Preparation of Octadecyl and Amino Mixed Group Modified Titanate Nanotubes and Its Efficient Adsorption to Several Ionic or Ionizable Organic Analytes

In this study, octadecyl (C-18) and aminopropyl group modified titanate nanotubes (C-18/NH2-TNs) were prepared successfully with a silylation method by introducing octadecyltrimethoxysilane (ODS) and aminopropyltrimethoxysilane (APS) simultaneously to the surface of titanate nanotubes (TNs). The properties of the products were characterized in detail. The results indicated that there might be steric hindrance effects to C18 groups on the nanoscaled interior and exterior space of TNs. Therefore, aminopropyl groups were more ready to immobilize on the surface of TNs than C18 groups when the organosilanes ODS and APS coexisted in reaction solution. C-18/NH2-TNs with the obvious characteristic of C-18 and aminopropyl groups were obtained as the ratio of ODS/APS in reaction solution was 4.0, and the corresponding proportion of the two groups (C-18/NH2) on the C-18/NH2-TN surface was about 0.25. This adsorbent had good water dispersibility, overcoming the bad deficiency of reverse phase nanosized materials in water and improving its accessibility to polar analytes. Due to the cooperative effect of alkyl and aminopropyl groups coupled with the residual hydroxyl groups on the surface, C-18/NH2-TNs exhibited excellent affinity efficiency to some ionic or ionizable organic analytes under a different solution pH, including anionic (perfluorooctane sulfonate, perfluorooctanoic acid, and sodium dodecyl sulfate), cationic (cetyltrimethylammonium bromide), and amphoteric compounds (sulfamethazine). The selective binding of these anionic, amphoteric, and cationic analytes could be achieved by adjusting solution pH to acid, neutral, and alkaline conditions, respectively. Since C-18/NH2-TNs possessed relatively large surface areas and strong affinity ability to ionic analytes, especially with long alkyl chains, it showed significantly higher adsorption capacity for perfluorooctane sulfonate than activated carbon and anion-exchange resin.