| Antibiotics, as a new type of pollutant in the environment, have been used widely in recent years. The concentration of antibiotics is also increased in the environment. This environmental hazards caused by antibiotics has give rise to widespread concern. As a typical antibiotic, Tetracycline (TC) is mainly used as a feed additive to inactivate diseases and improve the growth rate of livestock. However, it is very difficult to adsorbed TC in the digestive tract; about50%-80%of the initial TC dose was discharged in the livestock waste (urine or feces). Low concentration TC and their byproducts in the environment may increase the propagation of resistance gene in the microorganism, which can threat to the public health eventually.Most of the farm wastewater was discharged into the wastewater treatment plant (WWTP) with different forms. The activated sludge process is the most commonly used method in WWTP in China. Tetracycline is a kind of hydrophobic organic compounds, meanwhile the active sludge is considered to be the most effective adsorbent for hydrophobic organic compounds. Previous studies showed that adsorption is the principal removal method of TC by activated sludge. However, the complex composition of activated sludge results in the significant difference from other ordinary adsorbent. That maybe the reason why the mechanism of absorbed TC in activated sludge is rarely involved in previous studies. The objective of this study is evaluated the adsorption and degradation performance of TC on activated sludge. Furthermore, explore the adsorption mechanism of TC by using FTIR and XPS measurement.Degradation experiments indicated that there was no obvious degradation effect of TC on the activated sludge, with BOD5was nearly zero. The only way to improve the biodegradability of TC was the process of advanced oxidation technology, which can degraded the TC macromolecules into small organic molecules. After Fenton treatment in80min, the BOD5/COD of the wastewater increased from0to0.51, with the improved of biodegradability. On the other hand, the BOD5/COD increased0.32through the O3/H2O2oxidation process.Adsorption played a major role on TC removing by activated sludge for the untreatment wastewater. Adsorption experiments indicated that the initial concentrations of TC and activated sludge both have great influence on the adsorption rate. The adsorption process had better agreement with the pseudo-second order kinetics model (R2>0.997), which indicated that the adsorption onto activated sludge is a chemisorption process. The equilibrium adsorption capacity qe value obtained from pseudo-second order model was close to the measured values, with the relative error is within2.0%. Langmuir, Freundlich and Temkin models were chose to describe the adsorption isotherm. All of them could fit the adsorption process very well in the experimental pH rangePH had great influence on the adsorption process. In Langmuir adsorption model, the theoretical maximum adsorption qm changes from46.7mg·g-1to90.95mg·g-1with different pH. When the pH value is equal to5.5, the oxytetracycline was mainly exist as zwitterionic in the solution, with the maximum adsorption capacity is90.95mg·g-1on activated sludge. Using Freundlich model to fit the adsorption isotherm, the adsorption constant Kf values were strongly depended on the pH values.Different metal ions had different effects on the adsorption process. Na+, K+, Ca2+, Mg2+and Cd2+had obvious inhibitory effect on the adsorption process; however Cu2+impelled the adsorption process significantly. In addition, the effect of ionic valence states on adsorption process was obvious:the bivalent metal ions have stronger competitive capacity than monovalent metal ions.The FTIR spectra of activated sludge showed that a variety of functional groups on the activated sludge participated in adsorption reactions. Electronic induced effects occurred before and after adsorption indicated that cation-π interactions happened between the functional groups of the activated sludge and the oxytetracycline molecule. In the presence of Cu2+, Cu2+reacted with many function groups included the carboxyl, amide and phenolic groups, the inner complexes formed firstly and eventually formed a kind of ternary complexes "activated sludge-O≡Cu-OTC". Cu2+played a linking role in the TC adsorption process on activated sludge. By analysis of XPS spectra, the proportion of aldehydes, alcohols and ethers on the activated sludge changed significantly after the combination of activated sludge and oxytetracycline, which indicated that all of these three substances were involved in the adsorption process. When the adsorption process was completed, new chemical functional groups represented phenols appeared at403.2eV (conjugates of oxytetracycline molecule). Results indicated that the activated sludge and oxytetracycline were closely integrated into a new polymer. Furthermore, the proportion of-NH2(298.3eV) and-NH3+(399.9eV) on the activated sludge changed significantly compared to the native activated sludge, indicated that these two functional groups were participated in the adsorption process. In the presence of Cu2+new compounds appeared at404. leV in the oxytetracycline loaded sludge, which may the combination of metal ion and amino. This result was consistent with the FTIR analysis.Adsorption was the main way to remove TC from activated sludge. The main adsorption mechanism was ion exchange and surface complexation included metal bridging effect and cation-π. |
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