Sorption Characteristics Of Ofloxacin And Norfloxacin On Carbon Nanotubes

Environmental behavior and risk assessment of antibiotics have attracted special research attention due to the wide application and unavoidable discharge into the environments. These antibiotics have led to a global contamination problem because of their toxicity and risk to aquatic organisms and human beings. Adsorption of antibiotics on solid particles is a key process controlling their environmental fate in the subsurface. However, the sorption of antibiotics in natural adsorbents was complicated due to the variation of antibiotic species and the complication of adsorbent properties. As an important member of engineered nanomaterials, carbon nanotubes (CNTs) are good model adsorbents because of their definite structures and uniform surfaces. Previous studies have suggested that the main adsorption mechanisms involve hydrophobic, π-π electron-donor-acceptor mechanism, cation exchange and electrostatic interactions for antibiotic adsorption on CNTs. These interactions may exist simultaneously, while the contributions of these adsorption mechanisms are still unclear. Identifying the controlled mechanisms is vital in predicting antibiotic environmental behaviors and consequently understanding their environmental risks. This study used functionalized multi-walled CNTs (MWCNTs): hydroxylized (MH), carboxylized (MC), and graphitized (MG) MWCNTs; MWCNTs with three different diameters:15nm (M15),30nm (M30), and50nm (M50); and three functionalized single-walled CNTs (SWCNTs):hydroxylized (SH), carboxylized (SC), and purified (SP) SWCNTs, as well as activated carbon (AC) as adsorbents. Ofloxacin (OFL) and norfloxacin (NOR) were used as adsorbates to study the adsorption mechanisms of antibiotics on solid particles.Previous study indicated that hydrophobicity, species changes and solubility were the key factors that controlled the sorption of antibiotics. Therefore, these three contents were elaborated separately by a system design to investigate the sorption controlled mechanisms of antibiotics. Different interaction mechanisms controlled sorption processes in different conditions (such as different pHs and cosolvent systems). Thus, this study investigate the sorption controlled mechanisms of OFL and NOR on CNTs in antibiotic-CNT-water, antibiotic-CNT-pH solution, antibiotic-CNT-water/methanol cosolvent systems.In antibiotic-CNT-water systems, adsorption of OFL and NOR on CNTs was controlled by their structural and hydrophobic properties. A significant relationship between single-point adsorption coefficients (Kd) and specific surface area (highly hydrophobic) was observed, but not between Ads and oxygen content. These results suggested that site-specific sorption was not important but hydrophobic effect may have an important contribution to OFL and NOR adsorption on CNTs. However, normalizing the adsorption coefficients by OFL and NOR solubilities enlarged their adsorption difference indicating that hydrophobicity was not the only factor controlling the difference between OFL and NOR adsorption on CNTs. Their chemical structures show that both chemicals could interact with CNTs through an π-π electron-donor-acceptor mechanism. This mechanism was correlated with the different adsorption of OFL and NOR on functionalized CNTs (namely hydroxylized, carboxylized, and graphitized CNTs). These results revealed that OFL and NOR adsorption on CNTs in water was controlled by hydrophobic interactions because the positive relationship between sorption coefficients and surface area but this mechanism may not be able to differentiate OFL and NOR adsorption on CNTs.OFL and NOR exist three types of species (cationic, zwitterionic and anionic) under different pHs, pH is an important factor that affects the sorption characteristics of antibiotics. The relationship between sorption behavior and physicochemical properties of antibiotics or other affect factors is important because it is the fundament for understanding their environmental fate. The negative relationship between solubility and sorption was successfully used to predict the environmental behavior of hydrophobic organic contaminants (HOCs). However, this idea may not be applicable to antibiotics. Thus, solubilities and sorption of OFL and NOR on CNTs at different pHs were systematically measured. The solubilities of different OFL and NOR species were obtained based on the analysis of pH-dependent solubility. Cationic and anionic OFL or NOR showed much higher solubilities than zwitterionic OFL or NOR. The highest sorption was not observed at the pH with lowest OFL and NOR solubility, indicating hydrophobic interaction was not the dominant sorption mechanism at different pHs. The sorption decreasing in pH range of5-8was consistent with cationic OFL or NOR and specie distribution, suggesting cation exchange may play an important role. For NOR, the sorption of positively charged NOR molecule was higher than that of negatively charged NOR molecules due to the electrostatic interactions. In antibiotic-CNT-pHs systems, pH resulted in the change of the species and thus affected the solubility and sorption behaviors of OFL and NOR. Electrostatic interaction was the controlled interaction mechanism. Moreover, the environmental behavior of OFL and NOR may not be easily estimated using its solubility (as for HOCs).In addition, species of antibiotics will change in different solvent. Adsorption experiments in organic solvent or cosolvent could provide a new research perspective to understand antibiotic adsorption mechanisms. Solubility is another important factor for antibiotic sorption. Previous discussions haven’t separate the interactions of solute-adsorbent and solute-solvent. Therefore, this work investigated the effects of solubility and cosolvent for the sorption of OFL and NOR and sorption controlled mechanisms in water-methanol cosolvent further. In antibiotic-CNT-water/methanol cosolvent systems, sorption for OFL and NOR decreased as methanol volume fractions (fc) increased. But the log-linear cosolvency model could not be applied as a general model to describe the cosolvent effect on OFL and NOR sorption. OFL and NOR exist as neutral zwitterionic form in water, but as molecular form in methanol. The zwitterionic molecules may be adsorbed on the CNTs through electrostatic attraction. In addition, we computed the bond lengths of possible hydrogen bonds between solute and solvent and the corresponding interaction energies using Density Functional Theory. The decreased OFL solubility with increased fc ould be attributed to the generally stronger hydrogen bond between OFL and H2O than that between OFL and CH3OH. Solubility of NOR varied nonmonotonically with increasing fc, which may be understood from the stronger hydrogen bond of NOR-CH3OH than NOR-H2O at two important sites (-O18and-O21). The interaction energies were also calculated for the solute surrounded by solvent molecules at all the possible hydrogen bond sites, but it did not match the solubility variations with fc for both chemicals. These results revealed that in water-methanol cosolvent, sorbate-solvent interaction does not control sorption, electrostatic attraction played important role for the sorption of zwitterionic OFL/NOR on CNTs. Log-linear cosolvency model could not be applied as a general model to describe the cosolvent effect on antibiotics sorption.The controlled sorption mechanisms of OFL and NOR on CNTs at different environmental conditions were different. Moreover, hydrophobicity, species and solubility were the important factors that controlled the sorption of antibiotics. In water, sorption of OFL and NOR was controlled by hydrophobic interactions. Under different pHs, OFL and NOR exist different species, electrostatic interactions were the main mechanisms. In addition, species of OFL and NOR were changed in different solvent. In water-methanol cosolvent, electrostatic interactions was the main sorption mechanism for zwitterionic OFL and NOR molecules. Sorbate-solvent interaction does not control OFL and NOR sorption, the effect of cosolvent for their sorption could not be described by log-linear cosolvency. Important conclusions of sorption mechanisms for antibiotics have been posed. In addition, this study offered theoretical basis for pollution control and risk assessment. Firstly, CNTs, as a popular adsorbent, was widely used to remove antibiotics in wastewater treatment plant. On the other hand, the discharged CNTs in the environment can interact with antibiotics. The strong interactions may significantly alter their mobility, transport, fate and resulted in critical side effect for environment and ecology. Therefore, investigate the interaction of CNTs and antibiotics was significant for understanding the environmental behaviors and fate of both antibiotics and CNTs.