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Geoscience ›› 2021, Vol. 35 ›› Issue (05): 1450-1458.DOI: 10.19657/j.geoscience.1000-8527.2021.048

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

YANG Qiong1(), YANG Zhongfang1(), JI Junfeng2, LIU Xu1, JI Wenbing3, WANG Jue1, WU Tiansheng4, WANG Lei5   

  1. 1. School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
    2. Key Laboratory of Surficial Geochemistry, Ministry of Education, School of Earth Sciences and Engineering, Nanjing University, Nanjing,Jiangsu 210023, China
    3. State Environmental Protection Key Laboratory of Soil Environmental Management and Pollution Control, Nanjing Institute of Environmental Sciences, Ministry of Ecology and Environment, Nanjing,Jiangsu 210042, China
    4. Guangxi Institute of Geological Survey, Nanning,Guangxi 530023, China
    5. Geology Team No.4 of Guangxi Zhuang Autonomic Region, Nanning,Guangxi 530031, China
  • Received:2020-11-10 Revised:2021-04-06 Online:2021-10-10 Published:2021-11-04
  • Contact: YANG Zhongfang

Abstract:

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.

Key words: karstic soil, high geochemical background, ferromanganese nodule, heavy metal, diffuse reflection spectrum

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