| Metadata | Details |
|---|
| Publication Date | 2022-11-08 |
| Journal | Water |
| Authors | Mohammad Issa, Dennis Haupt, Thorben Muddemann, Ulrich Kunz, Michael Sievers |
| Institutions | Clausthal University of Technology |
| Citations | 2 |
| Analysis | Full AI Review Included |
- Core Technology: A combined Boron-Doped Diamond (BDD) anode and Gas Diffusion Electrode (GDE) cathode reactor was successfully used for the electrochemical mineralization of synthetic wastewater simulating high-concentration vacuum toilet sewage.
- Kinetic Modeling: The Chemical Oxygen Demand (COD) degradation followed first-order reaction kinetics, enabling the calculation of the observed rate constant (kobs) across various current densities (20-100 mA/cm2) and initial COD concentrations (1175-4410 mg/L).
- Predictive Model: A simple mathematical model was derived to predict kobs directly from the measured cell potential (Ecell): k = 4 x 10-6 * Ecell - 10-5 s-1, valid for Ecell ranging from 2.5 to 21 V.
- Performance Optimization: The highest current efficiency (CE), up to 48%, and the lowest specific charge demand (SCD), approximately 7 Ah/gCOD, were achieved at the lowest tested current density (20 mA/cm2).
- Efficiency Trade-off: Increasing current density significantly increased the total removed COD (TR-COD) but led to a substantial decrease in CE (down to ~23% at 100 mA/cm2) due to increased mass transfer limitations and energy loss via side reactions (e.g., oxygen evolution).
- Design Implication: The derived kinetic model is crucial for accurately designing continuous electrochemical reactors by pre-determining the required residence time and reactor volume based on real-time operational voltage.
| Parameter | Value | Unit | Context |
|---|
| Reactor Type | BDD Anode / GDE Cathode | - | No separator used. |
| Active Electrode Area (A) | 100 | cm2 | 10 cm x 10 cm reaction zone. |
| Flow Velocity | 0.23 | m/s | Highest tested flow rate. |
| Initial COD (C0,1) | 4410 Âą 17 | mg/L | Highest concentration tested. |
| Initial Conductivity (Ď0,1) | 8.58 Âą 0.24 | mS/cm | Corresponding to C0,1. |
| Current Density (j) Range | 20 to 100 | mA/cm2 | Tested operating range. |
| Cell Potential (Ecell) Range | 4.24 to 20.9 | V | Required potential to maintain j. |
| Observed Rate Constant (kobs) Range | 0.67 x 10-5 to 8.33 x 10-5 | s-1 | Measured across all conditions. |
| Highest Current Efficiency (CE) | 47.7 (up to ~48) | % | Achieved at 20 mA/cm2 (C0,1). |
| Lowest Specific Charge Demand (SCD) | ~7 | Ah/gCOD | Achieved at 20 mA/cm2. |
| COD Removal Increase (20 to 100 mA/cm2) | 163 | % | At C0,1 concentration. |
| SCD Increase (20 to 100 mA/cm2) | 90 | % | At C0,1 concentration. |
| Kinetic Model (k vs. Ecell) | k = 4 x 10-6 * Ecell - 10-5 | s-1 | R-squared = 0.9562. |
| Model Validity Range (Ecell) | 2.5 to 21 | V | For current densities 20-100 mA/cm2. |
- Synthetic Wastewater (SWW) Preparation: SWW simulating vacuum toilet sewage was prepared, using glucose as the primary organic source (96% of total COD). Three concentrations (C0,1, C0,2, C0,3) were tested by dilution, resulting in conductivities ranging from 2.49 to 8.58 mS/cm.
- Reactor Setup: A BDD-GDE reactor was used in a continuous flow setup. The BDD anode (DIA-CHEM) was coupled with a carbon-based GDE cathode (Printex L6 on Ag-plated Ni mesh) without a separator, separated by a 3 mm PTFE frame.
- Flow and Gas Supply: SWW was supplied at a constant flow velocity of 0.23 m/s. Air (oxygen source) was provided to the GDE back compartment at approximately 35 mbar to generate H2O2.
- Electrolysis Operation: Experiments were run for 4 hours at three constant current densities (20, 50, and 100 mA/cm2). Cell potential, pH (which dropped from 7 to 4), and temperature (which rose from 10 to 34 °C) were monitored.
- COD Analysis Pre-treatment: Wastewater samples were treated with NaHCO3 (22.5 mol-NaHCO3/mol-H2O2) and shaken for 24 hours to ensure complete decomposition of electrochemically generated H2O2, preventing overestimation of COD removal.
- Kinetic Determination: Experimental COD degradation data were plotted as ln(COD0/CODt) versus time. The slope of the resulting linear fit provided the observed first-order rate constant (kobs) for each condition.
- Model Derivation: The calculated kobs values were correlated with the corresponding initial cell potential (V0) and fitted to a linear equation to create a uniform predictive model for the rate constant.
- Decentralized Wastewater Treatment: Highly suitable for mobile or remote applications requiring compact, high-efficiency treatment, such as vacuum toilet sewage systems on trains, buses, or cruise ships.
- High-Strength Industrial Effluent: Applicable for the mineralization of concentrated organic waste streams where traditional biological methods are inefficient or space-prohibitive.
- Advanced Oxidation Processes (AOPs): Provides a robust electrochemical AOP solution, leveraging the synergistic effect of BDD-generated hydroxyl radicals (â˘OH) and GDE-generated reactive oxygen species (H2O2, â˘O2H).
- Zero-Sludge/Zero-Discharge Systems: Offers an environmentally favorable alternative by achieving complete mineralization of organics to CO2, minimizing sludge production and avoiding toxic by-products.
- Process Engineering and Scale-Up: The derived kinetic model allows engineers to accurately size and optimize continuous flow reactors, ensuring target removal efficiencies are met under varying wastewater conductivity and flow conditions.
View Original Abstract
A system of boron-doped diamond (BDD) anode combined with a gas diffusion electrode (GDE) as a cathode is an attractive kind of electrolysis system to treat wastewater to remove organic pollutants. Depending on the operating parameters and water matrix, the kinetics of the electrochemical reaction must be defined to calculate the reaction rate constant, which enables designing the treatment reactor in a continuous process. In this work, synthetic wastewater simulating the vacuum toilet sewage on trains was treated via a BDD-GDE reactor, where the kinetics was presented as the abatement of chemical oxygen demand (COD) over time. By investigating three different initial COD concentrations (C0,1 â 2 Ă C0,2 â 4 Ă C0,3), the kinetics was presented and the observed reaction rate constant kobs. was derived at different current densities (20, 50, 100 mA/cm2). Accordingly, a mathematical model has derived kobs. as a function of the cell potential Ecell. Ranging from 1 Ă 10â5 to 7.4 Ă 10â5 sâ1, the kobs. is readily calculated when Ecell varies in a range of 2.5-21 V. Furthermore, it was experimentally stated that the highest economic removal of COD was achieved at 20 mA/cm2 demanding the lowest specific charge (~7 Ah/gCOD) and acquiring the highest current efficiency (up to ~48%).
- 2011 - BDD anodic oxidation as tertiary wastewater treatment for the removal of emerging micro-pollutants, pathogens and organic matter [Crossref]
- 2011 - Electrochemical degradation of aromatic amines on BDD electrodes [Crossref]
- 2013 - Anodic oxidation of o-nitrophenol on BDD electrode: Variable effects and mechanisms of degradation [Crossref]
- 2006 - Anodic oxidation of Orange II on Ti/BDD electrode: Variable effects [Crossref]
- 2014 - Remediation of wastewaters containing tetrahydrofuran. Study of the electrochemical mineralization on BDD electrodes [Crossref]
- 2018 - Applicability of electrochemical methods to paper mill wastewater for reuse. Anodic oxidation with BDD and TiRuSnO2 anodes [Crossref]
- 2017 - Nitrate and carbon matter removals from real effluents using Si/BDD electrode [Crossref]
- 2019 - Recent developments and advances in boron-doped diamond electrodes for electrochemical oxidation of organic pollutants [Crossref]