Boqiang Li,Fei Yang,Baiyang Chen,Juan Li,Lijuan Zhu,Wen-Tao Li
Dissolved organic nitrogen (DON) plays a key role in many biogeochemistry and engineering processes such as forming nitrogenous disinfection byproducts . However, detecting aqueous DON at trace levels is challenging currently because conventional DON conversion methods have very high method detection limits (MDL). In addition, DON is measured indirectly by subtracting dissolved inorganic nitrogen (DIN) from total dissolved nitrogen (TDN), which can propagate analytical errors of each analyte. In order to solve these issues, we isolated DON from DIN with electrodialysis before and herein tested vacuum ultraviolet (VUV) in tandem with several oxidants to convert DON completely into nitrate for subsequent N analysis. Results showed that H 2 O 2 was more suitable than chlorine and persulfate because VUV/chlorine or VUV/persulfate is either inefficient to convert DON or subjected to nonnegligible N loss. To verify the efficiency, we evaluated the effects of typical water and operating variables on the conversions of four model DON compounds and their yields of nitrate. Under optimal conditions (pH 10.3 and 500 mg/L H 2 O 2 ), the process converted DON completely into nitrate within just 60 min. Compared to conventional TDN analytical methods, the VUV/H 2 O 2 method features not only better analytical precision but also lower MDL because the formed nitrate can be analyzed at very low MDL by ion chromatography (IC). So, this approach moves one step further to achieve a conceptually new DON analytical method by coupling electrodialysis, VUV/H 2 O 2 , and IC.
Ultrafiltration (UF) is widely used in drinking water plants; however, membrane fouling is unavoidable. Natural organic matter (NOM) is commonly considered as an important pollutant that causes membrane fouling. Herein, we proposed VUV/H 2 O 2 as a UF pretreatment and used UV/H 2 O 2 for comparison. Compared to UV/H 2 O 2 , the VUV/H 2 O 2 system presented superior NOM removal. In the VUV/H 2 O 2 system, the steady-state concentration of HO• was approximately twice that in the UV/H 2 O 2 system, which was ascribed to the promoting effect of the 185 nm photons. Specifically, 185 nm photons promoted HO• generation by decomposing mainly H 2 O at a low H 2 O 2 dose or by decomposing mainly H 2 O 2 at a high H 2 O 2 dose. The VUV/H 2 O 2 pretreatment also demonstrated better membrane fouling mitigation performance than did UV/H 2 O 2 . An increase in the H 2 O 2 dose promoted HO• generation, thereby enhancing the performance of NOM degradation and membrane fouling alleviation and shifting the major membrane fouling mechanism from cake filtration to standard blocking. The VUV/H 2 O 2 (0.60 mM) pretreatment effectively reduced disinfection byproducts (DBPs) formation during chlorine disinfection. Additionally, the oxidant H 2 O 2 affected the membrane surface morphology and performance but had no evident effect on the mechanical properties. In actual water treatment, the VUV/H 2 O 2 pretreatment exhibited better performance than the UV/H 2 O 2 pretreatment in easing membrane fouling, ameliorating water quality, and reducing DBPs formation and acute toxicity.
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