Effects Of Ferrate Preoxidation On Controlling The Formation Of Disinfection By-products From Algae Cells And Algal Organic Matter

Lakes and reservoirs are important drinking water sources.Cyanobacterial blooms have been frequently break out in recent years,which have brought many challenges to drinking water treatment.Algae cells and algal organic matter(AOM)were difficult to remove during conventional treatment processes and more organic nitrogen was contained in algal organic matter than natural organic matter(NOM).It was likely to form more toxic nitrogenous disinfection by-products(N-DBPs),thereby threatening the safety of drinking water supply.Potassium ferrate is a promising green water treatment agent which combined multiple functions of oxidation,disinfection,adsorption and aggregation aid etc.This paper firstly characterized the characteristics of algae cells and algal organic matter and the methods for high sensitivity determination of variety of disinfection byproducts(DBPs)were established.The relationships between the characteristics of algae cells and algal organic matter and the formation of chlorine/chloramine disinfection byproducts were investigated.The effects of ferrate preoxidation on the characteristics of algae cells and algal organic matter and the control of DBPs formation in subsequent chlorine/chloramine disinfection processes were investigated compared with widely used ozone preoxidation.The mechanisms of ferrate preoxidation on controlling the formation of DBPs from algae cells and algal organic matter were comprehensively discussed.The methods for high-sensitivity simultaneous determination of 15 halogenated volatile disinfection by-products and 9 N-nitrosamines were established respectively in this paper.The fluorescence response of algae cells/AOM were mainly focus on the regions of protein and microbial metabolites.The contents of characteristic organics including organic nitrogen amino acids,aliphatic amines and protein,and aldehydes and ketones in AOM were higher than that in NOM,while the polysaccharide content and SUVA value were lower than that of NOM.The high organic nitrogen content of algae cells/AOM resulted in more N-DBPs and N-nitrosamine formation in chlorine disinfection process.Compared with chlorination,the reactivity of chloramine with precursors was weak and the cell lysis efficiency was low.Chloramine could not react effectively with intracellular organic matter(IOM)due to the hindrance of cellular structure.And nitrogen-containing precursors in AOM are sufficient and additional nitrogen is usually not required.Therefore,the formation of carbonaceous disinfection by-products(C-DBPs)and N-DBPs from algae cells and AOM during chloramination were significantly reduced,These indicated that chloramination can be used as a technical measure for controlling the formation of DBPs from algae.But it promoted the formation of N-nitrosamines,especially N-nitrosodimethylamine(NDMA),which should be monitored carefully.The fluorescence response and the contents of aliphatic amines,amino acids and protein were decreased,and the contents of aldehydes and ketones were increased in IOM after ferrate preoxidation.The effects of ozonation on fluorescence response and characteristic organic matters were accordance with that of ferrate preoxidation but the impacts were more severe,indicating that more biomacromolecular organics were oxidized into small molecules.In addition,more organic nitrogen was transformed into inorganic nitrogen by ferrate.Ferrate preoxidation IOM only promoted the formation of N-nitrosamines during chlorination,and reduced the formation of C-DBPs and Nnitrosamines during chloramination.These indicated that ferrate preoxidation reduced the reactivity of IOM with disinfectant,especially chloramine,and ozone preoxidation increased the reactivity of IOM with disinfectant.The effects of two preoxidation way on the N-DBPs formation of IOM were not obvious and the formation of TOX were reduced with both ferrate and ozone preoxidation during chlorine/chloramine disinfection process.The cell lysis rate by ferrate was ranging from 24.66 M-1s-1 to 118.95 M-1s-1(1-10 mg/L ferrate)and the microcystin MC-LR released from ruptured cells could be degraded at the same time.The cell lysis efficiencies of common used oxidants in drinking water treatment process were ozone>ferrate>chlorine> monochloramine> permanganate.IOM was gradually released after ferrate preoxidation,and then the realeased IOM was sufficiently oxidized and the reactivity was decreased.While the algae cells were severely destroyed and a large amount of TOC,TON and characteristic organic matters were released after ozone preoxidation.The released IOM were converted into humic acid and fulvic acid like organics,and could be further oxidized to form small moleculars whose reactivity was high with disinfectant.For algae cells,only N-nitrosamines formation were promoted after ferrate preoxidation during chlorination and the contents of various DBPs were decreased significantly due to the oxidation of released IOM by ferrate during chloramination.For dissolved organic matter,the formation of DBPs were gradually increased after ferrate preoxidation due to the release of IOM during chlorination.The formation of DBPs increased first and then decreased with the increase of ferrate dosage during chloramination under the combined effects of IOM release and its reactivity decline after ferrate preoxidation.After ozone preoxidation,the formation of various DBPs were significantly increased during chlorine/chloramine disinfection processes of algae cells and dissolved organic matter,due to the severe release of IOM and its reactivity increase after ozone preoxidation.Therefore,ferrate preoxidation could effectively control the DBPs formation of algae cells and algal organic matter during chloramination.During coagulation process,the surface characteristics of algae cells were changed by ferrate preoxidation and the produced ferric nanoparticles were adsorbed onto the surface of algae cells.These effects increased Zeta potential and specific gravity of the algae cells and promoted flocculation of algae cells with coagulants which leading to the formation of more coarse flocs.The d(0.5)of algae floc size reached 288.9 ?m after coagulation with ferrate preoxidation and the residual individual alge cells were less.The algae cells and the released dissolved organic matter after ferrate preoxidation could be removed effectively during coagulation process with algae cells removal rate increased from 37.3% to 93.6%(7 mg/L ferrate).The effective remove of algae precursers led to the significant decline of subsequent DBPs formation.