Adsorption of Aromatic Compounds onto Activated Carbons: Effectsof the Orientation of the Adsorbates

摘要

The use of activated carbon in water and wastewater treatment is widespread due totheir high surface area, well-developed pore structure and surface properties. Theadsorption mechanism of these compounds is complex and a challenging topic for along time. One of the main theories suggested that the affinity of the aromaticcompounds toward the carbonaceous materials was a result of the π electron overlapbetween the sorbate and the graphene layers. This mechanism is based on thetraditional understanding of π-π interactions as proposed in classical organic chemistryliterature, where the aromatic systems were thought to lie face-to-face in a stackedmotif. However, our understanding of the mechanism of the specific attractionbetween graphite and aromatic solutes is still not clear. New theoretical models showthat the monolayer adsorption capacities should not correlate with molecules in astacked motif, instead the correlation should be somewhere between the stacked and Tshaped motifs. In this work, we have determined the preferred orientation motif fromcorrelating calculated surface coverage per molecule with monolayer adsorptioncapacities.In this work, adsorption of model aromatic compounds onto two untreated activatedcarbons with similar physical and chemical properties is investigated. The solution pHof all experiments was lowered so that all solutes were in their molecular forms. It isshown that the difference in the maximum adsorption capacities of the solutes wasmainly attributed to the difference in the sizes of the molecules. This new experimentalfinding is significant to gaining insight into the orientation of the adsorbed phase,hence the adsorption mechanism of aromatic compounds in aqueous solutions. It isshown that that the adsorption of aromatic compounds in a stacked motif for π-πinteractions is unlikely and in absence of physical restrictions such as pore width, a Tshaped motif is the preferred orientation.