Adsorption of Phenolic Compounds onto the Activated Carbon Synthesized from Pulp and Paper Mill Sludge: Equilibrium Isotherm, Kinetics, Thermodynamics and Mechanism Studies

New activated carbon was produced from pulp and paper mill sludge as a low cost precursor via chemical activation by zinc chloride. The activated carbon was successfully used for the adsorption of phenol and two of its derivatives, i.e. 2-chlorophenol and 4-nitrophenol from aqueous solutions. The characterization of the prepared activated carbon demonstrated high textural properties for the synthesized activated carbon. The results revealed superior adsorption performance of the prepared activated carbon in this study compared to the commercial one. The effect of main affecting parameters such as pH, contact time, concentration and temperature on adsorption of phenolic compounds was experimentally investigated in discontinuous system. The adsorption capacities followed the order of 4-nitrophenol > 2-chlorophenol > phenol which was attributed to the property of substituent groups in their chemical structure. Amongst the isotherm models used to describe experimental equilibrium data, the best match was obtained by Sips isotherm which revealed the heterogeneous behavior of the adsorption sites on the adsorbent surface. Kinetic study demonstrated that the adsorption kinetics followed pseudo-second-order kinetic model. Based on thermodynamical analysis, the negative values obtained for Gibbs free energy, enthalpy and entropy indicated spontaneous, exothermic and random behavior of the adsorption process.