Reaearch On Degradation Of Nitrosamine Precursor From Algae Organic Matter By UV/H₂O₂/O₃ Advanced Oxidation Technology

The problem of nitrosamines?NA_S?disinfection by-products is more prominent in China.According to a survey of 117 water samples from 40 cities in China,more than one kind of NA_S was detected in 94.8%of water samples,and the typical representative of NA_S,NDMA,had the highest detection concentration and detection rate.In order to solve the problem of NA_S disinfection by-products,we should focus on the front-end control,that is,in the water treatment process,we should remove the precursors as much as possible to achieve the purpose of controlling their production.The algae derived organic matter?AOM?released during the growth and propagation of cyanobacteria is a kind of complex mixture composed of protein,amino acid,amine organic matter,etc.the organic nitrogen content of AOMis high,which has an important contribution to the formation of NA_S.This paper focuses on the research and control of alginate NA_S precursors,the composition and formation characteristics of alginate NA_S precursors were studied,the control technology was selected and the operating conditions were optimized.Finally,the influencing factors and mechanism of the reaction were studied.Firstly,the composition of anabaena flos-aquae AOM and the characteristics of its NA_S production were studied.The results showed that AOM was mainly composed of dissolved microbial products,hydrophobic humic acid,fulvic acid and a small amount of aromatic protein,and the content of low molecular weight,hydrophilic and polar organic matter accounted for the majority.The small molecular components with MW<1kDa contributed the most to the formation of NA_S,accounting for 47.34%of the total NDMA-FP,while the large molecular weight components with MW>10kDa had a higher ability to generate NA_S than the small molecular organic compounds,which may be due to the fact that the large molecular weight components contained more organic nitrogen.The results of hydrophilicity and hydrophobicity analysis showed that the contribution of NA_S formation of different hydrophilic components was:hydrophilic component?HPI?>hydrophobic component?HPO?>transition hydrophilic component?TPI?,and the order of formation ability was:HPO>HPI>TPI;the contribution of nitrosamine formation of polar component was greater than that of non-polar component,while the generation ability was opposite.UV and H₂O₂ pre-oxidations could not effectively remove the precursor of NA_S from algae,while ozone alone only played a role under the condition of high dosage.UV-AOPs can remove the precursor of NA_S from algae,but at a low UV dose?20-50mJ/cm²?,UV-AOPs oxidation will greatly increase the amount of NA_S production.When the dosage of oxidant is reduced by half,the removal effect of UV/H₂O₂/O₃oxidation system on the precursor of NA_S is significantly better than UV/H₂O₂ and UV/O₃.The results showed that the best conditions of UV/H₂O₂/O₃ to remove the precursor of NA_S from algae were:UV dose 220 mJ/cm²,O₃ dosage 0.26 mg/L,H₂O₂dosage 2 mg/L.the order of the influence degree of each factor on the experimental results was:UV dose>O₃ dosage>H₂O₂ dosage.Alkaline environment will reduce the oxidation efficiency of UV/H₂O₂/O₃ technology,to some extent,acidic environment is conducive to its function,but the pH is too low will also have a negative impact;HCO₃^-?SO₄^2-?Cl^-three ions all play a role in inhibition,the order of effect was:HCO₃^->SO₄^2->Cl^-,and the presence of NO₃^-ions can significantly increase the content of NAs.The mechanism analysis shows that ozone oxidation in UV/H₂O₂/O₃ system only plays an oxidation role in the early stage of the reaction,and its content decreases by2 logs after five minutes of reaction,by analyzing the accumulation of hydroxyl radicals,it can be seen that under the double action of UV and H₂O₂,hydroxyl radicals?·OH?are formed after ozone decomposition,and the oxidation of hydroxyl radicals?·OH?runs through the whole reaction.The macro mechanism analysis shows that UV/H₂O₂/O₃ technology can destroy the unsaturated bond i n AOM,degrade macromolecular components and hydrophobic organics.EEM analysis showed that the content of dissolved microbial products and humic acids in the organic matter of algae source decreased,and the peak of aromatic protein disappeared.For the first time,LC-OCD was used to separate and analyze the AOM,the results showed that the biopolymers and humic acids with larger molecular weight in AOM were transformed into building blocks and LMW neutrals,and the biodegradability of AOM was improved.