The Langmuir monolayer adsorption model of organic matter into effective pores in activated carbon

A model of Langmuir monolayer liquid adsorption into effective pores was used to study the monolayer adsorption capacity by theory calculation. The activated carbon (AC) from rice husk with NaOH activation was selected as adsorbent to uptake phenol and methylene blue (MB). Materials were characterized by N2 adsorption, infrared spectroscopy (IR), and ultraviolet spectrophotometer (UV). In adsorption kinetics, it was observed that the experimental data were well explained by the pseudo second-order equation. Moreover, the Langmuir isotherm was more suitable to explicate results than Freundlich isotherm, implying the monolayer adsorption. Basing on the molecule diameter of 0.72 nm and cross-sectional area of 0.414 nm2 for phenol, the theoretical adsorption capacities were close to the actual values with small relative error (<7%). Due to the large molecule size of MB (0.98 nm, 0.749 nm2), the sample with wider pores activated at 900 °C exhibited the higher adsorption capacity than AC at 700 °C. And the capillary condensation increased the adsorption capacities, consistent with the order of average pore diameter for ACs. From the model, it indicated that the adsorbent was suitable for adsorption when its critical pore width was two times of adsorbate molecule diameter.

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► Langmuir monolayer adsorption model for matter into effective pores was built.
► Activated carbons from rice husk (600–900 °C) were selected as the adsorbent.
► Product was used to remove phenol (0.72 nm) and MB (0.98 nm) of 50–1000 mg/L.
► The maximum adsorption capacities of phenol and MB were 544.8 and 822.8 mg/g.
► Experiment data were close to theory values, indicating the applicability of model.