固化/稳定化修复后场地中铬的环境行为和归趋:以我国中部某修复后场地为例

为了探讨铬(Cr)在固化/稳定化修复后场地中的环境行为与归趋，通过浸出环境评价框架(LEAF)浸出评价、形态分级以及微观表征，对我国中部某修复后场地土壤中Cr的含量及形态进行3轮次跟踪监测. 结果表明:①跟踪监测试验发现，该修复后场地土壤中总Cr含量下降了18.92%. 在芥菜植株中Cr分布存在差异，其中，芥菜叶中Cr含量高达1.62 mg/kg (以干质量计)，芥菜茎中Cr含量为0.69~0.77 mg/kg (以干质量计). 芥菜植株吸收Cr的能力为0.44~3.10 mg/m2，对土壤中总Cr减量化的贡献率最高仅为0.05%；另外，新鲜芥菜植株中Cr含量为0.12~0.69 mg/kg，超过GB 2762—2017《食品安全国家标准 食品中污染物限量》中规定限值(0.5 mg/kg)，一旦食用该场地中的芥菜会对人体健康产生影响，增加了Cr在修复后场地中风险暴露的途径. ②土壤微观表征试验表明，修复后场地土壤中Cr主要以Cr₂O₃形式存在，土壤中的Mn_xO_y能起到氧化Cr的作用. 在LEAF试验中，固化/稳定化的Cr被活化溶出；而BCR分析发现，Cr的可还原态占比随修复后时间的延长逐渐增加，由1.13%增至4.02%. ③土壤中Mn、Fe及植物等环境因子均会参与Cr的氧化还原、沉淀与溶解，以及植物吸收与富集等环境行为，并导致已被固化/稳定化的Cr溶解或再氧化成Cr(Ⅵ)而溶出，从而增加Cr的迁移性和毒性，造成环境风险，其中Mn(Ⅳ)是Cr活化的关键因子. 研究显示，修复后场地中Cr的环境行为会受到多种环境因素共同影响，尤其是在Mn含量较高的区域更易形成氧化氛围导致Cr的重新活化. 

Environmental Behavior and Fate of Chromium in Solidification/Stabilization Post-Remediation Sites: A Case Study of a Remediation Site in Central China

In this study, the environmental behavior and fate of Cr in post-remediation sites in central China were studied. Three rounds of follow-up monitoring of Cr contents and forms in the soils and plants were carried out. The leaching characteristics and form transformation of Cr were explored using LEAF, BCR procedures, and micro-characterization. The results show that: (1) The total amount of Cr in the soils decreased significantly, with an average decrease of 18.92%. The content of Cr (in dry weight) in Brassica juncea leaves was as high as 1.62 mg/kg, Cr (in dry weight) in stems was 0.69-0.77 mg/kg, and Cr content in fresh Brassica juncea was 0.12-0.69 mg/kg, which exceeds the National Standard for Food Safety-Limits of Contaminants in Foods (GB 2762—2017) standard limit (0.5 mg/kg). Although the contribution of Cr removal via plant harvest was less than 0.05%, the potential health implication should be addressed in the post-remediation phase. (2) The key mechanism of Cr solidification/stabilization was the conversion of highly toxic and mobile Cr(Ⅵ) to less toxic and mobile Cr(Ⅲ). However, the co-existing Mn_xO_y could re-oxidize Cr, leading to high mobility of Cr as shown in the LEAF data. This finding was further supported by BCR analysis, which showed that the proportion of reducible Cr increased gradually from 1.13% to 4.02%. (3) Complicated environmental behaviors of Cr, such as oxidation/reduction and dissolution/precipitation in soils, as well as the migration and transformation associated with plants, were involved in the post-remediation site. The main influencing factors included pH, Mn, Fe and plants, while the Mn(Ⅳ) in soils played a crucial role, which leads to the dissolution/reoxidation of the solidified/stabilized Cr to Cr(Ⅵ) and the dissolution, thus increasing the mobility and toxicity of Cr and posing environmental risks. This study reveals that the environmental behavior of Cr in the post-remediation sites is affected by a variety of environmental factors, especially in the area with high Mn content, which is prone to the formation of oxidation atmosphere, leading to the reactivation of Cr.