Advances in Dye Degradation: Volume 2

By Paulpandian Muthu Mareeswaran (Editor) and Jegathalaprathaban Rajesh (Editor)

This series provides information on the nature of dyes, their harmful effects, and dye degrading techniques. The second volume focuses on sophisticated oxidation methods for dye degradation. The information on target-oriented dye mitigation is intended to give readers a better understanding of the dye degradation process to sustain a healthy environment. Chapters present referenced information and highlight novel industry breakthroughs.

Key topics:
The Fenton process for dye removal
MOF-based and graphene oxide photocatalysts for dye degradation
Novel photocatalysts active in visible light and IR spectra
Photocatalytic degradation of transition metal dichalcogenides
Environment friendly synthesis of photocatalysts for dye degradation
Metal oxide Nanomaterials for dye degradation

This volume is a comprehensive singular resource on photocatalytic dye degradation for researchers, apprentices and learners in chemistry and chemical engineering courses. It also serves as a reference for industry professionals who work with chemical dyes (for example in textile and plastic industries) and are engaged in the critical field of environmental remediation.

Readership
Scholars in chemistry and chemical engineering; professionals in manufacturing industries and environmental sustainability.

• Language: English
• Publisher: Bentham Science Publishers
• Release date: Jul 26, 2024
• ISBN: 9789815238150

Book preview

Fenton and Fenton-Like Processes for the Degradation of Dye in Aqueous Solution

R. Liju¹, Eswaran Rajkumar¹, *

¹ Department of Chemistry, Madras Christian College (Autonomous), Affiliated to University of Madras, Chennai-600059, Tamil Nadu, India

Abstract

Water is necessary for the growth of humans and all other living things. Water is becoming scarce due to industrialization and its rapid growth, and the water ecosystem is negatively impacted by the direct release of wastewater into the environment. The textile, tanning, coating, plastics, paint, printing, and other industries, discharge dyes and pigments into the environment. One major problem is to remove dyes and pigments from industrial wastewater in a inexpensive and environmentally friendly way. Before they are released into the environment, there are several ways to mitigate the situation, including chemical, biological, and chemical oxidation processes. The advanced oxidation process (AOP) is a widely employed technique for eliminating contaminants from water and wastewater. The dye molecules are broken down by a Fenton and Fenton-like mechanism, in which the breakdown of hydrogen peroxide produces hydroxyl radicals. This chapter focuses on the most current advancements and various strategies used in the Fenton and/or Fenton-like processes used to degrade the dye molecules.

Keywords: Decolourization, Dye degradation, Fenton process, Hybrid Fenton processes.

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* Corresponding author Eswaran Rajkumar: Department of Chemistry, Madras Christian College (Autonomous), Affiliated to University of Madras, Chennai-600059, Tamil Nadu, India; E-mail: [e-mail]

INTRODUCTION

According to Business Fortunes Insights, the global market for dyes and pigments is anticipated to expand at a compound annual growth rate (CAGR) of 4% from $40.7 billion in 2022 to $56.91 billion by 2029. Pollutants (dye molecules, which can be entirely organic, inorganic, or organometallic) pose a serious risk to human health and that of animals, plants, and aquatic life forms.

These substances also linger in water bodies for extended periods, deteriorating the ecosystem. Typically, the textile, tanning, coating, plastics, paint, printing, and

other industries discharge dyes and pigments into the environment; because of this, scientists, environmentalists, and academics are very interested in policies that can regulate and improve water quality. The Environment Protection Agency (EPA) categories these waste from industries are harmful under the Property, Conservation, and Recovery Act (RCRA). The production waste of the product is designated as EPA hazardous waste K181. When dyes and pigments are made and processed on-site, waste is produced at the manufacturing facilities. The only way to lessen the harmful effects of pollutants (like dyes) is to break down the dye molecules into innocuous compounds, which requires technological innovation to monitor the discharge levels of the pollutants. It would be quite challenging to control the amount of dye released into the environment due to the speed at which the industry is developing. However, dissolving these dangerous chemicals into safe, non-toxic compounds gives us a workable way to lessen the adverse effects. Before the dye molecules are released into the environment, they can be broken down or degraded by using various techniques, such as chemical treatment, biological treatment, and chemical oxidation.

Compared to other dye degradation techniques, chemical oxidation methods have several advantages. For example, biological treatment is more expensive and selective towards certain pollutants. In contrast, chemical treatment frequently risks becoming a pollutant in and of itself, making it unsuitable for the environment. On the other hand, chemical oxidation techniques are low-cost, efficient, and safe for the environment. The effluents of industrial dyes, notably textile dyes, have recently been shown to negatively affect the ecosystem's health and environment [1]. They have provided a concrete solution for the bioremediation process using microorganisms and enzymes and a summary of the many methods of reducing and eliminating pollutants. Nevertheless, there are several practical and sustainable constraints to these bioremediation techniques. By using extremely reactive hydroxyl radicals to break down organic pollutants (dye), the advanced oxidation process (AOP) produces carbon dioxide, water, and inorganic salts that are environmentally benign and do not endanger the environment [2]. Fenton/ Fenton-like processes, which are based on the breakdown of hydrogen peroxide to produce hydroxyl free radicals, have been studied and used more than other AOPs because of their straightforward operation, mild operating conditions, higher rate of •OH production, and redox potential of roughly 2.73 eV, which is sufficient to break down even resistant dyes.

Three types of Fenton processes can be distinguished: the hybrid Fenton process, the heterogeneous Fenton process, and the homogenous Fenton process [3]. Homogenous Fenton is the term for the conventional method of conducting Fenton reactions, in which the oxidizing agent (hydrogen peroxide), catalyst (metal ion), and reactant (organic dye) are all present in the liquid phase. On the other hand, reusable solid catalysts are utilized in heterogeneous Fenton reactions, including iron oxides, nano zero-valent iron, iron-loaded zeolites, and iron-immobilized clays. This makes oxidation reactions possible at neutral pH values and simplifies the separation of the liquid and solid components. Bi/tri metallic iron catalysts like NiFeMnO4 and copper-iron bimetallic CuFeO2 are also utilized in the heterogeneous system. Several review articles and book chapters are available to treat water and wastewater or to remove contaminants based on Fenton and Fenton-like processes. This chapter focuses on current advancements and the many strategies used for dye molecule degradation by Fenton and/or Fenton-like reactions.

CONVENTIONAL FENTON PROCESS

The Fenton process, a sophisticated oxidation method that uses hydrogen peroxide as an oxidant and ferrous ions as a catalyst to produce extremely reactive hydroxyl radicals in an acidic medium, is extensively researched for dye degradation. Below is a list of the reactions that comprise the Fenton process (Fig. 1).

Fig. (1))

Schematic mechanism of conventional fenton process [4].

Depending on many factors such as reaction conditions, catalytic effectiveness, pollutant (dye), and so on, the Fenton reagent or Fenton procedures have undergone numerous variations over time. One of the many disadvantages of traditional Fenton or Fenton-like dye degradation methods is the significant post-treatment of the iron sludge (Fig. 2).

Fig. (2))

Drawbacks of classical fenton processes and alternate approaches for the limitation [5]. Attribution 4.0 international (CC BY 4.0).

To overcome the Fenton process's shortcomings, various methods were used to generate the •OH radical. Researchers have suggested using magnesium, calcium, and sodium peroxide as substitutes for hydrogen peroxide. The primary advantage of this method is that it reduces the excess H2O2 generation in the reaction and prevents unwanted side effects by progressively releasing H2O2 from the reaction medium throughout the Fenton reaction. Other Fenton-like reagents, which are created by substituting lower valent metal ions for ferrous ions, as well as other assisting agents, such as sound, photons, electrons, etc., and combinations of multiple assisting agents, can be used to adjust the Fenton reaction's highly efficient degradation and selectivity towards different types of organic dyes (Fig. 3).

Recently, Zhang et al. reported the degradation of rhodamine B dye over a pH range of 5.60 to 12.21 utilizing three-dimensionally built 2D nanosheets of NiS as a catalyst [6]. This was achieved without the need for light. In addition to Ni³+ and •OH, the surface of the NiS reacted with H2O2 under acidic conditions to produce Ni³+ and SO4²- ions. In acidic environments, the Ni³+ ion becomes unstable and transforms into Ni²+. Fig. (4) illustrates a potential mechanistic pathway for using NiS catalyst at pH values of 5.60 and 12.21 to degrade RhB dye.

Fig. (3))

List of modifications applied to the fenton process [5].

PHOTO-FENTON PROCESS

The Photo-Fenton or photon-assisted Fenton process uses light as a tool to enhance the efficiency of regular Fenton-type reactions; they can be both homogeneous and heterogeneous Fenton types. The light-assisted Fenton processes are the widely used degradation methods of organic dyes and pollutants, owing to their simplicity, cost-effectiveness, and the possibility of exploiting solar irradiation. The Photon Fenton varies depending on the source and wavelength ranges of the light used in dye degradation and the nature of the dyes. The mechanism of the Photo-Fenton process is based on the photocatalytic property of the metal ions or metal nanoparticles. In the photodegradation of dyes, photons excite the electrons of the catalyst, thereby producing reactive hydroxyl radicals, which can oxidize organic dyes, causing degradation. The reaction mechanism of a conventional Photo-Fenton process is given below:

Fig. (4))

Schematic representation of the fenton process at different pH [6].

In Photo-Fenton processes, different UV regions, such as UVA (λ = 315–400 nm), UVB (λ = 285–315 nm), and UVC (λ < 285 nm), are utilized as sources of light energy. Therefore, the overall production of •OH free radicals varies depending on light intensity. The Photo-Fenton reactions are reportedly effective in dye degradations [3]. It has been revealed that the Fe²+ ions have maximum absorbance only in the UVB region, and solar irradiation holds only a fraction of UVB photons, thus reducing the quantum yield. In addition to the above limitation, Photo-Fenton reactions are effective around a pH of 3.0. In contrast, at neutral pH, the ferric ions would undergo precipitation and reduce the efficiency of the reaction. For a long time, homogenous photon Fenton-based dye degradation involving hydrogen peroxide and Fe²+ ions has been used. No additional steps, such as catalyst recovery and activation, are required for the homogeneous Fenton process.

Recently, Watwe et al. [7] investigated the homogeneous Fenton-type process using the Cr(VI) system for the degradation of methylene blue (MB) dye by wastewater containing chromate and recalcitrant pollutants, as shown in Fig. (5). The major industries like leather tanning, metal finishing, electroplating, and petroleum refineries generate large amounts of wastewater containing Cr(VI), which is highly toxic. The chromium(VI) contaminated water may degrade the dye present in the aqueous environment before the pre-treatment by adding H2O2. Numerous experimental conditions, including pH, dose, [MB], [H2O2], electrolytes, and temperature were examined for their effects. At room temperature and throughout the pH range of 3 to 8, the ideal concentrations for the MB process degradation were determined to be 19.4 mM H2O2, 3 mM Cr(VI), and 15.7 mM MB (the ratio of Cr(VI) to H2O2 is 1:6). With a total degradation of 62.8 M MB, the reaction's kinetics are reusable for up to four cycles and resemble pseudo-first order processes.

Fig. (5))

Degradation of MB dye using chromate and H2O2 as a fenton-like process [7].

The homogenous Fenton-like processes' catalyst loss and significant sludge development were avoided by using the heterogeneous Fenton-like processes. Using heterogeneous Fenton and modified Fenton-like processes, Shokri and Fard recently summarized the many advanced oxidation methods for removing contaminants in the aquatic environment [8]. A Fenton-type process with a variety of catalytic metals, like Co, Cr, Ce, Cu, Al, Ag, Mn, Ru, and polyoxometalates, has the following benefits: it can produce hydroxyl radicals effectively, access different oxidation states, operate best at a wider pH range (near neutral or alkaline pH), inhibit metal leaching, and improve the catalytic cycle (Fig. 6).

Fig. (6))

Heterogeneous fenton-like oxidation process [8].

ELECTRO-FENTON PROCESS

Conventional Fenton procedure (mixture of H2O2 and soluble ferrous salt) creates enormous ferric hydroxide sludge and a sluggish catalysis rate. Significant amounts of reagents are required to produce highly reactive hydroxyl radicals. The successful use of electrochemistry for dye degradation in an aqueous environment has garnered growing attention in recent decades due to its high efficiency and environmental compatibility. Because of its better mineralization efficiency, affordability, and convenience of use, the electro-Fenton methods have drawn more attention for the treatment of water and wastewater [9]. Using an in situ mechanism, this approach produces hydrogen peroxide. Externally introduced ferrous ions create hydroxyl radicals (•OH), which break down organic contaminants in the aqueous system until they reach the final mineralization step. The primary factors influencing the pace of pollutant degradation at the cathode are the regeneration of ferrous ions through the cathodic reduction of ferric ions and the two-electron reduction of oxygen in acidic circumstances. Various carbon-based materials were used as the cathode to create hydrogen peroxide. (Fig. 7).

Fig. (7))

Graphene in the electro-fenton-like process [10].

Electrons are the primary active element in the Electro-Fenton (EF) process, which may be run at room temperature and pressure. Known as a green reagent, electrons are thought to be less expensive per mole than chemical reagents [11]. The EF process includes electrochemically regenerating ferrous ions (Fe²+) to catalyze the Fenton reaction further and a regulated synthesis of hydrogen peroxide in situ in the reaction mixture. The likely response mechanism can be summarized as:

An electrochemical cell with an acidic pH of 2.5–3.5 is used for the EF process. Approximately 10-4 M of a catalytic amount of iron (Fe²+/Fe³+) salt solution is needed to support the Fenton reaction. Thus, Fe³+ ions produced by the Fenton reaction can be reduced at the cathode to produce Fe²+ ions again. As a result, the dye is degraded by the •OH generated at the anode without much iron sludge buildup or other problems related to the traditional Fenton reaction.

Numerous studies on the application of EF in dye degradation have been published in recent years. By utilizing several catalysts, Matyszczak et al. explored the electro-Fenton process to study the degradation of the azo dye Metanil Yellow. Various catalysts, including Fe²+, Co²+, Mn²+, Ni²+, and Ce³+, were employed, and their capacity to degrade Metanil Yellow was examined. According to reports, the Ni²+ catalyzed EF degraded Metanil Yellow with great efficiency. Therefore, the dyes' degrading efficiency can be significantly influenced by the adjustments made to the electrode systems, catalysts, and operational factors such as supporting electrolytes, pH, and applied potential [12]. Orange II is an azo dye that takes 90 minutes to fully decolorize when utilizing EF at pH 3 with different supporting electrolytes [11]. In related work, basic blue 3 in an aqueous medium was broken down by the EF process using a modified cathode system. A new cathode material called carbon sponge was introduced; previously, cathodes for EF were known to be employed in mercury pools, graphite, vitreous carbon, activated carbon fiber, etc. Three times as much H2O2 was produced electrochemically when using a carbon sponge electrode as other carbon electrodes [13].

SONO-FENTON PROCESS

High-frequency sound waves are used in sonochemistry to facilitate chemical reactions. The dye degradation has been the subject of extensive research in recent years on sonochemistry-based advanced oxidation processes [14]. Sonochemical oxidation can be employed alone or in conjunction with other AOPs in sonochemical Fenton or Sono-Fenton processes, which use high-frequency ultrasound (US) to generate extremely reactive hydroxyl free radicals to break down or degrade dyes (Fig. 8). The sonolysis of Fenton reagents produces both Fe²+ and H2O2 for the breakdown of organic dye, as seen in the reactions below [15]. Sonication works by a process known as acoustic cavitation, which is the creation, growth, and abrupt collapse of tiny bubbles in a liquid that results in advantageous chemical changes. Localized hot spots