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

Abstract

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, we have examined 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) (Fe(III) labile ), 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 of a Mt coprecipitate is observed in the transformation experiments with 5−10 μM citrate, when the Fe(II)/Fe(III) labile ratio on/near the Fh surface is close to 0.5, the stoichiometric Fe(II)/Fe(III) 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.

Further insights into the Fe(ii) reduction of 2-line ferrihydrite: a semi in situ and in situ TEM study

“…The rst visual evidence of the amorphous surface layer, previously hypothesized to be a "reactive-FH/labile-Fe(III) phase" 20,29 observed in our work been composed of the same chemical and structural composition as the bulk 2-line FH as well as the presence of a Fe(II)/Fe(III) interface at the surface of the reacted 2-line FH leads us to visually solidify previous hypotheses about their possible existence. 17,20,29,58,59 The precipitation of the well documented tabular LP crystals formed through the assemblage of elongated lamellae is a type of formation mechanism that has not yet been documented. [20][21][22][26][27][28][29][30][31] Detection of LP and GT end products as well as the observation of nano-wires dispersed and growing out of the surface of the reacted 2-line FH indicates that the Fe(II) (aq) catalysed the 2-line FH particles as intended.…”

“…C, N, and P) in natural and anthropogenic systems. [13][14][15][16][17][18][19] Under anoxic to sub-oxic, non-sulphide rich environments, microbial respiration leads to the generation of Fe(II) (aq) which can react with iron(III) oxy-hydroxides to form more thermodynamically stable phases or induce recrystallization and growth within hours to days. 13,[20][21][22][23] For bacterial mediated reduction of 2-line FH, various reaction mechanisms have been proposed that include: (1) electron transfer through direct contact between the cell wall and FH surface, (2) electron shuttling of soluble chelating agents such as humic acid, quinones, and avins with a "labile Fe(III) phase/reactive-FH" and/or 2-line FH and (3) through extracellular nano-wires.…”

“…13,[20][21][22][23] For bacterial mediated reduction of 2-line FH, various reaction mechanisms have been proposed that include: (1) electron transfer through direct contact between the cell wall and FH surface, (2) electron shuttling of soluble chelating agents such as humic acid, quinones, and avins with a "labile Fe(III) phase/reactive-FH" and/or 2-line FH and (3) through extracellular nano-wires. 13,17,24 In the case of the abiotic reduction of 2-line FH with Fe(II) (aq) , it has been generally agreed upon that the 2-line FH transformation to more thermodynamically stable phases (LP, GT, MG, and HT) is dependent upon several experimental variables. For example, the initial concentration of Fe(II) (aq) , pH, temperature, the presence of solid associated Fe(II) (aq) -FH, soluble ligands (e.g.…”

Thermal Stability and Decomposition Products of P-Doped Ferrihydrite

“…It plays an essential role as a sorbent of various major and trace elements and controls their availability in the environment [ 7 , 8 , 9 , 10 , 11 ]. Therefore, natural ferrihydrite always contains a variety of impurities that affect its chemical and physical properties [ 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 ]. Some impurities such as phosphate are important nutrients, while other impurities may be unwanted compounds, toxic metals and metalloids (e.g., Cd, Pb, As).…”

Impact of Zn Substitution on Fe(II)-induced Ferrihydrite Transformation Pathways

“…pH variations affect the formation of magnetite, 90 the relative amounts of goethite and lepidocrocite produced, and the rate of ferrihydrite 91 transformation (Hansel et al, 2005;Boland et al, 2014). Surface adsorbates, including silicate and organic matter, both slow ferrihydrite transformation and alter the final minerals that form (Jones et al, 2009;Wang et al, 2015;Thomasarrigo et al, 2018;Xiao et al, 2018;Zhou et al, 2018;Thomasarrigo et al, 2019;Sheng et al, 2020a). Aluminum substitution diminishes secondary mineralization and enhances preservation of ferrihydrite, requiring greater Fe(II) concentrations to induce transformation compared to Al-free ferrihydrite (Masue-Slowey et al, 2011;Hansel et al, 2011).…”