Preparation of activated carbon from sludge by 'double green activation' and adsorption capacity for Congo red dye

The sludge was transformed into activated carbon (SAC) by the green activation method. Citric acid (C6H8O7) and potassium ferrate (K2FeO4) were used as different activators, and four kinds of SAC (SAC(N); SAC(CA); SAC(PF); SAC(CA-PF)) were prepared by different activator combinations. The SAC(CA-PF) was produced with C6H8O7 and K2FeO4 by the 'double green activation' method, it had the highest specific surface area (136 m(2) g(-1)) and abundant functional groups (C=O, Si-C) attached to it. The adsorption capacity of SAC(CA-PF) for Congo red (CR) was 98.61 mg g(-1), faster and more efficient than the other SAC. The adsorption process was well described by the Langmuir model and quasi-second-order kinetic model. The adsorption mechanism of SAC(CA-PF) (available on the CR surface) was attributed to various interactions, such as hydrogen bonding and electrostatic attraction. Moreover, SAC(CA-PF) could be regenerated by Fenton reaction, and the removal rate was still more than 80% after five cycles of used. The results indicated that the impregnation of C6H8O7 and K2FeO4 can effectively improve the adsorption efficiency of SAC, prepared by the 'double green activation' method. This method can be applied for carbonized sludge and dye wastewater treatment in an environment-friendly way.