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目前,全国有近1/6的耕地土壤受到重金属污染,据《全国土壤污染状况调查公报》[1]显示,全国土壤污染总超标率为16.1%,其中耕地污染超标率高达19.4%,并以无机重金属污染为主。土壤重金属具有毒性,可通过食物链在人体内富集,严重威胁人群健康。重金属的动力学行为控制着其在土壤中的形态和反应活性,进而影响重金属的生物有效性。土壤重金属污染控制是目前我国环境污染治理的关键领域[2-3]。土壤溶解性有机质(DOM)由大量的小分子通过氢键和疏水作用结合在一起,主要包括脂肪、蛋白质/氨基糖、碳水化合物、酚类、多酚类和稠环芳烃等,在土壤众多的生物地球化学过程中起到重要的作用[4]。例如,土壤DOM与土壤中的碳循环密切相关,同时也是控制土壤中重金属环境行为的关键组分[5]。然而,土壤DOM的组成和特性受到环境条件控制,如气温和降雨[6]。研究环境条件变化对土壤DOM特性的影响对于预测土壤中的碳循环和污染物的环境行为十分关键。
干湿交替是重金属污染土壤修复过程中常见的环境条件变化,也是影响修复效果的重要环境因素。IPCC(Intergovernmental Panel on Climate Change)第五次评估报告(AR5)显示[7-9],在全球气候变化的背景下,干旱和极端降水等天气现象频发,直接影响降雨、蒸发、径流、土壤水等的时空分布,导致土壤出现频繁的干湿交替现象。土壤干湿交替会影响土壤中DOM的组成和特性,例如土壤干旱会增加土壤溶液中木质素、单宁酸和稠环芳烃相对含量,而土壤湿润会增加土壤溶液中脂肪、蛋白质及碳水化合物的相对含量[10];微生物起源的DOM分子的含量随着干旱度的增加而减少,而植物起源的酚类、多酚类和稠环芳烃分子的含量随着干旱度的增加而增加[11]。同时,有研究表明,周期性的干湿交替对土壤中Cd的释放有明显的抑制作用[12],土壤水分变化也影响重金属的有效性,其中土壤干燥可显著降低重金属Cu、Ni的有效性[13]。因此,土壤干湿交替是影响DOM和重金属环境行为的关键环境过程。然而,目前的研究主要集中于干湿交替对未污染土壤中DOM特性及土壤中重金属浓度和生物有效性的影响,对干湿交替如何影响重金属污染土壤中DOM的特性以及重金属的释放动力学过程还缺乏深入的研究。
本研究以广东省韶关市上坝村典型重金属污染农田土壤为研究对象,利用土壤培养实验、光谱学方法以及流动搅拌动力学实验来探究土壤干湿交替对重金属污染的农田土壤中DOM特性及Cu和Zn释放动力学行为的影响。本研究设置3个不同含水率的对照组和3个不同干湿交替条件的实验组进行土壤培养实验,利用紫外可见吸收光谱(UV-vis)和三维荧光光谱(3D-EEM)表征干湿交替培养过程中土壤DOM的芳香性和荧光组分的动态变化,同时利用流动搅拌反应器(Stirred-flow Reactor)分析不同培养条件下的土壤中Cu和Zn的释放动力学特性。本研究的结果将有助于理解干湿交替环境条件下土壤DOM的动态变化以及预测重金属在污染土壤中的环境行为,对重金属污染土壤修复工作的开展具有参考价值。
干湿交替对土壤DOM特性及重金属释放的影响
Effects of dry-wet cycles on the properties of soil DOM and the release of heavy metals
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摘要: 以典型Cu和Zn污染农田土壤为研究对象,利用土壤培养实验、光谱学方法以及流动搅拌实验,以阐明土壤干湿交替对重金属污染土壤中土壤溶解性有机质(DOM)特性及重金属释放动力学行为的影响。结果表明,土壤pH、DOM含量及其芳香性随着土壤含水率升高而增大,而干湿交替主要影响土壤中DOM的含量,对pH和SUVA254值的影响很小。重金属释放动力学研究表明,土壤含水率越高越有利于Cu和Zn的稳定,干旱过程则会促进其释放,但仅在初次湿润到干旱过程中对其释放特性产生影响,后期干湿交替循环影响很小,且不同干湿交替频率和强度具有类似影响。在同等条件下,Zn的释放比Cu更快,受干湿交替影响更小。本研究结果可为重金属污染土壤修复工作提供参考。Abstract: This study aims to elucidate the effects of dry-wet cycles on the properties of soil dissolved organic matter (DOM) and the release of heavy metals from a typical Cu- and Zn-contaminated agricultural soil, by employing the soil incubation experiments, optical analysis, and stirred-flow experiments. Results indicated that soil pH, DOM concentrations and DOM aromaticity increased with the increase of soil moisture contents. Soil dry-wet cycles had significant influence on DOM concentrations but had little impact on pH and SUVA254 values. Kinetic experiments of heavy metal release revealed that the release of Cu and Zn from the soil was more reluctant under higher soil moisture contents, while soil drying process promoted the release of Cu and Zn from the soil at the first wet-dry cycle. Further precipitation and drought would not affect the release of Cu and Zn, and different frequency and strength of wet-dry cycles had similar effects on the release of Cu and Zn. Moreover, under the same conditions, the release rate of Zn from soil was higher than that of Cu, and the effect of wet-dry cycles on the release of Zn was less compared to the release of Cu. The results of this study will contribute to providing basis for the effective remediation of heavy metal-contaminated soil.
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表 1 土壤培养含水率及取样点
Table 1. Soil water content and sampling points during soil incubation experiments
实验组别 含水率 干湿交替次数 土壤取样时间点(天数-含水率) 对照组1 75% 0 1、3、5、7、10、13、16、19、22、25、28、31 d-75% 对照组2 45% 0 1、3、5、7、10、13、16、19、22、25、28、31 d-45% 对照组3 30% 0 1、3、5、7、10、13、16、19、22、25、28、31 d-30% 实验组1 45%~15% 3 7 d-45%、10 d-30%、13 d-15%、16 d-45%、19 d-30%、22 d-15%、25 d-45%、28 d-30%、31 d-15% 实验组2 45%~15% 2 7 d-45%、10 d-30%、13 d-15%、20 d-45%、23 d-30%、26 d-15% 实验组3 45%~0 2 7 d-45%、10 d-30%、13 d-15%、16 d-0、23 d-45%、26 d-30%、29 d-15%、32 d-0 -
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