Removal Of Algae In Drinking Water Sources And Inhibition Mechanisms Of Disinfection Byproducts By Discharge Plasma Oxidation

Cyanobacterial blooms have been described as an“ecological cancer”,posing a serious threat to aquatic ecosystems,drinking water quality and human health.Although traditional pretreatment technologies,such as manganese oxidation,UV/H₂O₂,Fenton oxidation,etc.,can achieve the removal of algal cells,the massive use of chemical reagents will bring potential secondary pollution to the water body.As a green advanced oxidation technology with high efficiency,discharge plasma has been used for the inactivation of algal cells and the degradation of algal toxins.Meanwhile,algal organic matter（AOM）is also released accompanying with the algae inactivation.However,the information on the chemical molecular diversity of different algal-derived AOM is still lacking.Although DBPs resulting from the reaction of disinfectant chlorine with AOM in raw water has been widely discussed,most studies mainly focused on traditional DBPs（containing only 1-2 carbon atoms）such as trihalomethane（THMs）,haloacetic acid（HAAs）and haloacetonitrile（HHAs）.However,it is estimated that these conventional DBPs account for less than 30%of the total organic halides（TOX）which indicates that a large number of unregulated unknown chlorinated DBPs（defined as novel DBPs）have not yet been identified.So far,there are still some gaps in understanding the relationship between the molecular diversity evolution of different algal-derived AOM under plasma action and the generation of novel DBPs in the subsequent chlorination process.Therefore,this study starts with the removal of algal cells,algal toxins,and odor substances from water using the plasma oxidation process,exploring the inactivation laws and mechanisms of plasma oxidation on different types and forms of algal cells.The degradation laws and mechanisms of plasma oxidation on new algal toxins（BMAA）and odor substances（2-MIB and GSM）were also examined.The study also revealed the chemical diversity evolution laws of AOM derived from different algae species under the action of plasma oxidation.The potential for generating traditional DBPs and new DBPs induced by AOM before and after plasma pre-oxidation was evaluated,and the mechanism of new DBP generation after chlorination in AOM and characterizes the molecular characteristics of its potential precursors was also clarified.Furthermore,the reduction laws and action mechanisms of new DBPs from algal sources mediated by plasma pre-oxidation were also investigated.Based on these methods,the chemical molecular characteristics of AOM,the potential for generating DBPs,and the toxicological effects of DBPs were clarified.The main research results are as follows:（1）The inactivation rule and mechanism of plasma oxidation on algal cells with different types and forms were investigated.Using Anabaena spiroides（A.spiroides）as test species to evaluate the inactivation efficiency and mechanism of plasma oxidation,it was found that high voltage accelerated the deactivation of algal cells.The inactivation rate of A.spiroides cells with an initial algal density of 1.5×10⁸ cells m L^-1 reached 100%after 12 min treatment at 7k V voltage.The qualitative and quantitative experiments of active species showed that ¹O₂,·OH,O₃,H₂O₂ and NO₃^-produced during plasma process all played a role in the inactivation of algal cells,of which ¹O₂ and·OH contributed to the most.Transcriptomic analysis showed that some genes related to glutathione（GSH）（including glycine hydroxymethyltransferase,cythioneβ-synthetase and glutathione s-transferase）were significantly up-regulated and DNA repair protein genes were significantly down-regulated in A.spiroides cells at the initial stage of plasma oxidation treatment.It was indicated that plasma treatment inhibited phospholipid transport,damaged cell membrane,changed the permeability of cell membrane,and finally damaged cell integrity.The results of flow cytometry and confocal laser scanning microscope also confirmed this conclusion.In addition,physiological and biochemical indexes analysis showed that the antioxidant defense system of algal cells collapsed after plasma treatment for a long time,and the algal cells could not be revived in the process of reculture.Moreover,the inactivation efficiency of different species of cyanobacteria cells was also different,and the inactivation efficiency was C.racibprskii>A.spiroies>M.aeruginosa,which was related to the characteristics of cyanobacteria cells（such as shape,size and locomotion ability）.（2）The removal regularity and mechanism of plasma oxidation on novel algal toxins（BMAA）and odors（2-MIB and GSM）were investigated.For BMAA,the plasma degradation efficiency was 99.1%after 40 s when the applied voltage was 6 k V.However,when the applied voltage was 7 k V,the degradation efficiency decreased by 12.6%,which was attributed to the self-quenching reaction between ROS at high voltage.For the odor substances 2-MIB and GSM,the degradation efficiency of 2-MIB and GSM could reach 100%and 99.8%after 5 min plasma treatment at 6 k V.Meanwhile,p H posed significant influence on plasma oxidation of BMAA,2-MIB and GSM.The degradation effect of BMAA under alkaline condition was the best,while that of 2-MIB and GSM under acidic condition was the best.The presence of bicarbicarbose ion（HCO₃^-）,chloride ion（Cl^-）and algal organic matter（AOM）in water matrix inhibited the degradation efficiency of BMAA with the inhibition efficiency of AOM>HCO₃^->Cl^-.For GSM and 2-MIB,HCO₃^-and AOM could inhibit the degradation efficiency,while Cl^-had no significant difference on the degradation efficiency.Front-line orbit analysis showed that C-N key in BMAA was the most vulnerable site to plasma attack,C-O key in 2-MIB was the priority site to attack,and C-H key was the priority site to attack in GSM.The experimental results of the identification of the intermediates from the degradation of three substances were in agreement with the results of theoretical analysis.（3）The molecular diversity evolution of AOM under plasma-mediated interaction was revealed.It was found that at the initial stage of plasma treatment,the cell broke up accompanying with the release of AOM,which leaded to the increase of AOM content in algal water,but the ROS produced by Plasma was further oxidized and decomposed.Plasma treatment could reduce the aromaticity of AOM,with the SUVA₂₅₄ values of three kinds of AOM decreased by 66.1%～79.3%after 12 min treatment.EEM-PARAFAC analysis showed that AOM could be identified as chlorophyll-like substances,UVC-like humic acids,tryptophan-like substances and UVA humic substances.With the extension of plasma oxidation time,the four components of AOM increased firstly and then decreased gradually.The substance proportion with apparent molecular weight（MW>1 k Da）increased by7.1%～19.2%of AOM after 20 min Plasma treatment,while MW>10 k Da decreased by5.4%～15.1%.Fourier transform ion cyclotron resonance mass spectrometry（FT-ICR-MS）analysis showed that AOM was mainly composed of lipid,aliphatic/protein,carbohydrate,unsaturated hydrocarbon,lignin and tannic acid.After 20 min Plasma treatment,the relative abundance of the three AOM molecular formulas containing only CHO increased by11.5%～23.6%,while the relative abundance of the molecular formulas containing only CHNO decreased by 19.1%～21.8%.Plasma oxidation significantly changed the molecular composition and properties of AOM.The relative abundance of AOM compounds with molecular weight ranging from 100 to 250 Da increased by 6.1%～10.4%after 20 min treatment.Plasma oxidation also decreased the relative abundance of AOM containing 2～8oxygen atoms by 3.9%～11.5%,and increased the relative abundance of compounds containing16～20 oxygen atoms by 0.9%～1.7%.In addition,the unsaturated compounds and reducing compounds in AOM were preferentially decomposed during plasma oxidation process,leading to the decrease of molecular aromaticity and the increase of saturation and oxidation degree.（4）The reduction rule and mechanism of plasma preoxidation on algal DBPs were elucidated.Before plasma preoxidation,the potential of C-DBPs（THMs,HAAs and HKs）and N-DBPs（HANs）were 288.5～415.8μg L^-1,413.3～565.6μg L^-1,59.8～130.6μg L^-1 and83.2～87.0μg L^-1,respectively.After 20 min plasma preoxidation,the production of C-DBPs and N-DBPs decreased by 38.2%～80.7%and 37.7%～52.1%,respectively.Furthermore,direct chlorination of AOM derived from three different cyanobacteria produced approximately 2486novel C-DBPs and 1984 novel N-DBPs,with the relative abundance pairs of C₁₁H_nO_mCl_x and C₁₈H_nN_mO_zCl_x being large.After plasma preoxidation,the quantity of unknown C-DBPs and N-DBPs decreased by 63.3%and 62.9%,respectively.At the same time,it was found that the double bond equivalent（DBE）and aromaticity equivalent（Xc）values of novel C-DBPs and N-DBPs compounds decreased after plasma preoxidation,which may be the reason why the cytotoxicity and genotoxicity of novel DBPs decreased after plasma preoxidation.The analysis of the poor-quality network showed that the substitution of one or more Cl atoms was the main way to form the AOM-derived DBPs.The molecular properties of the potential precursors of the novel DBPs were characterized,of which result showed that the precursors of the novel DBPs had certain chemical characteristics（1.01≤H/C≤1.64;0.40≤O/C≤0.91;0.1≤N/C≤0.3）;Spearman correlation analysis showed that cytotoxicity was positively correlated with humification index（HIX）,FT-ICR-MS correlation parameters(Cos_wa),carbon standard oxidation state(NOSC_wa)and nitrogen-carbon ratio(N/C_wa).Genotoxicity was positively correlated with spectral indices（S_R）,Cos_wa and N/C_wa.In summary,this study systematically elucidated the effect of plasma oxidation on the removal of algal cells,algal toxins and odors in water and the inhibition mechanisms of algal source DBPs.We deeply understood the evolution law of chemical molecular diversity of AOM derived from different algae under plasma oxidation,and successfully identified a large number of new DBPs induced by AOM,which enriched the understanding of existing DBPs.This provides a certain guiding significance for the optimization and upgrading of the subsequent algal water treatment process.