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