The effect of surface chemistry and porosity of activated carbons on the adsorption of traces of odoriferous compounds

The odor of human sweat has its origin in such substances as acetaldehyde, valeric acid and ethylmethylamine (EMA). As adsorbents for these compounds, three activated carbons of coal origin and wood origin were used in this study. The surface of the initial samples was modified using oxidation with nitric acid or impregnation with urea followed by heat treatment at 723K and 1223K. Boehm and potentiometric titrations, thermal analysis and diffuse reflectance FTIR were used to characterize the surface chemistry. Adsorption of acetaldehyde from the vapor phase was studied using inverse gas chromatography (IGC) and thermal analysis (TA). The results showed that hydrogen bonding and dispersive interactions contribute to the heat of acetaldehyde adsorption. At very low surface coverage acetaldehyde tends to adsorb in the small pores whereas with increasing surface coverage, acetaldehyde starts to adsorb in larger pores where functional groups contribute to the adsorption process. Adsorption of valeric acid and ethylmethylamine from aqueous solutions was measured at 333 K and 299 K, respectively. The calculated isotherms showed good fits to the Freundlich equation. Specific acid-base interactions governed the adsorption of both compounds from aqueous solution at low concentrations. At higher concentrations, the volume of small micropores (<10 A) determined the capacity of carbons toward the valeric acid removal whereas the surface chemistry of activated carbons played the predominant role in case of EMA adsorption. The density of surface basic groups controlled amount of valeric acid strongly adsorbed whereas in the case of EMA the amount strongly adsorbed increased with an increase in the amount of strong acidic groups on the surface. In the case of adsorption from vapor phase under dry conditions, dispersive interactions and hydrogen bonding control the adsorption process of valeric acid while dipole-dipole, hydrogen bonding or specific acid-base interactions govern the mechanism for EMA adsorption. The presence of strong acidic groups not only increases the amount of EMA adsorbed but also promotes the incorporation of nitrogen into the surface when heat treatment is applied. In adsorption of EMA from humid air stream, water adsorbs preferably via hydrogen bonding and thus hinders EMA adsorption.