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

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

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)

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

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

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

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)

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

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