1. Assimilation of Iron in the Ocean: Acid dissolution of Micro and Nano Goethite in the Presence of Inorganic Oxy-anions
Patrick Kyei, Gayan R. Rubasinghege Grassian Group
Department of Chemistry, University of Iowa REU, Iowa City, IA 52242
Abstract
Iron dissolution in the ocean has been of great importance in recent studies due to its role in phytoplankton development. Although this process is influenced by other factors, acids and anions are of ample concern since they are readily available in the atmosphere. In this study, dissolved α-FeOOH particles were characterized, and examined in solutions of nitric acid and inorganic oxy-anions. This is to help understand the effects of these anions on Fe (II) reaction. To obtain this goal, 50% of the solution used was made up of acid with 0.02M concentration and the other 50% was nitrate, or carbonate or phosphate. Two iron source materials- nanorods and microrods of α-FeOOH- were used. The reactions were performed in the dark. Samples were taken at specific time intervals to measure Fe (II) and total iron dissolved. In general, it was observed that the amount of dissolved total iron and Fe (II) increased as a function of time. More importantly, there were higher amounts of dissolved iron from nanorods compared to microrods in the absence of inorganic ions. However, dissolution from nanorods quenched in the presence of inorganic anions due to particle aggregation at higher ionic strength.
Introduction
Results
Microscopic Evidence
Low iron concentrations have been shown to limit primary production rates, biomass accumulation, and ecosystem structure in a variety of open-ocean environments, including the equatorial Pacific, the subarctic Pacific and the Southern Ocean and even in some coastal areas. Anions have been noted to influence this process due to their abundance in nature. Here, α-FeOOH; nano and micro particles were reacted with nitric acid in the presence of inorganic anions such as nitrate, carbonate, and phosphate. The reactions were carried out in the dark so as to replicate the ocean and pacific night time environment.
Dissolution of Iron
(a) Unreacted nanorods
(c) NO3-
(e) PO43-
(d) CO32-
200 nm
500 nm
50 nm
Nanorods
Microrods
Time (h)
Dissolved Total Fe (M.g-1)
500
1000 10 20 30 40 50
Nanorods
Microrods
Time (h)
Dissolved Total Fe (M.cm-2) 3 6 10 20 30 40 50 9 B A
A comparison of the dissolution data of nanorods versus microrods at pH 2 using N HNO3 in the dark.
A – Mass normalized
B – Surface area normalized.
There was no Fe (II) formation.
50 nm
(b) Unreacted microrods
(d) NO3-
(f) PO43-
(d) CO32-
2 m
200 nm
1 m
500 nm
Total iron dissolution for the different anions can be related to the strength and mode of anion adsorption, and the ability of the complex to be further protonated. O
Surf Fe OH
+ A-
- OH- H+
Fe(OH)A+ A H+
NO3-
CO32-
PO43-
Fe2+
OH-
Fe3+
HNO3
Nanorods indicates an enhancement factor of ~3 excluding the surface area effect at pH 2 compared to microrods. (Fig. A & B)
Production of Fe (II)
H2O
Microrods
Time (h)
Dissolved Fe (II) (M.g-1)
2000 10 20 30 40 50
1000
PO43-
CO32-
NO3-
Nanorods
Time (h)
Dissolved Fe (II) (M.g-1)
2000 10 20 30 40 50
1000
PO43-
CO32-
NO3-
Wet / dry deposition H I
Effect of Oxy-anion
land
ocean
Microrods
The dissolution is quenched by a factor of ~1/120 at pH 1.0 for nanorods in dark.
Aggregation seem to suggest the reason for the very low dissolution of nanorods in PO43- and CO32-.
Nanorods
Time (h)
Dissolved Total Fe (M.g-1)
2000 10 20 30 40 50
1000
PO43-
CO32-
NO3-
Time (h)
Dissolved Total Fe (M.g-1)
2000 10 20 30 40 50
1000
PO43-
CO32-
NO3- C D
Experimental Setup
Time (h)
Dissolved Total Fe (M.cm-2) 40 10 20 30 50
PO43-
CO32-
NO3-
Time (h)
Dissolved Total Fe (M.cm-2) 40 10 20 30 50
PO43-
CO32-
NO3-
Dissolution was monitored for nanorods [75 ( 20) nm by 6 ( 2) nm; 119 m2/g] compared to microrods [1006 ( 55) nm by 25 ( 6) nm; 39 m2/g] of goethite. J K
Goethite
(2g / L)
pH probe
Temperature probe
Teflon air-tight cap
Glass window
Sample collecting port
N2 (g) purge
HNO3 acid solution (70 ml)
Stirred at ~220 r.p.m. at a constant temperature of 298 K.
Particle-size effect
Time (h)
Dissolved Total Fe (M.cm-2) 40 10 20 30 50
Microrods
Nanorods
NO3-
PO43-
Time (h)
Dissolved Total Fe (M.cm-2) 40 10 20 30 50
Microrods
Nanorods
Time (h)
Dissolved Total Fe (M.cm-2) 40 10 20 30 50
Microrods
Nanorods
CO32-
Provide evidence that differentiate between the surface chemistry of nano compared to micro goethite.
There was very low distribution of nanorods. This can be attributed to their particle size and surface area. (Fig. I & K) E F G
Microrods
Nanorods
Experiments were carried out in the dark, wrapped in aluminum foil.
The temperature was kept constant at 25 C.
Ferrous iron was measured calorimetrically with 1, 10-phenanthroline.
Conclusions and Environmental Implications
The surface area of micro goethite compared to nano goethite accounted for their higher dissolution. (Fig. E, F & G)
Fe (II) production was low since there was no photochemical redox chemistry of goethite α-FeOOH. But it could be observed that some significant amount of Fe (II) was found in PO43- and CO32-. This accounted for the high total iron in phosphate. The mechanism for this occurrence is yet to be known.
In the case of NO3-, the process quenched due to aggregation. This is to suggest that ionic strength cannot be considered as the role player. This is because all solutions had the same ionic strength.
Also it can be noted that particle size and surface area continued to affect the solubility of Iron.
Acknowledgement
Reference
Gayan Rubasinghege and Grassian Group.
This material is based on work supported by the NSF REU program (CHE ).
Iowa Center for Research by Uundergraduate
1Cwiertny, D. M.; Hunter, G. J.; Pettibone, J. M.; Scherer, M. M.; Grassian, V. H., J. Phys. Chem. C 2009, 113, (6),
2Jickells et. al., Science 2005, 308, (5718),
3Rubasinghege, G.; Lentz, R. W.; Park, H.; Scherer, M. M.; Grassian, V. H. Langmuir 2009.