The Effect Of Functionalized Multi-walled Carbon Nanotubes On Photodegradation Mechanism Of Typical Organic Contaminants In Water

Carbon-based nanomaterials (CNM) include fullerenes, carbon nanotubes and their derivatives with at least one dimension in the structure is less than100nm. Owing to their unique physical, chemical and electrical properties, CNM are widely used in medical, transportation and environmental protection. The huge production of commercial CNM and extensive application raise concerns about their environmental occurrence and risk. Although the synthesis and application of CNM have been frequently studied, their environmental fate and ecological risk are still largely unknown, especially for those water-dispersible CNM. Colloidal dispersible CNM could photochemically produce reactive oxygen species (ROS), such as hydroxyl radical (·OH) and singlet oxygen CO2), under UVA or solar irradiation, and promote the degradation of organic pollutants. Since ROS could damage the lipid chain, many studies in environmental chemistry and ecotoxicology focused on the mechanisms of ROS formation by CNM. Environmental behavior of CNM in natural waters is extremely complex because their dispersion and interaction with natural water constituents may be significantly affected by particle, structure and functional groups. Thus, there is a huge challenge for studing the environmental photochemistry of CNM.In the present study, the photoreactivity of functional multi-walled carbon nanotubes (FMWNTs) and their effects on the photochemical transformation of organic pollutant under simulated sunlight irradiation were investigated. In order to investigate the possible effects of hydroxylated multi-walled carbon nanotubes (MWNT-OH) on the transformation of organic pollutants upon irradiation, two emerging organic contaminants, atenolol (ATL) and florfenicol (FLO), were chosen as model compounds due to their widespread occurrence and relatively high level in natural aquatic environment. The main results are as follows:(1) Dispersion and photoreactivity of FMWNTsAqueous suspensions of the MWNT-OH and carboxylic multi-walled carbon nanotubes (MWNT-COOH) were prepared in DI water via intermittent and ice-bath sonication. There were good linear relationships between the concentrations and absorbances of the suspensions. The FMWNTs suspensions were stable since the absorbances remained unchanged (<5%) within30d and no phase separation was observed after100d.Furfuryl alcohol (FFA) and p-chlorobenzoic acid (PCBA) as molecular probes were used to identify the possible production of different ROS species, i.e.,1O2and·OH. The steady-state concentrations of ROS were measured to be10"14and10-15mol L-1level in8mgC L"1MWNT-OH and MWNT-COOH suspensions, respectively, under simulated sunlight source. The ability to generate1O2by MWNT-OH and MWNT-COOH was comparable, however, photoinduced generation of·OH by MWNT-OH was stronger than MWNT-COOH. The steady-state concentration of ROS increased with increasing concentration of carbon nanotubes (0.8-8mgC L"1). Compared with the naturally occurring1O2and·OH, the presence of FMWNTs in natural waters may provide addtional source of1O2and·OH, which would change the oxidative capability of water bodies and affect the environmental fate of many contaminants.(2) Photodegradation of atenolol in MWNT-OH suspensions Atenolol in DI water was photostable and hydrolysis resistant, but could be degradated in aqueous solution in the presence of NOMs, NO3-, Fe3+and MWNT-OH under simulated sunlight irradiation. The presence of natural water constituents, such as NOMs, NO3-and Fe3+, affected the photolysis rate constant of atenolol in MWNT-OH solution. Hydroxyl radical was identified as the predonminant reactive species in the photolysis process. SRFA and SRHA had both light screening and OH scavenging effect on atenolol photolysis, while NOFA and NOHA only showed light screening effect. NO3-and Fe(III) could promote the decay of atenolol in MWNT-OH solution. The photochemical transformation pathways of atenolol in MWNT-OH suspension involved hydroxylation, acetamide group oxidation, ether side chain cleavage and di-pohymerization of the intermediates.(3) Photodegradation of florfenicol in MWNT-OH suspensionsUnder solar and xenon lamp irradiation, direct photolysis contributed to the degradation of florfenicol in DI water, with a half-life of187.29h and22.4h, respectively. The photolysis for florfenicol in pure water and MWNT-OH suspensions under two irradiation sources follow pseudo-first-order kinetics. Hydroxyl radical (·OH) and singlet oxygen (1O2) were found to be involved in the photolysis process. The decays of florfenicol in MWNT-OH suspension were pH-dependent. The presence of MWNT-OH affected the photolysis of florfenicol in NO3-and NOMs solutions though light screening and radical quenching effect as well as photoinduced oxidization process. Under simulated solar irradiation, thiamphenicol was determined as the photohydrolysis product of florfenicol in MWNT-OH suspension. Others pathways included the electrophilic attacking of·OH at the aromatic ring, dechlorination, oxidation and cleavage of the side chain. The results of this study are not only important for better understanding the environmental fate of florfenicol and assessing the environmental risk of MWNT-OH, but also useful for further photochemical studies on carbon nanotubes.