Hematite-goethite ratios at pH 2–13 and 25–170 °C: A time-resolved synchrotron X-ray diffraction study

Chen, Si Athena; Heaney, Peter J.; Post, Jeffrey E.; Eng, Peter J.; Stubbs, Joanne E.

doi:10.1016/j.chemgeo.2022.120995

Cited by 11 publications

(9 citation statements)

References 63 publications

(121 reference statements)

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“…There is a tension in the literature about the affinity of AsO 4 3– and PO 4 3– for adsorption on iron oxyhydroxides (e.g., Gt). While some studies reported higher adsorption of AsO 4 , − , some other studies showed that PO 4 − ,,…”

Section: Results and Discussionmentioning

confidence: 95%

“…Antelo et al reported that PO 4 3– is more sensitive to changes in pH and ionic strength. They showed that PO 4 3– adsorption increases in acidic pH at different ionic strengths and decreases at basic pH and low ionic strengths, compared to AsO 4 . − Therefore, the AsO 4 3– and PO 4 3– adsorption on iron oxy-hydroxides depends on the system chemistry and may not be identical under different chemical conditions . In addition, Langmuir proposed that smaller ions with higher charges, that is, with a higher ionic potential (IP = Z/r), show a higher affinity for forming a strong covalent bond on the surface of solids.…”

Section: Results and Discussionmentioning

confidence: 99%

“…Ferrihydrite (Fh) is a nanosized iron oxyhydroxide that immobilizes contaminants and nutrients such as metal and metalloid oxyanions in natural environments through surface complexation reactions. − Immobilization may be temporary as Fh (Fe 1.0 O 0.67 (OH) 1.63 ) metastability causes it to eventually transform to larger crystalline products such as goethite (Gt; α-FeOOH) and hematite (Hm; α-Fe 2 O 3 ). − Transformation and growth reduce the reactive surface area and change the type and availability of surface functional groups, resulting in the desorption of preadsorbed oxyanions to solution. ,,, Elevated concentrations of oxyanions mobilized from mineral surfaces can cause severe environmental problems. , For instance, more than 100 million people worldwide are exposed to dissolved arsenic (e.g., arsenate; AsO 4 3– ) concentrations in groundwater that exceed acceptable drinking water standards. − Although sulfate (SO 4 2– ), phosphate (PO 4 3– ), and nitrate (NO 3 – ) are not regulated as being toxic, their excessive concentrations in aquatic environments from mining and agriculture activities are known to be detrimental, for example, eutrophication of surface water. − …”

Section: Introductionmentioning

confidence: 99%

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Effects of Oxyanion Surface Loading on the Rate and Pathway of Ferrihydrite Transformation

Namayandeh,

Borkiewicz,

Bompoti

et al. 2023

ACS Earth Space Chem.

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In natural environments, ferrihydrite (Fh) reacts readily with the contaminant and nutrient oxyanions through surface complexation. While previous experiments showed that the transformation of Fh to Gt and Hm under oxic conditions at 70 °C is controlled by the type and strength of oxyanion surface complexes, the impact of surface loading on this process is only partly understood. Synchrotron scattering methods and chemical analysis were used to develop a kinetic model that describes the impact of oxyanion surface loading on the rate and pathway of Fh transformation by using arsenate (AsO4 3–) and phosphate (PO4 3–). Kinetic modeling showed that AsO4 3– and PO4 3– adsorption decreased the rate of transformation and favored Hm formation over Gt. Higher surface loadings increasingly inhibited Fh transformation with a greater effect for PO4 3– compared with AsO4 3–. This information has implications for understanding the impacts of oxyanions on the transformation of natural Fe to Gt and Hm in environmental systems.

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Section: Results and Discussionmentioning

confidence: 95%

Section: Results and Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Effects of Oxyanion Surface Loading on the Rate and Pathway of Ferrihydrite Transformation

Namayandeh,

Borkiewicz,

Bompoti

et al. 2023

ACS Earth Space Chem.

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“…At pH 10, higher temperatures favor hydrohematite with fewer Fe vacancies, suggesting that highly Fe-deficient hematite, with Fe occ = 0.8-0.9, is evidence of precipitation at temperatures less than 200 °C. In a separate study, we showed that stoichiometric hematite forms in acidic solutions (pH ≤ 5) at high temperatures (T ≥ 180 ℃) (Chen et al, 2022).…”

Section: Fe Occupancy In Hydrohematitementioning

confidence: 90%

Vacancy infilling during the crystallization of Fe-deficient hematite: An in situ synchrotron X-ray diffraction study of non-classical crystal growth

Chen,

Heaney,

Post

et al. 2023

American Mineralogist

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The crystallization of hematite from precursor ferrihydrite was studied using timeresolved, angle-dispersive synchrotron X-ray diffraction in aqueous solutions at pH 10 and 11 and at temperatures ranging from 80 to 170 °C. Rietveld analyses revealed a nonclassical crystallization pathway involving vacancy infilling by Fe as defective hematite nanocrystals evolved. At 90 °C and pH 11, incipient hematite particles exhibited an Fe site occupancy as low as 0.68(2), and after 30 min, Fe occupancy plateaued at 0.84(1), achieving a metastable steady state with a composition corresponding to "hydrohematite". During crystal growth, unit-cell volume increased with an increase in Fe occupancy. The increase in Fe occupancy in hydrohematite was accomplished by deprotonation, resulting in a shortening of the long Fe-O(H) bonds and decreased distortion of the octahedral sites. Once the occupancy stabilized, the unit-cell volume contracted following further nanoparticle growth. Our study documented a variety of synthetic routes to the formation of "hydrohematite" with an Fe vacancy of 10-20 mol% in the final product.The structure refined for synthetic hydrohematite at 90 °C and pH 11 closely matched that of natural hydrohematite from Salisbury, CT, with a refined Fe occupancy of 0.83(2). Dry heating this natural hydrohematite generated anhydrous, stoichiometric hematite, again by continuous infilling of vacancies. The transformation initiated at 150 °C and was complete at 700 °C, and it was accompanied by the formation of a minor amorphous phase that served as a reservoir for Fe during the inoculation of the defective crystalline phase.

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“…Through hydrolysis and temperature changes, soil clays and silicate minerals weather to derive ions that form the basis to the Fe oxides (Kelly et al., 1998; Zhao et al., 2017a). Redox environment, parent material, pH and vegetation also constrain the suites of Fe oxides found in soils (Chen et al., 2022; Schwertmann, 1988).…”

Section: Introductionmentioning

confidence: 99%

Identifying Temperature and Moisture Controls on Fe Oxide Origins

Lepre

2023

Geophysical Research Letters

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Magnetism, redness, and Fe oxides are indicators of pedoclimatic conditions. However, uncertainties with observing how Fe oxides form within soils has led to debates about relationships between magnetic mineral assemblages, temperature, and rainfall. To address these issues, Fe oxides from the equatorial tropics of Kenya were examined in Pliocene soils that developed under orbital forcing of the monsoon. Results demonstrate that with warm‐wet monsoons, ferrimagnetic production was increased and correlated with hematite concentrations, in accordance with expectations that ferrimagnetic and hematite minerals codevelop from amorphous Fe oxides. With cool‐dry monsoons, hematite concentrations increased but ferrimagnetic production decreased and decoupled from hematite development. These findings suggest that decreased rainfall rather than temperature change favored the dehydration step required to catalyze hematite enrichment within soils. This study explains Fe oxides origins under variable monsoonal climates and recognizes moisture changes in comparison to temperature as stronger controls on the production of soil‐formed hematite.

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Hematite-goethite ratios at pH 2–13 and 25–170 °C: A time-resolved synchrotron X-ray diffraction study

Cited by 11 publications

References 63 publications

Effects of Oxyanion Surface Loading on the Rate and Pathway of Ferrihydrite Transformation

Effects of Oxyanion Surface Loading on the Rate and Pathway of Ferrihydrite Transformation

Vacancy infilling during the crystallization of Fe-deficient hematite: An in situ synchrotron X-ray diffraction study of non-classical crystal growth

Identifying Temperature and Moisture Controls on Fe Oxide Origins

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