Studies On The Sorption Characteristics Of Nitroaromatic Compounds And Antibiotics Onto Multiwalled Carbon Nanotubes

Carbon nanotubes (CNTs) have unique physicochemical and electrical properties and have widespread applications in field emission, in hydrogen storage, and as chemical sensors. Due to their high surface area and large micropore volume, CNTs are also considered to be extremely good adsorbents to remove many kinds of pollutants form water. The adsorption of hydrophobic organic compounds or heavy metals by carbon nanomaterials may affect the fate, transformation, and transport of pollutants in the environment. Nitroaromatic compounds (NACs) are widely used as pesticides, explosives, and intermediates in the synthesis of dyes and other high-volume chemicals. NACs are highly polar and often act as strong electron acceptors when interacting with adsorbentcontaining structures with high electron polarization or with electron donors such as oxygen-containing groups. Antibiosis is a natural chemical regulation mechanism among organisms, especially microorganisms. Today, a wide range of naturally occurring and of synthetic antibiotics is frequently used for the therapy of infectious disease in human and veterinary medicine.Due to their use pattern, they possess a potential for reaching the soil environment. Highly mobile antibiotics have the potential to leach to the groundwater and be transported with the groundwater, drainage water, and surface runoff to surface waters. There is thus a possible exposure pathway for aquatic organisms.There are numerous researches into the sorption of metals or organic contaminants separately in the environment. A single-solute system may not adequately represent the majority of mixed contaminanted systems commonly encountered in the environment, where both heavy metals and organic contaminants may coexist at many contaminated sites. Hence the effects of heavy metals on the adsorption and desorption processes of organic contaminants and conversely of organic contaminants on metal adsorption play an important role in revealing the environmental behaviors of contaminants.This research work focused on the sorption of nitroaromatic compounds and two kinds of antibiotics, sulfathiazole and ciprofloxacin, by multi-walled carbon nanotubes (MWCNTs), to examine the applications of MWCNTs in removing these pollutants from water. The effects of pH, temperature on the sorption were also investigated. Cu was selected to study the influence of heavy metals on the sorption of antibiotics to carbon nanotube.The kinetics of NACs sorption onto the as-grown and purified MWCNTs were investigated. The sorption rate of MWCNTs was very fast and the sorption equilibriums were obtained in 200 minutes. The sorption kinetics of NACs were well described by a pseudo-second-order rate model. The molecule size was an important factor that may affect the sorption rate of NACs on MWCNTs. The sorption of NB on MWCNTs was the fastest process compared to the other NACs studied which may be attributed to the smaller molar volume (Vs) and Van der Waals square (SVdw) for NB.The sorption rate of purified MWCNTs was lower than as-grown MWCNTs due to the introduction of oxygen containing groups.To study the sorption isotherms of NACs by MWCNTs, different MWCNTs with varying structure and surface chemistry were adopted using a batch equilibration technique. Langmuir, Freundlich and Dual mode model gave good fits to the sorption data of NACs. The sorption amouts of NACs to MWCNTs follow this order: DNB > MNT > PNP > NB, which in agreement with their molecule Van der Waals square (SVdw). The numbers of -NO2 also can affect the sorption affinities between NACs and MWCNTs. The BET surface areas and mesopore volumes of MWCNTs were the main factors that influence the sorption ability of MWCNTs at lower oxygen contents. There were no hysteresis of NACs desorption from MWCNTs.Surface oxygen functional groups significantly diminished the adsorption amount of NACs to MWCNTs. The mechanisms on the suppression effects of oxygen groups on the adsorption of organic contaminants were due to:1) oxygen may change the surface hydrophobicity of the MWCNTs, decreasing the hydrophobic effect between NACs and MWCNTs; 2) oxygen may change the electronic density of the graphene layers, depressing the EDA interaction between NACs and MWCNTs; 3) oxygen functional groups preferentially sorbs water molecules that competitively block sorption of NACs or sterically prevent their penetration into micropore space.The effects of pH and temperature on the sorption of NACs were also investigated. The pH values can affect the sorption amounts of PNP due to the changes of surface charge of MWCNTs at different pH, but no effect had been found for DNB. The values of enthalpy and entropy suggested that the sorption of NACs onto MWCNTs was exothermic and spontaneous. The accompany NACs depressed the sorption amount of NACs by MWCNTs and made the isotherms more linearity.The competitive ability of the NACs followed the order: DNB > PNP > MNT > NB, which related to their molecule structure and the linearity of the single solute sorption isothoms.MWCNTs10, MWCNTs10-O1 and MWCNTs10-O2, which contained different oxygen functional groups, were used to study the sorption of sulfathiazole (STZ) and ciprofloxacin (CIP) onto MWCNTs. The sorption kinetics of STZ and CIP were well described by a pseudo-second-order rate model. MWCNTs10-O1 have faster sorption rate due to its larger mesopore volume and less micropore volume. Both Langmuir and Freundlich model could well describe the sorption isothems of STZ and CIP onto MWCNTs-O. The oxygen groups depressed the sorption of STZ on MWCNTs-O which was similar to that of NACs sorption and maybe the same mechanism. But the oxygen functional groups of MWCNTs enhanced the sorption amount of CIP onto MWCNTs-O due to the formation of hydrogen bond between MWCNTs-O and CIP which was confirmed by micro Fourier transform infrared spectra. Due to the especial interaction, the desorption of CIP from MWCNTs-O had remarkable hysteresis. The accompany Cu had no remarkable effect on the sorption and desorption of STZ, but diminished the sorption amount of CIP on MWCNTs-O and promoted desorption of CIP from MWCNTs. Analysis of XAS results suggested that Cu complexation via formation of inner-sphere surface complex with oxygen functional groups of MWCNTs-O. Copper ion might compete with CIP for the sorption sites of oxygen groups on MWCNTs.