环境科学  2022, Vol. 43 Issue (12): 5560-5570   PDF    
引用本文
张宝豪, 武亚遵, 徐东昱, 高丽, 李艳艳, 王启文, 高博. 三峡水库典型消落带土壤DOM组分特征及其对有效态镉释放影响[J]. 环境科学, 2022, 43(12): 5560-5570.
ZHANG Bao-hao, WU Ya-zun, XU Dong-yu, GAO Li, LI Yan-yan, WANG Qi-wen, GAO Bo. Characteristics of Dissolved Organic Matters and Their Influence on Labile Cadmium Release from Soils of Typical Water Level Fluctuation Zones of Three Gorges Reservoir[J]. Environmental Science, 2022, 43(12): 5560-5570.
三峡水库典型消落带土壤DOM组分特征及其对有效态镉释放影响
张宝豪1,2, 武亚遵1, 徐东昱2, 高丽2, 李艳艳2, 王启文2, 高博2     
1. 河南理工大学资源环境学院, 焦作 454003;
2. 中国水利水电科学研究院流域水循环模拟与调控国家重点实验室, 北京 100038
收稿日期: 2022-03-04; 修订日期: 2022-04-10
基金项目: 国家自然科学基金项目（42007023）
作者简介: 张宝豪(1994~), 男, 硕士研究生, 主要研究方向为湖库水环境中重金属污染监测与评价, E-mail: bhzhang1223@163.com
通信作者: 徐东昱, E-mail: xudy@iwhr.com
摘要: 重金属镉（Cd）是三峡水库蓄水后消落带土壤中主要的重金属污染物之一.为研究三峡水库反季节调蓄条件下溶解性有机质（DOM）对有效态Cd在土壤固-液相间释放行为及其再补给过程的影响，以三峡水库香溪河消落带为研究对象，分别于2019年10月和2020年10月采集土壤样品，利用紫外-可见吸收光谱、三维荧光光谱技术和荧光区域积分法分析土壤DOM光谱特性，同时结合梯度扩散薄膜技术（DGT）和土壤扩散通量模型（DIFS）综合探究土壤有效态Cd及其释放动力学过程.此外，测定了消落带土壤基本理化特性以及土壤中Cd含量和赋存形态.结果表明，研究区土壤DOM以富里酸为主，腐殖质特征较弱，具有陆源为主和内源较弱的混合特征，与2019年相比，2020年土壤DOM中富里酸类物质占比更高；2019年和2020年土壤ω（Cd）平均值分别为（0.474±0.301）mg·kg-1和（0.249±0.058）mg·kg-1，土壤中Cd的赋存形态均以非残渣态为主；DGT测定结果显示2020年土壤中有效态Cd含量较2019年呈下降趋势，进一步通过DIFS模型拟合结果表明，土壤中Cd的吸附速率大于解吸速率，土壤对有效态Cd的再补给能力较弱，表明三峡消落带土壤中有效态Cd的潜在释放风险较低，低分子量DOM和DOM中富里酸类物质是抑制土壤中有效态Cd释放的重要因素.研究结果揭示了三峡水库水文情势变化下DOM对重金属Cd环境地球化学行为的影响，为三峡库区重金属污染防治提供科学理论依据.
关键词: 三峡水库(TGR)      消落带      溶解性有机质(DOM)      镉(Cd)      梯度扩散薄膜技术(DGT)      迁移释放     
Characteristics of Dissolved Organic Matters and Their Influence on Labile Cadmium Release from Soils of Typical Water Level Fluctuation Zones of Three Gorges Reservoir
ZHANG Bao-hao1,2 , WU Ya-zun1 , XU Dong-yu2 , GAO Li2 , LI Yan-yan2 , WANG Qi-wen2 , GAO Bo2     
1. School of Resources and Environment, Henan Polytechnic University, Jiaozuo 454003, China;
2. State Key Laboratory of Simulation and Regulation of Water Cycle in River Basin, China Institute of Water Resources and Hydropower Research, Beijing 100038, China
Abstract: Cadmium (Cd) is one of the main heavy metal pollutants in the water level fluctuation zone soils following the impoundment of the Three Gorges Reservoir (TGR). To investigate the influence of dissolved organic matter (DOM) on the release behavior and resupply process of labile-Cd in the soil solid and liquid interphase under the anti-seasonal regulation and storage mode of the TGR, soil samples were collected from the riparian soils along Xiangxi River in October 2019 and October 2020. The UV-visible absorption spectrum, three-dimensional fluorescence spectroscopy, and fluorescence regional integration (FRI) were conducted to analyze the spectral characteristics of DOM. The diffusive gradients in thin films technology (DGT) and DGT-induced fluxes in soils (DIFS) model were used to reveal the labile-Cd and its release kinetics. Further, the soil physical and chemical property, content, and occurrence form of Cd were also determined. The results showed that the DOM was dominated by fulvic acid, and the humification degree was low, the source of DOM was mainly input from the land, and the content of fulvic acid-like materials in the DOM was higher in 2020. The average contents of Cd in soils in 2019 and 2020 were (0.474±0.301) mg·kg-1 and (0.249±0.058) mg·kg-1, respectively, and the non-residual components of Cd were the dominant fraction in the soils. The DGT and DIFS model assay results showed that the content of labile-Cd in soils in 2020 was lower than that in 2019. The adsorption rate of Cd in soils was higher than the desorption rate, the soils had a weak ability to recharge labile-Cd, and the potential release risk of labile-Cd was low, mainly due to the potential inhibition of the release of labile-Cd in soils by low-molecular weight DOM and fulvic acid. These results revealed the influence of DOM on the environmental geochemical behavior of Cd under the change in hydrological situation of the TGR, providing a reference for the management of heavy metals in the TGR area.
Key words: Three Gorges Reservoir (TGR)      water level fluctuation zone      dissolved organic matter (DOM)      cadmium (Cd)      diffusive gradients in thin films (DGT)      migration and release     

三峡水库因采用“冬蓄夏泄”反季节调蓄模式, 形成了垂直距离30 m, 面积达350 km2的消落带[1].镉(Cd)作为三峡水库消落带土壤中的主要重金属污染物[2, 3], 其污染程度处于中度甚至重度水平[4, 5], 具有较高的潜在生态风险[6, 7].然而, 以往研究往往基于土壤中Cd的总量评价消落带土壤Cd污染程度和潜在风险, 无法客观反映土壤中重金属Cd的生物有效性[8, 9](即有效态), 同时研究采用的传统提取法在揭示Cd在土壤中的迁移释放过程方面也具有一定局限性[10].三峡水库特殊的反季节调蓄模式打破了库区消落带原有的水土环境[11], 导致土壤中重金属的环境行为发生改变[12].然而, 消落带水土环境和重金属Cd环境行为的变化是否会引起土壤中有效态Cd在土壤固-液两相之间释放行为的改变目前仍不清楚.梯度扩散薄膜技术(DGT)与土壤扩散通量模型(DIFS)作为揭示土壤重金属有效态及其释放过程的可靠分析技术手段[13, 14], 将二者结合使用, 可以综合探究三峡水库水位变化下土壤有效态Cd的变化和其在土壤固-液相间的释放动力学过程, 提升对三峡水库消落带土壤中Cd环境地球化学行为的认识.

溶解性有机质(DOM)是重金属重要的载体, 可以与重金属发生吸附、解吸和络合等一系列作用[15, 16].有研究表明DOM可以与土壤中Cd结合, 以Cd-DOM络合物形式存在[17], 促进Cd在土壤中的迁移[18].Huang等[19]通过模拟泄水实验证明土壤中DOM强化了Cd与土壤的结合, 抑制了Cd的解吸过程, 影响其在土壤中的迁移性.三峡水库消落带“干湿交替”使土壤中DOM组分和来源的分布受到影响[20, 21], 这势必会引起土壤中Cd的吸附和解吸行为发生变化[22], 而DOM的变化是否影响有效态Cd在土壤固-液相中的迁移释放过程目前尚不明晰.尽管已有研究讨论了DOM中的腐殖质组分与Cd结合后会降低有效态Cd的含量[23], 但目前仍不清楚三峡水库反季节调蓄模式下DOM差异性变化对土壤固-液相中有效态Cd迁移释放的影响.

本文以三峡水库香溪河消落带为研究对象, 分别于2019年10月和2020年10月采集土壤样品, 利用光谱手段并结合DGT技术和DIFS模型探讨DOM组分特征对土壤固-液两相间有效态Cd迁移释放过程的影响, 提升对重金属在三峡水库水环境中地球化学循环的理解, 以期为实现三峡库区重金属污染控制和水环境治理提供科学理论依据.

1 材料与方法 1.1 研究区概况与样品采集

研究区为香溪河上游峡口镇至下游河口处, 全长约15 km.分别于2019年10月和2020年10月, 对研究区上中下游165~175 m处土壤进行采样, 每块区域内均采用五点法取样并将样品混合均匀, 将收集好的土壤样品做好标记后带回实验室风干, 剔除土壤中植物根系和石块等, 研磨过2 mm尼龙筛网备用.采样点分布情况如图 1所示, 其中S1~S8为2019年采样点位, S9~S11为2020年采样点位.

图 1 研究区概况和采样点分布示意 Fig. 1 Overview of the study area and sample collection sites

1.2 样品理化性质测定

土壤pH采用美国Hach HQ40d多参数电化学分析仪测定, 水土比为5∶1.土壤阳离子交换量(CEC)采用氯化钡缓冲溶液法测定[24].土壤样品粒度采用日本Horiba LA-960A型激光粒度仪测定.土壤DOM提取方法采用水土振荡提取法[25], DOM浓度以溶解性有机碳(DOC)浓度表示, 采用德国vario TOC分析仪测定DOC浓度(mg·L-1)和土壤总有机碳(TOC)含量.DOM紫外可见吸收光谱采用北京TU-1810分光光度计测定, 扫描波长为200~800 nm, 三维荧光光谱采用日本Hitach F-4600荧光光谱仪测定, 激发波长为230~450 nm, 发射波长为250~620 nm.

土壤固相Cd采用硝酸-氢氟酸-双氧水法提取[26], 使用电感耦合等离子体质谱仪(ICP-MS, Agilent 7700x, 美国)测定, 通过分析空白样品和土壤标准物质(GSS-9)进行质量控制.土壤液相(孔隙水)中Cd采用离心法提取并使用ICP-MS测定.土壤Cd不同赋存形态采用改进的欧共体标准物质局(BCR)三步提取法提取[27], F1、F2、F3和F4依次为酸可提取态、可还原态、可氧化态和残渣态, 样品使用ICP-MS进行测定, BCR标准物质(BCR701)用于质量控制, F1、F2和F3的回收率分别为109.19%、91.51%和108.86%.实验过程中所有玻璃实验器皿均提前用20%硝酸溶液浸泡24 h, 实验用水为Milli-Q超纯水(电阻率18.2 MΩ·cm), 实验试剂均为分析纯.

1.3 土壤重金属有效态浓度的计算及动力学模型

采用标准圆扣式DGT装置测定土壤样品中Cd的有效态浓度, 其计算方法如式(1)和式(2)所示, 具体计算参数及操作步骤参考Gao等[28]的研究.

(1)
(2)

利用DIFS模型模拟Cd从土壤固相到液相的释放-再补给动力学过程, 计算方法如下[29]：

(3)
(4)
(5)

式中, R为重金属有效态浓度(cDGT)与液相中浓度(csol)的比值, 反映土壤从固相到液相再补给一种元素的能力, 值越大表明再补给能力越强[30]. Kdl为固液相分配系数(cm3·g-1); Cs为固相不稳定Cd含量; Pc为颗粒浓度; tc为Cd从土壤固相再补给到液相中的响应时间, 值越大说明响应时间越长; kf和kb分别为吸附速率常数和解吸速率常数(s-1), 其数值越高表明土壤吸附或解吸所需要的时间越短.

1.4 数据处理

采用Excel 2019和SPSS 26.0统计分析软件对原始数据进行相关统计分析, 相关图形采用ArcGIS 10.2和Origin Pro 2021进行绘制.

2 结果与讨论 2.1 消落带土壤基本理化性质及DOM组分特征

土壤样品基本理化性质如表 1所示.研究区土壤pH范围在6.72~7.95之间, 总体偏中性; 土壤中ω(TOC)范围在2.89~17.04 g·kg-1之间; 土壤粒度组成以砂粒(>20 μm)为主; 土壤阳离子交换量(CEC)介于8.87~13.40 cmol·kg-1之间.

表 1 研究区消落带土壤基本理化参数 Table 1 Soil physical and chemical properties in water-level fluctuation zone

土壤DOM特征参数如表 2所示. ρ(DOC)范围在18.2~27.4 mg·L-1之间, 不同位置与DOM浓度间无显著性差异(P>0.05).结合紫外-可见光谱参数与荧光参数分析可知, 所有采样点土壤DOM组分均以富里酸为主, 来源主要为陆源输入, 但夹杂微生物代谢等内源产生的输入; 土壤DOM的腐殖质特征较弱, 自生源特征较显著, 生物可利用性较高, 该结果与以往的研究结果类似[25, 31].各采样点土壤DOM的荧光光谱如图 2所示, 利用荧光区域积分法(FRI)对各采样点荧光光谱区域进行划分.荧光区域积分解析的5种组分的分布情况如图 3所示, 土壤DOM各荧光区域所占比例中区域Ⅲ所占比例最高, 说明土壤DOM以富里酸类物质为主, 与紫外-可见吸收光谱的分析结果保持一致.对比2019年和2020年土壤DOM中组分差异发现, 2020年土壤DOM具有更高比例的富里酸类物质.

表 2 研究区土壤DOM光谱参数 Table 2 Spectral parameters of DOM in soil at each sampling site

S1~S11分别为研究区土壤各采样点编号; 区域Ⅰ、Ⅱ、Ⅲ、Ⅳ和Ⅴ分别表示酪氨酸类物质、色氨酸类物质、富里酸类物质、微生物代谢产物和腐殖酸类物质 图 2 各采样点土壤DOM荧光光谱图 Fig. 2 DOM fluorescence spectra of soil at each sampling site

图 3 基于荧光区域积分法的土壤DOM组分分布 Fig. 3 Distribution of DOM components identified using FRI method in each soil sample

2.2 土壤Cd含量及其各赋存形态分布特征

对2019年和2020年香溪河消落带土壤固相中Cd含量(Ctotal-Cd)和液相中Cd浓度(csol-Cd)进行了调查, 结果如图 4所示. 2019年香溪河消落带土壤Ctotal-Cd平均值为(0.474±0.301)mg·kg-1, 2020年为(0.249±0.058)mg·kg-1, 均高于三峡库区土壤重金属背景值(0.134 mg·kg-1)[32], 香溪河沿岸工业废水及生活污水排放可能是造成土壤Ctotal-Cd值高的重要原因[33, 34]. 2019年和2020年土壤固相中Cd含量分布呈现相似的规律, 下游河口处Cd平均含量高于同年中上游处, 这可能与来自香溪河上游的泥沙运移沉积[35]和河口水动力条件的变化[36, 37]有关. csol-Cd范围介于0.12~2.40 μg·L-1之间, 其分布规律和Ctotal-Cd分布规律相似.土壤Cd不同赋存形态占比如图 5所示, 各采样点土壤中Cd的赋存形态均以非残渣态(F1+F2+F3)为主, 表明香溪河消落带土壤中Cd具有一定的迁移性和生物有效性[38, 39].

图 4 研究区土壤固相及液相中Cd分布情况 Fig. 4 Distribution of content of Cd in soil solid and liquid phases in the study area

图 5 研究区土壤Cd不同赋存形态占比 Fig. 5 Proportion of different fractions of Cd in the study area soil

2.3 土壤有效态Cd分布特征及释放动力学过程

研究区土壤有效态Cd浓度(cDGT-Cd)变化范围介于0.02~0.18 μg·L-1之间, 2019年平均值为(0.08±0.05)μg·L-1, 2020年cDGT-Cd平均值有所下降, 为(0.05±0.02)μg·L-1.与丹江口水库和嘉陵江[40, 41]土壤/沉积物中的有效态Cd浓度相比, 三峡库区消落带土壤中cDGT-Cd平均值较低.DIFS模型模拟结果如表 3所示, 三峡水库香溪河消落带土壤的R值均小于0.25, 在时间序列上表现出相似的变化趋势(图 6), 即R值迅速增加, 在15~30 min内达到最大值, 此后持续呈现下降趋势.该现象说明土壤Cd会从土壤固相解吸并释放到土壤液相中, 但不能使Cd稳定维持在初始状态, 并且随着时间推移Cd含量会越来越低.此外, 同一采样点吸附速率常数(kf)比解吸速率常数(kb)高2~3个数量级, 表明三峡水库消落带土壤中Cd从固相解吸的再补给速率远低于土壤液相中Cd的耗损速率[28].与2019年相比, 2020年土壤固液相分配系数(Kdl)更低, 说明该年份土壤固相中相对可补给有效态Cd的容量较低.对比分析各年份不同区域中土壤中有效态Cd的释放动力学参数可知, 研究区上游消落带土壤较中游和下游具有更大的再补给能力, 说明香溪河上游消落带土壤在自然环境变化和人类活动扰动下土壤中Cd具有相对较大的潜在释放风险.

表 3 DIFS模型模拟结果 Table 3 Simulation results derived from the DIFS model

图 6 研究区土壤R值随时间变化的最佳拟合曲线 Fig. 6 Best fitting lines of R values for the study area soil along with the variation time

通过对cDGT-Cd与Ctotal-Cd和Cd不同赋存形态进行线性回归分析发现, 土壤cDGT-Cd与Ctotal-Cd具有显著线性正相关关系(P＜0.01), 但二者分布规律并不完全相同(图 7).点位S6处Ctotal-Cd值最高, 但其cDGT-Cd值却不是最高.此外, cDGT-Cd与非残渣态CF1-Cd、CF2-Cd和CF3-Cd之间也具有显著相关性(P＜0.01), 该结果说明土壤中一些物质如有机质、铁/锰氧化物和硫化物等会吸附Cd或与之结合[38, 42], 形成不稳定结合物, 影响Cd在土壤固-液相间的释放.在环境变化和人类活动影响下, 又会再次将部分Cd从土壤固相中释放到液相中, 使cDGT-Cd值增加, 导致消落带土壤环境的二次污染.

(a)cDGT-Cd与Ctotal-Cd和CF1-Cd之间的关系, (b)cDGT-Cd与CF2-Cd和CF3-Cd之间的关系 图 7 研究区土壤cDGT-Cd与Ctotal-Cd和Cd不同赋存形态之间的关系 Fig. 7 Relationship among cDGT-Cd, Ctotal-Cd, and the different fractions of Cd

总体而言, 香溪河消落带土壤对Cd的再补给能力较弱, 土壤中有机质、铁/锰氧化物或硫化物等会与Cd发生吸附、络合等一系列反应, 影响有效态Cd的浓度分布及释放行为.此外, 各项结果表明三峡水库消落带土壤固相释放-再补给Cd的能力较低, 以往基于土壤Cd总量[4, 7]或赋存形态[43]的评价方法可能会高估三峡水库消落带土壤Cd的潜在生态风险.

2.4 土壤DOM对有效态Cd释放-再补给过程的影响

土壤DOM中不同组分对Cd在土壤中吸附解吸行为影响具有差异性[44, 45].主成分分析(PCA)显示DOM中类蛋白物质和富里酸类物质是影响有效态Cd释放的重要参数指标(表 4), 进一步通过Pearson相关性分析发现, cDGT-Cd值和DIFS模型模拟计算的Rdiff值与土壤DOM中富里酸类物质之间存在一定程度的负相关关系, 与类蛋白物质之间具有正相关关系, 该结果表明DOM中富里酸类物质对有效态Cd在固-液相中的释放存在潜在抑制作用.有研究表明, 富里酸类物质和类蛋白物质中的羧基和酚羟基类含氧基团对金属结合能力会产生影响[46]. Zhang等[47]也同样指出, 富里酸类物质与Cd的相互作用比类蛋白更稳定.土壤中Cd与富里酸类物质中主要活性基团(如羧基和酚基等)结合的稳定性比类蛋白物质中阳离子基团的结合稳定性更强[48], 这也限制了Cd从土壤固相中的解吸释放行为. 2020年三峡水库消落带土壤DOM具有较高比例的富里酸类物质(图 8), 这表示三峡水库消落带土壤经一年水位涨落调度后, 土壤DOM中富里酸类物质占比有所增加, 而类蛋白物质占比相对下降; 在富里酸类物质大量存在的情况下, 土壤中的Cd更容易被吸附而非解吸, 抑制了有效态Cd从土壤固相到液相间的迁移释放行为, 从而导致库区土壤有效态Cd浓度下降.

表 4 有效态Cd释放的主成分分析结果 Table 4 Principal component analysis results of labile-Cd release

图 8 不同年份土壤DOM组分占比和土壤有效态Cd浓度变化 Fig. 8 Changes in DOM component proportion and cDGT-Cd of the riparian soil in different years

光谱斜率比值SR被用来指示DOM相对分子质量, SR越大表明DOM相对分子质量越小[49, 50].Pearson相关性分析显示SR与Rdiff值之间具有负相关性(r=-0.733, P＜0.05), 并与土壤中Cd的总量及赋存形态具有显著负相关关系(P＜0.01), 和cDGT-Cd之间存在负相关趋势, 推测DOM分子量大小会影响有效态Cd的释放行为.有研究表明, DOM与Cd的结合能力会随其分子量降低而呈现增加趋势[51, 52].2020年土壤DOM的SR值略大于2019年, 表明2020年香溪河消落带土壤DOM相对分子质量略小于2019年, DOM与Cd的结合较为稳固, 土壤在受到环境扰动如pH或含水率变化后难以再次释放有效态Cd, 导致2020年土壤中有效态Cd浓度低于2019年; 此外, 低分子量DOM与Cd结合也会导致土壤固相中不稳定Cd的含量减少, 在一定程度上限制了有效态Cd来自土壤固相中的再补给, 抑制有效态Cd的释放, 进而影响有效态Cd的浓度大小.因此, DOM分子量是影响土壤中有效态Cd释放的重要因素, 且低分子量DOM对该过程具有潜在抑制作用.

3 结论

(1) 三峡水库香溪河消落带土壤DOM主要为富里酸, 来源以陆源输入为主, 并夹杂微生物代谢等内源输入.土壤DOM腐殖质特征较弱, 自生源特征较显著, 生物活性较强.2020年消落带土壤DOM较2019年具有更高的富里酸类物质和更低的类蛋白物质占比.

(2) 2019年和2020年香溪河消落带土壤Cd含量均高于三峡库区土壤重金属背景值, 土壤液相中Cd浓度与土壤Cd总量分布规律相似, 土壤中Cd主要赋存形态为非残渣态(F1+F2+F3).

(3) 2020年土壤有效态Cd浓度低于2019年, 土壤固相中Cd的吸附速率大于解吸速率, 土壤对有效态Cd从固相至液相的再补给能力较弱, 表明香溪河消落带土壤有效态Cd的潜在释放风险较低.

(4) 香溪河消落带土壤DOM中的富里酸类物质和土壤中低分子量DOM均潜在地抑制了有效态Cd在土壤固-液相中的释放-再补给动力学过程.

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