Competitive sorption used to probe strong hydrogen bonding sites for weak organic acids on carbon nanotubes

9 páginas.-- 3 figuras.-- 31 referencias.-- Supporting Information: characteristics of the three multiwalled CNTs; selected physicochemical properties of sorbates; Freundlich model fits to sorption and competitive sorption data of monaromatics by CNTs; adsorption kinetics of BA, DNP, and NB onto C−CNT at pH 7 and adsorption kinetics of MBPh, MBA, and PTA onto C−CNT at pH 6; comparison of sorption isotherms of a given adsobate on hydroxylated (H−), carboxylated (C−), and graphitized (G−)CNTs at pH 7.0; zeta potential measurements for the three CNTs; pH records of the NB adsorption on CNTs at initial pH 7.0; titration curves of the three CNTs from pH 3−12; consumption of OH− as a function of pH for the three CNTs from the titration curves; sorbed concentration of NB on three CNTs as function of sorbed competitor concentration at pH 7.0 and displacement efficiency (DE %) of BA or DNP as function of sorbed competitor concentration; sorbed concentration of MBPh on C−CNT as function of amount sorbed of competitors, MBA and PTA at pH 4 and pH 6 at fixed initial concentration of MBPh of 0.2 Mm; and solution pH change versus moles BA−/DNP− adsorbed for (A, C, E) H−CNT and (B, D, F) C−CNT without or with cosolute. This material is available free of charge via the Internet at http://pubs.acs.org

We recently proposed that weak acids (AH) adsorb to partially oxidized carbonaceous materials in part by forming strong hydrogen bonds with acidic surface groups, depicted by (A···H···O-surf)-, known as negative charge-assisted hydrogen bonds, (-)CAHBs. Here we use competition experiments to show that sorption of AH on carbon nanotubes (CNTs) can be described conceptually by a dual specific/nonspecific domain model, where one domain involves (-)CAHB sites that can become saturated. The trends observed in single-solute adsorption, including the stoichiometric release of hydroxide upon sorption of carboxyate or phenolate anions, were consistent with trends in the previous studies and pointed to the formation of (-)CAHB. 3,4-Dinitrophenolate formed (-)CAHBs more efficiently than did 2,6-dichloro-4-nitrophenolate because of alleviation of steric hindrance to approach by the ortho chlorines. Competition against a (-)CAHB-capable target compound was greater when the competitor was also (-)CAHB-capable than when it was not (e.g., benzoate as target vs 3,4-dinitrophenolate or nitrobenzene as competitor; mono-n-butyl phthalate as target vs methyl benzoate or p-tolyl acetate as competitor). Experiments also revealed competition between the nitroaromatic species for π-π electron donor-acceptor sites. The findings will contribute to a better understanding of the adsorption mechanism of ionizable compounds on carbonaceous materials.

(NSF) (CBET 1235459) and U.S. Department of Agriculture National Institute of Food and Agriculture (USDA NIFA) (MAS00475). B.G. thanks Junta de Andalucia for a postdoctoral contract linked to the project P07-AGR-03077

Peer reviewed