| Hydraulic fracturing is currently one of the most important methods of shale gas exploration,and the resulting water pollution control problems have received considerable research attention.The produced water from the hydraulic fracturing production process is highly saline and difficult to degrade,which contains many kinds of dissolved chemicals,heavy metals,natural radioactive substances,bacteria and water-soluble salts,etc.Scientific management of produced water is a topic that must be paid attention to.Most domestic produced water was discharged or reinjected after simple pretreatment,while the Fuling shale gas field produced water treatment station,as the first high-standard shale gas produced water treatment plant in China,has realized the standard.However,there are still many problems,such as the technologies for treating produced water are currently in their initial stages,the relevant standard system has not been perfected,and the transformation of pollutants during the treatment process is not yet clear.In recent years,the acute and chronic toxicity of the produced water has been confirmed,and its toxicity is closely related to the organic components contained in it.Therefore,this study about the transformation of dissolved organic matter(DOM)in the treatment process based on the Fuling shale gas field produced water treatment station can indirectly provide reference information on the environmental risk of complex organic matter in such type of disposal.In addition,transformation of DOM in electrochemical oxidation,the main organic matter and ammonia nitrogen reduction process,is important for the understanding of membrane contamination behavior in the subsequent reverse osmosis(RO)treatment.In this study,the DOM composition of water samples was obtained by fluorescence spectroscopy,Fourier transform ionic vibration syndrome(FT-ICR MS),nuclear magnetic resonance spectrum(NMR)and gas chromatography-mass spectrum(GC-MS),in order to investigate the DOM migration and transformation patterns.At the same time,the electrochemical oxidation was selected for in-depth analysis,and the organic nitrogenous by-products(N-DBPs)were investigated to identify their production and generation pathways,and to provide a research basis for controlling.The main findings are as follows.(1)The fluorescence spectroscopy results showed that the aromatic protein-like substances in the produced water were the first main peak and the most obvious removal effect during the treatment.The analysis results of FT-ICR MS showed that the chemical diversity and composition of DOM changed significantly during the pretreatment process,and RO was the main organic removal stage.The CHON-like organics also had high resistance in the RO and should be removed as much as possible in the pretreatment stage,considering their impact on membrane contamination.The main recalcitrant organics in the full process were CHO-like organics and aliphatic/proteins.(2)The reaction in the electrochemical oxidation was investigated by simulation experiments,and the results showed that free radicals would preferentially attack long-chain and aromatic compounds.The removed molecules were mainly Lignins/CRAM-like,and the produced molecules are mainly aliphatic/proteins,accompanied by the generation of halogenated organic compounds.Oxygen addition was probably the most dominant type of reaction occurring during the treatment,followed by dealkylation reactions,and low O/C DOM were more prone to halogenation reactions.The nuclear magnetic resonance hydrogen spectroscopy spectrum showed that the removal of aromatic hydrocarbons was better in electrochemical oxidation.(3)The in-depth analysis of N-DBPs in the electrochemical oxidation showed that the generated N-DBPs were mainly concentrated in the aliphatic/protein region,most of them containing one nitrogen atom,and their compositions were typically characterized by higher O/C and lower H/C.By speculating the structures of the organic nitrogen,it was inferred that their main precursors might be aldehydes and amine organics.In addition,the GC-MS results indicated that the DCAN and DBAN produced might be attributed to the organic by-products produced during the Fenton treatment and the higher concentration of halogen ions in the water. |
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