presentation-acs 2015 2-final

Resolving conflicting effects of surfaces, ligands, and

concentration on the evolution of reactive oxygen species

during iron oxidationShengnan Meng, Benson M. Solomon, and John L. Ferry

Department of Chemistry and BiochemistryUniversity of South Carolina

Tidally forced inundation of coastal marshes can be used as a repeatable model for measuring the effects of episodic oxygen restriction on other aquatic ecosystems. This study focuses on how the cycling of ferrous and ferric iron and the yield of associated reactive oxygen species (ROS) is affected by particulates and solution conditions.

Significance of Reactive Oxygen Species The direct reaction of atmospheric oxygen with organic carbon is spin-forbidden and therefore kinetically slow, despite its thermodynamic favorability.

The ROS are lower oxidation states of oxygen, some of which can react with organic carbon at rates that approach the diffusion controlled limit.

Ground state ROS include HOO/O2-, H2O2,

and HO.

Episodic inundation of soils restricts microbial access to atmospheric oxygen in pore waters.

Fe(III) can be used as an electron acceptor, resulting in solutions with locally high concentrations of Fe(II). This re-oxidizes when pore waters are exposed to air (Vereen Marsh, SC).

Emerging groundwater

Particulate iron oxides appear almost immediately upon mixing with surface water.

Connections and ChallengesWe know that: The direct and indirect reduction of Fe(III) by microbial processes leads to the production of Fe(II)

The oxidation of Fe(II) leads to the formation of ROS

The interaction of ROS with natural organic matter leads to the production of ROS and the reduction of Fe(III)

Some naturally occurring ligands promote the precipitation of Fe(III) species

But:The presence of Fe(III) containing suspensions may encourage the precipitation of Fe(III), but will it be more rapid than its reduction by ROS or by organic carbon?

Will the ROS that bridge the gap between carbon and oxygen be affected by the presence of surfaces?

How does the presence of surfaces and buffers affect this manifold?

Hypothesis: Conditions that favor Fe(III) precipitation limit ROS production; i.e. precipitation and loss is more rapid than ROS driven cycling of Fe(II)

Methods – Batch oxidation of Fe(II) in the presence of varying particulates

[Fe(II)]0 = 100 μMpH 7.5

No solidFe2O3

Fe3O4

FeOOH

Experimental design Analytical approaches

Fe(II), ferrozine method

H2O2, amplex red /horse radish peroxidase

HO, terephthalic acid

BO33-

HEPESHCO3

-

PO43-

air saturated


[Fe(II)]0 = 100 μMpH 7.5

Fe2O3

Fe3O4

FeOOH





BO33-

HEPESHCO3

-

PO43-

2-[4-(2-hydroxyethyl) piperazin-1-yl] ethanesulfonic acid

air saturated


[Fe(II)]0 = 100 μMpH 7.5

No solidFe2O3

Fe3O4

FeOOH





BO33-

HEPESHCO3

-

PO43-

air saturated

Solid and Surface

Area (m2/g)Surface Area

Loadings (m2/L)

HEPES(25mM)

BO33-

(25mM)HCO3

-

(2mM)PO4

3-

(1mM)

Fe(II)

100μM

pH 7.5

Fe2O3

(SA= 4.88)

0

0.0976 (0.02g/L)

0.1952 (0.04g/L)

0.2928 (0.06g/L)

0.3904 (0.08g/L)

Fe3O4

(SA= 4.658)

0

0.37264 (0.8g/L)

0.55896 (0.1g/L)

0.6987 (0.15g/L)

0.9316 (0.2g/L))

FeOOH(SA= 8.087)

0

0.64696 (0.08g/L)

0.8087 (0.1g/L)

1.6174 (0.2g/L))

2.02175 (0.25g/L)

In solution The net oxidation of Fe(II) is rapid and predictable at circumneutral pH

0 100 200 300 400 5000

102030405060708090

Net Fe(II) oxidation

Time (s)

Con

c. o

f Fe(

II) (μ

M) Reaction is first

order in Fe(II)

[Fe(II)]o = 100 μM; pH 7.5; [HCO3

-] = 2.00 mM, 25oC

H+

In solution Hydrogen peroxide evolves very rapidly under these conditions

0 100 200 300 4000

0.51

1.52

2.53

3.5

Time (s)H

2O2

(μM

)

Detected H2O2 as a function of time

[Fe(II)]o = 100 μM; pH 7.5; [HCO3

-] = 2.00 mM, 25oC

H+

In solution Hydrogen peroxide evolves very rapidly under these conditions

0 100 200 300 4000

0.51

1.52

2.53

3.5

Time (s)H

2O2

(μM

)

Detected H2O2 as a function of time

H+

[Fe(II)]o = 100 μM; pH 7.5; [HCO3

-] = 2.00 mM, 25oC

In solution The co-existence of Fe(II) and H2O2 leads to HO formation (Fenton Reaction)

0 100 200 300 4000

0.05

0.1

0.15

0.2

0.25

0.3

f(x) = 0.00059398 x + 0.011968458R² = 0.963411954305083

HOTPA accumulated rapidly

Time (s)

HO

TPA

(μM

)

[Fe(II)]o = 100 μM; pH 7.5; [HCO3

-] = 2.00 mM, 25oC

H+

0 500

1000

1500

00.5

11.5

22.5

33.5

4 bicarbonateHEPES

Time (s)

Det

ecte

d H

2O2

(μM

)

ΔH2O2

In solution Added electron donors appear to contribute to H2O2 formation after an initiation phase

[Fe(II)]o = 100 μM; pH 7.5; [HEPES]/[HCO3

-] = 25.0 mM/ 2.00mM, 25oC

H+

Peroxide formation was biphasic in the presence of the organic buffer

0 100 200 300 400

-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

Fe2O3 0.02g/L Linear (Fe2O3 0.02g/L)Fe2O3 0.04g/L Linear (Fe2O3 0.04g/L)Fe2O3 0.06g/L Linear (Fe2O3 0.06g/L)

Time (s)

lnFe

(II)/F

e(II)

0 Fe(II) oxidation reactions in the presence

of Fe2O3

[HCO3-] = 2.00 mM, pH: 7.5, Fe(II)0: 100μM

0 0.2 0.4 0.60.006

0.008

0.01

0.012

0.014

0.016

0.018

surface area loadings (m2/L)Iro

n ox

idat

ion

rate

s(k

obs,

s-1

)

H2O2 yield was inversely proportional to Fe(II) oxidation rate

0 50 100 150 200 250 300 350 400 4500

0.5

1

1.5

2

2.5

3

3.5

0.02g/L Fe2O3 0.04g/L Fe2O3 0.06g/L Fe2O30.08g/L Fe2O3 No added solid

Time (s)

Det

ecte

d H

2O2

conc

. (μM

)

[HCO3-] = 2.00 mM, pH: 7.5, Fe(II)0: 100μM

Hydroxyl Radical yield was directly proportional to Fe(II) oxidation rate

0 50 100 150 200 250 300 350 400 4500

0.050.1

0.150.2

0.250.3

0.350.4

Fe2O3 0.02g/L Fe2O3 0.04g/L Fe2O3 0.06g/LFe2O3 0.08g/L no added solid

time (s)

Det

ecte

d H

O c

onc.

(μM

)

[HCO3-] = 2.00 mM, pH: 7.5, Fe(II)0: 100μM

0 0.5 1 1.5 2 2.50

0.005

0.01

0.015

0.02

kobs as a function of surface area loading in the presence of iron oxides in bicarbonate buffer

Fe2O3Fe3O4FeOOH

Surface Area Loadings (m2/L)

Fe(II

) oxi

datio

n ra

te (s

-1)

0 0.5 1 1.5 2 2.50

0.0020.0040.0060.008

0.010.012

H2O2 degradation rate

Surface Area loadings (m2/L)

H2O

2 de

grad

atio

n ra

te (μ

M/s

)

0 0.5 1 1.5 2 2.50.00.51.01.52.02.53.03.5 Highest H2O2 yield


Hig

hest

H2O

2 Yi

eld

(μM

)

0 0.5 1 1.5 2 2.50

1

2

3

4

5

6

Fe2O3 in HEPESFe3O4 in HEPESFeOOH in HEPES


Det

ecte

d in

itial

[H2O

2] (μ

M)

Ranking H2O2 yield on buffer choice

0 0.5 1 1.5 2 2.50

1

2

3

4

5

6

Fe2O3 in borateFe3O4 in borateFeOOH in borateFe2O3 in HEPESFe3O4 in HEPESFeOOH in HEPES


Det

ecte

d in

itial

[H2O

2] (μ

M)


0 0.5 1 1.5 2 2.50

1

2

3

4

5

6

Fe2O3 in borateFe3O4 in borateFeOOH in borateFe2O3 in HEPESFe3O4 in HEPESFeOOH in HEPESFe2O3 in bicarbonateFe3O4 in bicarbonateFeOOH in bicarbonateFe2O3 in phosphateFe3O4 in phosphateFeOOH in phosphate


Det

ecte

d in

itial

[H2O

2] (μ

M)


Conclusions

• The rate of Fe(II) oxidation is dependent on the removal of Fe(III)

• Buffers that do not restrict Fe(III) solubility promote ROS formation

• Buffers that can serve as electron donors to reactive species like ROS can recycle Fe(III) to Fe(II) if it doesn’t precipitate

• Surfaces are important for accelerating Fe(II) oxidation but their action is chiefly as scavengers of Fe(III), not catalysts for supporting oxidation

Acknowledgement

• Department of Chemistry and Biochemistry, university of South Carolina

• NSF, CHE 1308801

presentation-acs 2015 2-final

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