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Ferrous-Assisted Corona Plasma Decolorization of Acid Black 194 and Auramine O in a Continuous-Flow Recirculation System

This study demonstrates that an atmospheric-pressure corona plasma system assisted by ferrous species in a continuous-flow recirculation setup effectively achieves over 97% decolorization of both the azo dye Acid Black 194 and the triarylmethane dye Auramine O within 90 minutes, with specific energy consumptions of approximately 40–42 kWh m⁻³.

Original authors: Erik Torres, Aarón Gómez, Pedro Guillermo Reyes, José Carlos Palomares, Joséfina Vergara, César Torres, Horacio Martínez

Published 2026-08-31

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CC BY 4.0

Original authors: Erik Torres, Aarón Gómez, Pedro Guillermo Reyes, José Carlos Palomares, Joséfina Vergara, César Torres, Horacio Martínez

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Water that has been used to dye fabric is one of the most stubborn types of pollution. The synthetic colors used in textiles are designed to be chemically tough so they do not fade on clothes, but this same durability makes them resistant to breaking down in rivers or treatment plants. When these dyes enter the environment, they block sunlight from reaching underwater plants and can harm aquatic life. Traditional methods to clean this water, such as filtering or using bacteria, often struggle with these stubborn molecules or create large amounts of sludge that must be disposed of. Scientists have turned to advanced oxidation processes, which use powerful chemical reactions to break these molecules apart, but finding a method that is both effective and energy-efficient remains a challenge. One promising approach involves using a type of electrical discharge called plasma, which creates a cloud of highly reactive particles capable of attacking and dismantling complex chemical structures.

In a recent study, researchers from universities in Mexico explored a way to make this plasma treatment even more effective by adding a simple chemical helper. They focused on two very different types of textile dyes: Acid Black 194, which contains a specific chemical bond known as an azo group, and Auramine O, which has a different structure based on three aromatic rings. The team set up a system where five liters of water containing these dyes were continuously pumped through a reactor. Inside this reactor, three sharp tungsten tips faced a stainless steel plate, creating a corona discharge—a steady, silent electrical glow in the air just above the liquid surface. This discharge generates reactive oxygen and nitrogen species, which are unstable molecules that seek out and break apart organic compounds. To boost this process, the researchers added a small amount of iron dissolved in an acidic solution before starting the treatment. The iron acts as a catalyst, helping to convert the reactive species generated by the plasma into even more powerful agents that can attack the dye molecules more aggressively.

The experiment ran for ninety minutes, with the water circulating back and forth through the treatment zone. The researchers monitored the color of the water using a light sensor that measured how much light the solution absorbed at specific wavelengths. They found that the addition of the iron solution caused an immediate, small drop in color, likely due to a quick chemical interaction between the iron and the dye. However, the real transformation happened once the plasma was turned on. Over the course of the treatment, the deep color of both dyes faded dramatically. By the end of the ninety minutes, the solution containing Acid Black 194 had lost 98.66 percent of its color, while the Auramine O solution lost 97.74 percent. The water became nearly clear, a change that was visible to the naked eye as the dark liquids turned into a transparent solution.

The speed of this cleaning process was also measured. The researchers calculated that it took about fourteen minutes for the Acid Black 194 to lose half of its color, and about sixteen minutes for the Auramine O to reach the same point. This suggests that while the two dyes have different chemical structures, the plasma system assisted by iron was able to degrade both of them with similar efficiency. The electrical system remained stable throughout the experiment, operating at an average power of roughly 139 watts for the black dye and 134 watts for the yellow dye. The energy required to treat a cubic meter of this water was calculated to be around 41 kilowatt-hours for the black dye and 40 kilowatt-hours for the yellow dye. These numbers indicate that the system is capable of handling the task without consuming excessive amounts of electricity, which is a critical factor for any technology intended for real-world use.

As the dyes broke down, other changes occurred in the water. The acidity of the solution increased slightly, dropping from a neutral level to a more acidic state, which is a common sign that new chemical byproducts are forming. The water also became temporarily cloudy when the iron was first added, but this cloudiness cleared up significantly as the treatment continued, dropping by more than 90 percent by the end. This suggests that the particles causing the cloudiness were either broken down or settled out of the water. By looking at the light emitted by the electrical discharge, the researchers confirmed that the plasma was generating the expected reactive particles, including hydroxyl radicals and excited nitrogen molecules, which are the agents responsible for the chemical breakdown.

The study concludes that combining a corona plasma discharge with a small amount of iron is a highly effective way to remove color from textile wastewater. The method worked well for two very different types of dyes, achieving near-total decolorization in a continuous flow system. However, the researchers note that while the color is gone, the water has not necessarily been completely purified of all organic matter. The breakdown of the dye molecules creates smaller fragments, and the study did not measure whether these fragments are toxic or how much of the original organic carbon was fully converted into harmless substances like carbon dioxide and water. The work demonstrates a powerful step toward cleaner water treatment, showing that plasma technology can be tuned to handle difficult industrial pollutants, but it also highlights that further steps are needed to ensure the treated water is safe for the environment.

Technical Summary: Ferrous-Assisted Corona Plasma Decolorization of Acid Black 194 and Auramine O

Problem Statement
Textile effluents present a significant environmental challenge due to the persistence of synthetic dyes, which are characterized by high chemical stability, low biodegradability, and intense coloration. Conventional treatment methods, such as coagulation-flocculation and adsorption, often face limitations regarding sludge generation, adsorbent saturation, and low efficiency against recalcitrant aromatic compounds. While Advanced Oxidation Processes (AOPs) offer a promising alternative through the generation of reactive oxygen and nitrogen species (RONS), many non-thermal plasma systems operate in batch modes or at small volumes, limiting their scalability. Furthermore, structurally diverse dyes, such as azo dyes and triarylmethane dyes, may respond differently to oxidation, necessitating comparative studies to evaluate system robustness.

Methodology
This study evaluated a continuous-flow recirculation system utilizing atmospheric-pressure corona plasma assisted by ferrous species. The experimental setup featured a three-tungsten-tip-to-stainless-steel-plate geometry operating in air at atmospheric pressure. The system treated 5 L of aqueous dye solution at a flow rate of 250 mL min⁻¹.

Two distinct dyes were selected for comparative analysis:

Acid Black 194 (AB-194): An azo dye with an –N=N– chromophoric group.
Auramine O (AO): A triarylmethane dye with conjugated aromatic systems.

Both solutions were prepared at an initial concentration of 0.1 mM using tap water. A ferrous catalyst, derived from ferrous sulfate heptahydrate (FeSO₄·7H₂O) in an acidic medium (sulfuric acid), was added to achieve a final Fe²⁺ concentration of approximately 0.98 mmol L⁻¹. This dosage was intended to promote ferrous-mediated radical pathways without external hydrogen peroxide addition, distinguishing the process from a conventional Fenton reaction.

Key parameters monitored included:

Decolorization: Measured via UV–Vis spectrophotometry at 594 nm (AB-194) and 432 nm (AO).
Physicochemical Properties: pH, electrical conductivity, and turbidity were tracked over a 90-minute treatment period.
Electrical Characterization: Voltage and current were recorded to calculate average power and ensure stable corona discharge operation.
Optical Emission Spectroscopy (OES): Used to identify excited species (e.g., ·OH, N₂, N₂⁺, Hα, Hβ) at the plasma–liquid interface.
Kinetic and Energy Analysis: Apparent pseudo-first-order kinetics were applied to determine rate constants ( $k$ ) and half-decolorization times ( $t_{50}$ ). Energy performance was assessed using Specific Energy Consumption (SEC) and Energy Yield at 50% decolorization ( $G_{50}$ ).

Key Results

Decolorization Efficiency: After 90 minutes of treatment, the system achieved high decolorization efficiencies: 98.66% for AB-194 and 97.74% for AO.
Kinetics: The apparent pseudo-first-order rate constants were 0.0486 min⁻¹ for AB-194 and 0.0433 min⁻¹ for AO. Consequently, the time required to reach 50% decolorization ( $t_{50}$ ) was 14.27 min for AB-194 and 16.00 min for AO. The azo dye (AB-194) exhibited a slightly faster apparent degradation rate (approx. 12% higher) than the triarylmethane dye (AO).
Electrical Performance: The discharge remained stable in the corona regime without transitioning to arc discharge. Average power consumption was 139.42 W for AB-194 and 133.71 W for AO.
Physicochemical Changes:
- pH: The addition of the acidic catalyst initially lowered the pH to ~6.4–6.6. During treatment, pH further decreased to 6.02 (AB-194) and 5.75 (AO), likely due to the formation of oxidized nitrogen species and organic byproducts.
- Turbidity: Initial turbidity spiked upon catalyst addition (due to iron hydroxide colloids) but decreased by approximately 92.2% (AB-194) and 93.4% (AO) by the end of the treatment, indicating the removal or transformation of scattering species.
- Conductivity: Conductivity increased initially due to ion introduction. Over 90 minutes, it decreased by ~5.9% for AB-194 but increased by ~3.3% for AO, reflecting complex changes in ionic composition.
Energy Metrics: The Specific Energy Consumption (SEC) was 41.83 kWh m⁻³ for AB-194 and 40.11 kWh m⁻³ for AO. The energy yield ( $G_{50}$ ) was 5.72 g kWh⁻¹ for AB-194 and 2.13 g kWh⁻¹ for AO. The higher $G_{50}$ for AB-194 is attributed to its shorter $t_{50}$ and significantly higher molar mass compared to AO.
Plasma Diagnostics: OES confirmed the presence of reactive species including hydroxyl radicals (·OH), molecular nitrogen (N₂), ionized nitrogen (N₂⁺), and hydrogen lines (Hα, Hβ), consistent with the generation of RONS at the interface. Apparent electron temperatures were estimated at 0.72 eV (AB-194) and 0.81 eV (AO).

Significance and Claims
The authors claim that atmospheric-pressure corona plasma, when assisted by ferrous species in a continuous recirculating-flow system, is an effective technology for the decolorization of structurally diverse textile dyes. The study demonstrates that the system can achieve near-complete decolorization (>97%) within 90 minutes for both azo and triarylmethane dyes.

The paper highlights that while the molecular structure influences the apparent degradation rate—with the azo bond of AB-194 appearing slightly more susceptible to oxidative attack than the conjugated system of AO—the overall process efficiency is high for both. The integration of ferrous species is presented as a method to intensify oxidation via secondary radical generation pathways without requiring external oxidants like hydrogen peroxide.

However, the authors maintain a modest stance regarding the extent of degradation. They explicitly state that the observed decrease in absorbance confirms the alteration of chromophoric groups but does not confirm complete mineralization or the absence of toxic transformation products. The study identifies key limitations, including the lack of Total Organic Carbon (TOC), residual iron, and toxicity measurements. Consequently, the authors conclude that while the system shows promise for decolorization, future work is required to optimize catalyst concentration, identify degradation products, quantify mineralization, and assess scalability under controlled industrial conditions.

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