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1. Introduction
All dyes generated from plants, animals, and minerals are referred to as natural dyes [1, 2]. Before the development of synthetic dyes, their primary use was in the coloring of natural fibers, leather, and food substances. Natural dyes are mostly derived from plants, and as many as 500 different plant species have been discovered as sources of natural dyes [3]. Using natural resources and natural ways are currently thought to provide value to a product and promote a sustainable lifestyle from sociological, economic, and environmental perspectives [4].
Due to strict guidelines and legislation for environmental and public health concerns regarding all production processes and products utilized, there is a rising demand for the use of natural dyes in all parts of the world today [3]. The rapid industrialization led to the use of several toxic chemical substances, causing serious environment pollution [5]. There are expected to be tens of thousands of tons of produced dyes used in textile and clothing manufacturing each year [6]. Due to their nonbiodegradable and carcinogenic character as they are released as a waste from the industries, synthetic compounds that are dyes and auxiliaries used in textile and apparel dyeing are one of the main pollutants of water and the environment [3].
The ability to continue existing is referred to as sustainability, and it is a significant global issue that refers to a social and ecological process that is characterized by the pursuit of common ideals [5]. It relates to how well human civilization and the ecosystem can coexist and describes the method by which people manage change in a dynamically balanced environment [7]. Systems, exploitation, future development, and other changes all require coordination in order to better serve present and future human needs and desires [8].
Natural indigo dye and natural dyes in general have lately been found to possess other useful qualities in addition to their biodegradability and environmental friendliness. These include antimicrobial activity [9], antifungal, antiviral activity [10], insect repellent [11], UV protection [12], and deodorizing agents [13]. The cosmetics industry today uses a lot of natural colours because they have less negative side effects than synthetic dyestuffs, but they can also offer additional benefits such as UV protection, skin moisturizing, and antiaging [14]. Natural dyes having antiseptic properties may reduce the likelihood of disease transmission, reduce the risk of infection brought on by trauma, prevent textile deterioration, and provide additional benefits [15]. Natural dyes are employed in a variety of applications today, including food and cosmetic coloring, cosmetic healing additives, pH indicators, and functional finishing (antimicrobial, antifeedant, deodorizing, or UV protection) for textile and garment [16].
In addition to potentially reducing health risks and environmental pollution associated with the textile and apparel manufacturing industries, the natural indigo dye also offers developing nations, particularly those located in tropical regions, opportunities for economic growth and ways to reduce poverty [17]. Owing to the fact that the tropical indigo species of Indigofera produces high-quality indigo dye with the highest purity ranges of 50–77% [18]. Because of their comparatively cheaper labor costs, developing nations play a significant role in the production of indigo plants and the dye extraction from them [18]. This helps offset the greater cost of the natural dyeing process additionally, and many developing nations already have a long history of using natural dyes and have the basic materials needed to extract dyes [18]. Thus, natural coloured goods present an excellent potential for value-added exports from nations that are already global leaders in the production of textiles and clothing.
Ethiopia as a developing country is suitable for tropical Indigofera indigo cultivation with its wide range of climates, wealth of land, and ample young work forces. In addition, Ethiopia puts priority on textile and cloth manufacturing sector industry development, and accordingly, these sector industries of the country are at their booming stage. These kinds of threatening that the potential pollutant industry are employing in manufacturing need possible prequestion measures in order to protect the environment and health hazard consequences from the textile and apparel manufacturing industry sector.
The availability of 78 species of wild indigo plants in Ethiopia has been reported. However, only a small number of botanists, phytologists, and painters are aware of this wild indigo plant species that are present in Ethiopia. At the time being, there is no indigo cultivation in Ethiopia and only wild indigo is presented [19]. Meanwhile, the indigo cake is being imported from India for small-scale dyeing, printing, and painting. Considering the Ethiopian potential for indigo cultivation, the research and development project on indigo plantation, extraction, and dyeing has been initiated at the Ethiopian Institute of Textile and Fashion Technology, Bahir Dar University, Ethiopia. The fundamental goal of this research is to transform wild indigo into a cultivable plant. The research also includes dye yield, extraction, and dyeing optimization. Thus, the purpose of this research is to provide clear information regarding indigo revitalization and Ethiopian status in this regard.
2. Revitalization of Natural Indigo
The current great concern for environmental sustainability and health hazards, fortunately, natural indigo is progressively gaining popularity around the world owing to the environmental benefits it provides over synthetic indigo and the growing demand for organic clothes [1]. In addition to the concern of environmental sustainability and health hazards, the revitalization of natural indigo also delivers economic and social welfares, particularly for developing countries.
2.1. Environment Pollution and Health Hazard Driving
The revival of natural dyes, especially indigo, is primarily motivated by the environment pollution and health risks from the textile and garment industries, with the hope that doing so will help reduce these problems [3]. There has been a resurgence of interest in natural indigo due to concerns over sustainability and consumer demand for products made from naturally sourced materials. As a result, extensive scientific and agronomic research has been carried out to maximize indigo production from the most promising plants [3]. The demand for natural indigo has increased recently in many nations due to the effects of pollution and health problems as well as a rebirth of interest in the relationship between colour and culture. Indigo is still farmed on a small scale and raised as a supplemental crop for dyeing purposes in India, as well as in various nations of Africa and Central America [20].
Natural dyes that are safe for the environment, nontoxic, long-lasting, and renewable are now a viable choice, including natural indigo dye [21]. Industries have a global responsibility to decrease waste and utilize nontoxic materials as soon as feasible in order to minimize the detrimental effects of materials on human health and the environment [21]. The prevalence of chronic pollution-related diseases such as asthma, cardiovascular disease, cerebrovascular accident, and cancer has increased along with increased industrial activity in emerging nations [22].
By contaminating the air, water supply, land, crops, cattle, fish, and other resources, manufacturing activities are also having an impact on public health [23]. By restricting the economic output of entire generations and damaging the social development of entire societies, the human health effects of pollution, such as induced cognitive impairment, can result in significant economic consequences. In addition, it has been discovered that climate change worsens the impacts of pollutant exposure in developing nations by raising levels of various chemicals in the water, air, and soil as well as sensitivity to pollution [24–26].
Owing to their lower body weight and increased health concerns during pregnancy, women in developing nations are probably more susceptible to particular pollution exposures [27]. Women may be more vulnerable and more prone to work in poorly regulated, and consequently polluting, manufacturing environments as a result of gender stereotypes and social institutions that limit their mobility, leisure time, and other employment opportunities [28]. Furthermore, research is urgently needed to determine how these (and other) concerns affect and how gender affects exposure to the environment and morbidity associated with pollution [23].
2.2. Economic Interest
While environmental concerns and rising demand for natural and sustainable dyes may contribute to an increase in natural indigo production, the world’s existing natural indigo dye production cannot keep up with the demand for synthetic dye [29]. In terms of the whole market, natural indigo represents just around 1% of the world’s annual indigo production, which is estimated to be in the range of hundreds of tons [18]. According to the unpublished market study, 5% of consumers said they preferred goods that were dyed with natural dyes. According to estimates, there is a $13 million market for indigo in Europe, and demand is only expected to rise [29]. For the basic quality product with a 45% purity of indigotin content, indigo dye is typically priced between $35/kg (with poor purity) and $40/kg on the worldwide market, with an extra premium of $0.85/kg added for every additional percentage point of indigotin content [29].
The primary fabrics for indigo-dyed clothes are cotton, linen, silk, or wool, and depending on how worn out the garment is, these materials can be recycled as fiber or resold, donated, repaired, altered, or redesigned [21]. Additionally, indigo dyeing can be performed with locally sourced materials that are repeatedly planted and made. The plant waste that is recovered can be turned into compost, and the dyeing water can be utilized for repeated irrigation [30]. In addition to being used for textile and clothing dyeing, the indigo extract also has a variety of functional finishing applications, such as antibacterial, antifeedant, UV protection, food and cosmetic colouring, cosmetic healing additives, pH indicators, pharmaceutical source, and more [31]. In the United States, indigo is classified as FD and C Blue No.2 Aluminum Lake for use as food colouring (https://Drugs.com via the internet). For the purpose of colouring textiles and other goods, natural indigo is produced in various amounts in African countries [31].
Since France’s and their team’s game jerseys are blue and Italy’s flag is light blue (or “Azure”), indigo dye is in great demand [32]. The cricket team from India and the soccer squad from Japan both wear blue as their official colours [32]. The colour blue was traditionally connected to riches, monarchy, and luxury in Egypt [33]. The most renowned sheikhs in Lebanon and Syria prefer to wear blue [34]. Owing to its desired characteristics and overall photo-stability, indigo dye was once known as the “colour of kings and the king of colours” [35].
Despite having characteristics that are not typical of synthetic vat dyes as a whole, natural indigo dye is still categorized as one [36]. Depending on the substrate or whether it is being used as a pigment or dye, indigo has been reported to have a moderate [37] to very high light fastness [31]. The synthetic vat dyes have better fastness qualities than natural indigo dye, especially against light, washing, and chlorine bleaching, but it is natural indigo’s fading of colour that has kept it so popular with those who wear jeans [31, 37]. Due to the widespread use of blue jeans, which are coloured with indigo, the dye’s present use is tremendous, and it is estimated that 33 million kg of indigo and other vat dyes are consumed yearly [29, 38]. A significant amount of indigo is necessary to dye denim fabric for blue jeans; a pair of blue jeans pants requires roughly 3–12 g of indigo [32].
Generally speaking, indigo has been revived with many more new applications than it had in the past, making it an economically significant cash crop for the colouration of food, pharmaceuticals, cosmetics, and so on. In order to create value chains with the textile, garment, and other indigo-consuming small and medium-level sectors (food, pharmaceuticals, cosmetics, and more), indigo cultivation and trading will therefore be advantageous in general for low-income individuals in developing countries.
3. Indigo Dye Yielding Plants
The world is home to several different species of the indigo plant, with the genus Indigofera supplying most of the world’s indigo dye [31]. Indigofera is a very broad genus that contains around 700 species that can be found in both the wild and in cultivation across the tropics and subtropics of Africa, Asia, and America [31]. Other indigo species such as I. coccinia, I. decora, and I. amblyantha are found in China, I. carolinians and I. lespotsepala are found in South Carolina, USA, and Indigofera suffruticosa and Indigofera arrecta in tropical America, and India is where I. tinctoria was domesticated [39].
For tropical Africa and other botanical taxa, over 300 species have been counted; the Southern Himalayas and Africa contain the most species overall, and for many years, China and Japan have cultivated, processed, and marketed dyer’s knotweed and the indigo variations of woad in Europe [31]. In general, the major plant families used as a major source of natural blue colour or indigo are as follows [35]:
(i) Leguminosae (pea family): the most well-known indigo producing plant is I. tinctoria, also known as I. sumatrana, which belongs to this family. It was a tropical shrub that was mostly found in Southeast Asia, India, and the Middle Eastern Region, and Indigofera suffruticosa from this family is native to tropical America. Furthermore, Indigofera incarnata (commonly named Chinese indigo) is a deciduous shrub native to Japan and East Asia.
(ii) Brassicaceae (cabbage family): it comprises Isatis tinctoria species, also known as woad. Woad is grown in Western Asia, the Mediterranean, across Europe, and North America.
(iii) Polygonaceae (dock family): it contains Persicaria tinctoria species commonly named as Japanese indigo.
(iv) Acanthaceae (acanthus family): under this family, the well-known species is Strobilanthes cusia found in Japan and Taiwan.
(v) Fabaceae family: it encompasses Indigofera, the third biggest genus of this family.
The largest family of medicinal plants is the Fabaceae, which has over 750 species and is utilized as a primary source of the blue color [40]. Among the Indigofera family, two species, namely, Indigofera tinctoria and Indigofera suffruticosa, are the plants that produce the blue colour in large quantities and they are the two economically important indigo dye-producing species known to exist today [41].
More than 78 Indigofera species have been recorded in Ethiopia, which makes sense given that tropical Africa is the native home of indigo species [42]. There are about 21 species in the genus Indigofera only in Benishangul Gumuz Regional State of Western Ethiopia [42]. However, most of the Indigofera species in Ethiopia including I. tinctoria are considered weeds [41], and they perhaps had no or low commercial importance so far.
There are only five species of Indigofera in Ethiopia, which were once important, particularly as the source of the internationally traded dye, indigo. These are as follows [43]:
(i) I. articulata Gouan: these are found in dry grassland and bush land in the Eritrea border, Afar (the North Eastern part of Ethiopia), Shewa (the middle part of Ethiopia), and Harerge (the eastern part of Ethiopia)
(ii) I. arrecta Hochst: they also originate in dry grassland and bush land and recorded from all parts of the country except for eastern Afar
(iii) I. coerulea Roxb: with two varieties (var. coerulea from the arid coastal plains and var. occidentalis Gillett and Ali from all drier parts of the country)
(iv) I. tinctoria L: this is found only in the Lower Omo Valley
Indigofera is a large genus in Ethiopia with 78 species in the most recent account of the family. Indigofera trigonelloides are found only in Namibia and Ethiopia. All indigo plant species found in Ethiopia are wild, and there is no known use for indigo dyeing.
4. Chemistry of Natural Indigo Dye
Natural indigo contains mostly inorganic materials, although the main active ingredients are organic materials [31]. Colourless glycosides of indoxyl (-hydroxy-indole), or more specifically, pale yellow, are indigo-producing compounds found in plants that are used to make indigo dye [31]. As shown in Figure 1, Indican, which takes its name from the Indigofera plants in which it was first identified, is the first of these glycosides to be characterized by Hoogewerff and Ter Meulenin 1900 [44]. For a long time, it had been suggested that it might be the only indigo precursor present in all indigo plants, but in the same year, the indigo precursor in woad (Isatis tinctoria L.) was shown by M.W. Beijerinck to be different from that in Indigofera spp. and was named isatan B [31]. Since then, it has been discovered that there are several different precursors of woad, and this may also be true of other incompletely studied indigo plants [31].
Indican (lH-indol-β-yl and B-D-glucoside) has been identified in many species of plants belonging to different botanical families [31]. The chemical precursor to indigo plants, or indican, is present throughout the plant but is concentrated almost entirely in the vacuoles of the leaves, as shown in Figure 1(a) [45]. The separation of indican was carried out into two parts following extraction and hydrolysis (achieved through heat and/or fermentation in water). The two parts are b-D-glucose and indoxyl, Figure 1(b). Enzymes that are naturally found in the indigo leaves and bacteria that probably seek for b-D-glucose as food both contribute to the hydrolysis. Indigotin, the active pigment in indigo dye, is produced by indoxyl’s reaction with a mild oxidizing agent such as air oxygen under highly alkaline pH circumstances, as shown in Figure 1(c). Indigotin is only one of the several oxidation products of indoxyl. Others, including isatin and indirubin, which are seen in Figures 1(d) and 1(e), might occur under less-than-ideal circumstances, which lowers the purity of indigo dyestuffs and reduces the amount of the desirable indigotin pigment [45].
[figure(s) omitted; refer to PDF]
Indigo does not exist in the leaves of indigo-producing plants. Instead, there are its antecedents, indican in Indigofera species, Polygonum tinctorium, and isatan B in addition to indican in Isatis tinctoria [46, 47]. Recently, it has been claimed that isatan A [48, 49] and isatan C [50], two more precursors, may also be found in woad. Since indican is bound to glucose and forms a stable indican glucoside in young, green leaves, it is stable. However, during fermentation, an enzyme called glucosidase, which is found in the leaves, breaks down indican to produce indoxyl, which quickly undergoes oxidative dimerization to become indigo [47]. This process is depicted in Figure 2.
[figure(s) omitted; refer to PDF]
Natural indigo typically comprises a variety of elements in variable amounts, along with mineral impurities and occasionally colourants of a different chemical origin, including flavonoids [47]. However, the existing methods of dye analysis, by high-performance liquid chromatography (HPLC), mostly detect indigotin and indirubin in fabrics colored with natural indigo but do not enable the accurate identification of the plant source(s) employed in each individual case [47].
The total quantity of indigo pigment produced and its composition depend on the different glycosides and enzymes present in indigo leaves and the reaction conditions (pH, temperature, and oxygen) in which the leaves are processed [31].
5. Dye Extraction
Harvested plants must be treated carefully before extraction, and leaf staining is to be avoided since the plant leaves contain the enzyme needed to hydrolyze the relatively stable indican into the unstable indigo precursor indoxyl. Indigo, a dark blue colourant, differs from all natural dyes since it is produced when the extract of the plant, crushed or macerated in water, reacts on exposure to oxygen in the air, and it is insoluble in its blue form which it cannot impregnate and become fixed to textile fibers [45]. To get rid of any soil that might be stuck to the indigo plants before extraction, it can be good to give them a thorough rinse [45]. Primary washing safeguards against soil compounds reacting with indoxyl to produce nonindigotin oxidation products, reducing yield, and it additionally protects against fine soil particles being carried through the process and contaminating the final pigment product, lowering purity [45].
5.1. Extraction Methods
The process of extracting indigo dye from plants is time-consuming and labor-intensive and should be started as soon as possible after harvesting and definitely no later than one day [51]. Even after just one day of plant storage, substantial reductions in indigo content were seen when plants were kept at a temperature of around 20°C before being extracted [51]. During extraction, complete submerging of the plant material to limit the excessive introduction of oxygen in the liquid and stirring the liquid minimally throughout the process is required to have maximum dye yield [45]. It is possible to carry out extraction using heated water for hours or days at a time or with a hybrid technique that employs warm water and takes less time than fermentation but more time than hot extraction [45].
5.1.1. Extraction through Fermentation
The time needed for fermentation extraction varies depending on the circumstances, but in temperate regions with warmed water, 3–5 days may be predicted, but some sources claim that timeframes in tropical climates might be as little as 14 hours [45]. Since indoxyl is relatively unstable, timing and careful extraction management are essential to maximizing yields and preventing undesirable oxidation products [46]. It is unclear exactly what mechanisms are at work when indigo crops are overfermented, and if fermentation is stopped too soon, indican will not be fully extracted, resulting in poor initial indigo yields. However, the plant material may be refermented, restoring the yields [45, 52].
Visual and olfactory signals are the most dependable signifiers of the extraction’s completion. When fermentation is nearly finished, the extraction liquid will develop a highly obvious neon blue-green colour, along with a few tiny bubbles and a metallic blue tint on the liquid’s surface [45]. At the same time, the vat begins to smell slightly fruity, which, despite possibly containing some unpleasant notes, is not very repulsive. The colour changes from the vibrant neon blue green to a duller army green after the fermentation progresses too far, and the fragrance shifts from fruity and brilliant to something that gets progressively worse [45]. Additional anecdotal signs as well as repeatable and reliable quantitative indicators, such as the leaves changing from green to yellow and growing a slimy covering, would be a good subject of further investigation [45, 52].
Cutting raw material into little pieces can lessen the offensive smell and make trash disposal easier, according to a study on tropical indigo kinds, but it is unclear what impact this has on indigo output or purity [52]. Separating the pigment from the leaves can be quite challenging because macerating and oxygen exposure to the leaves may form indigotin inside the plant cells before indican can exit the cells and enter the extraction solution [52]. Plant maceration could occur underwater or in an otherwise low-oxygen environment, but it is unclear whether this is feasible or whether the effort would be justified, and additionally, maceration may lead to a lower purity in the final product due to particles of plant material contaminating the pigment [33, 45].
When the leaves of the indigo plant are fermented in water, indican hydrolyzes to produce indoxyl, which then oxidizes to form insoluble indigo. The grade of indigo that results depends on how much of the major colouring ingredient (C16H10N2O2) is present in it [45]. After being combined with a strong base such as lye (a strong solution of caustic soda), the precipitate from the fermented leaf solution is then pressed into cakes, dried, and powdered [53]. The powder is then combined with a number of other ingredients to create various colours of blue and purple [53].
5.1.2. Hot Extraction
Getting reliably high yields of dye is difficult with cold fermentation extraction since controlling or optimizing the process is problematic [45]. Warm water can be added to the fermentation process to enhance it or even completely replace it if the extraction is finished in hot water [45]. Temperature, extraction time, and water quality are all process characteristics that can be easily handled and adjusted for reliability in hot water extraction [45, 54]. Indican degrades rapidly at temperatures of 79°C and higher; hence, any heated extraction must be carefully managed to stay below this threshold [45]. Additionally, beta-glucosidase, an enzyme from the leaves that hydrolyzes indican into indoxyl, is denatured at high temperatures and is entirely blocked at 60°C. At this temperature, the enzyme is unable to function, very little indican is hydrolyzed to yield indoxyl, and right little indigo is produced [45].
The appropriate temperature and duration for heated extraction must be defined in order to maximize yield and efficiency [45]. The temperature must be below 60°C, and the extraction time should preferably be as short as possible. Four hours at 40°C or twenty-four hours at 25°C are suggested extraction times in some sources [55]. Others conducted extractions at a temperature of 25°C for 72 hours or 50°C for 30 minutes [56]. After 20 years of testing heated extraction [45] and extraction above 57°C, a French natural dyer named Michel Garcia suggests extraction at a constant temperature of 50°C for two hours. This is because Garcia saw considerable yield losses when using heated extraction at higher temperatures. Extra saponins produced by soaking for longer than two hours lead to foam and prevent aeration during the next stage of processing. Generally speaking, indigotin, a precursor to indigo, can be recovered from the plant as a solution of indican, an indoxyl glucoside, which can be easily removed by steeping the leaves and stems in warm water. About 200 kg of leaves were needed to generate 1 kg of indigo dye [57].
5.1.3. Integrated Extraction
On a larger scale, the heated extraction process requires too much energy, and a hybrid method of extraction is applied with warm water going into the steeping tank [45]. As part of a seasoned hybrid or integrated extraction, the container was filled with water, heated to 47°C, and then left with the batches for around 24 hours. Hybrid extraction at 58°C is recommended for no more than 12 hours [45]. The climate will determine the precise amount of time required for hybrid extraction at a given starting temperature, and one cannot use the same signs as for fermentation to determine when the extraction is complete [45].
A useful topic for additional research would be generating accurate indicators for hybrid extraction. With warm or hot water, other compounds in addition to indican are drawn into the solution, and the resulting color is not neon green, but a deeper chestnut brown and the smell signals also differ [45, 58]. It is undoubtedly appealing to use warm or hot water during the extraction process to increase process control and dependability and shorten extraction times [45]. Fermentation, heating, or hybrid systems would all be appropriate for a garden-scale system, but for a bigger system, a fermentation strategy would require numerous tiny tanks or a few large tanks due to the lengthy batch periods (days to weeks) [45, 58]. The hybrid and fermented methods are more difficult to manage and anticipate than heated methods since they are both sensitive to environmental change (unless they are kept in a climate-controlled environment) [45, 58].
Even with the most advanced extraction technique, the purity of natural indigo dye is not as high as that of synthetic indigo. In addition to indigo, natural indigo also contains indirubin, indigo-brown, indigo gluten, and mineral matters. According to reports, the indigo purity ranges from 20 to 40% for woad indigo, up to 12% for P. tinctorium, and ranges from 50% to 77% for Indigofera indigo [51]. The effectiveness of the extraction process is also a concern; it has been found that the predicted yield of indigo production from indoxyl molecules is about 60% [38]. Therefore, contaminants such as isatin and indirubin as well as other reaction byproducts cause the process to lose 40% of the indoxyl [59].
5.1.4. Alkalization
Alkalization is a procedure that reduces the quantity of acid in a solution. Indoxyl oxidation and indigotin generation are basically nonexistent at a pH of 3, and conversion efficiency peaks at about pH 9.5 without appreciably decreasing at higher pH [45]. Alkaline pH speeds up the conversion of indoxyl to indigotin. When utilizing calcium hydroxide as an alkalizer, Vuorema states that a pH of 11 is best for indigo synthesis when both amount and purity are taken into account [59, 60]. It is possible to produce indigo without alkalization, albeit more slowly and possibly with a lower conversion efficiency [58, 60].
Calcium hydroxide (Ca(OH)2), ammonia (NH3), and sodium hydroxide (NaOH) are some of the more popular alkalizing agents that can be used to bring pH levels up to the appropriate range. Grey-water treatment is an important factor to take into account when selecting an alkalizing agent [60]. The use and production of benign materials will depend on the treatment strategy and the alkalizing agent. When utilizing ammonia as an alkalizing agent, grey water, for instance, might be neutralized with sulphuric acid (H2SO4) to create ammonium sulphate ((NH4)2, SO4), which is used as a synthetic agricultural fertilizer. However, the production and handling of both ammonia and sulphuric acid can be highly hazardous [60].
Because of its relatively safe manufacturing process, low price, and ability to serve as both an alkalize and a flocculant, calcium hydroxide (Ca(OH)2) is suggested as the perfect alkalizer [45]. Both the generally benign nature of grey-water production and the opportunities it affords for straightforward and efficient grey-water treatment make alkalinizing with Ca(OH)2 a viable option [45]. It is advised to aerate the extraction liquid before adding the alkalizer when employing Ca(OH)2. It is intended to raise the liquid’s carbonic acid concentration, which is a result of the atmospheric carbon dioxide (CO2) that is added during aeration [58]. In the subsequent phases, this carbonic acid will combine with Ca(OH)2 to form calcium carbonate, the presence of which promotes the flocculation and settling of the indigo [58]. This is carried out prior to the addition of alkali because alkalization will quickly cause indigo to form and flocculate, and excessive disruption of this indigo may cause the flocculate to break down into very tiny sizes that are challenging to settle and filter [58, 60].
5.1.5. Oxidation
The extraction vat’s indoxyl solution is ready to be converted into indigotin once it has been alkalized, and the oxidation step is then completed by simply exposing the liquid to oxygen in the air [45]. By adjusting aeration time, this can be accomplished in a variety of ways. The traditional large cement tanks used in India are aerated by a group of workers standing in the water and coordinating forceful kicks of their legs; this process typically takes about 2 hours [51]. Oxidation can occur with time alone, citing successful indigo formation for 4-5 days with intermittent stirring in open containers, albeit with low yields [60]. On the other hand, one can quickly oxidize the extraction liquid by bubbling pressured air (or even pure oxygen) through it [59]. Isatis tinctoria was subjected to pressured air experiments lasting 5, 15, 45, and 60 minutes; the results showed that 15-minute and even 5-minute periods were sufficient [60]. If the extraction liquid and the foam that forms at the surface take on a noticeably blue hue, the oxidation process is proceeding successfully. When a sample of liquid is allowed to settle, the oxidation is complete when the top liquid layer is reddish-brown and the bottom layer is settled in indigo pigment. The foam may have stopped turning blue at this time or vanished altogether [45, 58].
5.1.6. Filtration
Indigotin must first be generated before it can be isolated from the rest of the extraction fluid. It is advised to concentrate the pigment into slurry on a relatively small scale using a series of settling and decantation procedures before filtering with a fine cloth [45]. It is advised to use filter fabric made of that 8 mm habotai silk or even finer 16 mm habotai silk. The slurry is ready for final drying and storing after draining for 12 to 24 hours [45, 58]. On a larger scale, more elaborate filtration systems may be desirable, and there are so many filtration technologies used to separate indigotin from the liquid used for extract, including centrifugal concentration in addition or in place of filtration [45].
Using simple woven filters with mesh widths of 25 microns, significant volumes of indigo were effectively filtered; however, it took several days for the paste to become dry enough to collect and dry [45]. The indigo slurry can optionally be rinsed to eliminate water-soluble contaminants before final filtering and drying, ideally in an acidic solution, to get rid of any previously undeclared Ca(OH)2. In one study, the use of hydrochloric (HCl) acid improved pigment purity from an average of roughly 20 to 69% [61] by removing the calcium carbonate (CaCO3) that develops from Ca(OH)2 and aids in flocculation. While indigo might be kept as a paste with any remaining water for up to a year, thorough drying gives the pigment the longest shelf life and enables storage for limitless amounts of time [45].
6. Indigo Dyeing
Since indigo is a “vat dye,” it must first be converted into its water-soluble leuco-form before being used as a dye. The reduced form is taken up by the fibers or fabric and remains there when it is oxidized back to its blue form [62]. The indigo is permanently stained into the fiber when the fabric is removed because the air oxidizes it, bringing it back to its original, insoluble state [63].
6.1. Reduction
Since indigo is insoluble in water, it must be converted into a soluble substance that is nearly colorless and can be absorbed by fibers [45]. Leuco-indigo [64], which is soluble in water, is the reduced form of indigo dye produced by a reducing agent, which is used in the majority of indigo dyeing techniques. The leuco-indigo is oxidized in the textile substrate under the effect of oxygen from the air into indigotin or indigo blue [65] before the dyeing process is carried out. The textile substrate can absorb in soluble form.
Earlier, fermentation was used to accomplish the reduction and colouring [66]. Nowadays, sodium dithionite (Na2S2O4) has been used to perform the majority of reduction chemically. The breakdown products it creates, such as sulphite (SO32−), sulphate (SO42−), thiosulphate (Na2S2O3), and poisonous sulphides (S2−), contaminate the waste waters from the dyeing operations, making it undesirable to the environment [18]. Therefore, there has been interest in exploring new possibilities for indigo reduction. Bacterial-induced reduction and electrochemical reduction are potential substitutes for using dithionite as a reducing agent. At the University of Reading in the United Kingdom, a Gram-positive, aerobic, moderate, and thermophile bacterium (Clostridium isatidis) that can break down indigo dye was discovered in a woad vat [66]. With the electrochemical method, there are two options for electrochemical reduction: direct [67] or indirect [68] electrochemical reduction using various redox mediators.
In addition, organic reducing agents have been researched as potential substitutions for sodium dithionite [69]. Anthraquinones (C14H8O2) can help in the glucose-induced reduction of synthetic indigo by acting as catalysts in addition to reducing sugars alone as organic reducing agents. Pure bacterial cultures have previously been shown to be stimulated to reduce indigo by anthraquinones [70, 71], and madder powder rich in anthraquinones is known to have been a constant component of the medieval indigo dye vat. As noted in past research efforts, anthraquinones are also known to function as intermediaries in the indirect electrochemical reduction of indigo, where they transport electrons between the electrode and the dye molecule.
6.2. Dyeing
The main indigo dyeing processes are called “vats,” and from long-stayed dyeing experience and recommendations from research work, the optimal temperatures of the dyebaths usually vary around 50°C [60]. Dyeing can be carried out with extracts of fresh indigo leaves or indigo cakes [45]. Dyeing of fresh indigo leaves is done after complete fermentation or hydrolysis of the leaves by adding alkali, which helps neutralize the acids produced by cold fermentation extraction. At this stage, the dye thus remains in a colourless soluble form and can impregnate the textile substrates to be dyed [31, 65]; this dyeing technique is carried out after bacterial reduction of indican into indoxyl before its oxidation in the substrate. The substrate immersed in the vat remained for 1–12 hours depending on the dyeing conditions. When the substrate is taken out of the vat and placed in the open air, the indoxyl reacts with the oxygen to precipitate insoluble blue indigo on the fibers and inside the substrate [31]. This relatively simple and very ancient process has been used in the past by many different people all over the world and is still practised today in Africa, Madagascar, and Central America [31].
The scientific explanation for the indigo cake dyeing process has been clarified by microbiological research recently carried out at Reading University in the UK [66]. A cake of woad (3 kg) is ground into coarse powder and placed in a large vat (30 liters). Hot water should be kept in this to a temperature of 50°C, and it is filled to the brim. To give the bath the optimal alkalinity (pH 9) without getting too much sediment at the bottom of the vat, the concentrated potash alkali is used as reported by old literature, and the mixture was stirred well and then maintained at the right temperature for six hours.
The vat is then carefully stirred to introduce as little oxygen as possible and left to rest for further three hours at the end of which the pH of the bath is checked and potash added if needed, and then it is again stirred [31]. The bath is stirred, the temperature is checked, and the pH is checked repeatedly until the bath has taken on a yellowish green hue and a coppery coating has formed on the surface. It can take between 20 and 37 hours for all of the indigotin present in the indigo cake to be converted to indigo white, with the exception of the surface where the liquid is in touch with the air. However, it is advised to wait 24 hours before starting any dyeing to get the vat ready [31]. An anaerobic, somewhat thermophilic bacterium, Clostridium isatidis (strain Wv6T), has been named by its British discoverers as the cause of indigo reduction in this type of vat. It was the first of this category of bacteria to be recognized [31].
7. Indigo Cultivation and Dyeing Experience in Ethiopia
Indigo cultivation and noticeable dyeing practice from locally extracted indigo are not familiar in Ethiopia currently, even though at present the existence of many species of wild indigo have been reported by many authors. On the other hand, very long stayed blue colour mixed with old paintings is commonly seen in national museums and Ethiopian Orthodox Churches.
7.1. Indigo Dyeing Experiences
Like China and India in Asia and other ancient civilized countries in the world, Ethiopia is one of the oldest and most cultured nations in Africa. Indigo planting and dyeing may then have a place in the history of the nation’s craft. Five different species of Indigofera once played a significant role in this regard, particularly as the source of the worldwide marketed indigo dye. In relation to this, the availability of 78 wild indigo species in Ethiopia had been reported [72–75], and only I. tinctoria species has been found in cultivation in earlier times in the country. Meanwhile, indigo dyeing is practiced in response to the growing demand for stylish traditional Ethiopian clothing in the local market, among the diaspora, and for the niche export market by importing indigo cake, primarily from India. This happened for two reasons: first, there is no cultivated indigo currently in Ethiopia, and secondly, there is no experience extracting indigo dye from wild growing indigo. The colour of extraction water on the verge of complete fermentation is shown in Figure 3.
[figure(s) omitted; refer to PDF]
The National Museum of Africa’s technical analysis of six Ethiopian icons revealed that indigo was one of the colours utilized in the early country’s artworks. Therefore, blue and blue mixed coloured paintings are very common in national museums and Ethiopian Orthodox churches. In this regard, the Saint Mary Spiritual pictures at Saint George Church, Zegie, and Bahir Dar, Ethiopia, illustrated as an example in Figure 4, had been painted 700 years ago with blue mixed colours. This might indicate the existence of experience of using indigo dye for painting before manufacturing of synthetic dyes in Ethiopia.
[figure(s) omitted; refer to PDF]
7.2. Indigo Plantation in Ethiopia
The Gara and Yoruban species of indigo are found in Tropical Africa, particularly in West African countries including Senegal, Cameroon, Equatorial Guinea, Sierra Leone, Ghana, Benin, and Nigeria [31]. Furthermore, it had been reported that in these countries, indigo plants have been used for dyeing from wild and cultivated sources, and indigo served as the basis for several centuries-old textile customs throughout West Africa [39]. Africa, particularly, Ethiopia, is the major center of legume browse diversity fodder potential where Indigofera is one of them, although little work has yet been carried out on collection and evaluation. Additionally, indigo was one of the pigments employed in the early Ethiopian paintings, according to the technical investigation carried out on six Ethiopian icons at the National Museum of Africa [76].
One of the most significant genera of forage legumes native to Ethiopia is the Indigofera genus, which exhibits significant potential for forage development, particularly in drier regions where their deep roots offer some resistance to periods of little rainfall, according to old literature. In general, several species of indigo are growing widely in many parts of Ethiopia [42, 77–84]. According to Manuel Barradas, a Portuguese Jesuit, Northern Ethiopia also had indigo trees, especially Indigofera arrecta and Indigofera tinctoria. In addition, these two species, I. articulata Gouan and I. coerulea Roxb., also grow in Ethiopia as reported by old literature; however, there is not enough evidence to determine whether these indigo trees were used for painting and/or other applications in Ethiopia.
The widely growing of wild indigo of several species in Ethiopia is an opportunity in relation to the revitalization of natural indigo. This can be elucidated by contextualizing the Ethiopian situation that how this will be benefitted for the country in its environmental and health concerns and economic and social developments. Concerning economic affairs, the country is wealthy for its land and young workforce, which is important for cultivating indigo at a large scale; we supply it to local and international markets. The recent increasing demand for Ethiopian fashionable traditional clothing both in the local and niche international markets demands natural indigo for more value addition of the stylish traditional cloths to be considered a natural product. Furthermore, the global increasing demands for natural indigo will also benefit the country with export earnings if it is able to produce indigo with quality and quantity.
The environmental and health hazard traits of the country can be related to the recent booming of the textile and apparel sector industries in the country. Of course, the textile and apparel industries are important for developing countries since they create enormous job opportunities and accelerate economic development through export earnings. However, the textile and garment sectors are all part of major polluting industries. Therefore, textile and apparel production needs to be escorted by applying the methods that lead to minimize the environmental pollution and health hazard effects of the manufacturing processes of these industries. This can be achieved with several approaches, of which one of them is using alternative eco-friendly natural materials such as a natural indigo dye instead of using excessive and some toxic synthetic dyes for colourations of textile substrates in the manufacturing process. This needs effective research and development activities to bring the natural indigo application to an industrial scale.
The social condition of the country will be improved with indigo production and dyeing which probably create employment and/or self-employment opportunity for young citizens, particularly for women in the first place for those who are unemployed, that is one of the serious challenges of the country. Secondly, an additional economic segment will be created in the textile and apparel value chain that possibly link the local and international producers and traders. In general, the global revitalization of natural indigo dyeing is worth for countries such as Ethiopia in many aspects as discussed previously. Therefore, strengthening the production of indigo and in general other natural dye source plants in developing countries is important for economic development, minimizing environmental pollution and health hazards of the global challenges.
Therefore, contextualizing the issues into the potentials of the country in this regard is crucial if the nation is to profit from the global trend of indigo rejuvenation. As a result, research on indigo cultivation, extraction, and dyeing has begun at Bahir Dar University’s Ethiopian Institute of Textile and Fashion Technology (EiTEX). As illustrated in Figure 5, which depicts a recent experiment of indigo cultivation in Ethiopia, the indigo cultivation with the research project is underway.
[figure(s) omitted; refer to PDF]
As a result, this review is a component of a larger research effort that aims to understand the entire world situation with indigo revitalization as well as Ethiopia’s potential and history with indigo manufacturing. It is anticipated that the indigo cultivation will be increased in Ethiopia and throughout Africa as a result of the research project that has already begun and this review. This will benefit Ethiopia economically and socially while also helping reduce the problems associated with environmental pollution and health risks brought on by the textile and apparel manufacturing industries.
8. Conclusion
To draw attention to the health risks brought on by the excessive use of hazardous synthetic dyes in the production of textiles and apparel, researchers and development pioneers are pursuing natural indigo dye. The natural indigo dye has been revived for the aforementioned reasons as well as other demanding uses such as UV protection, practical textile finishing, cosmetic and food colouring, and industrial jeans fabric dyeing. As a result of its numerous uses, indigo dye is now traded as a significant cash crop on the global market. The revival of natural indigo could pave the way for the large-scale production of organic dye. Given that indigo is currently used on an industrial scale for the dyeing of blue jeans, this may have been performed through the manufacturing of natural indigo dye.
As a result, the global textile and clothing sector’s value chain will be expanded to include a production and business section that is environmentally friendly. The developing nations, especially those in tropical Africa, gain more economically and socially from this sector of the economy because they have labor that is relatively inexpensive, access to land resources, and climates that are suitable for cultivating high-quality Indigofera indigo species. Industrial-scale indigo dye production can be seen as a green economy development in the textile and apparel industries, thus it requires vigorous research and development operations to be strengthened. Generally speaking, the introduction of eco-friendly and natural material-based textile and apparel manufacturing processes by the newly developing countries into the sector industry manufacturing will allow for environmental sustainability, the reduction of health risks, and additional help in preventing worsening global climate change.
Acknowledgments
This work was carried out with financial support from Ethiopian-German cooperation project work funded by KfW Development Bank (KfW project no. 51235 and BMZ no. 201166305). Financial support was given for strengthening the Ethiopian Institute of Textile and Fashion Technology, Bahir Dar University, to be a center of excellence for textile and apparel education, research, and outreach activities. Therefore, the authors are thankful to KfW for the financial support provided in the preparation of this review study as a gate for the continuing the experimental research work of indigo. The authors also acknowledge Nicholas Wenner (FIBERSHED) for his recent, significant work on the cultivation, extraction, and dyeing of indigo and for allowing to use the photo from his article, which is referred to as Figure 3 in our review article [48]. The authors used his paper as a reference, especially for the review in Section 5 (indigo extraction). The authors have also used the article as reference for the review in section eight of the article and have listed it in reference list [25].
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Abstract
The recent increasing consciousness for environment sustainability and health hazards is the motive for looking for alternative environment-friendly materials and manufacturing processes. Therefore, environmental compatibility and safety to the health manufacturing process are a worldwide concern. The use of excessive and toxic synthetic dyes in textile and apparel manufacturing processes lineages environment pollution and health hazards to the surroundings where they are sited. Consequently, penetrating for alternative eco-friendly dyes has been initiated in recent times by researchers and development motivators in order to reduce the excessive use of synthetic dyes. In this regard, natural indigo dye from the indigo plant is getting attention, and thus, it is preferred as good as synthetic indigo dye, owing to its inherited dyeing property. With natural indigo dye, it can be achieved eco-friendly dyeing without the application of mordant attaining commercially acceptable fastness properties. Owing to the popularity of blue jeans around the world, especially those that are naturally dyed indigo blue, indigo dye consumption is now rising. Furthermore, currently, natural indigo dye is demanding for functional textile finishing, cosmetics and food colouration, UV protection and deodorizing agents, and other applications. In general, because of its increasing demand and wide applications, natural indigo has become an important cash crop and is traded in the international market with a price range of 35 to 40 USD per kilogram as per its purity level. Indigo dye has therefore been revived with more applications than it had before the advent of man-made colourants. The revival of indigo presents a chance for underdeveloped nations with incorporated economic and social benefits. Ethiopia as a developing country and situated in tropical Africa will benefit from indigo revival because of its potentials with having a suitable climate, a young workforce, enough land resource, and its booming textile and apparel industries.
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