太湖沉积物中重金属的垂向分布特征及迁移转化

为了解太湖沉积物中重金属污染的变化特征和迁移规律,选取太湖柱状沉积物为研究对象,测定Cd、Pb、Cu、Cr 4种重金属的总量及赋存形态,分析重金属在垂向上的分布状况及污染程度。结果表明,金属Cd、Pb、Cu、Cr的含量随着深度的增加而降低,并在1950年和1980年出现拐点,其中1980年污染最为严重;湖区分布特征表现为西北部梅梁湾、竺山湾和贡湖湾的重金属污染最为严重,湖心次之;重金属的赋存形态均以残渣态为主,Cd、Pb、Cu、Cr的残渣态分别占59.74%、81.67%、77.81%、97.56%,除残渣态外,Cd和Cu的可交换态含量较高,分别占15.81%、13.62%,而Pb的铁锰化物结合态所占比例较高,平均为11.36%。通过计算淋失比可知,重金属在上层沉积物中的迁移率较高,而下层沉积物中的重金属较稳定,迁移性较弱;分析重金属的形态迁移可知,表层沉积物中重金属的可交换态较多,由细颗粒物质等吸附直接沉淀在表层沉积物中,随着泥沙的运动逐渐向下迁移;随着沉积物中铁锰化物的积累、AVS(酸可挥发性硫化物)积累等环境条件的变化,可交换态向铁锰化物结合态、有机物及硫化物结合态转化。

Vertical distribution and migration of heavy metals in sediment cores of Taihu Lake

Studying the distribution of heavy metals in sediment profiles is of great significance for understanding the pollution history and migration of heavy metals in lake sediments. Here we collected sediment cores from Taihu Lake, and determined total content and fractions of Cd, Pb, Cu, and Cr. The total content of Cd, Pb, Cu, and Cr decreased with increasing depth, with the turning points appeared in 1950 and 1980, especially in 1980. Meiliang bay, Zhushan bay and Gonghu bay suffered from very severe pollution, followed by the central area. The residual fraction of the metals was dominant. The average values of residual Cd, Pb, Cu, and Cr accounted for 59.74%, 81.67%, 77.81% and 59.74% of their total, respectively. The exchangeable Cd and Cu fractions were respectively 15.81% and 13.62%, while the proportion of iron and manganese oxides bound Pb was 11.36%. Migration rates of heavy metals in the upper sediments were higher, whereas the heavy metals in bottom sediments were relatively stable and poorly mobile. Calculation of the form migration of heavy metals showed that exchangeable metals were higher in the surface sediments as they were adsorbed on fine particulate matter, settled directly in the surface sediments and moved downward along with the sediment movements. At deeper sediments where iron and manganese oxides and AVS(Acid Volatile Sulfide) accumulated, the exchangeable metals were transferred into iron and manganese oxides bound, organic matter and sulphide bound fractions.