Adsorption behavior and mechanisms of norfloxacin onto porous resins and carbon nanotube
Highlights
► Porous resin and carbon nanotube were employed together to remove norfloxacin. ► Adsorption was mainly controlled by pH with the zwitterionic form being dominant. ► CaCl2 and NaCl had different effect on the adsorption amounts of norfloxacin. ► Isotherms demonstrated different mechanisms responsible for norfloxacin removal.
Introduction
Norfloxacin (NOR), a fluoroquinolone antibacterial agent, has been widely used in human and veterinary medicine that targets both Gram-negative and Gram-positive bacteria through inhibition of DNA gyrase [1]. The synthetic antibacterial compound NOR is often discharged into aquatic environments via domestic wastewater effluents, disposal of expired pharmaceuticals, and excretion in its original or metabolized form [2]. The presence of NOR in wastewater and surface water, even in low concentrations, can lead to the development of antibiotic resistant bacteria [3]. However, the removal of pharmaceutical antibiotics by conventional water and wastewater treatment technologies is generally insufficient [4], [5]. Hence, there is an increasing demand for developing more effective ways to remove such organic contaminants from waters [6].
The adsorption mechanisms of fluoroquinolone antimicrobials to soils [7], [8], [9], clay minerals [8], [9], [10], natural aquifer material [11], oxide minerals [12], [13], activated carbon [1], [3], and carbon nanotubes [6] have been investigated by many researchers. Generally, primary adsorption interactions of fluoroquinolone antimicrobials with model sorbent phases have been documented to be cation exchange, cation bridging, surface complexation, hydrophobic effect, electrostatic interaction, and electron-donor–acceptor (EDA) interactions. These studies also deduced that the soil texture, free/crystalline oxide content, and exchangeable cation capacity were the most important determinants in adsorption process by the natural matrices while the surface area and surface functional groups of activated carbon and carbon nanotubes played important role. Moreover, adsorption was pH-dependent and involved diverse mechanisms that responded differently to solution chemistry. For instance, Hari et al. [11] found a 1–2 orders of magnitude variation in adsorption affinity with changing pH as the norfloxacins were converted from ionic forms to neutral/zwitterionic forms.
The above studies have mainly focused on the adsorption of fluoroquinolone antimicrobials from their aqueous solutions by natural and engineered adsorbents. Due to their large surface area and controllable pore size distribution, engineered adsorbents such as carbon nanotubes, activated carbons, and porous resins, have shown great potential in removing undesirable organic contaminants from aqueous solutions [14]. Among them, carbon nanotubes are considered to be a good choice because their structures are well defined and their surfaces are relatively uniform in contrast to activated carbons and porous resins [6]. Engineered carbon nanotubes are extensively employed to remove emerging organic contaminants from aqueous media and tend to adsorb antimicrobials very strongly [15], [16]. On the other hand, porous resin, a widely used adsorbent for purifying waters, has shown great promise in the recovery of organic compounds from waste steams [17]. Porous resin has high adsorption capacities, surface areas, and stable chemical structures. One of its main advantages is that it can be regenerated more easily by using organic solvents [18]. However, the adsorption behavior and mechanisms of NOR onto porous resin keeps largely unknown, thereby limiting the knowledge needed to assess its adsorption capacity and explore applications. Furthermore, the properties of resin are significantly different from carbon nanotubes and the adsorption mechanisms can be also different.
The main objective of this study was to investigate the mechanisms and predominant factors controlling the adsorption of NOR to porous resin and carbon nanotube. Norfloxacin, with both a carboxyl (anionic) and a piperazynyl (cationic) group, can exist in four forms (neutral, cationic, anionic, and zwitterionic) depending on the aqueous pH (Fig. 1), was examined as the adsorbates in batch experiments. Hypercrosslinked resin MN-202, aminated polystyrene resin MN-150, macroporous resin XAD-4, and carboxylated multiwall carbon nanotube C-MWCNT were selected as adsorbents. The impacts of solution chemistry conditions (pH, salt, and temperature) on adsorption were evaluated.
Section snippets
Adsorbates
The H-NOR (99.5%) was obtained from Sigma–Aldrich Chemical Co. (Shanghai, China) and used as received. Water used in the study was purified by distillation and the electrical conductivity is not more than 6 μS cm−1.
Adsorbents
Commercially available resin MN-202 and MN-150 were supplied by the Shanghai Office, Purolite International Co., Ltd. Commercial resin XAD-4 (Rohm-haas Co., USA) was purchased from the Shanghai reagent stations. The carboxylated multiwall carbon nanotube (C-MWCNT) (outer diameter 8–15
Characteristics of the adsorbents
Four adsorbents with different physicochemical properties were characterized (Table 1). The BET surface area of MN-202 is the largest, while that of C-MWCNT is the least. The three resins show a similar average pore diameter larger than that of C-MWCNT. The resin MN-202 and MN-150 present hybrid pore structure possessing a considerable portion of mesopores and micropores, which originated from the post-crosslinking chloromethylated poly-styrene-divinylbenzene in polymerization process. The
Conclusions
This is the first study on the adsorption of norfloxacin antibiotics by porous resins and carbon nanotube. The results demonstrate:
- (1)
The adsorption kinetics of the adsorbents follow the pseudo-second order model and the adsorption isotherm data fit well to the Langmuir model. Furthermore, the mono-molecular layer arrangement of molecules adsorbed to the surface of carbon nanotubes is much more compact than those of the other adsorbents.
- (2)
Adsorption of norfloxacins to the adsorbents is mainly
Acknowledgments
We gratefully acknowledge the generous support by the National Nature Science Fund (Grant No. 50978137), the Resources Key Subject of National High Technology Research & Development Project (2009AA06Z315). Additionally, a project funded by the priority academic program development of Jiangsu Higher Education Institutions is also appreciated.
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