典型有色金属冶炼区重金属污染的Cd同位素示踪研究——以白银市为例

Research about heavy metal pollution in suburb soil is one of pollution research hotspots.Baiyin city,Gansu Province,gives priority to mining,selection and smelting of non-ferrous metal.Suburb soil of Baiyin city has been polluted by Pb,Cu and Zn, with Cd the most significant，because of 40-year irrigation of suburb farmland by mixture of Yellow River water,municipal sewage and industrial wastewater discharged from North-West copper smeltry,zinc smeltry subordinate to Baiyin Corporation,and because of acid gas discharged from these enterprises and ash from municipal coal fire.Here we propose a novel dual isotopic tracing method of Cd and Pb isotopes to trace sources of heavy metals contamination. Fractionation of Cd isotope at high smelting temperature makes it a promising tracer for industrial contamination, while Pb isotope has been known as a powerful tool for tracing Pb sources for its "fingerprint" effect. Thus, this dual isotopic tracing method can hopefully unravel the sources and processes of local heavy metals contaminations. In brief, a comprehensive investigation on distribution characteristics of heavy metals in aerosols, waste water, farmland soils, and roots, stems, leaves and seeds of crops is planned for this study. Then, Cd isotope would be used to resolve transport, transform, and distribution of Cd within the atmosphere-soil-plant system. Moreover, the dual isotopic tracing system of Cd and Pb isotopes would be applied for tracing sources of heavy metals within the local environments. Therefore, this study can not only provide a basic dataset for understanding Cd evolution in surficial environments, but also set up a novel method for tracing soil contaminations, being valuable and meaningful for both scientific research and social development.

城郊土壤是当前土壤重金属污染研究的热点之一。甘肃省白银市是以有色金属采选冶为主的工业城市，城郊农田土壤用黄河水、生活污水和白银公司下属企业西北铜冶炼厂、铅锌冶炼厂等排放的工业污水混灌已近40年，加上上述企业排放的酸性气体及市政燃煤产生的粉尘，使白银市郊土壤中Cd、Pb、Cu和Zn等重金属污染严重，其中Cd污染最为严重。冶炼厂的高温生产会引起Cd同位素的分馏，使得用Cd同位素来示踪工业镉污染源成为可能；作为含铅物质的"指纹"，铅同位素可指示环境样品中铅的来源。本项目拟调查白银市区气溶胶、冶炼烟气、冶炼污水、农田土壤、植物根茎叶和籽实中重金属元素分布特征，用Cd同位素示踪法研究大气、土壤-作物体系中Cd的迁移转化及分配规律，用Cd和Pb同位素法追溯环境中重金属的来源。该项目的实施，为系统研究Cd在表生环境中的迁移转化过程提供基础数据支持，可丰富环境污染的示踪体系，具有重要的科学意义和现实价值。

以“白银厂”而闻名的甘肃省白银市是以Cu、Pb和Zn等有色金属采选冶金为主的工业城市。在长期工矿业排污及基岩区岩矿石风化等表生地球化学作用下，重金属元素等各类污染性物质大量进入市区周围的农田生态系统中。我们在白银市区采集了特征性土壤样品、成熟期玉米整棵植株样品、污水样品、白银公司铅锌冶炼厂、三冶炼厂和铜冶炼厂的飞灰、炉渣以及市区冬季燃煤供暖锅炉厂飞灰样品等，其中针对玉米植株分根、茎、叶、籽实分别测试其中的重金属元素含量。分析发现，冶炼厂周围土壤和植物中的重金属含量明显高于其他样品，比如元素Cd，在冶炼厂周围表层土壤中含量为75.8-109ppm之间，而高家台玉米地表层土壤含量为0.40-8.90ppm之间，可见冶炼厂对周围环境中金属元素的含量贡献不容忽视。非根系土样品的重金属含量高于根系土的。对玉米植株不同部位进行重金属元素含量分析，玉米植株根、茎、叶、籽实中，根部吸收的重金属要明显高于其余3个部位, 根据食品中污染物限量标准(GB 2672-2005)，玉米籽实中的重金属含量在安全范围内。通过Cd同位素分析，获得三冶炼厂铅锌渣中的δ114/110Cd为-4.9‰，三冶炼厂铅锌飞灰的δ114/110Cd为-1.14‰，燃煤锅炉飞灰中的δ114/110Cd为3.89‰。