Sensing of chemical oxygen demand (COD) by amperometric detection—dependence of current signal on concentration and type of organic species
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2023-05-02 |
| Journal | Environmental Monitoring and Assessment |
| Authors | Samira Lambertz, Marcus Franke, Michael Stelter, Patrick Braeutigam |
| Institutions | Friedrich Schiller University Jena |
| Citations | 9 |
| Analysis | Full AI Review Included |
Research Paper Analysis: Sensing of Chemical Oxygen Demand (COD) by Amperometric Detection
Section titled “Research Paper Analysis: Sensing of Chemical Oxygen Demand (COD) by Amperometric Detection”Executive Summary
Section titled “Executive Summary”- Core Value Proposition: Developed a fast, non-toxic, and instrumentally simple amperometric method using Boron-Doped Diamond (BDD) electrodes for determining Chemical Oxygen Demand (COD), replacing the harmful, slow K2Cr2O6 standard method.
- Mechanism Confirmation: The oxidation process relies on hydroxyl radicals (BDD(OH)) formed electrochemically. The reaction selectivity is governed by the ratio of hydroxyl radical formation rate (k1) to the organic species reaction rate (k2CR).
- Compound Dependence: The current signal is highly dependent on the type of organic species present, and this variance increases significantly with increasing COD concentration.
- Linear Working Range: A compound-independent linear working range was established only for low concentrations: 25-150 mg/L COD.
- Performance Metrics: Achieved a low detection limit of 25 mg/L COD and a precision of 30% within the compound-independent range.
- Optimization Strategy: To increase the linear range and reduce signal variance, the ratio of hydroxyl radicals to organic species must be increased, potentially via porous BDD electrodes or improved mixing (e.g., ultrasonic stirring).
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Working Electrode Material | Boron-Doped Diamond (BDD) | N/A | 5 µm diamond on niobium substrate |
| BDD Electrode Geometry | 8 | mm | Diameter, coated on both sides |
| Reference Electrode | Ag/AgCl (3 M NaCl) | N/A | Standard reference electrode (RE-1S) |
| Counter Electrode | Platinum wire | N/A | 0.5 mm diameter, 10 mm length |
| Electrochemical Cell Volume | 17 | ml | Custom-made Teflon cell |
| Activation Potential (Eact) | 3 | V vs Ag/AgCl | Applied for 30 s |
| Measurement Potential (Emeas) | 2.4 | V vs Ag/AgCl | Applied for 170 s |
| Stirring Speed | 100 | rpm | Used during measurement |
| Compound-Independent Working Range | 25-150 | mg/L COD | Linear fit range for all tested organics |
| Method Precision (p) | 30 | % | Calculated in the 25-150 mg/L range |
| Detection Limit | 25 | mg/L COD | Lower bound of the linear range |
| Electrolyte Composition | 0.1 M Na2SO4 / 0.1 mM H2SO4 | N/A | Aqueous solution |
| k1 (Formation Constant) | 0.027 to 0.152 | N/A | Similar order of magnitude for all organics |
| k2 (Reaction Constant) | 2.49x10-5 to 4.12x10-4 | N/A | Varies over different orders of magnitude, indicating selectivity |
Key Methodologies
Section titled “Key Methodologies”- Electrolyte and Sample Preparation: Electrolyte (0.1 M Na2SO4, 0.1 mM H2SO4) prepared using filtered ultrapure water (TOC < 5 ppb). COD samples (10 mg/L to 10,000 mg/L) prepared as dilutions of stock solutions using six diverse organic compounds (acetic acid, glucose, malonic acid, ascorbic acid, sucrose, citric acid) to simulate extreme compositional variations.
- Electrochemical Setup: A three-electrode cell (17 ml) was used with a commercial BDD electrode (DiaCCon) as the working electrode, Ag/AgCl as the reference, and platinum wire as the counter electrode.
- Amperometric Measurement Protocol:
- Cell filled with 12 ml electrolyte and stirred at 100 rpm.
- Activation step: 3 V vs Ag/AgCl applied for 30 s.
- Measurement step: Potential set to 2.4 V vs Ag/AgCl.
- Sample injection: 5 ml of sample solution added after a 110 s waiting period.
- Data Processing and Signal Calculation: Current-time curves were processed using a KNIME® workflow, involving smoothing (moving average, window 20), subtraction of the background current average, and calculation of the average reduced signal current.
- Statistical Validation: Analysis of Variance (ANOVA) and Tukey tests were performed to statistically determine if the current signals for different organic species showed significant differences at various COD concentrations (significance level p=0.05).
- Kinetic Modeling: Data was fitted using a non-linear model derived from the two-step oxidation mechanism (BDD(OH) formation and subsequent reaction with organic species) to calculate kinetic constants (k1 and k2).
Commercial Applications
Section titled “Commercial Applications”The development of a fast, non-toxic COD sensor based on BDD electrodes is critical for industries requiring continuous water quality monitoring:
- Wastewater Treatment: Enables real-time monitoring and control of sewage treatment plants (STPs), allowing for immediate process adjustments and ensuring compliance with discharge requirements (e.g., German Federal Law, Urban Waste Water Treatment Directive).
- Industrial Effluent Monitoring: Essential for industries generating complex organic waste (e.g., chemical, pharmaceutical, food and beverage manufacturing) to rapidly assess effluent load and avoid environmental penalties.
- BDD Electrode Manufacturing: Provides a high-value application for advanced BDD materials, particularly those optimized for high surface area (e.g., porous BDD) to enhance hydroxyl radical generation and extend the linear working range beyond 150 mg/L COD.
- Environmental Sensing Networks: Facilitates the deployment of robust, low-maintenance sensors for continuous, remote monitoring of natural water bodies, addressing global water quality threats.
- Process Optimization: The kinetic insights (k1 and k2 data) are valuable for engineers designing and optimizing electrochemical Advanced Oxidation Processes (AOPs) that rely on BDD for pollutant destruction.