Citrate Controls Fe(II)-Catalyzed Transformation of Ferrihydrite by Complexation of the Labile Fe(III) Intermediate

Journal Article · Tue Jun 16 00:00:00 EDT 2020 · Environmental Science & Technology

DOI:https://doi.org/10.1021/acs.est.0c00996· OSTI ID:1639335

Sheng, Anxu ^[1]; Li, Xiaoxu ^[2]; Arai, Yuji ^[3]; Ding, Yuefei ^[2]; Rosso, Kevin M.  ^[4]; Liu, Juan ^[5]

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1. UNIVERSITY PROGRAMS
2. Peking University
3. University of Illinois at Urbana-Champaign
4. BATTELLE (PACIFIC NW LAB)
5. PEKING UNIVERSITY

Ferrihydrite (Fh) is generally associated with dissolved organic matter (DOM) in natural environments due to a strong sorption affinity at circumneutral pH and its high specific surface area. In suboxic conditions, aqueous Fe(II) (Fe(II)aq) can catalyze transformation of Fh into more stable crystalline Fe(III) phases, but how DOM influences the transformation kinetics and pathway is still unclear. Using citrate as a surrogate, here we examine Fh transformation with 1 mM Fe(II)aq and 0-60 µM citrate at pH 7.2. We focus on quantifying the time-dependent concentrations of sorbed Fe(II), structural Fe(II), and a key intermediate species, labile Fe(III), resulting from interfacial electron transfer (IET), and how these species correlate with the evolution of lepidocrocite (Lp), magnetite (Mt), and goethite (Gt) products. Low concentrations of citrate significantly impact the proportions of Lp/Gt, and the collective results reveal that its effect is primarily through its ability to complex labile Fe(III) and thereby disrupt polymerization into product crystallites, as opposed to modifying the surface properties of Fh or inhibiting IET. The emergence and decline of a Mt coprecipitate between 5-10 µM citrate is shown, after correction for Fe(III)labile complexation by citrate, to correspond with occurrence of an unbound Fe(II)aq/Fe(III)labile ratio of 0.5, the stoichiometric Fe2+/Fe3+ ratio in Mt. At the molecular level, the findings suggest that citrate, and by extension DOM, can modify the relative rates of olation and oxolation reactions that assemble labile Fe(III) into various product minerals.

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1639335
• Report Number(s): PNNL-SA-151513
• Journal Information: Environmental Science & Technology, Vol. 54, Issue 12
• Country of Publication: United States
• Language: English