1. Heterogeneous Photocatalytic Reduction of Chromium (VI) by Fe3O4/TiO2 Core/Shell Nanoparticles
Chao Pan and Cheng-En Sung
April 29th, 2016
EECE 534 Environmental Nanochemistry
Good morning, everyone. I’m Chao Pan from Washington University. I’m glad to be here to talk about the Effect of humic acid on Cr(VI) removal from water by electrocoagulation. I would acknowledge my collaborators..and also my advisor Daniel Giammar. The project is funded by the national science foundation.

2. Stability diagram for Cr(VI)/Cr(III)
Chromium toxicity and New Drinking Water Regulations
Chromium is widely used in industry and has resulted in a tremendous contamination.
Chromium(Cr): Cr(VI) and Cr(III).
The current U.S. drinking water standard for total Cr is 100 µg/L.
California has a drinking water standard for total Cr of 50 μg/L and maximum contaminant level (MCL) of 10 μg/L for Cr(VI).
New treatment technology needs to be implemented in drinking water for potential Cr(VI)-specific regulation nationwide.
Cr(OH)2+
Thus the new water treatment technology needs to be implemented to comply with potential new regulation.
Stability diagram for Cr(VI)/Cr(III)
for 10-7 M total Cr

3. Current methods for Cr(VI) removal
Adsorption onto iron oxides (goethite, hematite, and ferrihydrite): The adsorption is highly dependent on water chemistry ( common anions , pH and Fe(III) dosage)
Cr(VI) reduction by:
Fe(II);
Greenrust
Pyrite
Magnetite (Fe3O4); (Fe(II) is highly susceptible to auto-oxidation; decrease in the magnetic properties with the reduction of Cr(VI))
Heterogeneous photocatalysis under UV/Visible light
TiO2 is the most studied semiconductor widely used for environmental decontamination for its high activity, chemical inertness, and low cost; However we need to propose suitable methods for collecting TiO2 after treatment due to the toxicity itself.

4. Megnetite ( 𝑭𝒆 𝟑 𝑶 𝟒 )
𝑭𝒆 𝟑 𝑶 𝟒
Iron magnetic nanoparticle possess high surface areas
Easy to separate and recover by applying external magnetic field
Mixture of Fe(II) and Fe(III)
Fe(II) initiate the reduction of Cr(VI) to Cr(III)
Cr(III) is chelated by OH group and form inner-sphere complex
Cr(III) form as Cr(OH)3 and FexCrx(OH)3

5. TiO2
Recyclable photocatalysis which utilize natural sunlight to induce redox reactions
High stability, nontoxicity, and redox potential to oxidize pollutants
Simultaneous adsoption and detoxification of Cr(VI) in aqueous condition
When expose to UV-light, electron ( 𝑒 − ) is promoted to conduction band, while generates hole ( ℎ + ) in valence band.
Reduction: 𝐶𝑟 𝑉𝐼 →𝐶𝑟 𝐼𝐼𝐼
Oxidation: 𝐹𝑒 𝐼𝐼 →𝐹𝑒 𝐼𝐼𝐼
Photo-induced hydrophilic conversion of TiO2 ; Photo-induced redox reaction of absorbed substances

6. Fe3O4-TiO2 benefit (Cr(VI) reduction)
TiO2+ h𝝂 = TiO2 ( 𝒆 − -- 𝒉 + ) 𝑪𝒓 𝟐 𝑶 𝟐− + 14 𝑯 𝒆 − = 2Cr3+ + 7H2O
Spherical shape ??

7. Objectives (1): Synthesis XRD : crystal lattice
Water stabilize with controlled size, composition, and surface coating
Synthesis
XRD : crystal lattice
TEM : particle size, and morphology
FTIR : characteristic bond

8. Objective 1: Fe3O4/TiO2 Core/Shell Nanoparticles
𝑭𝒆 𝟑 𝑶 𝟒 𝒏𝒂𝒏𝒐𝒕𝒖𝒃𝒆:
Mixture of 𝑭𝒆 𝑪𝒍 𝟑 , 𝑵𝑯 𝟒 𝑯 𝟐 𝑷𝑶 𝟒 ,𝒂𝒏𝒅 𝑵𝒂𝑺𝑶 𝟒 in water with continuous stirring at 220ºc, for 48h. Collect the precipitates when cooling down to room temperature.
𝑭𝒆 𝟑 𝑶 𝟒 / 𝑻𝒊𝑶 𝟐 nanotube:
𝑭𝒆 𝟑 𝑶 𝟒 immerse in DI water stirring, and then add in 𝑻𝒊(𝑺𝑶 𝟒 ) 𝟐 aqueous solution in room temperature for 3h. And collect the precipitates.

9. Solid characterizations
TEM enables direct 2-D imaging of particle size, shape, surface morphology characteristics and core-shell structure of 𝑭𝒆 𝟑 𝑶 𝟒 / 𝑻𝒊𝑶 𝟐
TEM image of the crystalline Fe3O4/TiO2 core/shell nanotubes
(a): crystalline core/shell nanotubes (b) bare Fe3O4 nanotubes
One difference is that the core/shell nanostructures exhibit morphologic characteristics of double walls

10. Solid characterizations
XRD can be used to for phase identification of a crystalline material and can provide information on unit cell dimensions.
Study crystal lattice of 𝑭𝒆 𝟑 𝑶 𝟒 core in 𝑭𝒆 𝟑 𝑶 𝟒 / 𝑻𝒊𝑶 𝟐 by the peak position and relative intensity.
Identify the transformation of magnetite ( 𝑭𝒆 𝟑 𝑶 𝟒 ) to maghemite ( 𝑭𝒆 𝟐 𝑶 𝟑 ) during removal of Cr(VI).
XRD patterns of the (a) Fe3O4 sphere (b) Fe3O4/Ti(OH)x core/shell spheres (c) Fe3O4/TiO2 hollow spheres
FTIR: The characteristic band of 𝑭𝒆 𝟑 𝑶 𝟒 (600 𝒄𝒎 −𝟏 ) and 𝑻𝒊𝑶 𝟐 ( 𝒄𝒎 −𝟏 ).

11. Objective 2: Investigation of aqueous stability of synthesized nanoparticles
Stability in different ionic strength, cation types and pH
Increase ionic strength will decrease stability. (1-50 mM NaCl, 𝟎−𝟑 𝒎𝑴 𝑪𝒂 𝟐+ ,𝟎−𝟓 𝒎𝑴 𝑴𝒈 𝟐+ ). And pH from 5 to 9.
DLS : zeta potential, hydrodynamic size of the particles

12. Objective 2: Investigation of aqueous stability of synthesized nanoparticles
Dynamic light scattering(DLS): particle size measurement; electrophoretic mobilities (EPM)
The zeta-potential of nanoparticles is an important factor for characterizing the stability of colloidal disperions and provides a measure of the magnitude and sign of the effective surface charge associated with the double layer around the colloid particles
Coagulation of nanoparticles in the presence of Ca2+
Theoretical analysis on stability of nanoparticles
Whether the net interaction between two particles is repulsive or attractive depends on the sum of these two forces, which is expressed as follows
DLVO theory is a well known method to evaluate the stability of particles in water by analyzing the interaction energy between particles.

13. Objective 3:Cr(VI) removal and the effect of water chemistry
Hypothesis 3A: Fe3O4/TiO2 Core/Shell nanoparticles could reduce Cr(VI) very fast under UV/Visible conditions.
Hypothesis 3B: The pH will strongly affect Cr(VI) removal by altering Cr(VI) reduction rates, reduction mechanisms, and the stability of the solid-associated Cr(III) species. The balance of these processes will result in optimal removal near neutral pH.
Hypothesis 3C: Common water constituents (sulfate， phosphate, dissolved silica and natural organic matter) will influence Cr(VI) removal by altering the interfacial reactions and controlling the rate and identity of the solids produced.

14. Objective 3:Cr(VI) removal and the effect of water chemistry
Batch experiments of Cr(VI) adsorption and reduction by Fe 3 O 4 /TiO2/UV-Vis .
Control experiments includes Cr(VI) adsorption in the dark and also the reduction system with pure anatase and rutile TiO2.
Parameters study:
liquid samples Cr(VI): DPC methods;
Fe(II): Ferrozine methods;
dissolved chromium and Fe: ICP-MS;
Solid samples XAS
Sample preparation for XAS consisted vacuum filtration

15. Objective 3:Cr(VI) removal and the effect of water chemistry
X-ray absorption spectroscopy (XAS) and Cr stable isotope measurements can distinguish among mechanisms of Cr removal and identify the resulting products
XANES: directly quantify Cr oxidation state, which can provide the Cr(VI) to Cr(III) ratio in a sample with an accuracy of 3 mol%.
EXAFS: the coordination environment of Cr, such as Cr(III) adsorbed onto an nanoparticle surface, can also be directly determined;
EXAFS can also quantify the abundance of Fe solid phases after Cr(VI) reduction

16. Environmental implication and expected results
The combination of the photocatalysis properties of TiO2 and the superparamagnetic property of Fe3O4 nanoparticles endows this material with a bright perspective in purification of Cr(VI) polluted water under visible light.
We believe that can provide an efficient, environmentally benign, and low-cost approach for the removal and photodegradation of recalcitrant contaminants present in water supplies.

17. Thanks! Questions?