劉廣龍,朱端衛(wèi),周易勇,曹秀云,宋春雷,華玉妹,趙建偉(.華中農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院,湖北 武漢 430070；.中國科學(xué)院水生生物研究所,淡水生態(tài)與生物技術(shù)國家重點(diǎn)實(shí)驗(yàn)室,湖北 武漢 43007)

硝酸根對(duì)有機(jī)磷光解釋放磷酸根的影響

劉廣龍1,2,朱端衛(wèi)1,周易勇2*,曹秀云2,宋春雷2,華玉妹1,趙建偉1(1.華中農(nóng)業(yè)大學(xué)資源與環(huán)境學(xué)院,湖北 武漢 430070；2.中國科學(xué)院水生生物研究所,淡水生態(tài)與生物技術(shù)國家重點(diǎn)實(shí)驗(yàn)室,湖北 武漢 430072)

為了解硝酸根的光化學(xué)活性對(duì)水體中有機(jī)磷間接光解釋放磷酸根過程的影響,采用室內(nèi)模擬實(shí)驗(yàn),研究了在草甘膦光解轉(zhuǎn)化為磷酸根中的作用及濃度對(duì)該反應(yīng)速度的影響,并以甲醇為羥基自由基(·OH)猝滅劑,印證了對(duì)富營養(yǎng)化湖泊水體中有機(jī)磷光解釋放磷的作用.結(jié)果表明,在紫外光(UV)照射下,草甘膦可以直接光解轉(zhuǎn)化為磷酸根,其磷酸根的釋放量隨著溶液初始pH和草甘膦濃度的增加而增大.加入處理,光照60min后,磷酸根的釋放量由空白處理的0.05mg/L增加到了0.43mg/L.添加Fe3+可促進(jìn)這一過程,而添加腐殖酸(HA)和碳酸氫根則可以顯著抑制磷酸根的釋放.在湖泊水體中,添加草甘膦,磷酸根的釋放量顯著高于空白,而增加湖水中濃度時(shí),磷酸根的釋放量則進(jìn)一步增大.在湖水/草甘膦/體系中添加·OH猝滅劑甲醇后,磷酸根的釋放量顯著降低.可見在紫外光條件下,的光化學(xué)作用對(duì)有機(jī)磷釋放磷酸根過程具有重要影響.

有機(jī)磷；磷酸根；硝酸根；光降解；自由基

湖水中有機(jī)磷通過生物礦化和化學(xué)分解轉(zhuǎn)化為無機(jī)磷,是在浮游藻類大規(guī)模持續(xù)生長時(shí),水體磷補(bǔ)償?shù)囊粋€(gè)重要途徑[1-2].一方面,湖泊水體中的磷可通過微生物或酶作用礦化為溶解態(tài)反應(yīng)性磷[3-4],如在風(fēng)浪擾動(dòng)下太湖再懸浮沉積物中磷的轉(zhuǎn)化過程中,懸浮顆粒物中有機(jī)磷生物礦化分解所釋放的溶解態(tài)磷酸鹽被認(rèn)為是太湖內(nèi)源磷動(dòng)態(tài)釋放量的最主要來源[5].另一方面,有機(jī)磷的化學(xué)分解在其形態(tài)轉(zhuǎn)化過程中也起著重要作用[6-7].最近研究證實(shí),再懸浮沉積物若暴露于光照下會(huì)釋放出水溶態(tài)營養(yǎng)鹽[7-8],表明光化學(xué)分解可能是一種潛在的營養(yǎng)元素遷移轉(zhuǎn)化機(jī)制.

研究發(fā)現(xiàn),經(jīng)過高壓滅菌處理的潮間帶沉積物(粒徑＜30μm)和經(jīng)過濾除去微生物的海水,在光照條件下沉積物溶解態(tài)磷酸鹽釋放通量顯著高于黑暗條件下的值.由于排除了微生物的影響,可以認(rèn)為海水中溶解態(tài)磷酸鹽的增量來源于沉積物中有機(jī)磷在持續(xù)光照下的光化學(xué)分解[10].后續(xù)工作證實(shí),富含有機(jī)質(zhì)的沉積物在再懸浮條件下,經(jīng)光照后更易釋放出溶解態(tài)磷酸鹽[11]. 在水環(huán)境中,部分有機(jī)磷可以直接吸收太陽能而進(jìn)行分解釋放溶解態(tài)磷酸鹽[12-13],其與有機(jī)磷結(jié)構(gòu)和形態(tài)具有直接相關(guān)性.同時(shí),自然水體中的天然光敏物質(zhì),如、Fe3+、溶解性有機(jī)質(zhì)(DOM)等,可以在太陽光照射下,誘導(dǎo)產(chǎn)生羥基自由基、超氧陰離子自由基、單線態(tài)氧等活性氧物質(zhì),進(jìn)而氧化降解水體的有機(jī)物[14].例如,紫外光照射下鐵-草酸絡(luò)合物可以氧化降解草甘膦,使其轉(zhuǎn)化為磷酸根[15].但對(duì)自然淺水湖泊而言,水體有機(jī)磷光解對(duì)溶解態(tài)磷酸鹽負(fù)荷的貢獻(xiàn)程度可能與有機(jī)磷光解的難易程度及其驅(qū)動(dòng)力有關(guān).然而,相關(guān)過程還缺乏深入研究.

1 材料與方法

1.1 試劑

草甘膦為色譜純,購自Sigma-Aldrich(美國);腐植酸(humic acid,HA),購買自國際腐殖質(zhì)協(xié)會(huì);硝酸鈉、氯化鐵、鹽酸、氫氧化鈉、甲醇均為分析純,購買自國藥集團(tuán)(上海).試驗(yàn)用的純水由優(yōu)普系列超純水機(jī)(UPH-I-40L)提供.

1.2 實(shí)驗(yàn)裝置

光解實(shí)驗(yàn)在PhchemIII型旋轉(zhuǎn)式光化學(xué)反應(yīng)儀(北京紐比特科技有限公司)中進(jìn)行.反應(yīng)器主要由一個(gè)雙層石英冷肼與周圍12支石英管構(gòu)成,光源為300W高壓汞燈,置于石英冷肼內(nèi).12支裝有反應(yīng)溶液的具塞石英管(200mm×□50mm, 75mL)垂直固定在冷肼外側(cè).將配制好的溶液置于石英反應(yīng)管中,按照不同要求進(jìn)行光照試驗(yàn).實(shí)驗(yàn)過程中通過光功率測(cè)定儀(北京紐比特科技有限公司)測(cè)得平均光功率為1.30mW/cm2.

1.3 實(shí)驗(yàn)方法

表1 南湖上覆水基本理化指標(biāo)Table 1 The physicochemical of overlying water of Nanhu Lake

2 結(jié)果與討論

2.1 pH值對(duì)草甘膦直接光解釋放磷酸根的影響

圖1 初始pH對(duì)草甘膦直接光解釋放磷酸根的影響Fig.1 The effect of pH on the phophsate release from glyphosate photooxidation

一般而言,環(huán)境中的有機(jī)物光化學(xué)轉(zhuǎn)化主要為直接光解和間接光解[20].在湖泊水體中,部分有機(jī)磷本身可以直接吸收太陽能而進(jìn)行分解反應(yīng),其與有機(jī)磷結(jié)構(gòu)和形態(tài)具有直接相關(guān)性[21].且水體 pH對(duì)這一過程具有重要影響,其主要通過改變有機(jī)物分子在水溶液中的存在形態(tài),從而影響其吸收光譜特性,最終影響光化學(xué)轉(zhuǎn)化過程

[22].本研究首先模擬水中初始 pH對(duì)草甘膦直接光解釋放磷酸根影響的結(jié)果,如圖1所示,當(dāng)草甘膦初始濃度為5mg/L,初始pH分別為2, 4, 7, 8, 10時(shí).光照反應(yīng)60min后,磷酸根濃度分別達(dá)到了0.04, 0.05, 0.08, 0.09, 0.21mg/L,可以看出隨著水體 pH的增高,草甘膦光解釋放磷酸根的速率顯著增快.Manassero等[23]研究草甘膦在H2O2/UVC體系中的降解過程中發(fā)現(xiàn),隨著反應(yīng)體系 pH的升高,草甘膦的降解速度顯著加快.這些研究結(jié)果均以表明,pH升高有利于草甘膦光解釋放磷酸根.在富營養(yǎng)化湖泊水體中,隨著藻類的大量生長,湖水 pH值顯著提升,這一過程則可顯著提升部分有機(jī)磷直接光解釋放磷酸根速度,從而增加了湖泊水體中溶解態(tài)反應(yīng)磷的含量.

2.2 草甘膦初始濃度對(duì)其光解釋放磷酸根的影響水體中有機(jī)磷含量對(duì)其直接光解釋放磷酸根的濃度具有重要影響[21].為探討草甘膦初始濃度對(duì)其光解釋放的影響,本研究設(shè)置草甘膦初始濃度分別為1, 5, 20mg/L,溶液pH為7,且對(duì)初始濃度為1mg/L的處理進(jìn)行進(jìn)行暗反應(yīng)實(shí)驗(yàn).由圖2可知,在草甘膦初始濃度為1mg/L的處理中,由于樣品的水解及保存等原因,初始樣品均可以測(cè)得痕量的磷酸根.但是在暗反應(yīng)60min后,反應(yīng)體系中磷酸根濃度基本不變,表明草甘膦在暗反應(yīng)條件下具有較好的穩(wěn)定性.但是經(jīng)光照反應(yīng)后,反應(yīng)體系中磷酸根濃度顯著提升.紫外光照射60min后,草甘膦的初始濃度為1, 5, 20mg/L體系中,磷酸根濃度分別為0.03, 0.05, 0.08mg/L,即草甘膦濃度越高,經(jīng)光照直接分解產(chǎn)生的磷酸根含量也就越多.另,草甘膦初始濃度 20mg/L與1mg/L相比,其濃度是后者20倍,經(jīng)紫外光照射后,前者磷酸根產(chǎn)量?jī)H為后者的 2.5倍左右.這一結(jié)果同樣表明,在自然環(huán)境中,少量有機(jī)磷即可在光照條件下快速釋放大量溶解態(tài)磷酸根.湖泊上覆水中溶解態(tài)有機(jī)磷的含量一般較低,如實(shí)驗(yàn)所用的南湖上覆水中溶解態(tài)有機(jī)磷的含量為0.12mg/L(表 1).但對(duì)淺水湖泊而言,當(dāng)發(fā)生再懸浮過程時(shí),其上覆水中溶解態(tài)有機(jī)磷的含量急劇增大.這部分溶解態(tài)有機(jī)磷光解釋放磷酸根過程尚未見報(bào)道,值得深入研究.

圖2 草甘膦初始濃度對(duì)其直接光解釋放磷酸根的影響Fig.2 The effect of initial concentration of glyphosate onthe phophsate release from glyphosate photooxidation

2.3 硝酸根對(duì)草甘膦間接光解釋放磷酸根的影響

已有研究表明,湖泊水體中的有機(jī)磷可以經(jīng)間接光解釋放磷酸根[24].作為自然水體中廣泛存在的光敏物質(zhì),能夠經(jīng)光誘導(dǎo)產(chǎn)生羥基自由基(·OH),從而氧化降解水體中的有機(jī)物.一般而言,天然水中濃度為10-5～10-3mol/L[25].本文選取了硝酸根濃度分別為0.2, 2, 20, 200mg/L,反應(yīng)體系中草甘膦濃度為5mg/L,pH為7,進(jìn)行紫外光照射下硝酸根濃度對(duì)草甘膦間接光解釋放磷酸根的實(shí)驗(yàn).如圖 3所示,紫外光照射 60min后,濃度為0.2, 2, 20, 200mg/L的體系中,磷酸根濃度分別為0.10, 0.31, 0.43, 0.22mg/L,均高于同等濃度草甘膦直接光解產(chǎn)生的磷酸根濃度(0.05mg/L),說明在存在下,可以顯著加快草甘膦由有機(jī)磷向無機(jī)磷的轉(zhuǎn)化過程.這一結(jié)果主要是由于能夠吸收光子生成·OH,·OH氧化降解草甘膦生成磷酸根所致,其主要過程如下[17-19]:

-對(duì)·OH的淬滅應(yīng)該是反應(yīng)體系中 NO3-濃度升高而磷酸根產(chǎn)量降低的主要原因.

圖3 硝酸根濃度為草甘膦間接光解釋放磷酸根的影響Fig.3 The effect of initial concentration ofon the phophsate release from glyphosate photodecomposition by

2.4 Fe3+對(duì)硝酸根驅(qū)動(dòng)草甘膦釋放磷酸根的影響

Fe3+在天然水體中的濃度約為1.2～ 16.8μmol/ L,其主要以Fe3+, Fe(OH), Fe(OH)2+和Fe2(OH)等形式存在,其在湖泊水體磷循環(huán)過程起著十分重要的作用[29].一方面,Fe3+可與溶解態(tài)反應(yīng)磷(SRP)生成不溶復(fù)合物而吸附大量的SRP,另外,羥基氧化鐵膠體也可以吸附一定量的SRP,從而減少水體中可利用性磷的含量[30].另一方面,在光照射條件下,Fe3+和羥基氧化鐵膠體,都可以吸收太陽光(λ＞290nm)發(fā)生反應(yīng)產(chǎn)生羥基自由基(·OH),進(jìn)而氧化水體中的有機(jī)物.其過程如下[31-32]:

Lesueur等[13]的結(jié)果證明,在紫外光照射下,Fe3+可以氧化降解三磷酸、草甘膦和氨甲基磷酸等磷酸酯氧化降解為磷酸根;Chen等[15]研究表明,UV/Fe(III)/oxalate體系也可以實(shí)現(xiàn)草甘膦快速降解為磷酸根.據(jù)此,本文進(jìn)一步探討了Fe3+濃度對(duì)硝酸根驅(qū)動(dòng)草甘膦釋放磷酸根的影響.反應(yīng)體系中,濃度為 20mg/L,草甘膦為 5mg/L.如圖4所示,外源加入Fe3+濃度至3, 5, 10mg/L,光照60min后,反應(yīng)體系中磷酸根濃度顯著增大,其磷酸根濃度分別達(dá)到0.83, 0.85, 0.90mg/L,均高于單一/UV體系.在紫外光照射下,Fe3+轉(zhuǎn)化為 Fe2+(如式(4)所示),消除了 Fe3+與磷酸根反應(yīng)形成沉淀而減少水體中磷酸根的過程.此外,Fe2+并沒有表現(xiàn)出過度消耗經(jīng)光照產(chǎn)生的·OH,表現(xiàn)出了 Fe3+存在可以顯著促進(jìn) NO3-驅(qū)動(dòng)有機(jī)磷向無機(jī)磷轉(zhuǎn)化的現(xiàn)象.此外,當(dāng)Fe3+濃度為3mg/L時(shí),紫外光照射60min后,其有機(jī)磷轉(zhuǎn)化率已經(jīng)達(dá)到了90%,證明少量Fe3+即可促進(jìn)有機(jī)磷向無機(jī)磷的快速光化學(xué)轉(zhuǎn)化.

圖4 Fe3+對(duì)硝酸根驅(qū)動(dòng)草甘膦釋放磷酸根的影響Fig.4 The effect of initial concentration ofon the phophsate release from glyphosate photodecomposition by

2.5 腐植酸對(duì)硝酸根驅(qū)動(dòng)草甘膦釋放磷酸根的影響

腐植酸(Humic Acid, HA)是動(dòng)植物殘?bào)w經(jīng)微生物分解和轉(zhuǎn)化及一系列化學(xué)過程所產(chǎn)生的一類有機(jī)物質(zhì),占土壤和水生態(tài)體系中有機(jī)質(zhì)的50%～80%.其可以吸收可見光被激發(fā),經(jīng)反應(yīng)形成各種活性氧自由基,包括·OH、1O2及H2O2等,從而誘導(dǎo)有機(jī)物發(fā)生光化學(xué)降解[33]. Thirumavalavan等[34]研究表明,HA 可以實(shí)現(xiàn)臺(tái)灣河流水體中微囊藻毒素(MC-LR)的快速降解,在254nm UV照射60min后,水體中的MC-LR完全降解.而Kim和Zoh[14]的研究結(jié)果證實(shí),隨著溶液中HA濃度的增大,反應(yīng)體系中的甲基汞降解速率顯著降低.二者實(shí)驗(yàn)結(jié)果的差異,主要是由于HA的·OH的“源”和“匯”作用所導(dǎo)致.一方面,HA可以光解生成·OH.而另一方面,HA又可以捕獲·OH進(jìn)而對(duì)其清除.圖5為HA濃度對(duì)驅(qū)動(dòng)草甘膦釋放磷酸根的影響.從圖5中可以看出,隨著反應(yīng)體系中HA濃度的增加,體系中磷酸根產(chǎn)量逐漸降低.不添加HA的體系中,光照60min后,反應(yīng)體系中的磷酸根濃度達(dá)到了0.43mg/L,而添加了2, 4, 6mg/L的HA體系中,經(jīng)紫外光照射60min后,反應(yīng)體系中的磷酸根濃度分別為0.27, 0.19, 0.10mg/L,這表明HA對(duì)驅(qū)動(dòng)草甘膦光解釋放磷酸根具有顯著的抑制作用,其主要是由于HA與草甘膦競(jìng)爭(zhēng)由吸收光而產(chǎn)生的·OH,從而抑制了草甘膦向無機(jī)磷的轉(zhuǎn)化過程所致.

圖5 HA對(duì)硝酸根驅(qū)動(dòng)草甘膦釋放磷酸根的影響Fig.5 The effect of initial concentration of HA on the phophsate release from glyphosate photodecomposition by

2.6 碳酸氫根對(duì)硝酸根驅(qū)動(dòng)草甘膦釋放磷酸根的影響

圖6對(duì)硝酸根驅(qū)動(dòng)草甘膦釋放磷酸根的影響Fig.6 The effect of initial concentration ofon the phophsate release from glyphosate photo-decomposition by

2.7 湖水中草甘膦光解釋放磷酸根

為研究湖水中硝酸根對(duì)草甘膦光解釋放磷酸根的影響,本研究將草甘膦溶于自然富營養(yǎng)化湖水.分別探討了湖水、湖水/草甘膦及湖水/草甘膦/3個(gè)體系光照前后無機(jī)磷的含量變化.從表 1中可以看出,實(shí)驗(yàn)所用湖水中濃度為0.56mg/L,為將湖水與實(shí)驗(yàn)室超純水體系中草甘膦光解釋放磷酸根過程進(jìn)行對(duì)比,因此,外源添加使其濃度至 20mg/L.從圖 8可以看出,湖水體系中,光照60min后,磷酸根濃度由0.14mg/L增加至0.16mg/L,其主要是由于湖水中有機(jī)磷的光解所致.Southwell 等[10]的工作已經(jīng)表明沉積物再懸浮過程中,光照可以實(shí)現(xiàn)更多磷酸根釋放,其增量主要來源于沉積物中有機(jī)磷在持續(xù)光照下的光化學(xué)分解.在我們的實(shí)驗(yàn)中,經(jīng)0.7μm濾膜過濾的水樣中仍含有有機(jī)磷,其光解釋放磷酸根是湖水體系中磷酸根含量上升的主要原因.在湖水/草甘膦體系中,經(jīng) 60min光照后,磷酸根濃度由0.15mg/L增加至0.26mg/L,其水體磷酸根濃度高于湖水體系中的0.16mg/L,主要原因是由于草甘膦的光解所致.從純水實(shí)驗(yàn)結(jié)果可以看出,草甘膦的直接光解對(duì)磷酸根釋放貢獻(xiàn)較低,但是從表 1可以看出,實(shí)驗(yàn)所用湖水中含有和Fe3+等光敏物質(zhì),其可以通過光敏作用實(shí)現(xiàn)水體中草甘膦的間接光解,進(jìn)而增加了水體磷酸根的含量.在湖水/草甘膦/體系中,光照 60min后,磷酸根濃度由0.16mg/L增加至0.32mg/L,顯著高于湖水/草甘膦體系,主要是由于外源添加所致,這一結(jié)果同樣表明,當(dāng)水體中含量增加時(shí),可以增大水體磷酸根的光化學(xué)釋放.同時(shí),值得注意的是,湖水/草甘膦/體系中磷酸根的釋放量要低于純水處理,這主要是湖水中的 HA和等物質(zhì),競(jìng)爭(zhēng)捕獲了·OH,進(jìn)而抑制了有機(jī)磷的光化學(xué)分解過程[35].

圖7 湖水體中硝酸根驅(qū)動(dòng)草甘膦光解釋放磷酸根的影響Fig.7 The effect ofon the phophsate release from glyphosate photo-decomposition in lake water

圖8 甲醇對(duì)湖水中硝酸根驅(qū)動(dòng)草甘膦光解釋放磷酸根的影響Fig.8 The effect of methanol on the phophsate release from glyphosate photo-decomposition byin lake water

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Photo-induced phosphate released from organic phosphorus decomposition by nitrate.

LIU Guang-long1,2, ZHU Duan-wei1, ZHOU Yi-yong2*, CAO Xiu-yun2, SONG Chun-lei2, HUA Yu-mei1, ZHAO Jian-wei1(1.College of Resources & Environment, Huazhong Agricultural University, Wuhan 430070, China；2.State key Laboratory of Freshwater Ecology and Biotechnology, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan 430072, China). China Environmental Science, 2016,36(12)：3657～3664

In order to understand the influence of nitrate (NO3-) photochemical activity on the phosphate released from organic phosphorus indirect photolysis degradation, the phosphate released from glyphosate photodegradation by NO3-was investigated in deionised and natural water under UV irradiation. Meanwhile, the methanol was used as the ·OH scavenger to confirm the role of NO3-on the phosphate released from organic phosphorus degradation in the natural eutrophication lakes. The results showed that the phosphate could be released from glyphosate photolysis under UV irradiation and the release amount of phosphate increased with pH and initial concentration of glyphosate increasing under UV light irradiation in deionised water. NO3-adding enhanced the phosphate released from glyphosate photodegradation in comparison with direct photolysis and the concentration of phosphate increased from 0.05mg/L to 0.43mg/L with the 20mg/L NO3-added to the solution under UV irradiation for 60min. The effect of environmental parameters including Fe3+, HCO3-and humic acid on the phosphate released from glyphosate photodegradation by NO3-was also performed. In the presence of Fe3+, the amount of phosphate released from glyphosate photodegradation increased significantly due to photosensitization by reactive species such as hydroxyl radical. The presence of humic acid and HCO3-inhibited phosphate released through a radical scavenging effect. The release amount of phosphate in the natural water with NO3-spiked is higher than that of control and the phosphate released was inhibited when the methanol was added to the reaction system. All these results reveal that NO3-plays an important role in phosphate released from organic phosphorus photodegradation.

organic phosphorus；phosphate；nitrate；photo-decomposition；radicals

X703.5

A

1000-6923(2016)12-3657-08

劉廣龍(1984-),男,山東即墨人,博士,副教授,主要從事水體污染控制與生態(tài)修復(fù)研究.發(fā)表論文20余篇.

2016-04-20

國家自然科學(xué)基金項(xiàng)目(41230748,41401547);霍英東青年教師基金(151078);中央高?；究蒲袠I(yè)務(wù)費(fèi)資助(2662016PY061);中國博士后基金項(xiàng)目(2013M540619,2015T80855)

* 責(zé)任作者, 研究員, zhouyy@ihb.ac.cn