Interactions of ferrous iron with clay mineral surfaces during sorption and subsequent oxidation. Issue 6 (6th May 2020)

• Record Type: Journal Article
• Authors: Van Groeningen, Natacha; ThomasArrigo, Laurel K.; Byrne, James M.; Kappler, Andreas; Christl, Iso; Kretzschmar, Ruben
• Is Part Of: Environmental science. Volume 22:Issue 6(2020)
• Page Start: 1355
• Page End: 1367
• Publication Date: 2020-05-06
• Subjects: Environmental monitoring -- Periodicals; Biological monitoring -- Periodicals; Environmental chemistry -- Periodicals; 363.7363
• DOI: 10.1039/d0em00063a
• Languages: English
• ISSNs: 2050-7887

Abstract

Abstract : Under anoxic conditions, clay minerals sorb dissolved Fe(ii ) predominantly by forming surface precipitates, which transform into Fe(iii )-phases upon aeration. Abstract : In submerged soils and sediments, clay minerals are often exposed to anoxic waters containing ferrous iron (Fe 2+ ). Here, we investigated the sorption of Fe 2+ onto a synthetic montmorillonite (Syn-1) low in structural Fe (<0.05 mmol Fe per kg) under anoxic conditions and the effects of subsequent oxidation. Samples were prepared at two Fe-loadings (0.05 and 0.5 mol Fe added per kg clay) and equilibrated for 1 and 30 days under anoxic conditions (O2 < 0.1 ppm), followed by exposure to ambient air. Iron solid-phase speciation and mineral identity was analysed by 57 Fe Mössbauer spectroscopy and synchrotron X-ray absorption spectroscopy (XAS). Mössbauer analyses showed that Fe(ii ) was partially oxidized (14–100% of total added Fe 2+ ) upon sorption to Syn-1 under anoxic conditions. XAS results revealed that the added Fe 2+ mainly formed precipitates (layered Fe minerals, Fe(iii )-bearing clay minerals, ferrihydrite, and lepidocrocite) in different quantities depending on the Fe-loading. Exposing the suspensions to ambient air resulted in rapid and complete oxidation of sorbed Fe(ii ) and the formation of Fe(iii )-phases (Fe(iii )-bearing clay minerals, ferrihydrite, and lepidocrocite), demonstrating that the clay minerals were unable to protect ferrous Fe from oxidation, even when equilibrated 30 days under anoxic conditions prior to oxidation. Our findings clarify the role of clay minerals in the formation and stability of Fe-bearing solid phases during redox cycles in periodically anoxic environments.