Yuhoon Hwang 1 , Young- Chul Lee 2,3 , Paul Mines 1 , Yun-Suk Huh 2 , Henrik Andersen 1

1. <8TH Annual Meeting of DWRIP 2014, January 30> Highly stable and reactive nano zero-valent-iron synthesis with Mg-aminoclay and aging characteristics for practical application Yuhoon Hwang1, Young-Chul Lee2,3, Paul Mines1, Yun-Suk Huh2, Henrik Andersen1 • 1Department of Environmental Engineering, DTU, Denmark • 2 Department of Civil and Environmental Engineering, KAIST, Korea • 3 Department of Chemical Engineering, Inha University, Korea

2. Introduction - nZVI for environmental application Heavy metals Cr(VI) As(III) ZVI ClO4- TCE Inorganic ions Chlorinated organics NO3- PCBs Main mechanism: Redox!

3. Introduction - nZVI injection into sub-surface Tratnyek and Johnson, Nanotoday 1, 44-48 (2006)

4. Introduction – nZVI aggregation (Phenrat et al., 2007) • Tendency to agglomerate of uncoated nZVI - Cluster size increased to 70 µm in 30 min (Phenrat et al., 2007) • Undeliverable to the target location for remediation - Uncoated nZVI could migrate only few inches to feet (Li et al., 2006) (Saleh et al., 2007) 1 min 3.75 min 9 min 30 min 25 µm (Li et al., 2006)

5. Introduction – Particle stabilization • Particle stabilization has been achieved by attaching stabilizers onto the NPs. - Surfactant, carboxymethylcellulose (CMC), starch, guar gum, etc. - Provide strong interparticle electrostatic and/or steric repulsion 1 min 3.75 min 9 min 30 min Tween-20 (conc.) Starch (conc.) (Chen et al., 2012) (Dong and Lo, 2013)

6. Introduction – Mg-aminoclay • Mg-aminoclay for stabilizing metal nanoparticles - Stacked lamellar structure, water solubilized by protonation of amine group - Reported as effective stabilizing agent for noble metal NPs (Au, Ag, Pt, Pd) (Datta et al., 2013)

7. Introduction – Aging effect (Kim et al., 2012) • Contact with air and water after nZVI synthesis - Simultaneous oxidation of Fe(0) core to iron oxide • For practical application: synthesis – (transport) – application (injection)  What is the essential step to maintain nZVI properties before application ??

8. Overall objectives • Stable and reactive nZVI synthesis in a Mg-aminoclay solution - Feasibility of Mg-aminoclay as stabilizer : stability in aqueous solution, reactivity (rate & capacity) - Stabilization mechanism - Optimal synthesis condition • Aging study of MgAC coated nZVI - Effect of preparation (washing and storage) procedures

9. Stable and reactive nZVI synthesis in a Mg-aminoclay solution Q. • Mg-aminoclay (MgAC):effective stabilizer for noble metal particles • Is it applicable for nZVI ? (stability & reactivity) • Then, what is the stabilization mechanism?

10. Materials and Methods 1. Synthesis of MgAC coated nZVI (borohydride reduction of ferrous salt) 2. Stability test - Sedimentation - Dynamic light scattering (Aggregate size & zeta potential) 3. Reactivity test - Reactive iron content (acid digestion followed by H2 measurement) - Nitrate reduction batch test(100 mg NO3-N/L, 1000 mg Fe/L) 4. Characterization (KAIST) - TEM, XRD, FTIR

11. nZVI stability Sedimentation test Dynamic light scattering (DLS) • MgAC/Fe ↑: higher stability & smaller particle size •  Feasible as stabilizer for nZVI synthesis

12. Stabilization mechanism Mg-aminoclay ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ nZVI nZVI nZVI nZVI ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ • MgAC/Fe ↑ • : Higher zeta potential (positively charged MgAC) • Stabilization mechanism • : Surface coating by MgAC by charge neutralization • : Inhibition of nuclei growth by steric hindrance of MgAC

13. Reactivity (NO3- reduction) • The low reaction rate of uncoated nZVI: Aggregation during preparation • Increase of Fe(0) normalized reaction rate • : Discrete distribution of individual nZVI particles

14. 2. Aging study of MgAC coated nZVI : Effect of washing & storage Residual species from synthesis (SO42-, Na+, H2BO3-) <nZVI synthesis in lab> MgAC nZVI Solid phase : MgAC-nZVI Reduction Separation MgAC + FeSO4 + NaBH4 Liquid phase : Extra stabilizer : Residual impurities from precursors 2Fe2++BH4−+3H2O → 2Fe0+H2BO3−+4H++2H2 Q. • Is washing step required for stability and reactivity? • Then, what is the reason?

15. Experiment – aging strategies • Aging strategies Set 1 Set 3 Set 2 nZVI MgAC Residual species from synthesis (SO42-, Na+, H2BO3-)

16. Effect of MgAC on stability of nZVI • Particle size: set 1 >> set 2, 3 • Zeta potential: set 1 < set 2, 3  Lower stability in set 1 (pre-washing)  nZVI stability was influenced by excess MgAC in solution

17. Effect of MgAC on stability of nZVI Fresh nZVI washed with NaHCO3 (set 1) • MgAC on the surface of nZVI • Not perfectly coated •  break down and loss MgAC particles Fresh nZVI washed with MgAC (set 3) • MgAC particles in solution

18. Effect of MgAC on stability of nZVI 1d aged sample • Iron oxide plate and needle like structure •  oxidation • Loosen structure Set 1 Washing Set 2 • Existence of surface coating Set 3

19. Effect of washing on reactivity of nZVI • Tendency to decrease: treatment 2 (post) >> treatment 1,3 (Pre) • Difference: existence of residual impurities increase of oxidation rate • No significant effect of excess MgAC on prevention of oxidation NO3- reduction capacity Fe(0) content

20. Increase of nZVI reactivity by electrolyte Antibiotic removal (Ghauch et al., 2009) • Reductant: ZVI powder, NZVI • Electrolyte: 355 mg Cl-/L • Reactivity: Increase! (kobs = 0.025/min  0.041/min) Explosives removal (Kim et al., 2007) • Reductant: ZVI • Electrolyte: 0.5 ~ 50 mM Cl-/L (29 ~ 2900 mg/L NaCl) • Reactivity: Increase! (kobs = 0.0037/min  0.17/min) (Kim et al., 2007) Due to pitting corrosion

21. Summary • Stable and reactive nZVI synthesis with Mg-aminoclay as stabilizer : Thin sheathed grape-like nZVI with high degree of crystallinity : Electrostatic repulsion offered by positively charged MgAC • Aging study of MgAC coated nZVI - Stability: w/ MgAC > w/o MgAC (pre-washing)  Loss of stability by washing of MgAC (surface coating) - Reactivity: Pre-washing > post-washing  The fast oxidation of nZVI due to residual impurities  Importance to consider estimated time frame of application

22. Thank you for attention Any questions & comments?

23. Experiment - analysis • Stability (treatment 1 vs. treatment 2,3) - Particle size & zeta potential (dynamic light scattering) - Morphology (TEM)  Effect of excess MgAC in solution • Reactivity (treatment 2 vs. treatment 1,3) - Aging time: 0, 1, 3, 7 d - Three reactivity assays : (1) Optical density, (2) Reactive iron content, (3) Nitrate reduction capacity : Assumption: relationship between iron content and contaminant reduction - Confirmation by XRD  Effect of residual species from synthesis

24. Aging test – Reactivity (correlation) • 3 factors to show reactivity: initial absorbance, reduced nitrate, Fe(0) • All 3 factors show high linear correlation (R2 > 0.95, P < 0.05) • - Especially, the initial absorbance and Fe(0) content show highest correlation • - Prediction of reactivity change during aging time by more simple analysis !!

25. XRD • Fresh sample showed sharp peaks of cubic α-Fe(0) • Set 2 (Post-storage washing): Fe(0) peaks were totally disappeared • Set 3 (Pre-storage washing w/ MgAC): Fe(0) peaks with co-existed iron oxides  Effect of residual impurities on aging of nZVI

26. Algae harvesting Lee et al. (2014) RSC Adv., 4, 4122-4127