污灌区土壤盐渍化对汞的转化及释放特征的影响

Soil salinization commonly occurs in wastewater-irrigated areas. With the growing problems of mercury pollution and soil salinization in wastewater-irrigated fields, the influence of soil salinization on the transformation and release of mercury is a matter of serious concern. However, relevant researches have been rarely reported. This study is designed to pinpoint the impact of salinization on the immobilization, transformation and release of mercury in wastewater-irrigated areas. Batch and kinetic experiments, sequential extraction procedure for fractionation, technique of multilabeling with stable isotopically enriched tracers and optimized flux chamber method will be performed to investigate adsorption/desorption, chemical speciation, methylation/demethylation and release characteristics of mercury in salinized soils from wastewater-irrigated area of Tianjin city. This study should offer a significant insight into the environmental behavior of mercury in salinized soils and hence provide a theoretical basis for its pollution control and ecological risk assessment. It would be helpful to appropriate management of soil environmental quality and contribute to the guidance of agriculture production in wastewater-irrigated farmlands.

土壤盐渍化是污灌区的重要特征。随着我国污灌区内土壤汞污染与盐渍化趋势日益严重，土壤盐渍化是否会影响累积在土壤中汞的转化及释放特征，从而加剧汞的污染危害及防治难度是一个迫切需要解决的问题，但是目前仍然缺乏相关的研究。本项目以天津污灌区盐渍化汞污染土壤为研究对象，以精确的测定手段为基础，通过吸附/解吸等温批处理试验和动力学试验、培养试验及连续提取法、多重标记浓缩稳定同位素示踪技术和优化设计的动态通量箱法，揭示污灌区土壤汞在盐渍化环境中的吸附/解吸特征、赋存形态及转变趋势、甲基/去甲基化及向水体和大气的释放特征。研究旨在阐明盐渍化对土壤汞的固持、转变及释放的影响，为污灌区土壤重金属汞的污染控制和生态风险评价提供理论依据，对于科学管理污灌区土壤环境质量，指导污灌区农业生产和盐碱地的开发利用有重要意义。

本项目通过吸附/解吸等温批处理试验和动力学试验、培养试验及连续提取法、多重标记浓缩稳定同位素示踪技术和动态通量箱法，研究了盐渍化环境中污灌区土壤汞的吸附/解吸特征、汞的有效性问题、加入有机肥后汞的形态变化情况、盐渍化生态系统（土壤、水稻、叶菜）的总汞、甲基汞、气态汞的污染特征。结果表明：（1）高浓度的NaCl环境极其不利于Hg(II)在污灌区土壤中的吸附及固持，用含NaCl较高的污水灌溉作物很有可能会引起汞的二次污染，土壤的盐渍化趋势会使汞污染和释放趋势更趋严重。（2）外源稳定同位素202Hg加入土壤后，悬浮液中同位素交换均在24 h后达到稳定状态。同位素交换动力学方程对悬浮液中同位素比值的变化及土壤同位素交换态含量Et值有比较理想的拟合效果，与化学方法相比较，用土壤同位素可利用态Hg含量（Ea）作为土壤中有效态Hg含量的表征可能偏高。（3）在连续提取法中结合同位素202Hg稀释技术，通过同位素比值RHg（202Hg/200Hg）的变化对Hg进行形态分析，相比传统的对Hg形态直接进行含量测定，可以更精确直观地反映出Hg形态分布的变化。不同来源的有机肥对Hg污染潮土中Hg的形态分布有显著影响，在Hg污染农田土壤上施用有机肥料需要格外谨慎。（4）在调查的29个污灌区内稻田中，土壤总汞含量为(367.04 ± 129.36) μg/kg，显著高于区域土壤Hg背景值73 μg/kg，甲基汞含量为(0.87 ± 0.77) μg/kg；稻米总汞含量为(12.80 ± 5.14) μg/kg，甲基汞含量为(2.09 ± 1.20) μg/kg，甲基化率均值超过10%。污灌区内稻米总汞及甲基汞暴露对居民健康风险总体仍在安全阈值内。（5）经过长期污灌，稻田和菜地大气环境已受到较为严重的汞污染，气态总汞均值分别为71.3 ng·m-3和39.2 ng·m-3，远高于北半球大气总汞含量的背景水平（1.5-2.0 ng·m-3），对照草地总汞含量平均为9.4 ng·m-3。叶菜汞含量与污灌区气态汞含量对数之间呈现极显著的线性关系，菠菜和苋菜Hg含量的中位值及平均值均超出食品卫生限量标准，其中菠菜的中位值及均值在20 μg·kg-1以上。污灌区内叶菜Hg来源主要通过叶片吸收气态汞进入植株体内，而非颗粒态汞。