1. Iron (III) Oxide Formation in solution with calcite and other species
Justin Mehl
10/2/15

2. Background
Research has shown that iron oxides can absorb arsenic from contaminated water.
This approach has been employed for water purification on small scale
Could provide a cheap, easy to use method for affected regions such as Bangladesh and countries in Eastern Africa
Normally, iron oxides are used as a coating on another substrate (such as silica or activated carbon).
Only one group (Uygur) has used a Fe coated calcite for Zn removal; others have used a conglomerate of Fe & CaCO3

3. Goals – Fe rx with CaCO3
Identify process of iron oxide formation in the presence of calcite
What type of iron oxide forms, and at what rate
Differentiate between influence of Ca2+, CO32-, chloride vs nitrate iron salts, hydroxide, and bicarbonate on formation
Determine the extent of adsorption of iron oxide on surface of calcite crystals (Iceland Spar)
Starting material
We did not use Fe(II)
Figure 1: Iron oxide formation under natural conditions (limited cofactors)

4. Experimental set up
We ran 6 experiments, each following the same basic procedure with a few variations:
Basic procedure
add a base (sodium hydroxide, sodium bicarbonate) to a solution of iron salt (ferric nitrate, ferric chloride) to cause iron crystals to precipitate.
pH held constant at 7.5
Variations:
Iron salt: ferric nitrate, ferric chloride
Base: sodium hydroxide, sodium bicarbonate
Substrate:
calcium nitrate
calcite powder
calcite grains (cubic, ~1mm side lengths);
large calcite crystals

5. Reactions in the presence of calcite
Take aways:
Experiment 1 showed goethite by day 1, disappeared by day 2
Exp 1-5 showed hematite (in raman spec) and no goethite regardless of salt or titrant
Fe salt used
Titrant
Calcium substance
Results
Experiment
FeCl3
Fe(NO3)3
NaOH
NaHCO3
Ca(NO3)2
Calcite powder
Exp 1 ✓
- Goethite on day 1, disappears by day 2
Exp 2
Exp 3
Exp 4
(Exp 3.1)
- Goethite formed by day 3
Exp 5
Sample 1-3
✓ (5, 15, 25 mM)
- Weak goethite formation by day 3
Sample 4
- Strong goethite formation by day 3
Calcium seemed to decrease goethite formation

6. Experiments 1-4 IR data
We used IR spectroscopy to detect goethite; and Raman spectroscopy to detect hematite
In samples 1-4, goethite converted to hematite rapidly (on timescale of hours)
The speed of this conversion was unanticipated (not known that calcite would have any effect on iron oxide formation kinetics)
Calcite, strong goethite
Calcite, medium goethite
Calcite, weak/no goethite
1000
950
900
850
800
750
700

7. ExpeFe(NO3)3 and calcite grains titrated with HCO32-
1 – 1.5 d: nanorod crystals (representative of goethite)
1.9 – 2.4 d: platy crystals (representative of hematite)
Proof that goethite is converting to hematite (and that there is an accelerating factor, likely calcite)

8. Conclusions Next Steps
If goethite is forming, it forms rapidly in the presence of calcite, and is gone within 2-3 days at both 70 and 90 C
Coatings of iron oxide on calcite grains begin as nanorods and convert to a smaller crystal within 2 days
Presence of Ca2+ and/or CO32- may be influencing the iron oxide formation
Next Steps
For As removal: repeat Experiment 6 using limestone grains, and determine both Fe oxide polymorph as well as extent of coating that can be achieved
Test arsenic adsorption by different polymorphs
Monitor effect of Ca2+ and/or CO32- on Fe oxide formation to determine impact

9. Thank you’s and Acknowledgements
Thank you to:
PEI, for funding my summer internship; and the Princeton Geosciences Department
My PI, Professor Satish Myneni; and researcher, Dr. Sarah Jane White
My partner Rebecca Lowy
Dr. Nan Yao, John Schreiber, Paul Shao; and the rest of the staff at Princeton Imaging and Analysis Center
Labmates Chris Habermann and Aparna Raghu