广西贵港岩溶地质高背景区富含铁锰结核土壤的矿物学与重金属地球化学特征

岩溶地质高背景区土壤中普遍存在的铁锰结核对重金属的赋存状态和有效性有重要影响。选择广西贵港覃塘岩溶地质高背景区富含铁锰结核的表层土壤(0~20 cm)为研究对象,筛分出不同粒径的铁锰结核(10~120目)和细粒径土壤(<120目)样品进行化学分析,针对以下三个方面开展研究:(1)重金属(As、Cd、Cr、Cu、Hg、Ni、Pb和Zn)在铁锰结核和细粒径土壤中的分布分配规律和铁氧化物矿物的组成;(2)铁氧化物矿物对富含铁锰结核的土壤中Cd等重金属富集的影响;(3)重金属在富含铁锰结核的土壤中的赋存机制。研究发现,铁锰结核中的Fe和Mn以及Cd等重金属含量随着粒径的增大而不断增加,说明Cd等重金属元素更倾向于在大粒径铁锰结核中富集;土壤中Cd等重金属总量的约90%赋存在结核中,表明研究区土壤中重金属主要以结核形式赋存;富含铁锰结核的土壤中赤铁矿和针铁矿的平均含量分别为0.61%和4.94%,且结核粒径越大,针铁矿和赤铁矿含量越高;除Hg外,Cd等重金属含量与针铁矿和赤铁矿的含量均呈现极显著正相关,与赤铁矿的相关性稍优于针铁矿,表明铁氧化物矿物与富含铁锰结核土壤中的Cd等重金属元素富集密切相关。铁锰结核的存在既能促进Cd等重金属在土壤中的富集,又能降低土壤中重金属的生物有效性,研究结果为解释岩溶地质高背景区土壤Cd等重金属元素高含量、低生物有效性提供了理论依据。

Characteristics of Mineralogy and Heavy Metal Geochemistry in Ferromanganese Nodule Rich Soils with High Geochemical Background from Guigang,Guangxi

Ferromanganese nodules commonly exist in high quantity in karstic soils with high geochemical background and thus play a crucial role in the occurrence and bioavailability of heavy metals. In this study, karstic topsoil (0-20 cm) samples rich in ferromanganese nodules from Qintang District, Guigang city, Guangxi were particularly collected, and ferromanganese nodules of different particle sizes (10-120 mesh) and fine-grained soils (<120 mesh) were screened prior to further chemical analysis. The objectives of this study were to investigate: (1) the distribution of heavy metals (As, Cd, Cr, Cu, Hg, Ni, Pb and Zn), and composition of iron oxide minerals in the nodules and fine-grained soils, (2) the effect of iron oxide minerals on the accumulation of heavy metals in the nodules and fine-grained soils, and (3) the mechanism of heavy metals occurrence in soils rich in ferromanganese nodules. The results showed that the concentrations of Fe, Mn and heavy metals (e.g., Cd) in ferromanganese nodules increased with the increment of particle sizes, which indicates that heavy metal elements tend to be enriched in the nodules with large size. Quantitatively, about 90% of heavy metals in the topsoil samples occurred in the ferromanganese nodules. It indicates that heavy metals mainly occurred in the form of nodules in the studied soils. The mean contents of hematite and goethite in the topsoil samples containing ferromanganese nodules were 4.94% and 0.61%, respectively. The larger the particle size of nodules, the higher the contents of hematite and goethite. Except for Hg, the concentrations of heavy metals were all significantly and positively correlated with the contents of goethite and hematite, and the correlation with hematite was slightly better than that with goethite. These results indicate that iron oxide minerals are closely related to the enrichment of heavy metals in soils rich in ferromanganese nodules. The presence of ferromanganese nodules can not only promote the enrichment of heavy metals, but also reduce the bioavailability of heavy metals in soils. These results provide a theoretical basis for explaining the high concentrations and low bioavailability of heavy metals in karstic soil with high geochemical background.