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6CCVD Research

The Removal of Organic Pollutants and Ammonia Nitrogen from High-Salt Wastewater by the Electro-Chlorination Process and Its Mechanism

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2024-12-18
JournalSeparations
AuthorsYujun Zhou, Ting Hou, Bo Zhou
InstitutionsNanjing Agricultural University, Nanjing University of Science and Technology
Citations3
AnalysisFull AI Review Included

The following analysis summarizes the research paper on the Electro-Chlorination (E-Cl) process for treating high-salt wastewater, tailored for an engineering audience.

Executive Summary

Section titled “Executive Summary”
  • Core Value Proposition: Electro-Chlorination (E-Cl) is an emerging, green Advanced Oxidation Technology (AOT) offering high efficiency and deep mineralization for complex, high-salinity wastewater, addressing the challenge of salt interference in conventional biological methods.
  • Mechanism Priority: Pollutant removal is primarily driven by indirect oxidation via in situ generated intermediate active species, specifically Reactive Chlorine Species (RCS: HClO, ClO-, Cl2) and Chlorine Radicals (¡Cl, ¡ClO), which possess strong oxidizing power and high selectivity for electron-rich groups (e.g., amino groups in ammonia).
  • Key Anode Materials: The most widely used anodes are Dimensionally Stable Anodes (DSA, e.g., Ti/RuO2-IrO2), which favor chlorine evolution due to low overpotential, and Boron-Doped Diamond (BDD) electrodes, which offer high stability and high oxygen evolution overpotential.
  • Optimal Reactor Design: Flow-through electrode reactors are considered the most promising configuration, utilizing porous, 3D structures to significantly enhance mass transfer and electron transfer efficiency, allowing for faster reaction rates at lower voltages.
  • Performance Metrics: E-Cl achieves high removal rates for challenging pollutants, including 100% removal of ammonia nitrogen (AN) and various antibiotics (e.g., Levofloxacin, Sulfadiazine) in salt-containing effluents.
  • Challenges: Future research must focus on developing stable, low-cost anode materials for scale-up and mitigating the formation of potentially toxic chlorinated by-products (e.g., ClO3-, ClO4-) generated by RCS over-oxidation.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Hydroxyl Radical (¡OH) Redox Potential2.8VNon-selective strong oxidant.
Chlorine Radical (¡Cl) Redox Potential2.4VHighly selective oxidant for electron-rich groups.
¡OH Half-Life Time10-9sExtremely short lifespan in aqueous phase.
¡Cl Half-Life Time3 x 10-6sLonger lifespan than ¡OH, enhancing contact time.
DSA (Ti/RuO2-IrO2) O2 Evolution Overpotential1.23V vs. Ag/AgClActive anode, low overpotential for Cl evolution.
BDD (Ti/BDD) O2 Evolution Overpotential2.66V vs. Ag/AgClNon-active anode, high stability.
AN Removal Efficiency (Pd-Sn-Ru-Ir)~100%Achieved within 40 min at 0.5 A current.
TN Removal Efficiency (Fe/TiO2)94%Achieved within 120 min at 2 V (vs. Ag/AgCl).
Levofloxacin Removal (Ti4O7)100%30 min, 39.6 mA/cm2, 4% NaCl electrolyte.
Rhodamine 6G Decolorization (Ti/RuO2)100%Achieved within 5 min under optimal conditions.
Energy Consumption (Ti/RuO2-IrO2)0.088kWh/m3Low consumption reported for active anode.
Energy Consumption (Ti/BDD)1.00kWh/gHigher consumption reported for non-active anode.

Key Methodologies

Section titled “Key Methodologies”
  1. Electrode Material Selection: Anodes are chosen based on their oxygen evolution overpotential (OER) and chlorine evolution overpotential (CER). Active anodes (DSA, Pt) favor CER, while non-active anodes (BDD, PbO2) favor OER, influencing the ratio of RCS to ¡OH generated.
  2. Reactor Design: Flow-through electrode reactors are preferred, utilizing porous electrode materials (e.g., fiber mesh, nanotube arrays) to create a high-porosity, large-pore-size 3D structure, maximizing the contact area and minimizing mass transfer distance.
  3. Oxidant Generation: Chloride ions (Cl-) in the wastewater are electrochemically converted at the anode surface to chlorine radicals (¡Cl) and subsequently to Reactive Chlorine Species (RCS: HClO, ClO-, Cl2).
  4. Pollutant Degradation Pathways: Pollutants are degraded via indirect oxidation mechanisms including substitution (especially for AN removal), addition, and general oxidation by RCS, leading to intermediates and eventual mineralization (CO2, H2O, inorganic ions).
  5. Process Optimization: Key operating parameters—applied current density, voltage, electrolyte (Cl-) concentration, and initial pH—are rigorously controlled. Lower pH values generally favor the Chlorine Evolution Reaction (CER) and the production of highly reactive HClO.
  6. Active Species Detection:
    • Radicals (¡Cl, ¡ClO, ¡OH): Detected qualitatively using Electron Paramagnetic Resonance (EPR) spectroscopy with trapping agents like DMPO.
    • RCS (Free Chlorine): Quantified using spectrophotometry with N, N-diethyl-1,4-phenylenediamine sulfate (DPD) as a chromogenic agent, or via Ion Chromatography (IC).
  7. Mechanistic Verification: Quenching experiments are performed using selective scavengers (e.g., Na2S2O3, TBA, MeOH) to isolate and quantify the contribution of specific oxidants (¡Cl, ¡OH, ClO-) to the overall degradation rate.

Commercial Applications

Section titled “Commercial Applications”
  • High-Salinity Industrial Wastewater Treatment: Direct application in treating effluents from petrochemical, textile dyeing, and chemical manufacturing industries characterized by high chloride content and complex organic loads (e.g., COD, TOC).
  • Pharmaceutical and Pesticide Effluent Remediation: Effective for the complete removal of persistent organic pollutants (POCs) such as antibiotics (e.g., Levofloxacin, Tetracycline) and pesticides (e.g., Atrazine) that resist biological treatment.
  • Denitrification of Aquaculture Wastewater: Used in marine and high-salt aquaculture systems for efficient conversion of ammonia nitrogen (AN) to inert N2, often integrated into hybrid systems (e.g., UV/E-Cl) for multifunctional purification.
  • Electrode Manufacturing: Drives the development and commercialization of high-performance, stable anode materials, particularly modified DSA (e.g., RuO2-IrO2 composites) and BDD thin-film electrodes, suitable for continuous electrochemical processes.
  • Landfill Leachate Treatment: Applicable for treating complex, highly contaminated landfill leachates, demonstrating drastic improvement in COD and AN removal efficiency with increasing current density.
View Original Abstract

Electro-chlorination (E-Cl) is an emerging and promising electrochemical advanced oxidation technology for wastewater treatment with the advantages of high efficiency, deep mineralization, a green process, and easy operation. It was found that the mechanism of pollutant removal by electro-chlorination mainly involves an indirect oxidation process, in which pollutant removal is mainly driven by the intermediate active species, especially RCS and chlorine radicals, with a strong oxidization ability produced at the anodes. In this work, we summarized the principles and pathways of the removal/degradation of pollutants (organic pollutants and ammonia nitrogen) by E-Cl and the major affecting factors including the applied current density, voltage, electrolyte concentration, initial pH value, etc. In the E-Cl system, the DSA and BDD electrodes were the most widely used electrode materials. The flow-through electrode reactor was considered to be the most promising reactor since it had a high porosity and large pore size, which could effectively improve the mass transfer efficiency and electron transfer efficiency of the reaction. Of the many detection methods for chlorine radicals and RCS, electron paramagnetic resonance (EPR) and spectrophotometry with N, N-diethyl-1,4-phenylenediamine sulfate (DPD) as the chromogenic agent were the two most widely used methods. Overall, the E-Cl process had excellent performance and prospects in treating salt-containing wastewater.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2021 - Recent developments in physical, biological, chemical, and hybrid treatment techniques for removing emerging contaminants from wastewater [Crossref]
  2. 2023 - Plant tannin foam anchored iron nanoparticles: Efficient and recyclable degradation of tetracycline antibiotics under high salt conditions [Crossref]
  3. 2024 - Review on ultrasonic technology enhanced biological treatment of wastewater [Crossref]
  4. 2015 - Challenges and Opportunities for electrochemical processes as next-generation technologies for the treatment of contaminated water [Crossref]
  5. 2009 - Electro-Fenton process and related electrochemical technologies based on Fenton’s reaction chemistry [Crossref]
  6. 2008 - The Synergistic Influences of OH- concentration and electrolyte conductivity on the redox behavior of Ni(OH)2/NiOOH [Crossref]
  7. 2021 - Current progress in electrochemical anodic-oxidation of pharmaceuticals: Mechanisms, influencing factors [Crossref]
  8. 2024 - Electrogenerated singlet oxygen and reactive chlorine species enhancing volatile fatty acids production from co-fermentation of waste activated sludge and food waste: The key role of metal oxide coated electrodes [Crossref]
  9. 2023 - Elimination of pesticide from high salinity wastewater by electrochlorination process: Active chlorine species and scale-up performance [Crossref]

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