长江中下游典型浅水湖泊沉积物水界面磷与铁的耦合关系

龚梦丹，金增锋，王 燕，林 娟，丁士明

(1：中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室，南京 210008)(2：中国科学院大学，北京 100049)

龚梦丹1,2，金增锋1,2，王 燕1,2，林 娟1,2，丁士明1

(1：中国科学院南京地理与湖泊研究所湖泊与环境国家重点实验室，南京 210008)(2：中国科学院大学，北京 100049)

目前普遍认为磷铁耦合关系是P迁移的主要机制，但大部分研究结果并未提供直接的原位证据. 为了探索沉积物剖面磷(P)与铁(Fe)的耦合关系，利用ZrO-Chelex薄膜扩散梯度技术(ZrO-Chelex DGT)，分别对太湖、巢湖、鄱阳湖和洞庭湖4个浅水湖泊沉积物有效态Fe和P进行高分辨采样和分析. 结果表明，不同湖区有效态Fe和P浓度在沉积物-水界面处开始增加，之后波动变化，垂向异质性较强，但两者浓度变化同步. 有效态P和Fe浓度的相关分析结果证明两者浓度具有显著的线性相关. 室内厌氧培养实验进一步表明，Fe3+的还原性促使Fe2+与铁结合态磷的释放，促使DGT有效态P与Fe同步变化. 该结果表明沉积物P的二次迁移和释放受Fe氧化还原过程的控制，为铁磷耦合关系提供了直接证据.

沉积物；薄膜扩散梯度；铁磷耦合；内源；有效性；长江中下游

湖泊沉积物是水体氮、磷等营养盐重要的汇，由于浅水湖泊易受风浪扰动作用发生再悬浮，造成沉积物氮、磷等营养物质释放到上覆水，因此比深水湖泊更易引起湖泊富营养化[1]. 目前太湖和巢湖等多个浅水湖泊面临水质恶化和湖泊富营养化等问题[2-4]. 研究表明，磷(P)是湖泊蓝藻水华产生的限制性因子[5]，内源P的释放是切断外源污染后,富营养化现象持续存在的重要原因[6-8].

在探讨内源P释放机制时，铁、铝、钙结合态磷(Ca-P、Al-P、Fe-P)等都纳入研究范围，其中Fe-P是沉积物中最易受环境影响而引起磷释放的形态[9]. 目前，大量研究围绕沉积物Fe和P的关系展开[10-12]. Bortleson等[13]研究发现湖泊沉积物中P和Fe呈现显著的相关性. 进一步的研究表明， Fe含量高的沉积物，NH4Cl-P、NaOH-P浓度相对也高[14]. Søndergaard等[15]对丹麦浅水湖泊沉积物进行P形态分析时，提出沉积物表层P浓度受外源P负荷和Fe浓度影响，且随着两者浓度增加而增加. 另外也有研究表明P的释放与P/Fe比值呈负相关，在P/Fe<1时，P的释放速度明显升高[16]. 这些发现都验证了Fe和P的循环对P的界面释放起重要作用，Fe3+的降低对P释放具有主导作用，在高P水平下沉积物中Fe与P的关系更加密切[17].

目前有关沉积物的研究主要基于主动采样技术，采样和运输过程会破坏沉积物原有性质，改变沉积物氧化还原条件[18]，且分辨率低(cm级别)，分析测定步骤繁琐且速度较慢. 新型原位被动采样技术ZrO-Chelex薄膜扩散梯度技术(ZrO-Chelex DGT)，可以在不破坏沉积物情况下，能够原位同步获取沉积物剖面有效态P和Fe的信息，分辨率达到毫米级. Xu等[19]和Sun等[20]通过室内模拟实验证明ZrO-Chelex DGT在自然界正常水体下能够同时测量沉积物剖面有效态P和Fe浓度，且其对这两种离子的检测极限浓度较高.

本研究选取长江中下游地区典型浅水湖泊太湖、巢湖、鄱阳湖和洞庭湖这4个湖泊为研究对象，利用ZrO-Chelex DGT同步获取现场沉积物-水界面有效态P和Fe浓度的剖面分布信息，并对两者的变化进行分析，同时进行室内模拟实验，进一步明确铁磷耦合关系，为内源磷释放机理提供直接证据.

1 材料和方法

1.1 研究区概况

1.2 ZrO-Chelex DGT技术原理和装置准备

薄膜梯度扩散技术(DGT)作为一种原位被动采样技术，由Davison等于1994年发明[23]. 该技术是以费克第一扩散定律为理论基础，当装置投放至水体或沉积物中，环境中自由态离子通过滤膜和扩散膜组成的扩散层，进而被固定膜捕获并累积，根据通量与费克第一定律公式计算出DGT有效浓度[24].

ZrO-Chelex DGT是在氧化锆DGT(Zr-oxide DGT)基础上发展起来的复合DGT，利用ZrO-Chelex凝胶层作为同步固定Fe、P的固定层[19,25]，其容量较高[19]，还可同时测定P和As5+、Cr6+、Mo6+、Sb5+、Se6+、V5+、W6+等重金属离子[26-27]. ZrO-Chelex DGT购置于南京智感环境科技有限公司，采用了新型平板DGT塑料外套[28]，固定膜的配制方法参考文献[19]，扩散膜为1.5%琼脂糖，厚度为0.8 mm，滤膜为PVDF膜(0.45 μm, 厚度为0.1 mm). 组装DGT装置时，首先将固定膜放置在底板上，之后依次放置扩散膜和滤膜，最后盖板固定3层膜，置于去离子水充氮去氧16 h备用.

1.3 样品采集与分析方法

1.3.1 样品采集 2015年5-7月，依次在太湖、巢湖、鄱阳湖和洞庭湖进行DGT装置投放和现场表层泥取样，其采样点分布见图1. 通过重力投放器，将DGT装置投放到采样点湖区沉积物中，24 h后回收DGT装置，标记沉积物-水界面，用去离子水冲洗装置表面沉积物，装入自封袋保持湿润，带回实验室进行分析. 采集太湖梅梁湾柱状样品和水样，低温保存带回实验室. 其中柱状样进行现场分层，用于室内培养实验.

1.3.2 室内培养实验 将太湖梅梁湾分层泥进行相同层次的混匀过筛，按沉积物分层顺序分装成4个平行沉积物柱状样，并添加上覆水(原位过滤水样)，恒温25℃下淹水培养. 室内模拟好氧-厌氧环境，首先将稳定后的沉积物间断曝气使其充分好氧，随后加盖子密封，使沉积物逐渐厌氧，每天定时监测上覆水溶解氧浓度，在好氧3 d，厌氧7、14、30 d时投放DGT装置，同步获取沉积物剖面P、Fe的信息.

沉积物分析方法：将不同湖区的沉积物带回实验室后，称取约2 g 沉积物样品烘干测定含水率. 剩余样品封袋后马上进行低温冷冻，再将冻土进行冷冻干燥，研磨过筛后装入自封袋中待分析. 沉积物各指标均按照标准分析方法分析[30]. 烧失量(LOI)通过在550℃环境下灼烧沉积物6 h测定；元素含量测定采用LiBO2消解法，提取液中总磷含量采用钼蓝显色法测定，其他金属离子含量用ICP-AES进行测定.

图1 采样点分布Fig.1 Distribution of sampling sites

1.4 数据处理和分析

(1)

式中，Ce为提取液中P和Fe的浓度，Ve和Vg分别为提取液和固定膜的体积，fe为提取率，ZrO-Chelex固定膜P和Fe的提取率分别为96%和88%.

有效态P和Fe浓度可依据公式(2)得到：

(2)

2 结果与讨论

2.1 沉积物理化性质

4个湖泊采样点表层沉积物的元素含量组成如表1所示， Al和Fe含量最高，分别为64.9～75.7 mg/g和31.8～48.4 mg/g，其数值超过其他元素一个数量级. Ca含量仅次于Al和Fe，为2.1～6.4 mg/g. Mn在沉积物表层中的含量最低，为0.82～1.3 mg/g. 沉积物总磷含量为0.54～0.84 mg/g，在各个湖泊的次序为洞庭湖>太湖>巢湖>鄱阳湖. LOI表征沉积物中有机质含量[33]，太湖、巢湖、鄱阳湖和洞庭湖的LOI分别为4.7%、6.8%、6.6%和4.9%. 研究表明，Fe/P比值可表征沉积物对内源磷负荷的控制能力[34]，当Fe/P比值为10～15时，由于铁(氢)氧化物对磷的控制作用，内源P不足以释放. 本研究4个湖泊Fe/P比值分别为71、52、59和46，表明所选采样点的沉积物内源磷的风险较小.

表1 4个湖泊表层沉积物的基本理化性质

2.2 沉积物有效态P和Fe的分布特征

4个湖泊沉积物剖面DGT有效态P和Fe浓度分布如图2所示. 太湖、巢湖、鄱阳湖和洞庭湖的有效态P浓度均在沉积物-水界面处增加，分别在-14、-10、-19和-26 mm处达到第1个峰值，且不同湖区沉积物峰值处有效态P的浓度不一，其中在鄱阳湖和洞庭湖浓度较高，分别为0.41和0.52 mg/L，与鄱阳湖和洞庭湖湖泊富营养化情况更为严峻一致[22]. 有效态P到达第一个峰值后继续波动变化，在不同深度达到峰谷值，且峰谷值数不一. 峰值区域表明间隙水中有效态P浓度的增加[35]. 相应地，谷值区域可能为有效态P的释放衰减，且可能与细菌等微生物对有机质的降解有关，这种减少在时间上会持续到细菌衰亡[36-37]. 另外太湖和巢湖沉积物剖面有效态P浓度的垂向变化更为明显，这表明这两个湖泊沉积物异质性较强[38]. 有效态P浓度分别在太湖、巢湖和洞庭湖沉积物剖面的-53、-79和-56 mm处趋于平缓，且浓度值较高，可能与下层的还原环境有关系[39].

图2 沉积物剖面有效态P和Fe浓度分布Fig.2 Vertical distributions of labile P and Fe in the sediments profile

4个湖泊沉积物剖面有效态Fe浓度范围变化较大，其增减幅度在峰谷处明显于有效态P浓度的增减幅度，这与Ding等[40]对太湖沉积物的原位测定结果一致. 与有效态P浓度在沉积物剖面分布类似，有效态Fe浓度在沉积物剖面的垂向分布规律与有效态P一致，两者均在沉积物-水界面处增加，并在相同深处波动变化，呈现同步升高或降低的趋势.

对上述4个剖面有效态P和Fe浓度变化明显区域(虚线界定区域)进行相关性分析，结果见图3. 4个湖泊沉积物剖面有效态P和Fe浓度具有显著的正相关关系(P<0.05)，表明沉积物中磷与铁均存在明显的耦合关系，磷释放受铁还原的控制，即好氧条件下Fe2+被氧化为Fe3+，所生成的(氢)氧化铁吸附固定磷，Fe和P同步减少；厌氧条件下Fe3+被还原为Fe2+，溶解释放Fe2+和P，有效态Fe和P同步增加[41]. 线性方程的斜率表征沉积物中有效态Fe对P迁移的能力[40]. 在太湖和鄱阳湖沉积物中，有效态Fe和P浓度比值的斜率较低(3.96 和2.21)，表明这两个湖泊沉积物中活性氧化铁浓度较低，沉积物剖面有效态P释放能力较强[40]. 而在巢湖和洞庭湖中，其斜率值较高(14.25和9.18)，说明沉积物中对P的固定能力较强.

图3 沉积物有效态P和Fe的相关性(*表示相关性显著，下同)Fig.3 Correlation analyses between labile P and Fe in sediments

2.3 沉积物-水界面磷铁同步释放机制分析

通过室内模拟好氧-厌氧环境，发现在沉积物厌氧过程中，有效态P和Fe浓度均在-10 mm左右处开始增加，于-40～-50 mm处达到峰值，且保持峰值的较高浓度，向下停止减少；随着厌氧过程的加剧，有效态P和Fe浓度逐渐同步升高(图4). 可见，随着沉积物内部还原环境的加强，铁不断还原，Fe3+被持续还原为Fe2+，铁结合态的磷也随之释放，导致了磷的释放量也逐渐增加. 进一步分析不同厌氧条件下沉积物剖面有效态P和Fe浓度变化明显区域(虚线界定区域)的相关性，发现均呈显著或极显著的正相关，即两者在沉积物剖面存在同步释放的规律(图5). 室内厌氧培养实验进一步证实了磷铁之间的耦合关系，即铁的还原导致磷的释放. Ding等[40]对太湖14个位点进行原位测定，均发现沉积物剖面有效态P和Fe的释放具有同步性，与本研究的结果一致，很可能说明磷铁同步释放普遍存在于长江中下游湖泊. Rydin等[42]对波罗的海海岸沉积物界进行P释放机理探索，发现潜在活性P的释放主要受Fe-P控制，即P的释放受Fe的氧化还原控制[41,43]. 可能的原因是Fe3+浓度的增加，会降低P的迁移性[44]. 另外，Fe2+浓度的减少，不仅仅受氧化还原环境的变化影响，也可能是硝酸或硫酸酶含量减少所致[45]，硝酸盐细菌呼吸作用的增强，能够增加沉积物中Fe-P络合物中P释放的速率[46]. 同时，硝酸盐细菌的分泌物也能够加速Fe3+的溶解，使吸附在Fe(OH)3中的P释放出来[47].

图4 好氧-厌氧过程沉积物剖面有效态P和Fe的浓度分布Fig.4 Distributions of labile P and Fe in the sediments during anaerobic-aerobic incubation

图5 好氧-厌氧过程沉积物有效态P和Fe的相关性Fig.5 Correlation analyses between labile P and Fe during anaerobic-aerobic incubation

3 结论

1) 不同湖泊沉积物理化性质差异较大，且磷铁含量在沉积物垂向分布具有较大的空间异质性，不同湖泊异质性也存在差异.

2) 沉积物中磷和铁存在耦合关系，磷、铁含量呈现同步增加或减少的趋势，4个湖泊沉积物均存在相同的变化规律.

3) 室内好氧-厌氧培养实验验证了铁磷耦合关系，即铁的还原可导致磷释放，沉积物磷的二次迁移主要受铁氧化还原过程的控制.

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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

国家水体污染控制与治理科技重大专项(2012ZX07103-005)资助. 2016-09-07收稿；2016-12-07收修改稿. 龚梦丹(1990～)，女，硕士研究生； E-mail: mengdan_rainbow@163.com.

； E-mail: smding@niglas.ac.cn.

DOI 10.18307/2017.0508