Advances in Dye Degradation: Volume 1

Dye Degradation - Basics and Necessity

Kiruthiga Kandasamy¹, Sheeba Daniel², Poovan Shanmugavelan³, Paulpandian Muthu Mareeswaran⁴, *

¹ Department of Chemistry, Vellammal College of Engineering and Technology, Madurai-625 009, Tamilnadu, India

² Department of Chemistry, Holy Cross College (Autonomous), Nagercoil-629 004, Tamilnadu, India

³ Department of Chemistry, School of Sciences, Tamilnadu Open University, Saidapet, Chennai-600 015, Tamilnadu, India

⁴ Department of Chemistry, College of Engineering, Anna University, Chennai-600025, Tamilnadu, India

Abstract

Without colour, life is incomplete. Dye refers to the compounds that give goods their colour. Even though natural dyes have been used for generations, their limitations have led to the development of synthetic dyes. By addressing the history and significance of natural dyes, the limitations of natural dyes, the introduction of synthetic dyes, the negative effects of synthetic dyes, and an overview of several techniques used for the treatment of disposed dyes in the environment, this chapter serves as a foundation for the discussion of the entire upcoming book. The goal of this chapter is to provide a brief overview of the need for and the concept of dye degradation.

Keywords: Colour index, Degradation, Natural dyes, Oxidation, Synthetic dyes.

* Corresponding author Paulpandian Muthu Mareeswaran: Department of Chemistry, College of Engineering, Anna University, Chennai-600025, Tamilnadu, India; E-mail: muthumareeswaran@gmail.com

INTRODUCTION

Dyes are coloured substances that adhere to the substrate and give items their colour. Otto N. Witt developed a dyeing theory in 1876 that was based on functional groups like auxochrome and chromophore. According to his idea, certain auxochromic groups, which are responsible for dyeing properties, and certain unsaturated chromophoric groups, which are responsible for colour, are present in all coloured organic compounds (also known as chromogens) [1]. Dyes absorb visible wavelength ranges of radiation, and the appearance of colour depends on the wavelength ranges that are both absorbed and reflected. The term visible was created since the human eye can perceive light between 380 nm

(violet) and 700 nm (red) [2]. Two distinct indices are used to represent the commercial dyes. First The first is called a colour index generic number (CIGN), and it is used by businesses. The second one is the colour index constitution number (CICN), which has to do with the chemical makeup of the dye and is primarily employed by producers and academics [3].

Based on the chemical components found in the compounds, which determine the colour of dye, dyes are divided into numerous categories (Fig. 1) [4].

Fig. (1))

Classification of dyes.

Natural Dyes

Natural dyes and synthetic dyes are the two main classifications, which are based on their manufacturing techniques. Animals and other plant components, including the root, bark, leaf, flower, fruit, and seed, are used to make natural colours [5]. All civilizations have a very long history associated with the dyeing industry. Chinese dyeing techniques have been used for 5000 years [6]. The Ajantha cave paintings from the Ellora caves in India date to between 600 and 1000 CE, and those at Sittanavasal belong to the seventh century [7, 8]. These paintings are painted with vegetable oil colours, which have been around for more than a thousand years, on lime plaster. In order to categorize natural dyes, several criteria are taken into consideration, including their chemical makeup (anthracenes, carotenoids, xanthophylls, flavonoids, betacyanins, tannis, indigo, and chlorophyll dyes), their sources (animal and plant sources), their application techniques (direct dyes, acidic dyes, and basic dyes), and their colour [4]. The portions of plants from which the colour is derived are used to further categorize them. Based on colour, one of the most common classes is made. They are listed in the Colour Index based on their uses and chemical makeup of natural dyes. According on the application category they fall under, natural dyes have their own area in the Colour Index. Most red colour dyes are made from plant bark [9]. The predominant colour that can be derived from most plant parts is yellow.

The plant Iindigofera tinctoria is the source of the significant blue dye known as indigo (Fig. 2). 6000 years ago, this dye was used for the first time in Peru [10]. Indigo uprising occurred in Bengal, a region of the Indian Subcontinent, as a result of the extensive cultivation of this dye during the colonial era [11]. Lichens, which are fungi-algae composite creatures, are some of the organisms used to colour clothes. They come in a variety of colours, including orange, red, pink, and yellow. Because these creatures must be grown in an environment free of pollution and impurities, industrial-scale production is not feasible [12].

Fig. (2))

Examples of natural colourants.

Advantages of Natural Dyes

Natural dyes are made from natural materials and are therefore not bad for the environment. Natural colours are biodegradable and renewable [2]. They can be used without risk, and there are no disposal issues. The plants used to make colours are frequently also used as medicines. Despite being used for generations, many of their therapeutic benefits have only recently come to light [12]. The dye made from henna, walnut, and alkanet, which is high in napthoquinone, also has antibacterial, antifungal, and anti-inflammatory properties. The use of natural dyes in dye-sensitive solar cells is one of their more recent applications. This is because a variety of dye compounds with plausible absorption mechanisms are readily available [13]. Natural dye-sensitized solar cells (NDSSC) have a high efficiency for converting solar energy [14]. For direct application of NDSSC, for instance, dye combinations like chlorophyll/anthocyanin and chlorophyll/betalain are used. The UV-Visible absorption spectra of natural dyes provide evidence of the cause. Anthocyanin dye absorbs light with a wavelength range of 480 to 580 nm. Chlorophyll dye absorbs light between the wavelengths of 500 and 600 nm, and by making synthetic alterations, this range can be increased to 600 to 700 nm. Betalain dye absorbs light between 470 and 600 nanometres (Fig. 3). As a result, for effective energy harvesting, the absorption rage range spans most of the visible zone [15]. The energy conversion efficiency is improved by the physical coating or chemical anchoring of these dyes on semiconductor materials like TiO2. Electrons can be injected into semiconductor materials by the stimulation of natural dye mixes with a wide range of absorption in the visible region [16]. Electron flow is created by the excited electrons from the dye that are injected to the materials (such ZnO, TiO2, and Nb2O5). Increased surface area and effective dye mixture dispersion on the surface lead to efficient solar energy production [12].

Fig. (3))

Normalized absorption spectrum of a mixture of fresh natural dyes in ethanol solvent.

Limitations of Natural Dyes

In terms of commercial applications, natural dyes also have significant limitations. The main drawbacks of natural dyes are as follows [17-19].

Cost

As there is a large demand for dyes, there should be a high level of production. To produce dye, a significant number of raw materials are required, from which natural dye can be recovered. For the commercial manufacture of dyes, it is cost-effective to produce large quantities of the raw materials used to make dyes, such as the collection of leaves, flowers, bark, etc. Because of this, natural colours are pricey.

Colour

Natural dyes have a strong photobleaching of colour and low selectivity. Only selective dyes can be derived from natural dyes for selective colours. Synthetic dyes, on the other hand, come in a wider range.

Availability

Due to their cultivation circumstances, the availability of stating materials is limited. Consequently, it is challenging to produce all the basic ingredients in one location.

Harmful Effects

Natural dyes can sometimes be dangerous. Along with the environmentally harmful mordants, natural colours are employed. The use of organic solvents is part of the synthetic colour extraction process. Consequently, they also have a sizable number of environmental issues.

Sustainability

For the manufacturing of raw materials, land must be set aside for the cultivation of plants and trees that produce natural colours. In order to produce goods efficiently, this procedure needs labour.

SYNTHETIC DYES

The limitations of natural dyes prompted researchers to look for alternatives to meet demand and to begin developing synthetic colours. Dyes are artificial organic substances that are used in many industries, including textiles. Perkin created the first synthetic dye, known as mauveine, in 1856 [6]. The advantages of synthetic dyes over natural dyes are several, including their wide colour range, low cost, and resilience to fading from sunlight, water, and perspiration [19-21]. Natural dyes have been supplanted by synthetic dyes, and their industrial scale manufacture has increased dramatically—nearly 8× 10⁵ tons of synthetic dyes are produced annually. The textile sector uses over 75% of the world's dyestuffs. More than ten thousand different dyes and pigments are used in the global market to colour garments. The main users of synthetic dyes include also other sectors including printing, painting, and cosmetics. In addition to the heavy and light industrial sectors, one significant industry using synthetic dyes within acceptable limits is the food industry. The intriguing food colouring is utilized in jams, creams, and baking goods [22].

Classification of dyes based on application

Direct Dyes

Direct dyes are colours that already have the ability to bond with fabric. Since most of them are water-soluble, direct dyes do not require mordants. The primary cause of the substantivity of direct dyes is the secondary valence bonding between dye and fabric. According to the Society of Dyers and Colourists (SDC), there are two categories for direct dyes: 1. Based on bleaching or leaving ability and 2. Chemical structure [23].

Reactive Dyes

Reactive dyes, a family of synthetic dyes that have had great success and outstanding fastness properties with most materials, are used in a variety of products. They have chromophores with pendant groups that can join with nucleophilic fibre material locations to generate covalent connections. The development of polyfunctional reactive dyes that can react or form bonds with two dye-fibre bonds is a breakthrough in reactive dyes [24-26].

Basic Dyes

Basic dyes are Cationic dyes. Most of them have a water-soluble nature and will adhere to the negative sites of the fibre materials. Due to electrostatic attraction, there is also a possibility of uneven dyeing. They are mostly used to dye fabrics made of wool, silk, and acrylic. Along with mordant, they are also used to color fabrics made of other materials, such cotton [27-29].

Acid Dyes

Acidic dyes are those that are frequently processed for dyeing under an acidic pH condition. The process through which these dyes interact with the fibres is crucial.

The ion exchange mechanism contributes to their strong binding. With the charged groups contained in the fibre materials, they are establishing ionic connections. The most popular fibre types for acid dying at low pH are protein fibres (such as wool and silk) and polyamides with amide groups [30-32].

Mordant or Chrome Dyes

Many dyes have a weak affinity for fabric. A mordant is a chemical that is used to increase the affinity of dyes for fabric. The word modere means to bite in Latin. Tannins, metallic mordants, and oil-mordants are the three different types of mordants. For stable coordination compounds with fibre materials, chrome dyes, which are acidic mordant dyes, can be used [33-37].

Disperse Dyes

These have a high substantivity to hydrophobic fibres, such as nylon, cellulose, cellulose acetate, and acrylic fibres, and are insoluble in water. Nonionic and water insolubleness are two crucial characteristics of dispersion dyes. Additionally, they are not altered chemically during the colouring process [38-40].

Vat Dyes

Vat dyes are insoluble in water. These dyes can be converted into a water-soluble form known as "Leuco form" by reducing them with inorganic ions. They were re-oxidized after the dyeing process to return them to their original state. The fabric receives stable colour as a result of the dyeing process. These dyes are insoluble in water due to their hefty conjugated structure. Additionally, the conjugation offers superior optical qualities that can be used in optoelectronic devices [41-43].

Sulphur Dyes

Sulphur colours are water-insoluble dyes that contain sulphur. They are significant in the dyeing business because of their inexpensive cost and affinity for cellulose. Alkali metals like sodium are used to decrease sulphur dyes to create water-soluble thiols. They are restored to their natural state after the colouring procedure [44].

Azoic Dyes

The most common synthetic dyes are azo dyes. Azo dyes make up 70% of synthetic organic dyes. They are easier to synthesize, have more structural variety, and high fastness. By diazotizing aromatic primary amine with amino and hydroxyl groups, azo dyes are synthesised. Azo dyes are categorized into monoazo, diazo, triazo, and polyazo dyes based on the number of azo links [45-47].

ADVANTAGES OF SYNTHETIC DYES

Crude oil (a fossil fuel) is the main source of synthetic dye because most synthetic dyes are made from petrochemicals. As a result, the raw resources are inexpensive and accessible. These dyes are effective at dying and produce a consistent, uniform colour. The availability of dyes for various types of materials is another crucial consideration. As opposed to natural dyes, synthetic dyes may be produced affordably and with less energy use by adjusting the hues and strength of the colours to suit the needs [19]. Since the colour is directly tied to the structural characteristics of dye molecules, synthetic dyes produce more striking coloration than conventional pigments. Whereas, the conventional pigments are mostly influenced by the physical properties [6]. For instance, dyes containing the azo, coumarin, and perylene groups are responsible for the vibrant colour [48]. Due to the commercial effect and the numerous benefits of synthetic dyes, two thirds of the synthetic dyes produced are commercial organic dyes with a wide variety of structural diversity and applications. Like the food sector and the pharmaceutical industry, synthetic dye-using industries are also growing. Safranine T, Thioflavin T, and other synthetic dyes make up most of the MRI contract agents and stains used for oncological investigation [49].

Toxic effect of synthetic dyes

Synthetic dyes released into the environment, including water bodies, untreated or partially treated, have negative environmental effects (Fig. 4). For dyeing, fixing, washing, and other processes, the textile industry uses enormous amounts of water, and 15% of the synthetic dyes used in these processes are released with the waste water [50, 51].

The effluents from textile factories contain a variety of organic and inorganic pollutants, including soaps, sequestering agents, dyes, pigments, chromium compounds, other heavy metals, chlorinated compounds, nitrates, sulphur, naphthol, formaldehyde, and benzidine [52]. Numerous harmful substances continue to exist in effluents even after the treatment process and cause multiple contaminants, such as soil, water, and air pollution [22]. The receiving water bodies (such as the sea, river, lake, natural ponds, and streams) are where the textile industry's effluents have the greatest impact on the living environment and ecosystem. Even at low dye concentrations (>1 mg/L), dyes can produce intense colours, but wastewater effluents typically have dye concentrations of 300 mg/L or higher along with other harmful substances. The pH impact of these effluents is their initial effect. The pH changes will cause a mass extinction of the animals and plants (planktons) present in the water bodies. The dark colour caused by the dyes prevents sunlight from penetrating, which significantly restricts the photosynthetic activity. As a result, the level of dissolved oxygen decreases [22].

Fig. (4))

The direct and indirect effect of synthetic dye on the environment [22].

Dye degradation techniques

Researchers have consequently paid a lot of attention to the degradation of colours from textile industry effluents (Fig. 5). Four categories—physical processes, chemical processes, biological processes, and combinatorial processes—are used to categorize the colour degradation approaches [53]. In addition to coagulation, reverse osmosis, photodegradation, ion exchange, oxidation, biodegradation, nanotechnology, an improved oxidation process, and adsorption, these procedures also include other elements [54]. The focus of current research is on safe, efficient, and environmentally acceptable methods for removing colours from contaminated water [55-57]. Microorganisms are likely to take a while for the chemical components of the dyes to biodegrade. All the traditional physical and chemical techniques used to eliminate dyes are frequently too expensive, only partially effective, and generate waste that is challenging to dispose of [58]. The majority of traditional techniques only successfully transfer dyes from one phase of water to another, which results in secondary contamination. Additional treatment is needed for the secondary contamination, and the procedure is not economical [59].

Fig. (5))

Methods for degradation of dyes.

PHYSICAL PROCESSES

Physical dye removal is the removal of dyes without chemical modification. There are three types of physical processes; 1. Adsorption, 2. Filtration and 3. Ion exchange [60].

Adsorption

The solid-state removal of colours and pigments is centuries old technique [28]. Although the adsorption mechanism was not fully known in the past, adsorption technology has been studied since the turn of the twentieth century [61]. In many different industries, water-soluble synthetic dyes are utilized; these colours are typical industrial effluent water pollutants [62]. Due to their structural makeup, most synthetic dyes are stable when exposed to light and heat as well as resistant to aerobic digestion and oxidizing chemicals [63]. Adsorption is the effective method for removing dyes before they are broken down into benign chemicals [64]. Activated charcoal, silica, zeolites, bone charcoal, alumina, carbon molecular sieves, polymeric polymers, carbonized materials, and other materials are all accessible as adsorbents (Fig. 6) [65]. The efficiency and selectivity of the adsorption materials can be adjusted by physical activation and chemical surface modification [60]. There are two types of adsorption processes: physical and chemical. An essential technique for activating surface adsorption is carbonization. The base materials for this approach range from biological wastes to synthetic polymeric materials [66, 67].

Fig. (6))

Various adsorbent materials and the processes of adsorption [65].

Filtration

Adsorbent materials are always used in conjunction with filtering techniques [68]. Coagulation and filtration are frequently combined. Using filters such as filter papers, membrane filters, etc., the suspended particles can be separated [69]. Adsorbent materials will be crucial