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

Journal Article · · Geochemical Transactions
 [1];  [2];  [1];  [3];  [1]
  1. Univ. of Delaware, Newark, DE (United States)
  2. US Dept. of Agriculture (USDA), Beltsville, MD (United States). Agricultural Research Service (ARS)
  3. Brookhaven National Laboratory (BNL), Upton, NY (United States). National Synchrotron Light Source II (NSLS-II)

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.

Research Organization:
Brookhaven National Laboratory (BNL), Upton, NY (United States)
Sponsoring Organization:
USDOE Office of Science (SC), Basic Energy Sciences (BES). Scientific User Facilities (SUF)
Grant/Contract Number:
SC0012704
OSTI ID:
2477947
Report Number(s):
BNL--226340-2024-JAAM
Journal Information:
Geochemical Transactions, Journal Name: Geochemical Transactions Journal Issue: 1 Vol. 25; ISSN 1467-4866
Publisher:
American Chemical Society, Geochemistry Div.Copyright Statement
Country of Publication:
United States
Language:
English

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