Water adsorption in activated carbons with different burn-offs and its analysis using a cluster model

This work presents the behavior of water adsorption in the activated carbons with different porous structure derived by varying the level of char burn-off. Water adsorption isotherms of activated carbons prepared from longan seed at three different burn-offs (19, 26 and 60%) were measured gravimetrically. These obtained carbons were dif- ferent in terms of their pore size distribution and also the surface functional group properties by showing an increasing of total pore volume and the concentration of surface functional groups with increasing in the burn-off level. The water adsorption isotherms showed that the behavior and amount of water uptake could be divided into three consecutive ranges of relative pressure, 0.0-0.3, 0.3-0.7 and 0.7-0.94, corresponding to adsorption in ultramicropores, supermi- cropores, and mesopores, respectively. The isotherm data were simulated by a cluster model proposed by Do and Do. The correlation was found to be satisfactory over the entire range of relative pressure only with the lowest burn-off carbon which contained mainly micropores. For higher burn-off carbons, which showed increasing proportions of mes- opores, the model needed to be modified by increasing the cluster size of the adsorbed water molecules from 5 to 20 for adsorption at relative pressures greater than about 0.7.