Synergistic mineralization of ofloxacin in electro-Fenton process with BDD anode - Reactivity and mechanism

At a Glance

Metadata	Details
Publication Date	2023-05-09
Journal	Separation and Purification Technology
Authors	Weilu Yang, Nihal Oturan, Jialin Liang, Mehmet A. Oturan
Institutions	Zhongkai University of Agriculture and Engineering, Université Gustave Eiffel
Citations	24
Analysis	Full AI Review Included

Executive Summary

This research explores the enhanced performance and mechanism of the electro-Fenton (EF) process utilizing a Boron-Doped Diamond (BDD) anode for the mineralization of the antibiotic Ofloxacin (OFLO).

Synergistic Mechanism: The EF-BDD system leverages the simultaneous generation of homogeneous hydroxyl radicals (•OH) in the bulk solution and highly potent heterogeneous hydroxyl radicals (BDD(•OH)) on the BDD anode surface, leading to synergistic degradation.
Performance Improvement: EF-BDD improved the decay kinetics of OFLO by 1.4-2.6 times compared to the standard Anodic Oxidation (AO) process.
Cost Efficiency: Energy consumption (EC) for OFLO removal was reduced by 28%-41% in EF-BDD compared to AO, demonstrating superior cost-effectiveness, especially under low current density conditions (4.2 mA cm^-2).
Current Density Effects: High current densities (16.6 mA cm^-2) accelerate BDD(•OH) production, causing the heterogeneous reaction pathway to dominate. This results in the synergistic factor (SF) decreasing or becoming negative (SF = -0.08 at 16.6 mA cm^-2).
Complete Mineralization: The process achieved 100% Total Organic Carbon (TOC) removal, confirming complete mineralization of OFLO and its refractory short-chain carboxylic acid intermediates (like oxalic acid).
Toxicity Reduction: Acute toxicity (LC₅₀) and bioaccumulation factors of all identified transformation products (OTPs) were significantly lower (up to 85 times less toxic) than the parent OFLO compound.

Technical Specifications

Parameter	Value	Unit	Context
Anode Material	Boron-Doped Diamond (BDD)	N/A	Non-active anode type
Cathode Material	EEGr-CF	N/A	Electrochemically exfoliated graphene-based carbon felt
Initial OFLO Concentration	0.1	mM	Corresponding to 21.6 mg L^-1 TOC
Optimal Fe²⁺ Concentration	0.2	mM	Catalyst concentration for EF process
Solution pH	3	N/A	Optimized for Fenton reaction
Electrolyte Concentration	50	mM	Na₂SO₄ supporting electrolyte
Anode Surface Area	24	cm²	4 x 6 cm² effective area
Cathode Surface Area	6	cm²	2 x 3 cm² effective area
Max Current Density Tested	16.6	mA cm^-2	Achieved 100% OFLO degradation in 10 min
Max TOC Removal (EF-BDD)	100	%	Achieved at 16.6 mA cm^-2 (240 min)
Lowest Energy Consumption (EC)	0.5	kWh (gTOC)^-1	EF-BDD process at 4.2 mA cm^-2
Absolute Rate Constant (k_OFLO)	3.86 x 10⁹	M^-1s^-1	Oxidation of OFLO by homogeneous •OH
Maximum Synergistic Factor (SF)	0.28	N/A	Observed at 4.2 mA cm^-2
H₂O₂ Accumulation Rate (Max)	6.7	mg h^-1 cm^-2	Observed at 8.3 mA cm^-2

Key Methodologies

The study employed the electro-Fenton (EF) process in a single-chamber electrolytic cell, comparing BDD (EF-BDD) against a Dimensionally Stable Anode (DSA) (EF-DSA) and Anodic Oxidation (AO) using BDD.

Cathode Preparation (EEGr-CF): Graphene (EEGr) was synthesized via electrochemical exfoliation. EEGr and carbon black (1:4 mass ratio) were mixed with PTFE binder and solvents, coated onto carbon felt (CF), and annealed at 360 °C for 30 min under N₂ atmosphere.
Electrolysis Setup: Experiments were conducted in 250 mL cells using BDD as the anode and the prepared EEGr-CF as the cathode (2 cm separation). The solution was maintained at pH 3 and continuously aerated (0.75 L min^-1) to ensure O₂ saturation for H₂O₂ generation.
Kinetic Analysis: Pseudo-first-order rate constants (k_app) were determined by monitoring OFLO decay via HPLC. The Synergistic Factor (SF) was calculated using the kinetic constants of EF-BDD, AO, and EF-DSA processes (Eq. 7) to quantify the combined effect of •OH and BDD(•OH).
Absolute Rate Constant Determination: The second-order rate constant (k_OFLO) for OFLO oxidation by •OH was determined using the competition kinetic method, employing 4-hydroxybenzonic acid (4-HBA) as the known standard competitor.
Mineralization and By-product Tracking: Total Organic Carbon (TOC) removal was measured to assess mineralization efficiency. Carboxylic acids (e.g., oxalic, oxamic, formic) and inorganic ions (NH₄⁺, NO₃^-, F^-) were monitored using HPLC and Ion Chromatography (IC).
Toxicity Assessment: Acute toxicity (LC₅₀), developmental toxicity, and bioaccumulation factors of OFLO and its transformation products (OTPs) were predicted using quantitative structure-activity relationship (QSAR) modeling via the Toxicity Estimation Software Tool (T.E.S.T.).

Commercial Applications

The findings are highly relevant to industries requiring robust and cost-effective methods for treating refractory organic pollutants, particularly those containing pharmaceutical residues.

Pharmaceutical Wastewater Treatment: Direct application for the complete destruction and mineralization of persistent antibiotics (like fluoroquinolones) in effluent streams, ensuring compliance with strict environmental discharge limits.
Advanced Water Purification: Integration of EF-BDD technology into tertiary treatment stages for municipal or industrial wastewater to remove trace organic contaminants (TOCs) and micropollutants.
BDD Electrode Technology: Validation of BDD anodes as superior, non-active electrode materials for electrochemical advanced oxidation processes (EAOPs), driving demand for high-quality BDD film manufacturing.
Cost-Optimized Remediation: Implementation of EF-BDD at optimized low current densities (e.g., 4.2 mA cm^-2) to maximize the synergistic effect and minimize energy consumption (EC), offering a competitive advantage over high-energy AO processes.
Environmental Safety: The demonstrated ability to reduce the toxicity and bioaccumulation potential of degradation products is crucial for environmental risk management in water reuse and discharge scenarios.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.seppur.2023.124039

References

2010 - Drugs degrading photocatalytically: kinetics and mechanisms of ofloxacin and atenolol removal on titania suspensions [Crossref]
2011 - Study of the toxicity of sulfamethoxazole and its degradation products in water by a bioluminescence method during application of the electro-Fenton treatment [Crossref]
2019 - Occurrence, fate and environmental risk assessment of the organic microcontaminants included in the watch lists set by EU decisions 2015/495 and 2018/840 in the groundwater of Spain [Crossref]
2014 - Electrochemical advanced oxidation processes: today and tomorrow. A review [Crossref]
2009 - Electro-Fenton process and related electrochemical technologies based on Fenton’s reaction chemistry [Crossref]
2021 - Nanostructured electrodes for electrocatalytic advanced oxidation processes: From materials preparation to mechanisms understanding and wastewater treatment applications [Crossref]
2022 - Recent advances in electro-Fenton process and its emerging applications
2017 - Catalytic activity of MOF (2Fe/Co)/carbon aerogel for improving H2O2 and OH generation in solar photo-electro-Fenton process [Crossref]
2016 - Continuous bulk FeCuC aerogel with ultradispersed metal nanoparticles: an efficient 3D heterogeneous electro-Fenton cathode over a wide range of pH 3-9 [Crossref]
2021 - Electro-Fenton beyond the degradation of organics: Treatment of thiosalts in contaminated mine water [Crossref]