Competitive Sorption Used To Probe Strong Hydrogen Bonding Sites for Weak Organic Acids on Carbon Nanotubes

• Published in: Environmental science & technology 2015-02, Vol.49 (3), p.1409-1417
• Main Authors: Li, Xiaoyun, Gámiz, Beatriz, Wang, Yiquan, Pignatello, Joseph J, Xing, Baoshan
• Format: Article
• Language: English
• Subjects: Acids; Adsorption; Competition; Experiments; Hydrogen Bonding; Hydrogen bonds; Nanotubes; Nanotubes, Carbon - chemistry; Organic chemicals; Organic Chemicals - chemistry; Oxidation-Reduction; Trends
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/es504019u

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.