Observing in-situ transformations of ferrihydrite in redox-active paddy soil microcosms
Abstract
Iron minerals are abundant in soils, have a major influence on the cycling of trace elements and undergo transformations that have the potential to immobilise or remobilise trace elements in redox dynamic soils (Borch et al., 2010). Iron mineral transformations have been extensively studied in laboratory-based model studies. However, assessing the relevance of the observed pathways and transformation rates in soil environments can be experimentally challenging. Here, we studied ferrihydrite transformation in soil microcosms, and demonstrate that different soil environments influence the rate and nature of iron mineral transformations. Five rice paddy soils from south, east and south-east Asia were flooded in laboratory microcosms to simulate soil conditions at the start of a rice growing period. We introduced synthetic ferrihydrite samples to the soil in permeable polyethylene terephthalate fabric sachets (max interior dimension approx. 30 mm 12 mm 3 mm, pore size 52 µm), and followed the changes in the mineral composition inside the mineral sachets over time using X-ray diffraction (bulk) and Raman spectroscopy (spatially resolved). Mineral transformations were then analysed in the context of the soil and pore water properties.
Within two weeks, lepidocrocite and goethite partially replaced ferrihydrite in all bulk samples, except ferrihydrite that was incubated an acid-sulfate sub-soil from Thailand. However, ferrihydrite transformation rates and product composition varied widely, reflecting variations in the concentrations of key parameters of the soil solution, including dissolved metal concentrations, oxyanion concentrations and pH. Despite very high concentrations of Fe(II) and sulfate in some soil pore water (>10 mM Fe(II) and >20 mM SO42- after 2 weeks), and evidence of Fe and S reduction, neither magnetite nor sulphide minerals were observed, as might have been predicted from the results of laboratory-based ferrihydrite transformation experiments (e.g. Hansel et al., 2015; Troc et al., 1992). Still, higher iron and sulphate concentrations in solution were associated with faster transformation rates and higher ratios of lepidocrocite to goethite in the product, contradicting the findings of previous laboratory studies (Hansel et al., 2005). Local transformation is likely to have been much faster than bulk transformation rates, as observed in the colour distribution (Figure 1) and spatially resolved Raman spectroscopy of mineral identity across mineral sachet cross-sections. Mineral transformation kinetics and pathways are highly dependent on the location within the mesh bag with reference to the soil. The distribution of minerals in the mesh bags demonstrates the importance of the local environment to ferrihydrite transformation processes in soil, and contributes to understanding how to improve the in-situ investigation of soil mineral transformation processes. Show more
Publication status
publishedPublisher
Swiss Academy of Science (SCNAT); ETH ZurichEvent
Organisational unit
03541 - Kretzschmar, Ruben / Kretzschmar, Ruben
Notes
Poster presented on November 7, 2020. Due to the Coronavirus (COVID-19) the conference was conducted virtually.More
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