Post on 14-Apr-2017
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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
Methods – Batch oxidation of Fe(II) in the presence of varying particulates
[Fe(II)]0 = 100 μMpH 7.5
Fe2O3
Fe3O4
FeOOH
Experimental design Analytical approaches
Fe(II), ferrozine method
H2O2, amplex red /horse radish peroxidase
HO, terephthalic acid
BO33-
HEPESHCO3
-
PO43-
2-[4-(2-hydroxyethyl) piperazin-1-yl] ethanesulfonic acid
air saturated
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
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
Surface Area loadings (m2/L)
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
Surface Area Loadings (m2/L)
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
Surface Area Loadings (m2/L)
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 HEPESFe2O3 in bicarbonateFe3O4 in bicarbonateFeOOH in bicarbonateFe2O3 in phosphateFe3O4 in phosphateFeOOH in phosphate
Surface Area loadings (m2/L)
Det
ecte
d in
itial
[H2O
2] (μ
M)
Ranking H2O2 yield on buffer choice
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