Influence of alkaline activation temperature on the porous structure of the activated brown coal and its capacity to adsorb phenol

Abstract

The purpose of this work was to evaluate the influence of alkaline activation temperature (t) on the pore structure of activated carbons produced from brown coal (ACs) and their adsorption capacity towards the sorption of phenol from aqueous solutions. The ACs were prepared by heating (t=400–8500C, 1 hr) of brown coal impregnated with KOH; the prepared samples were designated as AC(t). Based on the nitrogen adsorption-desorption isotherms (77 K), the total volume and surface (S) of all pores and separately micro- and subnanopores were determined. Maximum phenol adsorption capacities of ACs (Am) were registered at the temperature of 250С. The activation temperature was stated to be the most significant factor influencing the ACs capacity to adsorb phenol. The growth of temperature from 4000С to 8500С in the case of AC(800) results in an exponential increase in the value of Аm from 27 mg g–1 to 240 mg g–1 (a maximal value) according to the following equation: Аm=6.038exp(0.0045t) (R2=0.952). The kinetics of phenol adsorption obeys the second order model (R20.982). The AC(800) adsorption isotherm is described by the Freundlich equation (R2=0.988) within the equilibrium concentrations Ce=0.001–2.5 mg cm–3 with the heterogeneity factor nF=3.23 indicating physical adsorption. The same model for Ce0.001 mg cm–3 (R2=0.951) gives nF=0.52, which suggests chemical adsorption. The specific adsorptive capacity АS=Аm/S sharply decreases from 2.11 mg m–2 to 0.21 mg m–2 with increasing the temperature from 4000С to 5500С and remains constant (0.200.01 mg m–2) for ACs prepared at higher temperatures. The invariability of AS indicates the temperature independence of adsorption centers concentrations of ACs prepared within 550–8500С.