| The increasing level of life state make more and more individuals begin to think much of their health, leading to the rising consumption of pharmaceutical and personal care products(PPCPs). PPCPs generally have strong polarity, high solubility, low volatilization and resistance to biodegradate for the reason of pharmaceutical design. These properties bring on low removal effenciency of PPCPs by general waste water treatment systems, thus the huge amount of produced and consumed PPCPs would inevitably cause the continual releasing of PPCPs to the environment. Most pharmaceuticals have the structure of phenyl, heterocycle, phenol, therefore direct photodegradation may proceed because these function groups can absorb sunlight. Natural water also contains photoactive components such as nitrate, humic substance, iron ion and so on, which can be excited by sunlight generating reactive oxygen species. Therefore, photodegradation may be the main transforming pathway of those PPCPs which have low adsorption, high solubility and resistance to biodegradate. The wide usage of nanomaterials translates into an increasing presence in the environment. Whether these namomaterials have photochemical action and what effects they may pose on the transformation of PPCPs are the questions needed to be answered.In this study, the photodegradation of beta-blocker atenolol (ATL) which has low adsorption, high solubility and resistance to biodegrdate was performed. Humic substances were added into ATL solution to observe the impacts of them on the photolysis of atenolol. We also investigated the photochemical action of single-wall nanotubes (SWNTs). Furthermore, the influence of nitrate, iron ion and SWNTs on the direct and indirect photodegradation of ATL in humic substances was conduced. The main conclusions are as follows:(1) The effect of humic substances on the photolysis of atenolol. ATL (10.0μM) is photostable and has strong resistance to be oxidized. A DOM-induced decay of ATL was observed, which followed pseudo first-order dynamics with the half-life of45-70h. Different origined humic substances have the same photodegradation efficiency of ATL. Through quenching experiments, we found the loss of atenolol in humic substances solutions was due to the hydroxyl radical generated from excited humic substances. The steady state concentrations of-OH were determined at the range of5.0-10.4×10-6mol L-1:(2) The effect of humic substances on the photolysis of atenolol in the presence of nitrate. ATL can be well photodegradated in the presence of nitrate (1.0mM) alone, which followed pseudo first-order dynamics with the half-life of2.5h. When5.0mgC L"1humic substances were added, the rate constants were decreased to half of that with nitrate alone. The inhibition effect is due to DOMs screening the photoactive light absorbed by nitrate and competing with ATL for nitrate-derived·OH. Suwannee River derived humic substances have a stronger screening effect rather than quenching effect while the Nordic Lake derived humic substances are converse. The same origined DOMs have the similar screening and quenching effects. Three pathways of ATL photodegradation were found in the nitrate solution alone:O-dealkylation, addition of oxygen to the lost amide group and addition of hydroxyl radical to the aromatic ring of ATL. SRFA can promote addition of·OH on aromatic ring, decomposing intermediates containing phenolic group.(3) The effect of humic substances on the photolysis of atenolol in the presence of iron ions. Iron ion had strong photoaction to the photodegradation of ATL when pH was less than5.0. In most natural water pH values (6.0-8.0), the dominant species of iron is the photostable Fe(OH)2+, so iron-mediated photodegradation of ATL is unlikely occurring. When5.0mgC L-1humic substances were added to30.0μM iron (III) solutions at pH6.3, iron-humic substance complexes could be formed. Excited iron-humic substance complexes could generate ferrous ion and hydroxyl radical during ligand-to-metal charge transfer process, which accelerate decay rate of ATL. The pH has little effect on Fe (III)-DOM complex mediated ATL degradation in the range of pH5.3to pH7.2. At natural water level concentration of iron ion and humic substances, the rate controling factor of ATL photodegrdation is the concentration of iron ion. The photodegradation of ATL in Fe3+solution have three similar pathways with that in nitrate solution. SRFA can promote reducing ketone to alcohol.(4) The effect of single-wall carbon nanotubes on the photolysis of atenolol in simulated natural water. The suspensions of SWNTs (0.8-3.2mgC L-1) can generate singlet oxygen and hydroxyl radical under irradiation of simulated sunlight. The capacity of producing singlet oxygen by SWNTs suspensions was proportional to the concentration of SWNTs, only1/9-1/5of that of SRFA. The capacity of producing hydroxyl radical by SWNTs suspensions was adversely proportional to the concentration of SWNTs, only2/5-1/2of that of SRFA. No adsorption of atenolol by SWNTs was observed, while significant degradation of atenolol was found under irradiation, following pseudo first-order dynamics with the half-life time of11-41h. In nitrate solution, SWNTs inhibited photodegradation rate of atenolol through screening effect while the quenching hydroxyl radical effect was not obvious. In humic substances or iron solutions, SWNTs prohibited these photoactive components to produce reactive oxygen.Beta-blocker atenolol can not be photodegradated directly, even in the presence of singlet oxygen, but it can be oxidized by the hydroxyl radical generated by humic substances. Humic substances have quenching effect on hydroxyl radical in nitrate solutions, while in the presence of iron ions, more hydroxyl radical were yielded by the Fe(III)-DOM complex. SWNT can produce singlet oxygen and hydroxyl radical under irradiation, but the level of these ROS are far below those generated by humic substances at the same concentrations. SWNT also have strong inhibiting effect on the ability of humic substances to produce ROS. The paper found the photodegradation law of atenolol in simulated natural water and also discover the interaction between humic substances and other natural water components such as nitrate, iron ions and SWNT. |
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