Coagulation Mechanism of Dissolved Organic Nitrogen Based on 3DEEM-FRI
The removal characteristics of dissolved organic nitrogen(DON)by aluminum sulfate,aluminum chlorohydrate,and ferric chloride were examined.The relationship between the DON components and polarity was analyzed.The DON components and transformation characteristics were investigated through excitation-emission matrix spectroscopy(3DEEM)and fluorescence regional integration(FRI).Aluminum sulfate(27 mg/L),aluminum chlorohydrate(40 mg/L)and ferric chloride(70 mg/L)demonstrated the average removal efficiencies of 25.7％,45.3％and 48.6％respectively for DON.When the dosage of cationic-type polyacrylamide ranged from 0 to 4 mg/L,the DON removal efficiency of aluminum salt initially increased and then decreased,while the DON removal efficiency of ferric chloride decreased gradually.When pH was within the range of 4 to 10,it significantly influenced the DON removal efficiency by aluminum sulfate and ferric chloride(with the removal efficiencies varying by 17.6％and 27.1％respectively),and the DON removal efficiency of aluminum chlorohydrate showed the smallest change(≤3.2％).After undergoing sand filtration treatment,the coagulation removal efficiency of DON rose to a certain degree.The removal of DON by aluminum chlorohydrate and ferric chloride showed certain correlation with the initial concentration of DON(R2 was 0.64 and 0.57,respectively).Ferric chloride achieved the highest removal efficiencies for dissolved organic carbon(DOC)and orthophosphate,approximately 42％and 96％respectively.The turbidity could be reduced to less than 0.1 NTU through coagulation with aluminum chloride and ferric chloride,and to(0.8±0.3)NTU by means of coagulation with aluminum sulfate.The average concentration of DON with particle size less than 0.45μm was(1.88±0.12)mg/L,the average concentration of DON with molecular weight less than 3 ku was(0.89±0.09)mg/L,and the low molecular weight of DON(＜3 ku)accounted for 47.3％of the total DON.In the water samples with DON particle size less than 0.45 μm,DON particle size less than 0.1 µm,DON molecular weight less than 10 ku and DON molecular weight less than 3 ku,the FRI of region Ⅰ and region Ⅱ accounted for 28％and 30％respectively,while that of region Ⅳ and region Ⅴ accounted for 12％.After coagulation,the DON removal efficiency in water samples within the range of 0.1-0.45 µm and 10 ku-0.1 μm exceeded 80％,while the DON removal efficiency in water samples within the range of 3-10 ku and less than 3 ku was less than 20％.The removal efficiency of hydrophobic DON was higher compared to that of hydrophilic DON.The removal efficiencies of FRI in water samples with particle sizes less than 0.45 μm and 0.1 µm,and molecular weight less than 10 ku and 3 ku were all less than 15％,suggesting that tyrosine and tryptophan organics in the effluent from the secondary treatment process were the key components for enhancing coagulation and increasing the DON removal efficiency.
effluent from the secondary treatment processdissolved organic nitrogencoagulationexcitation-emission matrix spectroscopyfluorescence regional integration
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