Surface features of exfoliated graphite/bentonite composites and their importance for ammonia adsorption

Commercial graphite intercalated with sulfuric or nitric acid was exfoliated by rapid heating at 800 degrees C, homogenously mixed with bentonite water suspension and then dried at 120 degrees C. Dynamic ammonia adsorption was then measured on the prepared samples, the as received intercalated graphite, the expanded graphite, and the as received bentonite. The surface of the materials before and after exposure to ammonia was characterized using adsorption of nitrogen, XRD, SEM, FTIR, TA, and potentiometric titration. The results showed that mixing exfoliated graphite with clay leads to formation of layered composites on which enhanced amount of ammonia is retained. It is likely there are three types of active sites for ammonia adsorption. The first group consists of acidic functional groups formed on the edges of graphite flakes during exfoliation. The second groups are Bronsted or Lewis acidic centers of clay origin, and the third category includes unique features of the composite structure and chemistry. When a composite is formed metal cations from the clay interlayer space react with carboxylic groups of graphite flakes, as a result, the layer charge has to be balanced by additional adsorption of ammonia, which results in an enhancement in the performance of these materials as ammonia removal media. (C) 2008 Elsevier Ltd. All rights reserved.