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Electrochemical Oxidation for Water Treatment and the Limitation of Hazardous Byproducts

Electrochemical Oxidation for Water Treatment and the Limitation of Hazardous Byproducts. AWRA Meeting Philadelphia, PA March 21, 2013 Adrienne Donaghue Brian P. Chaplin Villanova University Department of Civil & Environmental Engineering . Introduction.

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Electrochemical Oxidation for Water Treatment and the Limitation of Hazardous Byproducts

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  1. Electrochemical Oxidation for Water Treatment and the Limitation of Hazardous Byproducts AWRA Meeting Philadelphia, PA March 21, 2013 Adrienne Donaghue Brian P. Chaplin Villanova University Department of Civil & Environmental Engineering

  2. Introduction Electrochemical oxidation has become promising for treatment of recalcitrant and biorefractory waste streams Advantages: • Easy installation and operation • Cost effective • Environmentally friendly

  3. Environmental Applications Oxineo ® Electrochemical Oxidation Pilot plant for landfill leachate in Cantabria, Spain.

  4. Electrochemical Reactions _ Power Supply + e- OH OH OH OH OH OH OH Anode Cathode H2O  OH + H+ + e-

  5. Electrochemical Oxidation Destruction of pollutants occurs through 2 mechanisms: 1. Direct electron transfer (DET) 2. Indirect oxidation via hydroxyl radicals (OH●) * Electrochemical material plays important role in the effectiveness of oxidation! R Direct electrochemical oxidation RO current R or RO e- Free •OH Indirect electrochemical oxidation CO₂ + H₂O •OH Adsorbed •OH Oxygen Evolution Anode Zhu et. al, 2008.

  6. Boron Doped Diamond Electrode • Boron-doped diamond (BDD) film grown on p-silicon substrate using CVD (Advanced Diamond Technologies). • Boron doping @ ppm levels provides electrical conductivity. • Inert surface and low adsorption properties • Remarkable corrosion satiability • Produces large amount of OH● (weakly adsorbed) • Emerging AOP technology. • Can oxidize perfluorinated compounds Note! These compounds can not be degraded by other AOP technologies

  7. PerfluorooctaneSulfunate (PFOS) (C₈F₁₇SO₃⁻) Farrell et al. (2008)

  8. By-product/Perchlorate (ClO4-) Formation • Is a multi-step process • Hazardous to human health • EPA set an advisory limit of 15 ppb for drinking water sources • CA and MA drinking water limits of 2 and 6 ppb Rate-limiting step Cl- OCl- ClO₂- ClO₃- ClO₄-

  9. By-product/ClO4- Formation Cont. 2 step process: • Rate-limiting step ClO₃⁻ Reaction Zone ClO3● 1. e- Cl- OCl- ClO₂- ClO₃- ClO₄- OH● 2. Anode ClO4- Azizi et. al, 2011

  10. Research Objectives • Understand how the reactivity of certain organics effect perchlorate formation at the anode surface • Use “model” p-substituted phenols to determine the importance of each step in the two step process of perchlorate formation. • Model organic behavior with in the diffuse and reaction zones to understand mechanisms of inhibition of ClO4- at the anode surface 1) 2)

  11. Experimental Setup Batch Reactor Rotating Disk Electrode (RDE) Organic compounds p-nitrophenol (p-NP) p-methoxyphenol (p-MP) p-benzoquinone (p-BQ) Oxalic acid (OA) Solutions were tested at: Kinetically Control: 1.0 mA/cm² Mass-transfer Control: 2.4 mA/cm², 10.0 mA/cm²

  12. Results: ClO₄⁻ Formation Initial Organic Concentration = 250 μM * Buxton et al. 1988

  13. Results: Organic Reactivity Anode Diffuse Layer ReactionZone Diffusion Zone C OH● RB Anode Surface ClO₃● x/L 2 μm 5μm COMSOL ®

  14. Results: ClO₄⁻ Formation

  15. Conclusions: Limiting ClO₄- formation • Rate limiting step is a two step process • Reactions occur right at surface • Organic reactivity is important ClO₃⁻ Step 1 e- ClO3● Anode Step 2 OH● For Low Current Densities: Scavenging occurs on surface For High Current Density: Location becomes important ClO4-

  16. Conclusion Cont. • Operating under MT conditions is the most effective means to limit ClO4- formation. • In addition, operating at these conditions is cost effective. • EC is viable technology for refractory organic pollutants but in order for it to be integrated into environmental applications, ClO4- must be inhibited below advisory levels.

  17. Acknowledgements This research was funded by Advanced Diamond Technologies (ADT) in Romeoville, IL via NSF SBIR Phase II grant. Special thanks to my advisor Dr. Brian P. Chaplin

  18. Questions?

  19. Results: LSV of p-substituted phenols 1.0 mM 0.75 mM 0.25 mM Blank 0.50 mM Blank 10 mM 5 mM 1 mM Blank

  20. Measured Rates vs. Mass Transfer

  21. R R R OH● OH● OH● OH● Anode ClO3● ClO3● ClO3● ClO₄⁻ ClO₄⁻

  22. Reaction Zone ClO₃⁻ e- ClO3● Anode OH● ClO4-

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