The adsorption characteristics of an anionic azo dye (new coccine) onto digested sludge have been studied. Results show that the dye can effectively be removed by sludge. It was found that the adsorption rate is very rapid and the equilibrium can be reached in 10 min. The adsorption kinetics can be expressed by the modified Freundlich equation. Also, the solution pH, ionic strength, and temperature are the key factors affecting the adsorption. The effect of electrical double layer thickness on the adsorption was also discussed. The adsorption standard free energy, enthalpy, and entropy were determined. The adsorption follows a nonlinear multilayer adsorption isotherm. The specific surface area of sludge was determined as 82-150m2/g on the basis of its monolayer dye adsorption capacity.

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