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土壤重金属污染是全球日趋严重的环境问题,污染所引起的土壤理化性质、功能和生态结构变化,严重危及着环境质量、人类健康和国家粮食安全[1-2]。厘清介质中污染迁移过程及机制是实现污染精准控制、修复技术优化及修复全过程效评所需解决前提,也是推进污染修复技术朝可持续化发展,实现“双碳”目标的关键[3]。
分离技术可通过促进重金属离子的定向迁移,实现污染物的迁出及净化,是目前重金属污染土壤研究的主要趋势[4-6]。电场和温度场是主要作用方式,栾常慧[7]利用人工电场探究高岭土和农田土壤中重金属的迁移,发现Cu的平均去除率可达50.98%;李亚林等[8]对复合污染土壤施加直流电压,发现当施加电压在10 V时,As的迁移率为85%,修后土中As的含量低于第一类建设用地的筛选值;杨宁芳和牛富俊[9]研究了重金属元素在冻土和融土中迁移情况,发现温度越高越有利于重金属迁移,其迁移性与污染物类型有关;郭文凯[10]开展温度作用下非饱和高岭土中水分及重金属迁移实验,发现温度驱动下土柱中液态水分迁移能够有效促进重金属运移扩散,且与重金属的对流作用密切相关。
透明土是由与土体性质相似的透明固体及孔隙流体 (与透明固体折射率相匹配) 所组成的复合物,利用透明土可以模拟天然土体并进行土体内部的可视化观测[11-12]。目前,透明固体材料有无定型硅粉、无定型硅胶和熔融石英砂,主要化学成分为二氧化硅,可模拟天然黏土和砂土2大类[13-14]。孔纲强等[15]利用熔融石英砂和有机混合油配制透明土,测定分析其基本力学性质与福建标准砂相近,并具有较好的光学透明性;ISKANDER等[16]利用无定型硅粉与溴化钙溶液合成透明土,其岩土工程性质与天然黏土一致。透明土用于地基变形和承载力等力学性质研究的可行性已得到证实。SONG等[17]将透明土应用于锚板嵌入黏土的锚固力损失分析;NI和HIRD[18]、王秀华等[19]和陈亚东等[20]利用透明土进行成桩过程中桩与周围土体相互作用研究。同时,透明土可用于研究多孔介质中孔隙液的流动过程。梁越等[21]利用人工配置的透明多孔介质来测定其内部孔隙液体的流动特性;王凯剑[22]和贾宇鹏等[23]对透明多孔介质进行了内部单向流动可视化研究;SEN等[24]研究了微观尺度下多孔介质内的过渡流和湍流的分布规律;马戎荣等[25]借助透明土的可视性,在自由水中滴加红墨水,直观得出毛细水上升高度。大多数重金属污染物具有显色效应,利用透明土理念对重金属污染物在多孔介质中的迁移扩散过程进行可视化过程监测具有理论可行度。
以电场和温度场作用为驱动力,以Cu(Ⅱ)、Cr(Ⅵ)污染为研究对象,以重金属的质量分数及波动率为指标,论证透明土在研究污染物界面迁移实施的可靠度,并借助污染质量分数的多点位重组,构建电场与温度场作用下污染土体中重金属污染物的迁移机制,以期为土壤污染预测及修复提供参考。
基于可视角度下重金属污染物在介质中的迁移规律
Migration rule of heavy metal contamination in media based on visual angle
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摘要: 介质中污染物迁移的可视化研究有助于厘清污染物及介质间微观作用机制。依托透明土的可视化理念,以Cu(Ⅱ)、Cr(Ⅵ)污染土为研究对象,分析电场及温度场驱动作用下重金属污染物在透明土及实际土体中的迁移分布规律,探讨可视化研究的可靠性并建立重金属污染物的界面迁移机制。结果表明,透明土可用于研究多孔介质中重金属污染物的迁移扩散性,电场及温度场作用下同点位污染物质量分数的波动率均在10%以内;电场作用下污染物的迁移由离子赋存状态所带电荷决定,Cu(Ⅱ)、Cr(Ⅵ)分别以阳离子、酸根阴离子状态存在,迁移方向相反。Cu(Ⅱ)因铜胶体絮凝物生成易产生聚集效应,迁移受阻;温度场作用下污染物的迁移取决于温度梯度,因土颗粒高低温处结合水厚度的差异及水分子间的吸力会促使孔隙流体迁移以达平衡,重金属污染物在对流和分子扩散的协调作用下,均表现为向低温处迁移趋势。在同外力驱动作用下,重金属污染物间的迁移存在差异性,Cr(Ⅵ)的迁移性均强于Cu(Ⅱ)的,化学沉淀物质的产生及重金属阳离子静电吸附作用是影响迁移效率的内在关键。本研究结果可为土壤污染预测及修复研究提供参考。Abstract: The visualization of contaminant migration in media can help clarify the microscopic mechanism of contaminants and media. Based on the visualization concept of transparent soil, Cu(II) and Cr(VI) contaminated soil was used as the research object to analyze the migration law of heavy metal contamination in the transparent soil and the actual soil under the driving force from electric field and temperature field. The reliability of visualization study and the interface migration mechanism of heavy metal contamination were investigated. The results showed that the transparent soil could be used for the study of migration and dispersion of heavy metal contamination in porous media, and the fluctuation rate of contamination mass fraction at the same point were all within 10% under the electric field and temperature field. The migration of contamination under the action of electric field was determined by the charge of ion endowment state, and the migration direction of Cu(II) and Cr(VI) was opposite as they exist in the state of cation and acid anion respectively. Cu(II) was easy to be flocculated by copper colloid, and then its migration was blocked; the migration of contamination under the effect of temperature field depended on the setting of temperature gradient, and pore fluids migrated to reach equilibrium under the effect of thickness difference of bound water at high and low temperatures of soil particles and the suction between water molecules. Heavy metal contamination all migrated to low temperatures area under the coordinated effect of convection and molecular diffusion. Under the action of external driving force, there was variability in migration among heavy metal contamination, and the migration of Cr(VI) was stronger than that of Cu(II). The generation of chemical precipitation substances and the difference of electrostatic adsorption were the intrinsic key to the migration efficiency. The results can provide a reference for soil contamination prediction and remediation research.
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Key words:
- heavy metal contaminated soil /
- contaminant migration /
- visibility /
- electric field /
- temperature field
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