城镇周边土壤中燃煤成因磁性微球粒的环境特性研究

Environmental characteristics of magnetic micro-particles derived from coal combustion in soil of peripheral areas of cities

通过对我国13个中心城市及其周边地区重金属污染土壤中磁性微球粒的形貌结构和矿物成分的研究,确定其具有明显的燃煤成因特征和示踪土壤重金属污染物来源的功能。化学成分分析结果表明,8种主要重金属元素在磁性微球粒中的含量普遍都高于土壤生态地球化学基准值,可判断磁性微球粒也是污染土壤中重金属元素的主要载体之一。磁性微球粒对土壤重金属累积的贡献率大多不超过1%,但在微球粒含量高的情况下可达10%,甚至60%。由于以磁性微球粒形式存在的重金属元素相对比较稳定,因此在进行土壤地球化学生态效应评价时有必要考虑其环境意义。

Magnetic micro-particles in soil polluted by heavy metals around 13 cities were collected for the purpose of determining their environmental significance.They can be used as tracers for the source of heavy metals in soil because the microstructure and mineral component characteristics show evidently their derivation from coal combustion.The magnetic susceptibility of falling dusts containing abundant magnetic micro-particles is much higher than the average value of urban soil,which explains clearly the reason why soil polluted by heavy metals always has high magnetic susceptibility.Finer particle sizes of magnetic micro-particles have higher magnetic susceptibilities,which facilitates the diffusion of heavy metal pollution for a long distance.The occurrence rates of the magnetic micro-particles in soil collected from 13 investigated cities all exceed 80%,which indicates the universal distribution of magnetic micro-particles in soil of urban and surrounding areas.The average content of the magnetic micro-particles in 5 kg soil sample collected from Beijing varies in the range of 30～7 070 mg with the highest being 267 g.Magnetic micro-particles can be regarded as carriers of heavy metals in soil because all the eight heavy metals could be detected in chemical component analysis and the content is 2～25 times higher than the soil eco-geochemical reference value of China＇s eastern plain region.Results of correlation analysis show that in most cases there exists a significant correlation between content of heavy metals in magnetic micro-particles and that in soil polluted by coal combustion.The contribution rate of magnetic micro-particles to heavy metal accumulation in soil is generally lower than 1% though it could reach 10% or 60% in soil with much more magnetic micro-particles.It must be emphasized that the contribution rate mentioned above only shows the trend of the environmental effect of coal combustion because magnetic micro-particles do not constitute all the pollution components produced in the process of coal combustion and the content of magnetic particles can not be quantified due to the unavoidable loss in the process of elutriation.Nevertheless,the above phenomena could at least prove that magnetic micro-particles can be used as an easy means for connecting media that contact heavy metal pollution in soil of cities with atmosphere pollution produced by coal combustion.As heavy metals in magnetic micro-particles are commonly very inactive,the evaluation criterion range of heavy metal content in eco-geochemical assessment can be properly broadened according to the content of magnetic micro-particles in areas around coal combustion factories.At the same time,it should be taken into account that heavy metals in magnetic micro-particles can also be released in some extreme conditions such as low pH or high salt content.Forecast should be conducted so as to prevent the environmental disaster produced by activated heavy metals in magnetic micro-particles.