Formation of chlorate and perchlorate during electrochemical oxidation by Magnéli phase Ti4O7 anode - inhibitory effects of coexisting constituents
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-09-23 |
| Journal | Scientific Reports |
| Authors | Lu Wang, Yaye Wang, Yufei Sui, Junhe Lu, Baowei Hu |
| Institutions | University of Georgia, Shaoxing University |
| Citations | 12 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”- Anode Performance Comparison: Electrochemical oxidation (EO) of chloride (Cl-) was evaluated using Magnéli phase Ti4O7 and Boron Doped Diamond (BDD) anodes, focusing on the formation of toxic by-products, chlorate (ClO3-) and perchlorate (ClO4-).
- Slower Formation on Ti4O7: The formation rates of both ClO3- and ClO4- were consistently and significantly slower on the Ti4O7 anode compared to the BDD anode, regardless of the supporting electrolyte used (Na2SO4, NaNO3, Na2B4O7, Na2HPO4).
- Perchlorate Yield Reduction: Using NaNO3 as the supporting electrolyte on Ti4O7 resulted in only 9% of the initial Cl- being transformed to ClO4- after 4.0 h, demonstrating superior selectivity compared to BDD.
- Effective Inhibition Strategy: The addition of co-existing constituents—Methanol (MeOH), Hydrogen Peroxide (H2O2), and Potassium Iodide (KI)—was shown to effectively mitigate or eliminate ClO3- and ClO4- formation on the Ti4O7 anode.
- Optimal Inhibitor Identified: Near-complete inhibition of ClO3- and ClO4- formation was achieved with 100 mM KI, which is considered stable, accessible, and inexpensive for practical EO system implementation.
- Mechanism: Inhibitors function by either consuming reactive chlorine species (like HOCl) or competing with Cl- for oxidation (e.g., I- is oxidized to the benign iodate, IO3-).
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Anode Materials Tested | Ti4O7 & Si/BDD | N/A | Electrochemical Oxidation (EO) |
| Initial Chloride Concentration | 1.0 | mM | Standard solution concentration |
| Constant Current Density | 10 | mA cm-2 | Standard operating condition |
| Max ClO3- (BDD) | 276.2 | µM | Reached at 0.5 h (Na2SO4 electrolyte) |
| Max ClO3- (Ti4O7) | 350.6 | µM | Reached at 4.0 h (Na2SO4 electrolyte) |
| ClO4- Formation Time (BDD) | 4.0 | h | Time to reach approx. 1000 µM ClO4- (Na2SO4) |
| ClO4- Formation Time (Ti4O7) | 8.0 | h | Time to reach 780.0 µM ClO4- (Na2SO4) |
| KI Concentration for Inhibition | 100 | mM | Achieved near-complete inhibition of ClO3-/ClO4- |
| MeOH Concentration for Inhibition | 1000 | mM | Achieved complete inhibition of ClO3-/ClO4- |
| H2O2 Concentration for Inhibition | 1000 | mM | Significantly limited ClO3-/ClO4- formation |
| Fitted Rate Constant k1 (Ti4O7, Control) | 9.78 x 10-5 | s-1 | ClO3- formation rate (Na2HPO4 electrolyte) |
| Fitted Rate Constant k1 (Ti4O7, 100 mM KI) | 0 | s-1 | ClO3- formation rate (Near-complete inhibition) |
Key Methodologies
Section titled “Key Methodologies”- Reactor Setup: Experiments utilized an undivided rectangular acrylic cell (10 cm x 5 cm x 2.5 cm) containing 200 mL of electrolyte solution, continuously stirred at 700 rpm.
- Electrode Materials: Magnéli phase Ti4O7 ceramic plates and Si/BDD plates (both sides coated) were used as anodes (78 cm2 total geometric surface area). 304 stainless steel plates served as cathodes.
- Electrochemical Operation: A constant current density of 10 mA cm-2 was supplied using a controllable DC power source. All tests were conducted at ambient room temperature.
- Electrolyte Variation: Experiments tested the impact of various 100 mM supporting electrolytes, including Na2SO4, NaNO3, Na2B4O7, and Na2HPO4, all containing 1.0 mM initial Cl- concentration.
- Inhibitor Dosing: Methanol (10-1000 mM), Potassium Iodide (20-100 mM), and Hydrogen Peroxide (100-1000 mM) were spiked into the electrolyte solution to assess their inhibitory effects on ClO3-/ClO4- formation.
- Potential Measurement: Anodic potential was monitored using a CHI 660E electrochemical workstation with an Ag/AgCl reference electrode, employing iRs compensation.
- Analytical Techniques: Free chlorine (HOCl) was quantified spectrophotometrically (510 nm, DPD method). ClO3- and ClO4- concentrations were determined using Ultra-High Performance Liquid Chromatography coupled with Electrospray Ionization Mass Spectrometry (UPLC-MS/MS).
Commercial Applications
Section titled “Commercial Applications”- Advanced Oxidation Processes (AOPs): Implementation of Ti4O7 anodes in industrial and municipal wastewater treatment facilities utilizing EO, particularly where high chloride concentrations are present.
- PFAS Remediation: Application in treating concentrated wastes (e.g., still bottoms) from Ion Exchange Resin (IXR) regeneration used for Per- and Polyfluoroalkyl Substances (PFASs) removal, leveraging the inhibitory effect of co-solvents like MeOH.
- Effluent Quality Control: Using Ti4O7 anodes combined with KI dosing to ensure EO effluent meets strict regulatory limits for perchlorate (ClO4-), which is difficult to remove post-treatment.
- Electrochemical Reactor Design: Informing the design of EO reactors by selecting Ti4O7 over BDD when minimizing toxic halogenated by-products is a primary operational goal, trading off slightly slower overall oxidation kinetics for improved selectivity.
- Water Disinfection Systems: Utilizing the slower kinetics of Ti4O7 to control the conversion of beneficial free chlorine (HOCl) used for disinfection into undesirable, stable oxyhalides (ClO3-, ClO4-).