Porous properties of activated carbons from waste newspaper prepared by chemical and physical activation

Activated carbons were prepared from old newspaper and paper prepared from simulated paper sludge by chemical activation using various alkali carbonates and hydroxides as activating reagents and also by physical activation using steam. In the chemical activation, the influence of oxidation, carbonization, and activation on the porous properties of the resulting activated carbons was investigated. The specific surface areas (S_BET) of the activated carbons prepared by single-step activation (direct activation without oxidation and carbonization) were higher than those resulting from two-step activation (oxidation-activation and carbonization-activation) and three-step activation (oxidation-carbonization-activation) methods. The S_BET values were strongly dependent on the activating reagents and the activating conditions, being > 1000 m²/g using K₂CO₃, Rb₂CO₃, Cs₂CO₃, and KOH as activating reagents but <1000 m²/g using Li₂CO₃, Na₂CO₃, and NaOH. These differences in S_BET values are suggested to be related to the ionic radii of the alkalis used as activating reagents. The microstructures of the higher S_BET samples show a complete loss of fiber shape but those of the lower S_BET samples maintain the shape. In the physical activation, the porous properties of the activated carbons prepared by the single-step method were examined as a function of the production conditions such as activation temperature, activation time, steam concentration, and flow rate of the carrier gas. The maximum S_BET and total pore volume (V_P) were 1086 m²/g and 1.01 ml/g, obtained by activation at 850°C for 2 h, flowing 20 mol% of steam in nitrogen gas at 0.5 l/min. A correlation was found between S_BET and the yield of the product, the maximum S_BET value corresponding to a product yield of about 10%. This result is suggested to result from competition between pore formation and surface erosion. Compared with chemically activated carbons using K₂CO₃, the porous properties of the physically activated carbons have lower S_BET and V_P values because of the smaller size and lower volume of their micropores. On the other hand, they retain the original fiber shape and the paper sheet morphology after activation.