Earth Science Frontiers ›› 2020, Vol. 27 ›› Issue (5): 218-226.DOI: 10.13745/j.esf.sf.2020.5.49

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Effect of apatite on the stability of ferrihydrite in lacustrine sediments

LUO Shaoyong1(), ZHOU Yuefei2, LIU Xing1,*()   

  1. 1. Faculty of Land Resource Engineering, Kunming University of Science and Technology, Kunming 650093, China
    2. School of Resources and Environmental Engineering, Hefei University of Technology, Hefei 230009, China
  • Received:2020-04-20 Revised:2020-06-28 Online:2020-09-25 Published:2020-09-25
  • Contact: LIU Xing

Abstract:

In this study, we set out to investigate the mechanism of apatite-mediated dissolution and transformation of ferrihydrite in lacustrine sediments, and the synergic environmental effect of both minerals. We carried out in-situ ferrihydrite transformation experiments in the Dianchi Lake, Yunnan Province, China. The results show that ferrihydrite remained stable in 1-month cultured samples, whereas in apatite-added samples we observed obvious mineral phase transformation after 3-month culturing. The change of newly formed iron mineral assemblage composition with depth in the top 50 cm sedimentary layer were goethite+magnetite/maghemite → goethite+lepidocrocite → goethiten in the winter experiments, or goethite+lepidocrocite+magnetite/maghemite → goethite+lepidocrocite → no new mineral in the summer experiments. Transmission electron microscopy observations show that magnetic minerals formed in winter are nanosized magnetite and maghemite, while in summer they are mainly magnetite. X-ray photoelectron spectroscopy analysis only found P in 3-month cultured samples in winter at a sedimentary depth of no more than 20 cm. The experimental results suggest that the progressive steps in the apatite-mediated ferrihydrite transformation process in lacustrine sediments are as follows: (1) apatite is dissolved by microorganisms and P is released during dissolution; (2) dissolved P promotes iron-reducing bacterial growth; (3) iron-reducing bacteria promote ferrihydrite reduction and release of dissolved Fe2+; and (4) dissolved Fe2+ catalyzes the transformation of ferrihydrite to goethite, lepidocrocite, and magnetite. In winter at the sedimentary redox boundary, the functional microorganisms responsible for apatite dissolution and ferrihydrite reduction can grow better, thus the risk of endogenous phosphorus release is much greater.

Key words: ferrihydrite, apatite, lacustrine sediment, redox boundary, stability, transformation

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