Mechanism Of UV Photolysis For Accelerating Sulfadiazine Biodegradation

Sulfadiazine(SD) is one of the most important sulfonamides, which are detected in environment every year. The components may alter ecosystems in the natural environment, and pose a risk to human health when they bio-accumulate through the food chain, since many pharmaceuticals existed in natural environments are recalcitrant to be biodegraded by microorganisms. Currently, most activated sludge systems for treatment of municipal wastewater are mainly designed for conventional nutrients removals, such as BOD, COD, nitrogen, and phosphorus etc., but they are ineffective for pharmaceuticals removal. Therefore, the conventional activated sludge systems would be easily broken, once pharmaceuticals, like antibiotic drugs, enter into the systems by the way of municipal wastewater. It is significant to use some effective methods for alleviating the inhibition of antibiotic drugs to microorganism, and accelerating their removal rate.The current study found that wastewater containing sulfonamides can be treated by chemical oxidation process effectively, but can not be degraded completely, which may also generate intermediate products that have adverse impact to the environment. Biodegradation process solely has low efficiency, and more often, lead to low bioactivity. However, use UV photolysis or biological photocatalytic method as a pretreatment method can significantly improve the biodegradability of sulfonamides, and accelerate the rate of biodegradation.In this study, we will focus on the mechanism of how UV photolysis can accelerate SD biodegradation. Kinetics will be used to discuss SD degradation, analyze the resulting intermediate product under UV irradiation, and generate a UV photolysis pathway of SD. Especially, based on distribution of electron flow during SD biodegradation, we will compare with impacts that different intermediate products will have on SD biodegradation, when they were used for endogenous electron donors. On the basis of existing literatures, we try to find out how electrons can affect SD biodegradation and the allocation of. The results showed that:(1) We compare the degradation rate of SD under different UV intensity, the results correspond to 0.6-order kinetics. Pearson coefficient larger than 0.99 means it fit very well. When UV intensity increases gradually from 0.3 m W/cm2 to 0.45 m W/cm2, 0.6 m W/cm2 and 0.9 m W/cm2, the degradation rate of SD was increased by 29%, 120% and 238% respectively. The results showed that stronger UV intensity can significantly accelerate SD degradation. Stronger UV intensity can accelerate the progress of the experiment so as to save time and resources. Hence, we use 0.9m W/cm2 as best UV intensity for subsequent experiments.(2) According to the literature, results analysis and mass balance calculation, we generate a UV photolysis pathway. Given the results from HPLC, we get quantitative measurements of the two major intermediate products: p-aminobenzenesulfonic acid(4-ABS) and 2-aminopyrimidine(2-AP), which can be used to verify the UV photolysis way. Meanwhile, we analyze N and S removal during SD degradation process. The results showed that S is completely removed from SD as SO42-, which further verified the UV photolysis way.(3) We conduct a series of experiments under three protocols: 2-AP+ SD; 4-ABS + SD; 2-AP + 4-ABS + SD, and the results described by a kinetic model. We find out that 2-AP cannot accelerate SD biodegradation because it also has the inhibition to microorganisms. While 4-ABS can significantly accelerate SD biodegradation, which indicated that UV photolysis can accelerate SD biodegradation is mainly because the formation of 4-ABS.(4) According to the research results, suggesting that electrons is the reason that accelerate SD biodegradation process. By calculating mass balance and electron balance, we generated SD photolysis/biodegradation pathway. Results showed that, the mechanism is that 4-ABS formulated during UV photolysis which released enough electrons to accelerate SD initial oxygenation reaction and biodegradation rate is proportional to the number of electronic equivalents.In summary, UV irradiation not only can minimize SD inhibition to microorganism, but also can accelerate biodegradation. The results can provide further experience and reference for the degradation of sulfadiazine and other similar refractory organics.