Sludge-based activated carbon experiment design, char properties, and evaluation of methyl orange adsorption

Paper mill sludge is a type of industrial waste that, if improperly disposed of, can cause significant environmental pollution and resource wastage. This study aims to utilize paper mill sludge (as feedstock) and agricultural waste peanut shells (as a carburizer) to produce sludge-based activated carbon for the adsorption of dye wastewater. Orthogonal experiments were conducted to compare the adsorption properties of activated carbon prepared through physical, chemical, and microwave activation methods. The resulting activated carbon samples were then characterized using BET, SEM, XRD, and FTIR analyses. Additionally, the adsorption properties of methyl orange on the optimized activated carbon were investigated under various influencing factors, and the adsorption kinetics were studied. The findings revealed that the activated carbon prepared through chemical activation exhibited the highest performance, with an iodine adsorption value of 913.86 mg/g and a methylene blue adsorption value of 153 mg/g. BET, SEM, and FTIR analyses demonstrated that the chemically activated carbon possessed a larger specific surface area, a more developed pore structure, and a higher presence of oxygen-containing functional groups on its surface. XRD analysis indicated the presence of quartz-phase Si in the activated carbon, suggesting a higher degree of graphitization in the activated samples. The adsorption equilibrium time of optimum activated carbon for methyl orange solution was 120 min. At a solution pH of 7, the removal efficiency and adsorption capacity of methyl orange reached their highest values of 86.37% and 215.93 mg/g, respectively. Furthermore, as the activated carbon dose increased, the removal efficiency of methyl orange significantly increased while the adsorption capacity decreased. When the solution concentration reached 500 mg/L, the adsorption capacity gradually reached saturation. The adsorption kinetic behavior was better described by the quasi-second-order kinetic equation, with a high fit degree (R² > 0.999). The fit degree was high during the external diffusion stage but poor during the internal diffusion stage. These results provide valuable insights for the development of activated carbon with enhanced adsorption performance for wastewater treatment.