Silicate coprecipitation reduces green rust crystal size and limits dissolution-precipitation during air oxidation

Authors: BETTS, AARON - Us Environmental Protection Agency (EPA); Fischel, Matthew; EVERS, ANNA - North Carolina State University; TAPPERO, RYAN - Brookhaven National Laboratory; SPARKS, DONALD - University Of Delaware

Submitted to: Geochemical Transactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/8/2024
Publication Date: 10/29/2024
Citation: Betts, A., Fischel, M.H., Evers, A., Tappero, R., Sparks, D.L. 2024. Silicate coprecipitation reduces green rust crystal size and limits dissolution-precipitation during air oxidation. Environmental Science and Technology. https://doi.org/10.1186/s12932-024-00093-9.
DOI: https://doi.org/10.1186/s12932-024-00093-9

Interpretive Summary: Green rusts are iron minerals found in flooded soils that will transform into different iron minerals when exposed to air. However, the green rusts scientists use in laboratory studies do not represent the minerals found in nature which contains some silica. We formed green rusts with different amounts of silica and studied how they transformed when exposed to air. When the green rust contains more than 10% silica, it changes into iron minerals with smaller crystal sizes and increased stability. These results matter because they transform our understanding of how green rusts change in the environment, and these minerals alter nutrient and contaminant movement in soils. Stakeholders that will benefit include parties involved in remediating contaminated sites like the Environmental Protection Agency and scientists who study how nutrients like phosphorus move in soil and groundwater.

Technical Abstract: Green rusts (GR) are mixed-valence iron (Fe) hydroxides which form in reducing redox environments like riparian and wetland soils and shallow groundwater. In these environments, silicon (Si) can influence Fe oxides’ chemical and physical properties but its role in GR formation and subsequent oxidative transformation have not been studied starting at initial nucleation. Green rust sulfate [GR(SO4)] and green rust carbonate [GR(CO3)] were both coprecipitated from salts by base titration in increasing % mol Si (0, 1, 10, and 50). The minerals were characterized before and after rapid (24 h) aqueous air-oxidation by x-ray diffraction (XRD), scanning electron microscopy (SEM), Fe extended x-ray absorption fine structure spectroscopy (EXAFS), and N2-BET surface area. Results showed that only GR(SO4) or GR(CO3) was formed at every tested Si concentration. Increasing % mol Si caused decreased plate size and increased surface area in GR(CO3) but not GR(SO4). GR plate basal thickness was not changed at any condition indicating a lack of Si interlayering. Air oxidation of GR(SO4) at all % mol Si contents transformed by dissolution and reprecipitation into lepidocrocite and goethite, favoring ferrihydrite with higher % Si content. Air oxidation of GR(CO3) transformed into magnetite and goethite but increasing Si caused GR to oxidize while retaining its hexagonal plate structure via solid-state oxidation. Our results indicate that Si has the potential to cause GR to form in smaller particles and upon air oxidation, Si can either stabilize the plate structure or alter transformation to ferrihydrite.