed that pseudo-second-order model better described the adsorption with 2.46 mg g−1 at pH 3.5 for PF and 2.61 mg g−1 at pH 5.4 for FS. Langmuir isotherm model was the best fitted to the adsorption equilibrium data with monolayer maximum adsorption capacities of 14.69 mg g−1 for PS and 46.81 mg g−1 for FS at optimal conditions. The positive ΔH° results indicated an endothermic process (4.73 kJ mol−1 and 8.02 kJ mol−1). Thus, it was concluded that the sewage sludge can be used as an alternative low-cost adsorbent in the removal of RR 120.