1. Application of Iron Oxide Nanoparticles/silica Composites on the Removal of Uranium(VI) for Drinking Water Treatment and the Impact of Water Chemistry
Michelle Heredia and Zezhen Pan
Energy Environmental and Chemical Engineering Washington University

2. Outline Background Knowledge Gap Objectives and Hypothesis
Experimental Design
Implications
Energy Environmental and Chemical Engineering Washington University

3. Background: Why U(VI) and Iron Oxide Nanoparticles
Widespread uranium contamination
Increasing attention on the removal of uranium from drinking water systems
New approach for the removal with iron oxide nanoparticles (IONPs): High surface area/chemical affinity and reduction capability
Uranium contamination over the U.S.
Research by University of Nebraska-Lincoln 2015
Energy Environmental and Chemical Engineering Washington University

4. Background: IONPs in Water Treatment system:
Synthesized with the presence of organic acid (oleic acid and steric acid) to form organic acid coated nanoparticles
Sorption at pH 5.6, 7 and 8.5 showed almost the highest reported U(VI) sorption capacity
U(VI) reduced to U(IV) species
Li et al., Environ Sci.: Nano. 2015
Energy Environmental and Chemical Engineering Washington University

5. Background:
Impact of Synthesis of IONPs and the effect on U(VI) removal
Influence of magnetite stoichiometry on uranium(VI) removal
Magnetite stoichiometry causes uranium(VI) to undergo reduction to uranium(IV), correlated with the oxidation of Fe2+ in magnetite.
In stoichiometric and partially oxidized magnetite, a higher bulk ratio led to more reduced uranium(IV) while more oxidized magnetite had less reduction process.
Reference
Energy Environmental and Chemical Engineering Washington University

6. Background: What is not clear?
The study focused on the sorption performance but not thorough investigation on the water chemistry
- broad pH impact on U(VI) species
no control experiments to exclude the carbonate content
need detailed information for the sorption mechanisms
U(VI) speciation diagram: TOTU=0.01 mM,
open to atmosphere
Energy Environmental and Chemical Engineering Washington University

7. Background: IONPs cooperated with other substrates:
Most studies on IONPs and silica focused on IONPs:
Deposited onto amorphous silica (not strong bind)
Effects of initial pH, amount of adsorbent, shaking time, and initial concentration of uranium (VI) on uranium sorption efficiency
Incorporated with silica nanoparticles (remain as nanoscale sorbents)
U(VI) adsorption by silica nanoparticles were enhanced with the presence of centered IONPs and surface functional groups
What is not clear?
Influence of amorphous silica on the sorption capacity of IONPs
Influence of IONPs on silica nanoparticles
Role of surface functional groups (CTAB)
- Mechanisms of the impact of water chemistry on sorption performance
Egodawatte., Langmuir. 2015
Energy Environmental and Chemical Engineering Washington University

8. Background: Summary of remaining questions
For real water treatment system, nanoparticles will not be loaded into columns for large amount of water treatment because of the low permeability
Stronger IONPs/silica composites will be stable sorbents for application instead of IONPs deposited to silica surface (formation of IONPs on the silica surface instead of deposit IONPs onto silica)
The sorption capacity of each component in the composite and the impact of each component to the other
Sorption mechanisms remain unclear, the sorption sites and the distribution of U(VI) have not been investigated thoroughly: e.g. Whether inner-sphere or outer-sphere complexes
The impact of water chemistry needs more investigation and explanation: e.g. when reduction or precipitation happen that account for the sorption capacity
Energy Environmental and Chemical Engineering Washington University

9. Objectives and Hypothesis
Develop, synthesize and characterize surface modified iron oxide nanoparticles/silica composites as sorbent media
Hypothesis:
During the synthesis, the addition of iron solutions and the concentration could largely impact the formation of IONPs on silica surface and the coverage of silica surface.
Cover of nanoparticles on the silica surface
Energy Environmental and Chemical Engineering Washington University

10. Objectives and Hypothesis
Investigate the sorption of uranium(VI) by each component and by sorbent composites in various water chemistry conditions
Hypothesis:
There will be different optimal sorption conditions for each component.
The overall sorption by the sorbent composite is contributed but not a simple summation of each component; the sorption behavior of the component in the composite is affected by others.
Modified coordination environments may be observed between uranium(VI) with sorbents
Energy Environmental and Chemical Engineering Washington University

11. Objectives and Hypothesis
Evaluate the stability of sorbed uranium and the approach for the desorption of uranium from sorbent media.
Hypothesis:
Different response of sorbed uranium to changing water chemistry (ionic strength and pH) will be expected for desorption.
Energy Environmental and Chemical Engineering Washington University

12. Task 1: Synthesize IONPs/Silica Composites
Sorbent media synthesis:
Materials:
- A mixture of FeCl3 and FeCl2
- NaOH and NaCl
Silica for IONPs/Silica composites
Post surface modification with CTAB by sonication
Optimal synthesis conditions:
- Concentration of FeCl3 and FeCl2
- Concentration of NaOH and NaCl
Addition speed of NaOH
- Ratio of silica to solution
Coverage and efficiency of nanoparticle/silica composites
Energy Environmental and Chemical Engineering Washington University

13. Task 1: Synthesize IONPs/Silica Composites
Characterization of IONPs and IONPs/Silica composite
X-ray Diffraction (XRD)
X-ray Absorption near edge structure (XANES)
- Fe(III) and Fe(II)
Transmission Electron Microscopy (TEM):
- Determine particle size
3. Dynamic Light Scattering (DLS):
- Quantify hydrodynamic diameter
4. Scanning Electron Microscopy (SEM):
Investigate the morphology of sorbent
BET
- Surface Area
To see the silica surface with NP growth
Energy Environmental and Chemical Engineering Washington University

14. Task 1: Outcome From task 1, we can determine:
Optimal synthesis conditions
Morphology and coverage of IONPs on silica surface
Reference for solid:water ratio for Task 2 and 3
Energy Environmental and Chemical Engineering Washington University

15. Task 2: Quantify Sorption Capacity
Batch Sorption Experiments:
Sorbents:
Silica
CTAB-IONPs
IONPs/Silica Composite
CTAB-IONPs/Silica composite
Conditions:
Procedure:
Prepare sorbent solution
Add UO2(NO3)2
Open/close to atmosphere
Adjust to target pH
Rotator for 24 hours
Variable
Range of values
Significance to study pH
5 - 9
Water chemistry
Ionic Strength (M)
10-6 ~ 10-2
U(VI) loadings (mg/L)
0.1 ~ 10
Energy Environmental and Chemical Engineering Washington University

16. Task 2: Characterize Sorption Mechanism
NPs/Silica-U(VI) suspension
Settled solids
Ultracentrifuge
0.22 µm Filter
Filtrate
[U(VI)]
Liquid analysis:
ICP-MS
Elemental concentration
DIC
Carbonate content
Extended X-ray Absorption Fine Structure (EXAFS):
- Coordination of U(VI) with sorbents
2. X-ray Absorption Near Edge (XANEs):
- Reduction of U(VI)
3. Laser-induced Florescence Spectroscopy (LIFS):
- Precipitation of U(VI)
Energy Environmental and Chemical Engineering Washington University

17. Task 2: Outcome Liquid analysis
- Determine the optimal sorption condition
Solid characterization
- Detailed knowledge about the sorption mechanisms
Thorough information of the impact of U loading, pH influence the reduction, precipitation amount of U
Evidence of the coordination environments for U(VI) with IONPs, Silica and IONPs/Silica
Energy Environmental and Chemical Engineering Washington University

18. Task 3: Investigate the Desorption Process
Batch Desorption Experiments:
Material:
- U(VI) associated sorbents
- Strong acid, surfactants
Procedure:
Add acid/surfactants
Rotator for 24 hours
Determine the U(VI) concentration in the liquid phase
Outcome:
- Determine the recovery efficiency and separate sorbed U(VI) into a small volume of contaminated sorbent
Recovery of U(VI)
Energy Environmental and Chemical Engineering Washington University

19. Implications
An enhanced understanding about the scientific information on the removal of metals have similar properties with U(VI) in drinking water to improve public health, determine which method out of many is optimal for wide scale implementation of the metal removal process in water systems worldwide, but also benefit other environmental technologies related to field remediation.
Elucidate the mechanisms responsible for the sorption process on the relatively stable composites compared to existed studied materials;
Identify the coordination structures for uranium(VI) associated sorbent media;
Determine the main factors in water chemistry that would influence the sorption capacity and play as a reference for selecting the optimal treatment conditions;
Provide basis for future application in large scale.
Energy Environmental and Chemical Engineering Washington University

20. Thank you
Energy Environmental and Chemical Engineering Washington University