Reduction of Phyllomanganates by Organic Acids:...
Transcript of Reduction of Phyllomanganates by Organic Acids:...
Reduction of Phyllomanganates by Organic Acids: Effects on Mineral Structure and Trace Metal Fate
Jeffrey G. Catalano, Elaine D. Flynn, Margaret A. G. Hinkle
Acknowledgements
Earth and Planetary Sciences • Washington University
Financial SupportUS NSF Geobiology and Low-Temperature Geochemistry ProgramUS NSF CAREER ProgramUS NSF Graduate Research Fellowship ProgramWashington University in St. Louis
AssistanceLyndsay Troyer (Wash. U.)Katherine Becker (Wash. U.)Jennifer Houghton (Wash U.)Dale Brewe (APS)Qing Ma (APS)Benjamin Reinhart (APS)Sungsik Lee (APS)Ryan Davis (SSRL)
User FacilitiesAdvanced Photon SourceStanford Synchrotron Radiation LightsourceNano Research Facility (Wash. U.)Institute of Materials Science & Engineering (Wash U.)
Cover images of Mn oxides in a cave system from: Frierdich and Catalano (2011) Chem. Geol. 284, 82-96
Prior Studies Show that Mn Oxides are Readily Reduced by Small Organic Molecules
■ A wide range of organic molecules reduce Mnoxides– Rates vary with compound
and pH■ Organic acids generate
CO2 and various compound-specific products
■ Mn oxides produce dissolved Mn(II)
Earth and Planetary Sciences • Washington University
Products of Citrate ReductionWang and Stone (2006a) GCA
Rate Dependence on pHWang and Stone (2006b) GCA
Despite these Reactions, Mn Oxides Persist in the Environment
■ Mn oxides are common in soils and other settings where organic acids are produced
■ Incomplete reduction is thus likely a common feature of natural systems
■ Mn(II) produced by partial reduction may alter the structure of the mineral that remains
Earth and Planetary Sciences • Washington UniversityLeft Image from European Geosciences Union
Manganese Oxides are Common in Soil Systems
Prior Studies Indicate that Dissolved Mn(II) Affects Mn Oxide Structure
■ Mn(II) causes reductive transformations to Mn(III) oxyhydroxides and Mn(II,III) oxides■ At lower concentrations, dissolved Mn(II) binds to phyllomaganates, altering the
symmetry of their sheets– These changes are generated by a comproportionation-disproportionation mechanism
Earth and Planetary Sciences • Washington University
Reductive Phase TransformationsElzinga et al. (2013) ES&T
Change in Mn Oxide Sheet Symmetry
Zhu et al. (2010) ES&T
Mn(II)-Mn(IV) Atoms ExchangeElzinga (2016) ES&T
Diverse Organic Acid are Reductants of Manganese Oxides
■ Mn(II) generated via reduction by organic acids may also alter structure■ Our work focuses on reduction by three representative small organic acids:
– Oxalate: Abundant in the environment, shows ready oxidation and strong metal complexation
– Citrate: Displays autocatalytic oxidation via Mn(II)-citrate complexes*– PHBA: Aromatic acid found in soil, reactive with Mn oxides but likely has weak
metal complexation in solution
Earth and Planetary Sciences • Washington University
Oxalate(pKa = 1.25, 4.27)
Citrate(pKa = 3.13, 4.76, 6.40)
p-Hydroxybenzoate (PHBA)(pKa = 4.58, 9.42)
*Wang and Stone (2006) GCA 70, 4463–4476Images from Wikimedia Commons
Effect of Mn(II) on Mn Oxides Expected to Vary with Mn(III) and Vacancy Content
■ Mn(II) likely primarily reacts at vacancy sites in Mn oxide sheets, either adsorbing or forming Mn(III) via comproportionation
■ Birnessite-type Mn oxides have a range of vacancy and Mn(III) contents; Reaction products with Mn(II) or other species likely vary
■ Aging may alter Mn oxide structures (e.g., Grangeon et al., 2014, Acta Cryst.)
Earth and Planetary Sciences • Washington University
δ-MnO2AMOS = 3.99
Mn(III) = 1±1 mol.%
Hex. Birnessite*AMOS = 3.80
Mn(III) = 10±2 mol.%
*c-Disordered H+-Birnessite of Villalobos et al. (2003)
Extent and Products of Phyllomanganate Reduction Vary with pH and Organic Acid
■ Complete oxidation of oxalate generates structure Mn(II/III)
■ Citrate and PHBA also fully consumed, cause greater Mnreduction– At pH 4: Produced
dissolved Mn(II) and solid-phase Mn(II/III)
– At pH 7: Only solid-phase Mn(II/III) form
Earth and Planetary Sciences • Washington University
pH 4 pH 7
10 mM NaCl, 1 mM Organic Acid, 28 days2.5 g/L δ-MnO2 = 22 mM Mn as Mn oxide
Extensive Electron Transfer by Organic Acids and Their Oxidation Products
■ OAs fully consumed during aging, indicating completion of an initial 2e- oxidation
■ Further reduction occurs from oxidation products of citrate and PHBA
■ Citrate shows full 18e-
transfer at pH 4– Only 31-44% of full reduction
capacity seen at pH 7■ PHBA produces 38-42% of
its theoretical reduction capacity (28e-) at pH 4, 14-22% at pH 7
Earth and Planetary Sciences • Washington University
pH 4 pH 7
Electron Balance (±1 e-) Calculated from Dissolved and Solid-Phase Manganese Speciation
10 mM NaCl, 1 mM Organic Acid, 28 days2.5 g/L Mn oxide
Max e-: 2 18 28 2 18 28
Organic Acids Have a Muted Impact on Manganese Oxide
Structure■ XRD shows only subtle
changes for δ-MnO2 structure– Increased vacancy capping at
pH 4– Improved sheet stacking at pH 7
■ Hexagonal birnessite shows additional ordering at pH 4
■ For both minerals, structural changes are more muted than expected based on the extent of Mn(IV) reduction and Mn(II)aqgeneration
Earth and Planetary Sciences • Washington University*Aging with 0.75 mM Mn2+aq from Hinkle et al. (2016)
pH 4 δ-MnO2 pH 7 δ-MnO2
pH 4 Hex Birn pH 7 Hex Birn
Anecdotal Evidence that Dissolved Mn(II) Affects Trace Metal Uptake by Mn Oxides in Nature
■ Mn oxides formed at groundwater seeps in a cave system show morphological and compositional variations that correlate with [Mn(II)aq]
Earth and Planetary Sciences • Washington UniversityFrierdich et al. (2011) Chem. Geol.
Mn(II) Generated during Mn Oxide Reduction by Organic Acids Liberates Trace Metals
■ Reduction of Mnoxides by organic acids causes release of sorbed Cu
■ Attributed to:– Mn(II)-Cu competition
for binding sites– Net reduction
decreasing available sites
Earth and Planetary Sciences • Washington University
Copper Mobilization during Reduction by Organic AcidsGodtfredsen and Stone (1994) ES&T
Effects of Organic Acids on Mn(II)aq Production and Trace Metal Retention
■ Mn(II) formed by oxalate binds to mineral, has negligible effect on metal retention– Amount of reduction
inadequate to cause Mn(II)-Metal competition
■ At pH 4, Citrate and PHBA produce substantial Mn(II)aq, liberate large fraction of sorbed metals
■ At pH 7, all organic acids have little effect on metal binding and Mn(II)aq
Earth and Planetary Sciences • Washington University
pH 4 pH 4
pH 7 pH 7
10 mM NaCl, 1 mM Organic Acid, 28 days2.5 g/L Mn oxide, 230 μM Ni or Zn
Organic Acids Alter Ni and Zn Binding to
Phyllomanganates
■ EXAFS spectra vary in the number and distance of Mn neighbors– Indicates a changing in
adsorption mechanism■ Effects vary among the
OAs and with pH■ The ratio of IVZn to VIZn
also changesEarth and Planetary Sciences • Washington University
Ni EXAFS Spectra of δ-MnO2 Aged with Organic Acids
Metal Binding Mechanisms on Phyllomanganates■ Vacancies are favorable binding
sites, yielding two possible complexes:– Triple Corner Sharing (TC): Capping
vacancies– Incorporated (Inc): Vacancy-filling,
only for Ni■ Sheet edges can also serve as sites
for metals, forming similar structures but with fewer Mn neighbors:– Double Corner Sharing (DC):
Structurally similar to TC– Tridentate Edge Sharing (TE):
Similar location as Inc, but longer Metal-Mn distances
Earth and Planetary Sciences • Washington University
Molecular-scale picture of trace metal binding mechanisms on
phyllomanganates revealed in studies by Peña, Manceau, Villalobos, Lanson,
Peacock, and other
Citrate and PHBA Promote Ni Binding to Edges, Oxalate Promoted Ni Binding at or in Vacancies
■ Low NTC/DC values at pH 4 indicate citrate and PHBA drive Ni to form DC complexes on mineral edges
■ At pH 7, citrate favors TE complexed on Mn(III)-rich edge sites■ Oxalate promotes Ni adsorption on and incorporation into vacancies
Earth and Planetary Sciences • Washington University10 mM NaCl, 1 mM Organic Acid, 28 days2.5 g/L Mn oxide, 230 μM Ni
Organic Acids and Mn(II) Produce Similar Yet Distinct Effects on Phyllomanganate Structure and Trace Metal Binding
■ Organic acids undergo complete initial oxidation (2e-), and citrate and PHBA oxidation products react further
■ Organic acids and dissolved Mn(II)* both increase solid-phase Mn(III) and Mn(II) and cause structural changes– These vary among the organic acids studied– Citrate and PHBA inhibit structural changes
produced by high Mn(II)■ Dissolved Mn(II) produced by reduction of Mn
oxides by OAs inhibits trace metal uptake■ Citrate and PHBA generally drive Ni and Zn to
edge binding site■ Oxalate enhances metal binding at vacancies
– It may opens up these sites before fully oxidizing
Earth and Planetary Sciences • Washington University
Altered Metal
Binding
Layer Ordering
Increased Mn(II/III)
Vacancy Capping
Reaction with Dissolved Mn(II)Reaction with Organic Acids
*See Hinkle et al. (2016) GCA