龔夢(mèng)丹，金增鋒，王 燕，林 娟，丁士明

(1：中國(guó)科學(xué)院南京地理與湖泊研究所湖泊與環(huán)境國(guó)家重點(diǎn)實(shí)驗(yàn)室，南京 210008)(2：中國(guó)科學(xué)院大學(xué)，北京 100049)

龔夢(mèng)丹1,2，金增鋒1,2，王 燕1,2，林 娟1,2，丁士明1

(1：中國(guó)科學(xué)院南京地理與湖泊研究所湖泊與環(huán)境國(guó)家重點(diǎn)實(shí)驗(yàn)室，南京 210008)(2：中國(guó)科學(xué)院大學(xué)，北京 100049)

目前普遍認(rèn)為磷鐵耦合關(guān)系是P遷移的主要機(jī)制，但大部分研究結(jié)果并未提供直接的原位證據(jù). 為了探索沉積物剖面磷(P)與鐵(Fe)的耦合關(guān)系，利用ZrO-Chelex薄膜擴(kuò)散梯度技術(shù)(ZrO-Chelex DGT)，分別對(duì)太湖、巢湖、鄱陽(yáng)湖和洞庭湖4個(gè)淺水湖泊沉積物有效態(tài)Fe和P進(jìn)行高分辨采樣和分析. 結(jié)果表明，不同湖區(qū)有效態(tài)Fe和P濃度在沉積物-水界面處開(kāi)始增加，之后波動(dòng)變化，垂向異質(zhì)性較強(qiáng)，但兩者濃度變化同步. 有效態(tài)P和Fe濃度的相關(guān)分析結(jié)果證明兩者濃度具有顯著的線性相關(guān). 室內(nèi)厭氧培養(yǎng)實(shí)驗(yàn)進(jìn)一步表明，F(xiàn)e3+的還原性促使Fe2+與鐵結(jié)合態(tài)磷的釋放，促使DGT有效態(tài)P與Fe同步變化. 該結(jié)果表明沉積物P的二次遷移和釋放受Fe氧化還原過(guò)程的控制，為鐵磷耦合關(guān)系提供了直接證據(jù).

沉積物；薄膜擴(kuò)散梯度；鐵磷耦合；內(nèi)源；有效性；長(zhǎng)江中下游

湖泊沉積物是水體氮、磷等營(yíng)養(yǎng)鹽重要的匯，由于淺水湖泊易受風(fēng)浪擾動(dòng)作用發(fā)生再懸浮，造成沉積物氮、磷等營(yíng)養(yǎng)物質(zhì)釋放到上覆水，因此比深水湖泊更易引起湖泊富營(yíng)養(yǎng)化[1]. 目前太湖和巢湖等多個(gè)淺水湖泊面臨水質(zhì)惡化和湖泊富營(yíng)養(yǎng)化等問(wèn)題[2-4]. 研究表明，磷(P)是湖泊藍(lán)藻水華產(chǎn)生的限制性因子[5]，內(nèi)源P的釋放是切斷外源污染后,富營(yíng)養(yǎng)化現(xiàn)象持續(xù)存在的重要原因[6-8].

在探討內(nèi)源P釋放機(jī)制時(shí)，鐵、鋁、鈣結(jié)合態(tài)磷(Ca-P、Al-P、Fe-P)等都納入研究范圍，其中Fe-P是沉積物中最易受環(huán)境影響而引起磷釋放的形態(tài)[9]. 目前，大量研究圍繞沉積物Fe和P的關(guān)系展開(kāi)[10-12]. Bortleson等[13]研究發(fā)現(xiàn)湖泊沉積物中P和Fe呈現(xiàn)顯著的相關(guān)性. 進(jìn)一步的研究表明， Fe含量高的沉積物，NH4Cl-P、NaOH-P濃度相對(duì)也高[14]. S?ndergaard等[15]對(duì)丹麥淺水湖泊沉積物進(jìn)行P形態(tài)分析時(shí)，提出沉積物表層P濃度受外源P負(fù)荷和Fe濃度影響，且隨著兩者濃度增加而增加. 另外也有研究表明P的釋放與P/Fe比值呈負(fù)相關(guān)，在P/Fe<1時(shí)，P的釋放速度明顯升高[16]. 這些發(fā)現(xiàn)都驗(yàn)證了Fe和P的循環(huán)對(duì)P的界面釋放起重要作用，F(xiàn)e3+的降低對(duì)P釋放具有主導(dǎo)作用，在高P水平下沉積物中Fe與P的關(guān)系更加密切[17].

目前有關(guān)沉積物的研究主要基于主動(dòng)采樣技術(shù)，采樣和運(yùn)輸過(guò)程會(huì)破壞沉積物原有性質(zhì)，改變沉積物氧化還原條件[18]，且分辨率低(cm級(jí)別)，分析測(cè)定步驟繁瑣且速度較慢. 新型原位被動(dòng)采樣技術(shù)ZrO-Chelex薄膜擴(kuò)散梯度技術(shù)(ZrO-Chelex DGT)，可以在不破壞沉積物情況下，能夠原位同步獲取沉積物剖面有效態(tài)P和Fe的信息，分辨率達(dá)到毫米級(jí). Xu等[19]和Sun等[20]通過(guò)室內(nèi)模擬實(shí)驗(yàn)證明ZrO-Chelex DGT在自然界正常水體下能夠同時(shí)測(cè)量沉積物剖面有效態(tài)P和Fe濃度，且其對(duì)這兩種離子的檢測(cè)極限濃度較高.

本研究選取長(zhǎng)江中下游地區(qū)典型淺水湖泊太湖、巢湖、鄱陽(yáng)湖和洞庭湖這4個(gè)湖泊為研究對(duì)象，利用ZrO-Chelex DGT同步獲取現(xiàn)場(chǎng)沉積物-水界面有效態(tài)P和Fe濃度的剖面分布信息，并對(duì)兩者的變化進(jìn)行分析，同時(shí)進(jìn)行室內(nèi)模擬實(shí)驗(yàn)，進(jìn)一步明確鐵磷耦合關(guān)系，為內(nèi)源磷釋放機(jī)理提供直接證據(jù).

1 材料和方法

1.1 研究區(qū)概況

1.2 ZrO-Chelex DGT技術(shù)原理和裝置準(zhǔn)備

薄膜梯度擴(kuò)散技術(shù)(DGT)作為一種原位被動(dòng)采樣技術(shù)，由Davison等于1994年發(fā)明[23]. 該技術(shù)是以費(fèi)克第一擴(kuò)散定律為理論基礎(chǔ)，當(dāng)裝置投放至水體或沉積物中，環(huán)境中自由態(tài)離子通過(guò)濾膜和擴(kuò)散膜組成的擴(kuò)散層，進(jìn)而被固定膜捕獲并累積，根據(jù)通量與費(fèi)克第一定律公式計(jì)算出DGT有效濃度[24].

ZrO-Chelex DGT是在氧化鋯DGT(Zr-oxide DGT)基礎(chǔ)上發(fā)展起來(lái)的復(fù)合DGT，利用ZrO-Chelex凝膠層作為同步固定Fe、P的固定層[19,25]，其容量較高[19]，還可同時(shí)測(cè)定P和As5+、Cr6+、Mo6+、Sb5+、Se6+、V5+、W6+等重金屬離子[26-27]. ZrO-Chelex DGT購(gòu)置于南京智感環(huán)境科技有限公司，采用了新型平板DGT塑料外套[28]，固定膜的配制方法參考文獻(xiàn)[19]，擴(kuò)散膜為1.5%瓊脂糖，厚度為0.8 mm，濾膜為PVDF膜(0.45 μm, 厚度為0.1 mm). 組裝DGT裝置時(shí)，首先將固定膜放置在底板上，之后依次放置擴(kuò)散膜和濾膜，最后蓋板固定3層膜，置于去離子水充氮去氧16 h備用.

1.3 樣品采集與分析方法

1.3.1 樣品采集 2015年5-7月，依次在太湖、巢湖、鄱陽(yáng)湖和洞庭湖進(jìn)行DGT裝置投放和現(xiàn)場(chǎng)表層泥取樣，其采樣點(diǎn)分布見(jiàn)圖1. 通過(guò)重力投放器，將DGT裝置投放到采樣點(diǎn)湖區(qū)沉積物中，24 h后回收DGT裝置，標(biāo)記沉積物-水界面，用去離子水沖洗裝置表面沉積物，裝入自封袋保持濕潤(rùn)，帶回實(shí)驗(yàn)室進(jìn)行分析. 采集太湖梅梁灣柱狀樣品和水樣，低溫保存帶回實(shí)驗(yàn)室. 其中柱狀樣進(jìn)行現(xiàn)場(chǎng)分層，用于室內(nèi)培養(yǎng)實(shí)驗(yàn).

1.3.2 室內(nèi)培養(yǎng)實(shí)驗(yàn) 將太湖梅梁灣分層泥進(jìn)行相同層次的混勻過(guò)篩，按沉積物分層順序分裝成4個(gè)平行沉積物柱狀樣，并添加上覆水(原位過(guò)濾水樣)，恒溫25℃下淹水培養(yǎng). 室內(nèi)模擬好氧-厭氧環(huán)境，首先將穩(wěn)定后的沉積物間斷曝氣使其充分好氧，隨后加蓋子密封，使沉積物逐漸厭氧，每天定時(shí)監(jiān)測(cè)上覆水溶解氧濃度，在好氧3 d，厭氧7、14、30 d時(shí)投放DGT裝置，同步獲取沉積物剖面P、Fe的信息.

沉積物分析方法：將不同湖區(qū)的沉積物帶回實(shí)驗(yàn)室后，稱(chēng)取約2 g 沉積物樣品烘干測(cè)定含水率. 剩余樣品封袋后馬上進(jìn)行低溫冷凍，再將凍土進(jìn)行冷凍干燥，研磨過(guò)篩后裝入自封袋中待分析. 沉積物各指標(biāo)均按照標(biāo)準(zhǔn)分析方法分析[30]. 燒失量(LOI)通過(guò)在550℃環(huán)境下灼燒沉積物6 h測(cè)定；元素含量測(cè)定采用LiBO2消解法，提取液中總磷含量采用鉬藍(lán)顯色法測(cè)定，其他金屬離子含量用ICP-AES進(jìn)行測(cè)定.

圖1 采樣點(diǎn)分布Fig.1 Distribution of sampling sites

1.4 數(shù)據(jù)處理和分析

(1)

式中，Ce為提取液中P和Fe的濃度，Ve和Vg分別為提取液和固定膜的體積，fe為提取率，ZrO-Chelex固定膜P和Fe的提取率分別為96%和88%.

有效態(tài)P和Fe濃度可依據(jù)公式(2)得到：

(2)

2 結(jié)果與討論

2.1 沉積物理化性質(zhì)

4個(gè)湖泊采樣點(diǎn)表層沉積物的元素含量組成如表1所示， Al和Fe含量最高，分別為64.9～75.7 mg/g和31.8～48.4 mg/g，其數(shù)值超過(guò)其他元素一個(gè)數(shù)量級(jí). Ca含量?jī)H次于Al和Fe，為2.1～6.4 mg/g. Mn在沉積物表層中的含量最低，為0.82～1.3 mg/g. 沉積物總磷含量為0.54～0.84 mg/g，在各個(gè)湖泊的次序?yàn)槎赐ズ?太湖>巢湖>鄱陽(yáng)湖. LOI表征沉積物中有機(jī)質(zhì)含量[33]，太湖、巢湖、鄱陽(yáng)湖和洞庭湖的LOI分別為4.7%、6.8%、6.6%和4.9%. 研究表明，F(xiàn)e/P比值可表征沉積物對(duì)內(nèi)源磷負(fù)荷的控制能力[34]，當(dāng)Fe/P比值為10～15時(shí)，由于鐵(氫)氧化物對(duì)磷的控制作用，內(nèi)源P不足以釋放. 本研究4個(gè)湖泊Fe/P比值分別為71、52、59和46，表明所選采樣點(diǎn)的沉積物內(nèi)源磷的風(fēng)險(xiǎn)較小.

表1 4個(gè)湖泊表層沉積物的基本理化性質(zhì)

2.2 沉積物有效態(tài)P和Fe的分布特征

4個(gè)湖泊沉積物剖面DGT有效態(tài)P和Fe濃度分布如圖2所示. 太湖、巢湖、鄱陽(yáng)湖和洞庭湖的有效態(tài)P濃度均在沉積物-水界面處增加，分別在-14、-10、-19和-26 mm處達(dá)到第1個(gè)峰值，且不同湖區(qū)沉積物峰值處有效態(tài)P的濃度不一，其中在鄱陽(yáng)湖和洞庭湖濃度較高，分別為0.41和0.52 mg/L，與鄱陽(yáng)湖和洞庭湖湖泊富營(yíng)養(yǎng)化情況更為嚴(yán)峻一致[22]. 有效態(tài)P到達(dá)第一個(gè)峰值后繼續(xù)波動(dòng)變化，在不同深度達(dá)到峰谷值，且峰谷值數(shù)不一. 峰值區(qū)域表明間隙水中有效態(tài)P濃度的增加[35]. 相應(yīng)地，谷值區(qū)域可能為有效態(tài)P的釋放衰減，且可能與細(xì)菌等微生物對(duì)有機(jī)質(zhì)的降解有關(guān)，這種減少在時(shí)間上會(huì)持續(xù)到細(xì)菌衰亡[36-37]. 另外太湖和巢湖沉積物剖面有效態(tài)P濃度的垂向變化更為明顯，這表明這兩個(gè)湖泊沉積物異質(zhì)性較強(qiáng)[38]. 有效態(tài)P濃度分別在太湖、巢湖和洞庭湖沉積物剖面的-53、-79和-56 mm處趨于平緩，且濃度值較高，可能與下層的還原環(huán)境有關(guān)系[39].

圖2 沉積物剖面有效態(tài)P和Fe濃度分布Fig.2 Vertical distributions of labile P and Fe in the sediments profile

4個(gè)湖泊沉積物剖面有效態(tài)Fe濃度范圍變化較大，其增減幅度在峰谷處明顯于有效態(tài)P濃度的增減幅度，這與Ding等[40]對(duì)太湖沉積物的原位測(cè)定結(jié)果一致. 與有效態(tài)P濃度在沉積物剖面分布類(lèi)似，有效態(tài)Fe濃度在沉積物剖面的垂向分布規(guī)律與有效態(tài)P一致，兩者均在沉積物-水界面處增加，并在相同深處波動(dòng)變化，呈現(xiàn)同步升高或降低的趨勢(shì).

對(duì)上述4個(gè)剖面有效態(tài)P和Fe濃度變化明顯區(qū)域(虛線界定區(qū)域)進(jìn)行相關(guān)性分析，結(jié)果見(jiàn)圖3. 4個(gè)湖泊沉積物剖面有效態(tài)P和Fe濃度具有顯著的正相關(guān)關(guān)系(P<0.05)，表明沉積物中磷與鐵均存在明顯的耦合關(guān)系，磷釋放受鐵還原的控制，即好氧條件下Fe2+被氧化為Fe3+，所生成的(氫)氧化鐵吸附固定磷，F(xiàn)e和P同步減少；厭氧條件下Fe3+被還原為Fe2+，溶解釋放Fe2+和P，有效態(tài)Fe和P同步增加[41]. 線性方程的斜率表征沉積物中有效態(tài)Fe對(duì)P遷移的能力[40]. 在太湖和鄱陽(yáng)湖沉積物中，有效態(tài)Fe和P濃度比值的斜率較低(3.96 和2.21)，表明這兩個(gè)湖泊沉積物中活性氧化鐵濃度較低，沉積物剖面有效態(tài)P釋放能力較強(qiáng)[40]. 而在巢湖和洞庭湖中，其斜率值較高(14.25和9.18)，說(shuō)明沉積物中對(duì)P的固定能力較強(qiáng).

圖3 沉積物有效態(tài)P和Fe的相關(guān)性(*表示相關(guān)性顯著，下同)Fig.3 Correlation analyses between labile P and Fe in sediments

2.3 沉積物-水界面磷鐵同步釋放機(jī)制分析

通過(guò)室內(nèi)模擬好氧-厭氧環(huán)境，發(fā)現(xiàn)在沉積物厭氧過(guò)程中，有效態(tài)P和Fe濃度均在-10 mm左右處開(kāi)始增加，于-40～-50 mm處達(dá)到峰值，且保持峰值的較高濃度，向下停止減少；隨著厭氧過(guò)程的加劇，有效態(tài)P和Fe濃度逐漸同步升高(圖4). 可見(jiàn)，隨著沉積物內(nèi)部還原環(huán)境的加強(qiáng)，鐵不斷還原，F(xiàn)e3+被持續(xù)還原為Fe2+，鐵結(jié)合態(tài)的磷也隨之釋放，導(dǎo)致了磷的釋放量也逐漸增加. 進(jìn)一步分析不同厭氧條件下沉積物剖面有效態(tài)P和Fe濃度變化明顯區(qū)域(虛線界定區(qū)域)的相關(guān)性，發(fā)現(xiàn)均呈顯著或極顯著的正相關(guān)，即兩者在沉積物剖面存在同步釋放的規(guī)律(圖5). 室內(nèi)厭氧培養(yǎng)實(shí)驗(yàn)進(jìn)一步證實(shí)了磷鐵之間的耦合關(guān)系，即鐵的還原導(dǎo)致磷的釋放. Ding等[40]對(duì)太湖14個(gè)位點(diǎn)進(jìn)行原位測(cè)定，均發(fā)現(xiàn)沉積物剖面有效態(tài)P和Fe的釋放具有同步性，與本研究的結(jié)果一致，很可能說(shuō)明磷鐵同步釋放普遍存在于長(zhǎng)江中下游湖泊. Rydin等[42]對(duì)波羅的海海岸沉積物界進(jìn)行P釋放機(jī)理探索，發(fā)現(xiàn)潛在活性P的釋放主要受Fe-P控制，即P的釋放受Fe的氧化還原控制[41,43]. 可能的原因是Fe3+濃度的增加，會(huì)降低P的遷移性[44]. 另外，F(xiàn)e2+濃度的減少，不僅僅受氧化還原環(huán)境的變化影響，也可能是硝酸或硫酸酶含量減少所致[45]，硝酸鹽細(xì)菌呼吸作用的增強(qiáng)，能夠增加沉積物中Fe-P絡(luò)合物中P釋放的速率[46]. 同時(shí)，硝酸鹽細(xì)菌的分泌物也能夠加速Fe3+的溶解，使吸附在Fe(OH)3中的P釋放出來(lái)[47].

圖4 好氧-厭氧過(guò)程沉積物剖面有效態(tài)P和Fe的濃度分布Fig.4 Distributions of labile P and Fe in the sediments during anaerobic-aerobic incubation

圖5 好氧-厭氧過(guò)程沉積物有效態(tài)P和Fe的相關(guān)性Fig.5 Correlation analyses between labile P and Fe during anaerobic-aerobic incubation

3 結(jié)論

1) 不同湖泊沉積物理化性質(zhì)差異較大，且磷鐵含量在沉積物垂向分布具有較大的空間異質(zhì)性，不同湖泊異質(zhì)性也存在差異.

2) 沉積物中磷和鐵存在耦合關(guān)系，磷、鐵含量呈現(xiàn)同步增加或減少的趨勢(shì)，4個(gè)湖泊沉積物均存在相同的變化規(guī)律.

3) 室內(nèi)好氧-厭氧培養(yǎng)實(shí)驗(yàn)驗(yàn)證了鐵磷耦合關(guān)系，即鐵的還原可導(dǎo)致磷釋放，沉積物磷的二次遷移主要受鐵氧化還原過(guò)程的控制.

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Coupling between iron and phosphorus in sediments of shallow lakes in the middle and lower reaches of Yangtze River using diffusive gradients in thin films (DGT)

GONG Mengdan1,2, JIN Zengfeng1,2, WANG Yan1,2, LIN Juan1,2& DING Shiming1**

(1：StateKeyLaboratoryofLakeScienceandEnvironment,NanjingInstituteofGeographyandLimnology,ChineseAcademyofSciences,Nanjing210008,P.R.China)(2：UniversityofChineseAcademyofSciences,Beijing100049,P.R.China)

It is universally accepted that the coupling relationship between phosphorus (P) and iron (Fe) is responsible for the migration of P, but there is little directinsituevidence. In order to investigate the coupling relationship between P and Fe in sediments of shallow lakes, the concentrations of labile P and Fe in the sediments in Lakes Taihu, Chaohu, Poyang and Dongting were measured using ZrO-Chelex diffusive gradients in thin films (ZrO-Chelex DGT). The results showed that both labile Fe and P began to increase downward below the sediment-water surface followed by fluctuation up to the bottom of the sediment profiles. Their changes were consistent along the profiles, which were further supported by the positively linear correlations among them. Anaerobic incubation experiment further showed that the reductive dissolution of iron oxides led to the releases of ferrous Fe and P associated with iron oxides. The results proved that the remobilization of P in sediments was dominated by Fe redox.

Sediment; diffusive gradients in thin films; Fe-P coupling; internal loading; availability; middle and lower reaches of Yangtze River

國(guó)家水體污染控制與治理科技重大專(zhuān)項(xiàng)(2012ZX07103-005)資助. 2016-09-07收稿；2016-12-07收修改稿. 龔夢(mèng)丹(1990～)，女，碩士研究生； E-mail: mengdan_rainbow@163.com.

； E-mail: smding@niglas.ac.cn.

DOI 10.18307/2017.0508