Enhanced Oxidation of Cr(III)-Fe(III) Hydroxides by Oxygen in Dark and Alkaline Environments: Roles of Fe/Cr Ratio and Siderophore.

The current understanding of Cr(III)-Fe(III) hydroxide (Cr1-xFex(OH)3) oxidation in the dark is primarily focused on strong oxidants, yet the role of oxygen has generally been overlooked. Meanwhile, the effects of organic ligands on the Cr(III) oxidation are poorly known. Herein, we determined the kinetics of Cr1-xFex(OH)3 oxidation by oxygen in the dark as a function of pH and Fe/Cr ratio with/without the presence of a representative organic ligand-siderophore. Results showed that the Cr(III) oxidation rate increased linearly with increasing pH and Fe/Cr ratio. Thermodynamic calculations suggested that the enhanced Cr1-xFex(OH)3 oxidation with increasing pH was primarily due to the decreased ΔG value (i.e., the Gibbs free energy change) at higher pH. The decreased redox potentials (Eh) of Cr1-xFex(OH)3 suspensions with increasing Fe/Cr ratio accounted for the enhanced Cr(III) oxidation of iron-rich Cr1-xFex(OH)3. The siderophore greatly accelerated the Cr1-xFex(OH)3 oxidation at alkaline pH by promoting the formation of soluble organically complexed Cr(III), which can be oxidized readily by oxygen via mineral-surface catalyzed oxidation. Overall, this study highlights the specific role of oxygen and its synergistic role with the siderophore in the oxidation of solid Cr1-xFex(OH)3, which should be taken into consideration in assessing the long-term stability of Cr(III)-Fe(III) hydroxides.

Effect of Stoichiometry on Nanomagnetite Sulfidation.

Magnetite (Mt) has long been regarded as a stable phase with a low reactivity toward dissolved sulfide, but natural Mt with varying stoichiometries (the structural Fe(II)/Fe(III) ratio, xstru) might exhibit distinct reactivities in sulfidation. How Mt stoichiometry affects its sulfidation processes and products remains unknown. Here, we demonstrate that xstru is a master variable controlling the rates and extents of sulfide oxidation by magnetite nanoparticles (11 ± 2 nm). At pH = 7.0-8.0 and the initial Fe/S molar ratio of 10-50, the partially oxidized magnetite (xstru = 0.19-0.43) can oxidize dissolved sulfide to elemental sulfur (S0), but only surface adsorption of sulfide, without interfacial electron transfer (IET), occurs on the nearly stoichiometric magnetite (xstru = 0.47). The higher initial rate and extent of sulfide oxidation and S0 production are observed with the more oxidized magnetite that has the higher electron-accepting capability from surface-complexed sulfide (S(-II)(s)). The FeS clusters formed from magnetite sulfidation can be oxidized by the most oxidized magnetite with xstru = 0.19 but not by other magnetite particles. A linear relationship between the Gibbs free energy of reaction and the surface area-normalized initial rate of sulfide oxidation is observed in all experiments under the different conditions, suggesting the S(-II)(s)-magnetite IET dominates magnetite sulfidation at high Fe/S molar ratios and near-neutral pH.

Fe(II)-Catalyzed Transformation of Ferrihydrite with Different Degrees of Crystallinity.

Natural occurring ferrihydrite (Fh) nanoparticles have varying degrees of crystallinity, but how Fh crystallinity affects its transformation behavior remains elusive. Here, we investigated the Fe(II)-catalyzed transformation of Fh with different degrees of crystallinity (i.e., Fh-2h, Fh-12h, and Fh-85C). X-ray diffraction patterns of Fh-2h, Fh-12h, and Fh-85C exhibited two, five, and six diffraction peaks, respectively, indicating the order of crystallinity: Fh-2h < Fh-12h < Fh-85C. Fh with the lower crystallinity has a higher redox potential, corresponding to the faster Fe(II)-Fh interfacial electron transfer and Fe(III)labile production. With the increase of initial Fe(II) concentration ([Fe(II)aq]int.) from 0.2 to 5.0 mM, the transformation pathways of Fh-2h and Fh-12h change from Fh → lepidocrocite (Lp) → goethite (Gt) to Fh → Gt, but that of Fh-85C switches from Fh → Gt to Fh → magnetite (Mt). The changes are rationalized using a computational model that quantitatively describes the relationship between the free energies of formation for starting Fh and nucleation barriers of competing product phases. Gt particles from the Fh-2h transformation exhibit a broader width distribution than those from Fh-12h and Fh-85C. Uncommon hexagonal Mt nanoplates are formed from the Fh-85C transformation at [Fe(II)aq]int.= 5.0 mM. The findings are crucial to comprehensively understand the environmental behavior of Fh and other associated elements.

Understanding the Importance of Labile Fe(III) during Fe(II)-Catalyzed Transformation of Metastable Iron Oxyhydroxides.

Transformation of metastable Fe(III) oxyhydroxides is a prominent process in natural environments and can be significantly accelerated by the coexisting aqueous Fe(II) (Fe(II)aq). Recent evidence points to the solution mass transfer of labile Fe(III) (Fe(III)labile) as the primary intermediate species of general importance. However, a mechanistic aspect that remains unclear is the dependence of phase outcomes on the identity of the metastable Fe(III) oxyhydroxide precursor. Here, we compared the coupled evolution of Fe(II) species, solid phases, and Fe(III)labile throughout the Fe(II)-catalyzed transformation of lepidocrocite (Lp) versus ferrihydrite (Fh) at equal Fe(III) mass loadings with 0.2-1.0 mM Fe(II)aq at pH = 7.0. Similar to Fh, the conversion of Lp to product phases occurs by a dissolution-reprecipitation mechanism mediated by Fe(III)labile that seeds the nucleation of products. Though for Fh we observed a transformation to goethite (Gt), accompanied by the transient emergence and decline of Lp, for initial Lp we observed magnetite (Mt) as the main product. A linear correlation between the formation rate of Mt and the effective supersaturation in terms of Fe(III)labile concentration shows that Fe(II)-induced transformation of Lp into Mt is governed by the classical nucleation theory. When Lp is replaced by equimolar Gt, Mt formation is suppressed by opening a lower barrier pathway to Gt by heterogeneous nucleation and growth on the added Gt seeds. The collective findings add to the mechanistic understanding of factors governing phase selections that impact iron bioavailability, system redox potential, and the fate and transport of coupled elements.

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

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.

Three-Dimensional Multi-Doped Porous Carbon/Graphene Derived from Sewage Sludge with Template-Assisted Fe-pillared Montmorillonite for Enhanced Oxygen Reduction Reaction.

Three-dimensional multi-doped porous carbon/graphene (Fe-Mt-SS-C) was prepared by carbonization of sewage sludge with template-assisted Fe-pillared montmorillonite. The material consisted of nanosheet- and particle- carbon had a high specific surface area (784.46 m2·g-1) and hierarchical porous structure of micro-, meso- and macropores. The prepared Fe-Mt-SS-C had a high degree of graphitization and large amount of defect atoms. The pyrolysis process made full use of the C, N, Fe, and S by turning them into the carbon framework of the as-obtained material in situ. Template-assisted Fe-pillared montmorillonite contributed to more characteristics of morphology and composition on Fe-Mt-SS-C than other three materials (SS-C, Mt-SS-C and Fe-SS-C), and enhanced the electrocatalytic ORR activity by providing more adsorption sites and the electronic structure, resulting in the increase of conductivity and electrochemical activity. The ORR activity performance of Fe-Mt-SS-C, including the value of onset potential (0.03 V) and E1/2 (-0.09 V), was better than that of commercial 20 wt% Pt/C (-0.02 V and -0.18 V, respectively). Moreover, the Fe-Mt-SS-C possessed excellent durability and outstanding immunity toward methanol crossover effects. Therefore, the resultant Fe-Mt-SS-C has great potential to applied as a high-efficiency ORR electrocatalyst, more importantly, it realizes the utilization of the sludge at the same time.