Adsorption of organic pollutants from the aqueous phase using graphite as a model adsorbent

Although graphite is not effective as an adsorbent in water treatment, it provides a homogenous, non-porous, carbonaceous structure that is ideal for studying fundamental adsorption mechanisms. High-purity graphite powder (C content 99.5%) was oxidized in an ozone stream, producing a near-surface oxygen content of 5.9 at.%, and was used together with the virgin material to establish adsorption isotherms for organic compounds in aqueous solutions. We examined how the aromaticity and substituents of the adsorptives affect adsorption on the model-activated carbon surface. For both virgin and oxidized graphite, the adsorption capacity for the aromatic compounds decreased in the order 1-naphthol > 2-methoxynaphthalene > naphthalene > anisole > phenol, with significant differences in the adsorption capacities of the two graphite species observed only for anisole, naphthalene, and 1-naphthol. The Freundlich constants (KF) for the five compounds on virgin graphite were 23.9, 10.3, 5.5, 1.4, and 0.8 (nmol mg−1 )/(µmol L−1)n, respectively. Naphthalene and 1-naphthol were slightly more adsorbed on the virgin material, whereas oxidized graphite had marginally better adsorption properties for anisole. The results underline the importance of dispersive and ππ interactions in the adsorption of organic compounds on carbonaceous adsorbents; a second aromatic ring in 1-naphthol and 2-methoxynaphthalene greatly increased the adsorption capacity for these compounds compared with their one-ring counterparts phenol and anisole. Differences were also observed in the adsorption of compounds containing hydroxyl or methoxy substituents, which have electron-donating properties (a resonance effect) but different electron-withdrawal characteristics (caused by induction). Two amino acids occurring as zwitterions, l-tryptophan and l-tyrosine, were also tested as adsorptives. l-Tryptophan, which has a larger aromatic system, achieved higher loading on graphite, suggesting an adsorption mechanism primarily governed by dispersive and ππ interactions for these two ionic compounds as well.


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