Combined Analytical Study on Chemical Transformations and Detoxification of Model Phenolic Pollutants during Various Advanced Oxidation Treatment Processes

At a Glance

Metadata	Details
Publication Date	2022-03-16
Journal	Molecules
Authors	Aleksander Kravos, Andreja Žgajnar Gotvajn, Urška Lavrenčić Štangar, Borislav N. Malinović, Helena Prosen
Institutions	University of Ljubljana, University of Banja Luka
Citations	11
Analysis	Full AI Review Included

Executive Summary

This study provides a comprehensive chemical and ecotoxicological comparison of four Advanced Oxidation Processes (AOPs) for degrading persistent phenolic pollutants (Phenol, 2,4-Dichlorophenol, Pentachlorophenol).

Ozonation (OZ) is the Optimal AOP: OZ demonstrated the fastest target removal (TT_>95% in minutes) and achieved rapid, complete detoxification (0% Daphnia magna inhibition in <1 min for chlorophenols), driven by efficient aromatic ring rupture and complete dechlorination.
Photocatalysis (PC) is Slow and Toxic: PC using immobilized N-doped TiO₂ films was significantly slower (TT_>95% in hours). It failed to efficiently open the aromatic ring, leading to the accumulation of highly oxidized and toxic aromatic transformation products (TPs), resulting in increased ecotoxicity.
Sequential Method (SQ) Improves PC: Combining a short OZ flash followed by PC significantly accelerated dechlorination and detoxification compared to PC alone, demonstrating a viable strategy to overcome PC limitations.
BDD Anodes Outperform MMO: In Electrooxidation (EO), Boron-Doped Diamond (BDD) anodes were generally more effective than Mixed Metal Oxide (MMO) anodes for degradation progressivity.
Electrolyte Selection is Critical for EO: Using NaCl electrolyte resulted in the fastest PHN removal but generated highly toxic chlorinated TPs, leading to 100% Daphnia magna inhibition. Using Na₂SO₄ resulted in slower removal but produced more favorable, less toxic TPs (e.g., hydroquinone, catechol).
Analytical Depth: The study utilized a wide array of complementary analytical techniques (HPLC-DAD, UHPLC-MS/MS, GC-MS/MS, IC, and ecotoxicity testing) to provide a holistic evaluation of chemical fate and biological impact.

Technical Specifications

Parameter	Value	Unit	Context
Target Concentration Range	10 to 50	mg/L	Initial concentration of PHN, DCP, or PCP in ultrapure water (MQ).
Ozonation Time (TT_>95%)	0.1 to 4	min	Time required for >95% removal, dependent on substrate load (0.1 min for PCP, 4 min for PHN/DCP at 50 mg/L).
OZ Pseudo 1^st Order Constant (k_r)	0.6 to 2.1	min^-1	For PHN and DCP (50 mg/L) in separate solutions.
PC Treatment Time (TT_>95%)	>180	min	Time required for >95% removal using N-TiO₂^im films.
PC Pseudo 1^st Order Constant (k_r)	0.005 to 0.03	min^-1	Significantly slower than OZ, indicating mild degradation.
Mineralization (OZ)	40 to 50	%	Total Organic Carbon (TOC) reduction after 10 min treatment.
Mineralization (PC)	60 to 70	%	TOC reduction after >180 min treatment.
Dechlorination (OZ)	100	%	Complete conversion of organic chlorine to Cl^- achieved in <4 min.
EO (BDD/NaCl) Time (TT_>95%)	<35	min	Fastest electrooxidation removal of PHN (50 mg/L).
EO (MMO/NaCl) Time (TT_>95%)	120	min	Slower electrooxidation removal of PHN (50 mg/L).
EO Electrolyte Concentration	2	g/L	NaCl or Na₂SO₄ supporting electrolyte used for electrooxidation.
Ecotoxicity (MMO/NaCl)	100	%inh	Acute inhibition of Daphnia magna mobility after 120 min treatment due to chlorinated TPs.
Ecotoxicity (OZ)	0	%inh	Complete detoxification achieved in <1 min for CPs (10 mg/L).

Key Methodologies

The study compared four AOP approaches using specific material configurations and a comprehensive analytical suite:

Ozonation (OZ):
- Setup: Gaseous O₂/O₃ mixture continuously introduced into the reactor.
- Observation: Characterized by a sudden pH drop (8 to 3-4) due to rapid organic acid formation.
Photocatalysis (PC) and Photooxidation (PO):
- Catalyst: N-doped TiO₂ (N-TiO₂^im) thin films synthesized via sol-gel method from a TiCl₄ precursor.
- Immobilization: Dip-coating technique used to immobilize films on glass plates.
- Reactor: Continuous flow photocell with O₂ supply, illuminated by UVA light.
- PO: Conducted using the same setup but without the N-TiO₂^im catalyst (UV/O₂ system).
Sequential Method (SQ):
- Process: Short “flash” ozonation (0.2 min) followed by prolonged photocatalysis (180 min).
Electrooxidation (EO):
- Anodes: Boron-Doped Diamond (BDD) mesh or Mixed Metal Oxide (MMO: RuO₂-IrO₂ mesh).
- Cathode: Stainless steel.
- Electrolytes: 2 g/L NaCl or 2 g/L Na₂SO₄, influencing the formation of chlorinating agents or hydroxyl radicals, respectively.
Instrumental Analysis Suite:
- Target/TP Quantification: High-Performance Liquid Chromatography coupled with Diode-Array UV Detection (HPLC-DAD).
- Specific TP Identification: Ultra-High-Pressure Liquid Chromatography coupled with Tandem Mass Spectrometry (UHPLC-MS/MS) using negative electrospray ionization (ESI).
- Volatile/Semipolar Analysis: Gas Chromatography coupled with Mass Spectrometry (GC-MS/MS) following Solid-Phase Microextraction (SPME) or Liquid-Liquid Extraction (LLE).
- Acid/Ion Analysis: Ion Chromatography (IC) for quantifying organic acids (oxalic, formic) and chloride (Cl^-).
- Ecotoxicity Assessment: 48-h acute mobility inhibition tests on Daphnia magna (OECD Guidelines No. 202).

Commercial Applications

The findings directly support the optimization and selection of AOP technologies for industrial and municipal water treatment, particularly concerning persistent organic pollutants (POPs).

Industrial Wastewater Treatment: Applicable for treating effluents from chemical, pharmaceutical, and textile industries containing high concentrations of phenolic compounds and chlorophenols.
Drinking Water Purification: Relevant for removing trace levels of genotoxic phenolic pollutants resulting from historical use or chlorination byproducts.
AOP System Design and Optimization: Provides critical data for selecting the most effective AOP (OZ vs. PC vs. EO) based on required treatment speed, mineralization goals, and detoxification requirements.
Electrochemical Reactor Engineering: Justifies the use of BDD anodes over MMO for robust degradation and highlights the necessity of careful electrolyte selection (e.g., avoiding NaCl if minimizing toxic chlorinated TPs is paramount).
Catalyst Development: Validates the use of immobilized N-doped TiO₂ thin films for PC, offering a reusable, non-powder alternative, especially when coupled with pre-ozonation.
Environmental Risk Assessment: The ecotoxicological data (Daphnia magna inhibition) provides a crucial metric for validating AOP efficacy beyond simple pollutant removal, ensuring the treated water is biologically safe.

View Original Abstract

Advanced oxidation processes (AOPs) have been introduced to deal with different types of water pollution. They cause effective chemical destruction of pollutants, yet leading to a mixture of transformation by-products, rather than complete mineralization. Therefore, the aim of our study was to understand complex degradation processes induced by different AOPs from chemical and ecotoxicological point of view. Phenol, 2,4-dichlorophenol, and pentachlorophenol were used as model pollutants since they are still common industrial chemicals and thus encountered in the aquatic environment. A comprehensive study of efficiency of several AOPs was undertaken by using instrumental analyses along with ecotoxicological assessment. Four approaches were compared: ozonation, photocatalytic oxidation with immobilized nitrogen-doped TiO2 thin films, the sequence of both, as well as electrooxidation on boron-doped diamond (BDD) and mixed metal oxide (MMO) anodes. The monitored parameters were: removal of target phenols, dechlorination, transformation products, and ecotoxicological impact. Therefore, HPLC-DAD, GC-MS, UHPLC-MS/MS, ion chromatography, and 48 h inhibition tests on Daphnia magna were applied. In addition, pH and total organic carbon (TOC) were measured. Results show that ozonation provides by far the most suitable pattern of degradation accompanied by rapid detoxification. In contrast, photocatalysis was found to be slow and mild, marked by the accumulation of aromatic products. Preozonation reinforces the photocatalytic process. Regarding the electrooxidations, BDD is more effective than MMO, while the degradation pattern and transformation products formed depend on supporting electrolyte.

Tech Support

Original Source

DOI: https://doi.org/10.3390/molecules27061935

References

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2013 - Efficient Anodic Degradation of Phenol Paired to Improved Cathodic Production of H2O2 at BDD Electrodes [Crossref]