水庫(kù)溫躍層氧最小值區(qū)域天冬氨酸的轉(zhuǎn)化研究:規(guī)律及機(jī)制

趙 娜,曹瑞華,黃廷林,文 剛

水庫(kù)溫躍層氧最小值區(qū)域天冬氨酸的轉(zhuǎn)化研究:規(guī)律及機(jī)制

趙 娜,曹瑞華,黃廷林,文 剛*

(西安建筑科技大學(xué)環(huán)境與市政工程學(xué)院,西北水資源與環(huán)境生態(tài)教育部重點(diǎn)實(shí)驗(yàn)室,陜西省環(huán)境工程重點(diǎn)實(shí)驗(yàn)室,陜西 西安 710055)

選取典型含氮消毒副產(chǎn)物前體物天冬氨酸(Asp)為研究對(duì)象,研究了不同溶解氧(DO)濃度、不同壓力MOM條件下Asp的轉(zhuǎn)化規(guī)律及消毒副產(chǎn)物生成勢(shì)(DBPFPs)變化,進(jìn)一步闡明了MOM條件下影響Asp轉(zhuǎn)化的主要環(huán)境因素及潛在機(jī)理.結(jié)果表明:不同MOM條件下,Asp水樣的溶解性有機(jī)碳(DOC)、溶解性有機(jī)氮(DON)和T-DBPFPs均隨著反應(yīng)時(shí)間的增加逐漸降低;反應(yīng)第3d,與厭氧(常壓)條件相比較,壓力為0.30MPa、DO為0.50mg/L的MOM條件下DOC和DON的下降程度(16.40%～25.50%)以及T-DBPFPs的下降程度(30.34%～59.81%)較低,這與該條件下微生物代謝過程中產(chǎn)生更多可溶性生物代謝物(C2組分)有關(guān);此外,壓力為0.30MPa、DO為3.00～7.00mg/L的MOM條件下DOC、DON和T-DBPFPs的下降程度(DOC和DON:70.27%～95.00%;T-DBPFPs:61.50%～98.88%)高于DO為0.50mg/L的MOM條件下的下降程度.綜上所述,較低DO濃度和壓力下的MOM條件不利于Asp水樣的轉(zhuǎn)化.冗余分析表明,DO濃度是MOM條件下影響Asp轉(zhuǎn)化的主要水環(huán)境因素;細(xì)菌群落分析進(jìn)一步證實(shí),低DO濃度MOM條件下細(xì)菌群落多樣性顯著下降,從而影響細(xì)菌對(duì)Asp水樣的代謝產(chǎn)物和轉(zhuǎn)化程度,不利于DBPFPs的降低.因此,研究MOM條件下Asp水樣的轉(zhuǎn)化規(guī)律與機(jī)制對(duì)于保障飲用水安全具有重要意義.

溫躍層溶解氧最小值；溶解氧；壓力；天冬氨酸；微生物

水庫(kù)水作為城市生活用水和飲用水的主要供給水源,其供水水質(zhì)直接影響用水安全[1].大多數(shù)深水庫(kù)在夏秋季會(huì)同時(shí)面臨熱分層和藻類爆發(fā)的問題,衰亡的藻細(xì)胞可能會(huì)釋放有機(jī)物并消耗大量溶解氧(DO),從而造成在水面下5.00～40.00m之間容易出現(xiàn)溫躍層溶解氧最小值(MOM)現(xiàn)象[2-5].水廠取水口設(shè)置在溫躍層及其附近深度會(huì)加劇MOM現(xiàn)象的發(fā)展,使飲用水水質(zhì)惡化[6].以往的報(bào)道中表明,在全球至少有51個(gè)湖泊和水庫(kù)中出現(xiàn)了MOM現(xiàn)象[7].早在1937年,Wiebe[8]發(fā)現(xiàn)美國(guó)田納西州的Norris水庫(kù)存在DO在9.14～15.24m之間急劇下降, 出現(xiàn)MOM現(xiàn)象; 2010年和2017年,吳志旭[9]和劉雪晴[10]等人分別監(jiān)測(cè)到我國(guó)千島湖的湖泊區(qū)和李家河水庫(kù)在10.00～20.00m處形成MOM.已有研究表明,MOM現(xiàn)象不利于藻類釋放的復(fù)雜有機(jī)物(包括氨基酸、蛋白質(zhì)、多糖等)的變化[11],對(duì)氨基酸(AAs)的影響尤為明顯[12],這可能會(huì)破壞水生生態(tài)系統(tǒng)的正常分區(qū),造成生態(tài)失衡,增加飲用水處理的難度,應(yīng)當(dāng)引起重視[6].

近年來(lái),水源藻華明顯,這些水源水通常具有較高的溶解性有機(jī)氮(DON)水平,其中AAs是DON化合物的重要組成部分.研究表明,天冬氨酸(Asp)作為天然水中主要的AAs之一,廣泛存在于湖泊、水庫(kù)和河流中,在長(zhǎng)江和漢江中占總AAs的40.00%～ 60.00%,這與Asp主要存在于大型水生植物和維管植物組織中(占15%左右)有關(guān)[13-14].在水體富營(yíng)養(yǎng)化時(shí)期,AAs檢出量最高可達(dá)到6.00mg/L[15],銅綠微囊藻在穩(wěn)定期釋放到水體中的Asp的平均濃度為27.00μmol/L,占總?cè)芙庑訟As的比例達(dá)到7.00%左右[16].此外,Asp作為重要的消毒副產(chǎn)物(DBPs)前體物質(zhì),在氯化消毒過程中容易形成DBPs,并且由于其側(cè)鏈結(jié)構(gòu)及其取代基的位置[17],導(dǎo)致較高消毒副產(chǎn)物生成勢(shì)(DBPFPs),尤其是二氯乙酸和二氯乙腈[18].目前,對(duì)水體中Asp的變化規(guī)律及其DBPFPs的研究主要集中在正常條件下,研究表明[11],藻類胞內(nèi)有機(jī)物(IOM)主要受MOM層低DO濃度的影響使水質(zhì)惡化,增加后續(xù)水處理難度,Asp是IOM的組成成分,且是典型含氮消毒副產(chǎn)物(N-DBPs)前體物質(zhì),而Asp在MOM條件下的轉(zhuǎn)化規(guī)律與機(jī)制仍不清楚,MOM條件影響Asp變化的主要水環(huán)境因素也尚不清晰,因此,有必要對(duì)不同MOM條件下Asp水樣的變化展開一系列研究.

本研究選取了水體中常見的、含量較高的、典型N-DBPs前體物質(zhì)Asp作為研究對(duì)象,通過改變MOM的DO濃度和壓力,探究不同MOM條件下Asp樣品的水化學(xué)特性及其DBPFPs變化規(guī)律;分析Asp水樣特征參數(shù)與DBPFPs之間的關(guān)系;研究不同MOM條件下Asp水樣的細(xì)菌群落變化,解釋Asp水樣的轉(zhuǎn)化機(jī)理;對(duì)于明確水源水庫(kù)水在MOM條件下影響Asp轉(zhuǎn)化的水環(huán)境因素具有較為重要的意義,為飲用水處理及其風(fēng)險(xiǎn)控制提供理論參考.

1 材料與方法

1.1 試驗(yàn)試劑

Asp購(gòu)自阿拉丁有限公司(中國(guó)上海),主要性質(zhì)見表1.DBPs標(biāo)準(zhǔn)品購(gòu)自Sigma-Aldrich.氯消毒劑為次氯酸鈉溶液(NaClO,5.00%,國(guó)藥控股化學(xué)試劑有限公司,中國(guó)),置于4℃陰暗處棕色試劑瓶中保存,試驗(yàn)前測(cè)定實(shí)際有效氯(Cl2)含量后立即使用.用磷酸鹽制備pH=7.00的緩沖溶液.采用硫代硫酸鈉(Na2S2O3)溶液(0.10mol/L)作為淬滅劑.本研究中使用的所有化學(xué)品至少是分析純,所有溶液都采用美國(guó)Millipore公司的Milli-Q超純水制備.

表1 Asp的物理化學(xué)性質(zhì)

1.2 試驗(yàn)方法

本研究使用7個(gè)容積為5.00L的自制密封不銹鋼容器(圖1),模擬不同MOM條件,首先在不銹鋼容器的A口、B口和減壓閥均打開的狀態(tài)下,通過B口向容器連續(xù)注入氮?dú)膺M(jìn)行曝氣將DO排出到所需設(shè)置濃度,模擬水體中不同DO濃度,設(shè)置完成DO濃度后,關(guān)閉B口和減壓閥,通過A口向密封不銹鋼容器注入氮?dú)庹{(diào)節(jié)設(shè)置不同的壓力,模擬不同水深.①設(shè)置初始DO濃度為0.50mg/L模擬水體DO最小值,通過向密封容器(A口)注入氮?dú)庠O(shè)置不同的壓力,分別為0.30和0.70MPa(分別模擬30和70m水深),常壓條件(模擬水體表層)即未注入氮?dú)鉅顟B(tài),以探究固定水體DO最小值條件下,不同壓力MOM的影響;②設(shè)置壓力為0.30MPa模擬水源水庫(kù)中MOM常發(fā)生的30m水深,通過向容器(B口)連續(xù)注入氮?dú)鈱O排出到設(shè)置濃度,分別為7.00, 5.00, 3.00, 0.50mg/L,以探究固定壓力條件下,不同DO濃度MOM的影響;③設(shè)置無(wú)菌水對(duì)照組DO濃度為0.50mg/L,壓力為0.30MPa,模擬水源水庫(kù)MOM條件,通過滅菌使容器達(dá)到無(wú)菌狀態(tài),且向容器中加入20.00mg/L氯化汞防止無(wú)菌水反應(yīng)過程中微生物生長(zhǎng),以探究MOM條件下微生物的作用.在上述7個(gè)不銹鋼容器中分別加入初始質(zhì)量濃度為2.00mgN/L的Asp溶液.本研究的接種物從中國(guó)西安的一個(gè)水庫(kù)中提取,除對(duì)照組外,其他初始微生物濃度均為(2.00±0.50)×107個(gè)/mL.室溫(25.00±2.00)℃下持續(xù)反應(yīng)14d.分別于第0、1、3、5、7、10和14d取樣并對(duì)樣品進(jìn)行分析,取第14d樣品進(jìn)行微生物分析.本研究的所有參數(shù)均重復(fù)3次(= 3)進(jìn)行計(jì)算.

1.3 分析方法

溶解性有機(jī)碳(DOC)采用TOC分析儀(TOC-L, Shimadzu,Japan)測(cè)定;DO濃度采用溶解氧儀(HQ30d,Hach,USA)測(cè)定;總?cè)芙庑缘?TDN)、硝酸鹽氮(NO3--N)和氨氮(NH4+-N)分別采用過硫酸鉀消解紫外分光光度法、紫外分光光度法和納氏試劑分光光度法測(cè)定,由于水樣中的亞硝態(tài)氮濃度低于檢測(cè)限,下文中不再進(jìn)行闡述,DON由式(1)計(jì)算得出[19]; UV254的吸光度采用紫外分光光度計(jì)(UV2600A, Unico,USA)測(cè)定;細(xì)菌濃度和膜完整性采用流式細(xì)胞儀(AccuriC6,BD,USA)測(cè)定[20];3D-EEM熒光光譜采用熒光分光光度計(jì)(F-7000,日立,日本)測(cè)量[21].從反應(yīng)器中取40.00mL水樣進(jìn)行氯化(72h),加氯消毒劑的劑量按式(2)計(jì)算[22].根據(jù)美國(guó)環(huán)保署方法551和552,采用氣相色譜/電子捕獲檢測(cè)器(GC/ECD)測(cè)定樣品中的DBPFPs.

1.4 細(xì)菌群落分析

采集反應(yīng)第14d的Asp水樣進(jìn)行細(xì)菌群落分析,并與反應(yīng)前(第0d)的樣品進(jìn)行對(duì)照.每個(gè)樣品經(jīng)真空過濾器(0.22μm)過濾,溶液中的細(xì)菌保留在膜上,樣品的所有膜保存在-20℃,直到進(jìn)一步分析[22].使用E.Z.N.A.?土壤DNA試劑盒(Omega Bio-Tek公司,美國(guó))提取和純化細(xì)菌DNA,使用Nano Drop 2000紫外-可見分光光度計(jì)(美國(guó)賽默飛)檢測(cè)DNA濃度和純度,采用引物338F(ACTCCTACGGGAGGCAGCAG)和806R(GGACTACHVGGGTWTCTAAT)進(jìn)行PCR擴(kuò)增,測(cè)序工作在美吉生物公司(中國(guó)上海)Illumina MiSeq平臺(tái)上進(jìn)行的.

1.5 數(shù)據(jù)分析

曲線圖和柱狀圖主要使用Origin 2018繪制;使用CANOCO(4.5版)進(jìn)行冗余分析(RDA);采用帶有DOMFluor工具箱的Matlab軟件進(jìn)行PARAFAC分析;使用MOTHUR(v.1.30.2)對(duì)細(xì)菌群落多樣性進(jìn)行計(jì)算分析;利用TBtools(1.068版本)繪制細(xì)菌屬水平的群落結(jié)構(gòu)圖.

2 結(jié)果與討論

2.1 不同MOM條件下Asp的轉(zhuǎn)化規(guī)律

2.1.1 溶解氧的影響 如圖2所示,Asp水樣在不同DO(7.00, 5.00, 3.00, 0.50mg/L)條件下反應(yīng)過程中DOC和DON均呈下降趨勢(shì),反應(yīng)前3d, DOC分別降低了84.53%、76.00%、70.27%、17.87%, DON分別降低了95.00%、85.50%、82.00%、25.00%,之后緩慢變化.由此得出,DO濃度越低,微生物對(duì)Asp的轉(zhuǎn)化越慢.這是因?yàn)榈虳O濃度影響了微生物的代謝和活性所導(dǎo)致[11].值得關(guān)注的是,當(dāng)DO為7.00mg/L時(shí),DOC和DON在反應(yīng)第3～14d呈略微升高的趨勢(shì),這可能是由于較高的DO濃度促進(jìn)了微生物的生長(zhǎng)繁殖和新陳代謝[23],從而增加了樣品中有機(jī)物的濃度.由圖2(c)可見, Asp水樣的DON主要在細(xì)菌的礦化作用下被轉(zhuǎn)化為NH4+-N,且DO濃度越低轉(zhuǎn)化越慢.由圖2(d)可見,不同DO條件下的UV254均隨著反應(yīng)時(shí)間的增加呈略微升高趨勢(shì),反應(yīng)結(jié)束時(shí)分別為0.027, 0.008, 0.014, 0.033cm-1,間接說明Asp水樣在微生物降解過程中產(chǎn)生了微量的芳香性物質(zhì)[21],且厭氧條件下表現(xiàn)出更強(qiáng)的芳香性.

圖2 不同MOM條件下Asp水樣中DOC、DON、NH4+-N及UV254變化

采用三維熒光-平行因子(EEM-PARAFAC)分析,得到Asp水樣在不同DO條件下反應(yīng)過程中四種熒光組分(C1,C2,C3,C4)的EEM圖譜和熒光峰位置(表2).以往研究表明,C1、C2、C4與類蛋白物質(zhì)相關(guān): C1(x/m 280/330和230/330)和C2(x/m 275/348和225/348)被鑒定為類色氨酸物質(zhì),C4(x/m 220/294和265/294)被描述為類酪氨酸物質(zhì);C3(x/m 275/422和335/422)被劃分為類腐殖質(zhì)物質(zhì)[24].如圖3(a-d)所示,檢測(cè)到Asp水樣在不同DO條件下反應(yīng)過程中四種組分的熒光強(qiáng)度隨時(shí)間的變化.反應(yīng)前主要檢測(cè)到C1和C4組分,以及微量的C3組分.隨著反應(yīng)時(shí)間的增加,有氧(7.00, 5.00, 3.00mg/L)條件下C1與C4組分之和在第5～7d降低到最小熒光強(qiáng)度,降低了29.45%～ 58.67%,之后又逐漸增加,這是因?yàn)闇y(cè)得的熒光強(qiáng)度為凈產(chǎn)量[25],反應(yīng)前期消耗為主導(dǎo),而當(dāng)有機(jī)物被降低到較低濃度時(shí)則以產(chǎn)生為主導(dǎo).厭氧(0.50mg/L)條件下C1組分降低較慢,直到第14d才降低了60.00%.此外,不同水樣的反應(yīng)過程中均出現(xiàn)了C2組分,研究表明,這是一類具有較強(qiáng)熒光特性的可溶性生物代謝物[26-27],且在DO濃度較高時(shí)產(chǎn)生較少.因此,低DO濃度導(dǎo)致微生物對(duì)Asp水樣的轉(zhuǎn)化效率降低,且轉(zhuǎn)化過程中還會(huì)產(chǎn)生不利于水樣中有機(jī)物降解的C2物質(zhì).

表2 三維熒光—平行因子分析得到的四組分 (C1、C2、C3、C4) 熒光光譜圖

2.1.2 壓力的影響 如圖2所示,Asp水樣在不同壓力(0.70MPa、0.30MPa、常壓)條件下反應(yīng)第14d時(shí),DOC分別降低了55.87%、59.87%、88.27%,表明有壓條件下DOC降低較慢;觀察DON的變化,發(fā)現(xiàn)水樣在0.70MPa和常壓條件下分別反應(yīng)到第10d和第5d達(dá)到90.00%以上的降解率,進(jìn)一步表明了較高的壓力可能會(huì)延長(zhǎng)Asp水樣被微生物降解的時(shí)間.由圖2(d)可見,不同壓力條件下的UV254均隨著反應(yīng)時(shí)間的增加呈略微升高趨勢(shì),反應(yīng)第14d時(shí)分別為0.034, 0.033, 0.009cm-1,說明有壓條件促使水樣在微生物降解Asp的過程中表現(xiàn)出比常壓條件下更高的芳香性,這可能是由于壓力增大了微生物的細(xì)胞膜通透性,胞內(nèi)有機(jī)物泄露增加了部分芳香類物質(zhì)[28],從而使DOC和DON降低較慢.

不同壓力條件下Asp水樣反應(yīng)過程中的四種熒光組分見表2,四種組分的熒光強(qiáng)度隨時(shí)間的變化如圖3(d)～(f)所示.隨著反應(yīng)時(shí)間的增加,常壓條件下C1組分在第5d降低到最小熒光強(qiáng)度,降低了75.51%,之后又有所增加,這是因?yàn)橛袡C(jī)物在微生物作用下消耗和產(chǎn)生同時(shí)發(fā)生[25],初期消耗占主導(dǎo),而當(dāng)有機(jī)物被降低到較低濃度時(shí)以產(chǎn)生為主導(dǎo).0.30MPa和0.70MPa條件下C1組分降低較慢,分別在反應(yīng)的第14d和第10d才降低了60.00%和50.00%,達(dá)到最小值.此外,常壓條件下新產(chǎn)生的C2物質(zhì)熒光強(qiáng)度較低.因此,有壓條件不僅不利于微生物對(duì)Asp的轉(zhuǎn)化,而且會(huì)產(chǎn)生更高的C2熒光強(qiáng)度,進(jìn)一步增加有機(jī)物被降解時(shí)間,但是, 0.30MPa和0.70MPa之間無(wú)明顯差異,說明發(fā)生在不同深度的MOM均會(huì)對(duì)Asp水樣的轉(zhuǎn)化產(chǎn)生類似影響.

圖3 不同MOM條件下Asp水樣各熒光組分的最大熒光強(qiáng)度

2.1.3 微生物作用的影響 為了探究微生物在Asp水樣降解過程中的作用,選擇在0.30MPa,0.50mg/L DO條件下采用無(wú)菌水添加Asp進(jìn)行對(duì)照,而且在反應(yīng)過程中添加氯化汞以防止微生物生長(zhǎng).如圖2所示,0.30MPa,0.50mg/L DO條件下,添加微生物的Asp水樣反應(yīng)14d時(shí),DOC從7.50mg/L降低到3.01mg/ L,DON降低了100%;無(wú)菌水條件下反應(yīng)14d時(shí),DOC和DON僅降低了1.50%～5.75%,整個(gè)反應(yīng)過程中均未發(fā)生明顯變化,且DON也未向氨氮發(fā)生轉(zhuǎn)化.觀察無(wú)菌水條件下熒光強(qiáng)度的變化(圖3(g)),發(fā)現(xiàn)C1組分在整個(gè)反應(yīng)過程中基本無(wú)變化,說明Asp水樣未發(fā)生轉(zhuǎn)化.C4組分逐漸升高,可能是由于氯化汞破壞了微量難滅活微生物的結(jié)構(gòu),使其胞內(nèi)具有較強(qiáng)熒光特性的部分物質(zhì)泄露所導(dǎo)致.因此,無(wú)菌水條件下Asp自身不能發(fā)生轉(zhuǎn)化,表明微生物在Asp水樣的降解過程中起主要作用,這與張瑞華等人[22]之前的報(bào)道一致.

2.2 不同MOM條件下DBPFPs的變化

探究不同DO條件下和不同壓力條件下Asp水樣在微生物作用過程中形成的DBPFPs變化,并與無(wú)菌水條件進(jìn)行對(duì)照,結(jié)果如圖4所示,檢測(cè)到的DBPs的種類包括三氯甲烷(TCM)、二氯乙酸(DCAA)、三氯乙酸(TCAA)、二氯乙腈(DCAN)和三氯硝基甲烷(TCNM).

圖4 不同MOM條件下Asp水樣中DBPFPs的變化

由圖4(a)可見,不同MOM條件下Asp水樣的T-DBPFPs均隨著反應(yīng)時(shí)間的增加逐漸降低,反應(yīng)至第3d,不同DO(7.00, 5.00, 3.00, 0.50mg/L)條件下分別降低了98.88%、66.96%、61.50%、30.34%,不同壓力(0.70MPa、0.30MPa、常壓)條件下分別降低了59.81%、30.34%、83.98%,但反應(yīng)14d時(shí),均達(dá)到了約99.00%的降低程度.結(jié)果表明,較低的DO濃度及有壓條件(￡0.70MPa)均不利于T-DBPFPs的降低,但增加反應(yīng)時(shí)間可以達(dá)到與有氧和常壓條件下相同的效果.對(duì)于TCMFP(圖4(b)),反應(yīng)14d時(shí),有氧(7.00, 5.00, 3.00mg/L)條件下分別降低了30.14%、2.71%、8.13%,厭氧條件下增加了23.27%,這是因?yàn)锳sp在生物降解過程中容易形成更多的TCM前體物質(zhì)[22],從而導(dǎo)致TCM控制效果不好,并且低DO濃度影響微生物代謝[23],從而不利于Asp水樣的降解,進(jìn)一步增加了TCMFP.對(duì)于DCAAFP和TCAAFP(圖4(c)和(d)),DO濃度越高,降低速率越快,反應(yīng)至第3d, 7.00mg/L DO條件下分別降低了99.18%和69.96%, 0.50mg/L DO條件下DCAAFP反而增加了7.17%, TCAAFP僅僅降低了2.84%,常壓條件下DCAAFP比有壓條件下的降低程度至少高15.52%.這可能與有氧和常壓條件下Asp水樣的DON向氨的轉(zhuǎn)化較快有關(guān),當(dāng)氯與高濃度的氨反應(yīng)時(shí)生成氯氨,氧化能力降低[15],從而使DBPFPs降低較快.對(duì)于DCANFP (圖4(e)),反應(yīng)至第3d,有氧(7.00, 5.00, 3.00mg/L)條件下分別降低到了1.32, 0.34, 1.83μg/L,達(dá)到了99.95%以上的降低程度,厭氧條件下降低了90.37%,常壓條件下比有壓條件下的降低程度至少高8.51%.以往研究表明,DCAN的減少一方面是由于DON在Asp水樣中的氨化[29],另一方面是因?yàn)镈CAN可以水解生成DCAA[30].另外,檢測(cè)到不同MOM條件下TCNMFP均在2.00 μg/L以下,有氧條件下逐漸降低,厭氧條件下略有升高,但由于濃度較低,不同水樣間的差異并不明顯.無(wú)菌水條件下Asp水樣的DBPFPs在反應(yīng)過程中基本無(wú)變化,說明微生物在DBPs前體物的降解過程中起主導(dǎo)作用[22].

綜上所述,Asp水樣在微生物作用過程中,DO濃度越高,DBPFPs降低速度越快,低DO濃度抑制微生物的代謝活性,不利于DBPs前體物質(zhì)的降解,但延長(zhǎng)反應(yīng)時(shí)間可以達(dá)到更好的DBPs控制效果.有壓條件(￡0.70MPa)不利于DBPFPs的降低,但0.30MPa和0.70MPa之間并無(wú)明顯有規(guī)律的差異,說明發(fā)生在不同水深的MOM均會(huì)對(duì)DBPFPs的降低產(chǎn)生類似影響.所以,飲用水處理廠取水口位置有必要避開MOM層,以降低飲用水處理難度.

2.3 綜合毒性風(fēng)險(xiǎn)值的變化

采用Zhang等[29]研究中所述的綜合毒性分析方法,探究了Asp水樣在不同MOM條件下反應(yīng)過程中氯化產(chǎn)生的DBPs(TCM、DCAA、TCAA、DCAN和TCNM)的潛在毒性.毒性指標(biāo)主要包括細(xì)胞毒性和遺傳毒性.綜合毒性值計(jì)算公式(3)如下所示:

ITRV=S(CTV′C) (3)

C為的濃度,mol/L;CTV為的聯(lián)合毒性值, L/mol;ITRV為的綜合毒性值;為TCMFP、DCAAFP、TCAAFP、DCANFP和TCNMFP, (μg/L).

如圖5所示,Asp水樣在不同MOM條件下的綜合毒性值均呈逐漸降低趨勢(shì).不同DO(7.00, 5.00, 3.00, 0.50mg/L)條件下,反應(yīng)前3d綜合毒性值分別降低了99.20%、78.37%、74.80%、51.06%,表明DO濃度越低,綜合毒性值的降低速率越慢.不同壓力(0.70MPa、0.30MPa、常壓)條件下,反應(yīng)前3d綜合毒性值分別降低了59.84%、51.06%、89.05%,說明0.30MPa和0.70MPa條件在一定程度上削弱了綜合毒性值的降低速度.反應(yīng)至第14d,發(fā)現(xiàn)隨著反應(yīng)時(shí)間的增加,不同MOM條件下各樣品的綜合毒性值均降低到了較低水平.無(wú)菌水對(duì)照試驗(yàn)發(fā)現(xiàn),綜合毒性值始終保持在較高水平,說明微生物在降解DBPs前體物質(zhì)以降低綜合毒性過程中發(fā)揮著主要作用.

圖5 不同MOM條件下Asp水樣在反應(yīng)過程中氯化DBPs引起的綜合毒性變化

2.4 相關(guān)性分析

采用冗余分析(RDA)評(píng)估不同DO和不同壓力條件下Asp水樣的特性參數(shù)與DBPFPs之間的關(guān)系(圖6).不同DO和不同壓力條件下,RDA的前兩個(gè)軸分別解釋了總方差的88.2%和94.2%,壓力、DOC、DON和C1組分與DBPFPs(TCMFP、TCNMFP、DCANFP、TCAAFP、DCAAFP和T-DBPFPs)以及綜合毒性值均具有正相關(guān)性,分布在RDA1的正軸上,其中壓力與DBPFPs的相關(guān)性極其微弱.DO分布RDA1的負(fù)軸上,與DOC、DON、C1組分、DBPFPs均呈負(fù)相關(guān).Wang等[31]和Ma等[32]之前的研究表明,DON和類色氨酸物質(zhì)與來(lái)自藻類有機(jī)物的DBPFPs有很強(qiáng)相關(guān)性.綜上所述,MOM條件下主要是由于低DO濃度影響了微生物的代謝和活性,不利于Asp水樣的降解,從而對(duì)DBPFPs的降低有較大影響.DOC、DON和類色氨酸蛋白樣熒光參數(shù)可以作為預(yù)測(cè)含Asp水樣的DBPFPs的替代指標(biāo).

圖6 不同MOM條件下Asp水樣特性與DBPFPs的RDA分析

2.5 MOM條件下Asp轉(zhuǎn)化率降低的機(jī)理

2.5.1 細(xì)菌數(shù)量及膜完整性變化 不同MOM條件下Asp水樣在微生物作用過程中的細(xì)菌數(shù)量和膜完整性的變化如圖7所示.DO為5.00, 3.00, 0.50mg/L條件下,反應(yīng)第1d各樣品中的細(xì)菌數(shù)量迅速增加到了4.89×107, 4.71×107和3.24×107個(gè)/mL,之后的反應(yīng)過程中基本保持平衡,并且膜完整性均保持在90.00%以上.DO為7.00mg/L條件下,反應(yīng)第5d樣品中的細(xì)菌數(shù)量達(dá)到了最大值7.00×107個(gè)/mL,之后持續(xù)下降,與此同時(shí),反應(yīng)14d時(shí),膜完整性也降低到了81.95%.由此表明,較高的初始DO濃度有利于細(xì)菌的生長(zhǎng)繁殖和對(duì)有機(jī)物的迅速利用,從而加快有機(jī)底物濃度和DBPFPs的降低速率,這與韓靜茹等[11]之前的報(bào)道一致.也正因?yàn)槿绱?DO濃度在反應(yīng)第1d急劇下降(圖8),較低有機(jī)質(zhì)濃度也無(wú)法為大量細(xì)菌的繁殖提供充足的營(yíng)養(yǎng)[33],從而導(dǎo)致細(xì)菌發(fā)生內(nèi)源性降解,數(shù)量逐漸降低并且膜完整性受到破壞,以至于細(xì)胞內(nèi)容物流出使樣品中的有機(jī)物濃度增加,這也解釋了7.00mg/L DO條件下DOC和DON在反應(yīng)后期升高的現(xiàn)象.不同壓力(0.70MPa、0.30MPa、常壓)條件下,反應(yīng)第1d各樣品中的細(xì)菌數(shù)量增加到了3.59×107, 3.24×107和2.79×107個(gè)/mL,之后的反應(yīng)過程中緩慢增加,并且膜完整性也基本沒有被破壞,說明有壓條件并不會(huì)對(duì)細(xì)菌的生長(zhǎng)產(chǎn)生明顯影響.

2.5.2 細(xì)菌群落門水平的變化 為了考察不同MOM條件下細(xì)菌群落對(duì)Asp轉(zhuǎn)化的貢獻(xiàn),取反應(yīng)前(第0d)和反應(yīng)第14d的水樣分析了alpha 多樣性參數(shù)(Simpson, Shannon, Chao, Ace, Coverage) (表3)和門水平(圖9)以及屬水平(圖10)上的細(xì)菌群落.所有樣本的Coverage 指數(shù)值均超過99.90%,不同DO和不同壓力條件下反應(yīng)14d的樣品中Shannon、Chao、Ace均有所增加,表明細(xì)菌多樣性及豐富度有所增加.

圖7 不同MOM條件下Asp水樣中細(xì)菌濃度和完整細(xì)胞的變化

圖8 不同MOM條件下溶解氧濃度的變化

表3 不同MOM條件下Asp水樣反應(yīng)0d和14d的細(xì)菌alpha多樣性指數(shù)

注:A: DO=7.00mg/L (0.30MPa); B: DO=5.00mg/L (0.30MPa); C: DO=3.00mg/L (0.30MPa); D: DO=0.50mg/L (0.30MPa); E: DO=0.50mg/L (0.70MPa); F: DO=0.50mg/L (常壓).

由圖9(a)可見,初始樣品中主要檢測(cè)到變形菌門(Proteobacteria)和擬桿菌門(Bacteroidota),相對(duì)豐度分別為94.40%和4.61%.不同DO和不同壓力條件下反應(yīng)14d后,6個(gè)樣品中共檢測(cè)到變形菌門、擬桿菌門、放線菌門(Actinobacteriota)、厚壁菌門(Firmicutes)和髕骨菌門(Patescibacteria)5個(gè)主要細(xì)菌門,不同樣品中的細(xì)菌多樣性在反應(yīng)過程中均會(huì)有不同程度的增加.與初始樣品相比較,有氧(7.00, 5.00, 3.00mg/L)條件下,變形菌門的相對(duì)豐度有所降低,分別為43.22%,89.59%、85.28%,擬桿菌門的相對(duì)豐度有所升高,分別為37.64%、5.69%、11.66%,此外,還出現(xiàn)了放線菌門,相對(duì)豐度分別為15.71%、3.25%、2.01%;厭氧(0.50mg/L)條件下,雖然細(xì)菌多樣性有所增加,但仍以變形菌門為優(yōu)勢(shì)門,相對(duì)豐度為95.02%,另外還出現(xiàn)了厚壁菌門(2.11%).有壓(0.70和0.30MPa)條件下,變形菌門的相對(duì)豐度保持在95.00%左右的較高水平,常壓條件下降低到了83.68%,擬桿菌門和放線菌門增加到了7.94%和7.00%.據(jù)報(bào)道,高DO和高游離氨水環(huán)境適宜變形菌門和擬桿菌門的生長(zhǎng)繁殖[34],其主要參與碳水化合物代謝和碳固定[35],是降解碳水化合物、蛋白質(zhì)、AAs等有機(jī)物的潛在貢獻(xiàn)者[36-37].另外,擬桿菌門富含碳水化合物活性水解酶,可以有效降解生物大分子物質(zhì),在有機(jī)質(zhì)降解和C/N循環(huán)中發(fā)揮著重要作用[38-39].Guo等人[40]研究發(fā)現(xiàn),放線菌在碳水化合物和AAs的代謝中起主要作用,這可能是有氧條件下Asp水樣被迅速降解的部分原因.厚壁菌門是一種共養(yǎng)細(xì)菌,具有在厭氧條件分解蛋白質(zhì)和AAs,降解揮發(fā)性脂肪酸的重要作用[41].綜上所述,DO濃度越高,細(xì)菌群落多樣性越高.有氧條件下的優(yōu)勢(shì)門依次為變形菌門、擬桿菌門、放線菌門,有利于對(duì)Asp及細(xì)菌代謝所產(chǎn)生的有機(jī)質(zhì)的降解,從而減少DBPs前體物質(zhì)的濃度,降低DBPFPs,厭氧條件下的優(yōu)勢(shì)門僅有變形菌門,大大降低了Asp水樣被細(xì)菌降解的速率,從而不利于對(duì)DBPFPs的降解.不同壓力條件下群落多樣性的變化并不明顯,但有壓條件(￡0.7MPa)會(huì)降低部分新生優(yōu)勢(shì)菌群的相對(duì)豐度.

圖9 不同MOM條件下反應(yīng)0和14d Asp水樣的細(xì)菌群落結(jié)構(gòu)

A: DO=7.00mg/L (0.30MPa); B: DO=5.00mg/L (0.30MPa); C: DO=3.00mg/L (0.30MPa); D: DO=0.50mg/L (0.30MPa); E: DO=0.50mg/L (0.70MPa); F: DO=0.50mg/L (常壓)

2.5.4 MOM條件下Asp轉(zhuǎn)化降低的原因分析 不同MOM條件下,反應(yīng)前期細(xì)菌數(shù)量均迅速增加,膜完整性基本未受到破壞,表明MOM條件并未影響細(xì)菌正常的生長(zhǎng)繁殖;低DO條件下細(xì)菌群落多樣性及豐富度明顯降低,從而影響細(xì)菌的代謝產(chǎn)物,這是導(dǎo)致Asp轉(zhuǎn)化速率降低的主要原因,韓靜茹等[11]的研究表明,低DO條件下細(xì)菌代謝活性也會(huì)降低;其次,與常壓相比較,有壓條件下部分細(xì)菌群落的相對(duì)豐度略有降低,這也是影響Asp轉(zhuǎn)化速率的部分原因.

0.30MPa不同DO條件下,DO濃度越高,細(xì)菌群落多樣性及豐富度越高,有氧條件下主要有變形菌門、擬桿菌門和放線菌門3種優(yōu)勢(shì)菌門,卡氏伯克霍德菌屬、紅球菌屬和金黃桿菌屬在Asp水樣降解過程中發(fā)揮著重要作用,促進(jìn)氨基酸類蛋白物質(zhì)(C1和C4組分)熒光強(qiáng)度的降低,反應(yīng)第3d水樣的DOC和DON降解率均達(dá)到70.27%以上;厭氧條件下只有變形菌門一種優(yōu)勢(shì)菌門,主要以卡氏伯克霍德菌屬和草螺菌屬為優(yōu)勢(shì)屬,細(xì)菌群落多樣性降低,不僅減緩了氨基酸類蛋白物質(zhì)(C1和C4組分)熒光強(qiáng)度的降低速率,而且代謝產(chǎn)生的可溶性生物代謝物熒光強(qiáng)度更高,反應(yīng)第3d DOC和DON降解率均在25.00%以下.0.50mg/L DO不同壓力條件下,細(xì)菌群落多樣性差異并不明顯,均以變形菌門為優(yōu)勢(shì)菌門,以卡氏伯克霍德菌屬和草螺菌屬為優(yōu)勢(shì)屬,有壓條件下擬桿菌門、放線菌門和多形單胞菌屬的相對(duì)豐度比常壓條件下有所降低,這可能會(huì)影響細(xì)菌代謝速率,從而導(dǎo)致反應(yīng)第3d常壓條件下DOC和DON降解率分別為54.67%和62.00%,而有壓條件下僅為16.40%～25.50%.綜上所述,Asp的轉(zhuǎn)化速率:厭氧有壓<厭氧常壓<有氧有壓,低DO濃度抑制細(xì)菌群落多樣性及豐富度是導(dǎo)致MOM條件下Asp轉(zhuǎn)化降低的主要原因.

3 結(jié)論

3.1 不同DO條件下,DO濃度越低,Asp水樣的轉(zhuǎn)化及T-DBPFPs的降低越慢.反應(yīng)第3d,有氧條件下DOC和DON降低了70.27%～95.00%,T-DBPFPs降低了61.50%～98.88%,厭氧條件下DOC、DON和T-DBPFPs僅降低了17.87%、25.00%和30.34%,且微生物代謝過程中產(chǎn)生了更多的可溶性生物代謝物(C2組分),這種物質(zhì)的大量產(chǎn)生可能是導(dǎo)致低DO濃度下水樣轉(zhuǎn)化效率降低的主要原因.

3.2 不同壓力條件下,反應(yīng)第3d,有壓Asp水樣的DOC、DON和T-DBPFPs的最大降低程度比常壓條件下小36.80%、36.50%和24.17%,0.70MPa和0.30MPa間的差異并不明顯.有壓水樣的轉(zhuǎn)化較慢,可能與其C1類色氨酸熒光強(qiáng)度降低較慢,C2可溶性生物代謝物熒光強(qiáng)度較高有關(guān).

3.3 RDA分析結(jié)果表明,不同MOM條件下,DO的濃度對(duì)Asp水樣的特性及DBPFPs變化有較大影響;DOC、DON和類色氨酸蛋白樣熒光與DBPFPs相關(guān)性較強(qiáng).

3.4 Asp水樣的降解過程中微生物發(fā)揮著主要作用.有氧條件下優(yōu)勢(shì)門依次為變形菌門、擬桿菌門、放線菌門,厭氧條件下只有變形菌門一種優(yōu)勢(shì)菌門,由此可見,低DO濃度顯著抑制細(xì)菌群落多樣性,進(jìn)而影響細(xì)菌代謝程度,大大降低了Asp水樣被細(xì)菌降解的速率,不利于DBPFPs的降低;有壓條件與常壓相比較僅降低了部分新生菌群的相對(duì)豐度.因此,厭氧有壓的MOM條件不利于Asp水樣的轉(zhuǎn)化及后續(xù)DBPFPs的控制.

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Transformation study of aspartic acid in under metalimnetic oxygen minimum region of reservoirs: regularity and mechanism.

ZHAO Na, CAO Rui-hua, HUANG Ting-lin, WEN Gang*

(Key Laboratory of Northwest Water Resource, Environment and Ecology, Ministry of Education, Shaanxi Key Laboratory of Environmental Engineering, School of Environmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China)., 2023,43(10)：5529～5542

Aspartic acid (Asp), a typical precursor of containing nitrogen disinfection byproducts, was selected as the research object to investigate the transformation of Asp and the variation of disinfection byproduct formation potential (DBPFPs) under MOM condition with different concentration of dissolved oxygen (DO) and pressure. The main environmental factors and potential mechanism of Asp transformation was further clarified. The results showed that the dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and T-DBPFPs of Asp water samples decreased gradually with the increase of reaction time under different MOM condition. Compared with anaerobic condition (atmospheric pressure), a lower decrease was occurred in DOC and DON (16.40%～25.50%) and T-DBPFPs (30.34%～59.81%) under MOM condition with 0.30MPa pressure and 0.50mg/L DO, which was related to more soluble biometabolites (C2 component) were produced during microbial metabolism under such condition. In addition, the decrease in DOC, DON, and T-DBPFPs (DOC and DON: 70.27%～95.00%; T-DBPFPs: 61.50%～98.88%) under MOM condition with 0.30MPa pressure and 3.00～7.00mg/L DO aerobic condition was higher than those under MOM condition with 0.50mg/L DO at 3thday of reaction. Combining the above discussion, it can be concluded that the MOM condition with pressure and lower DO concentration was not conducive to the transformation of Asp water samples. The redundancy analysis showed that DO concentration was the main water environmental factor affecting Asp transformation under MOM condition. Bacterial community analysis further confirmed that the diversity of bacterial community decreased significantly under MOM condition with lower DO concentration, affecting the metabolites and transformation degree of bacteria to Asp water samples and the reduction of DBPFPs. Therefore, it is of great significance to investigate the transformation and its mechanism of Asp water sample under MOM condition for ensuring drinking water safety.

metalimnetic oxygen minimum；dissolved oxygen；pressure；aspartic acid；microorganisms

X524

A

1000-6923(2023)10-5529-14

2023-03-31

國(guó)家自然科學(xué)基金資助項(xiàng)目(51978557);陜西省重點(diǎn)科技創(chuàng)新團(tuán)隊(duì)項(xiàng)目(2023-CX-TD-32)

* 責(zé)任作者, 教授, hitwengang@163.com

趙 娜(1997-),女,山東菏澤人,西安建筑科技大學(xué)碩士研究生,主要從事消毒副產(chǎn)物方向研究.發(fā)表論文1篇. zhaona17865565537@163.com.

趙 娜,曹瑞華,黃廷林,等.水庫(kù)溫躍層氧最小值區(qū)域天冬氨酸的轉(zhuǎn)化研究:規(guī)律及機(jī)制 [J]. 中國(guó)環(huán)境科學(xué), 2023,43(10):5529-5542.

Zhao N, Cao R H, Huang T L, et al. Transformation study of aspartic acid in under metalimnetic oxygen minimum region of reservoirs: regularity and mechanism [J]. China Environmental Science, 2023,43(10):5529-5542.