The textile industry uses large amounts of chemicals, artificial dyes and a large volume of water in its process, being considered one of the most polluting industries in the world [1, 2]. Its process pollutes both water and the environment with toxic waste. The World Bank considers that 17 to 20% of industrial water pollution comes from textile dyeing and from the finishing treatment given to the fabric [3,4,5]. It is estimated that 150 L of wastewater is generated to produce one kilogram of the finished product in the textile industry and certain synthetic dyes present are not biodegradable or remain in the environment for a long period, causing pollution. Therefore, it is highly necessary to remove the dyes present from industrial effluents to avoid water pollution . Synthetic dyes are made in such a way so that there is no fading away by any of the physical, chemical and biological agents . It is estimated that 20% of used dyes are dumped into the environment because of their low level of affinity with the fabrics . On the other hand, natural dyes come from renewable, ecological, safe, non-carcinogenic, non-allergenic and biodegradable sources [4, 9].
Currently there is a variety of applications for natural dyes such as antimicrobial, antifungal, UV protection, cosmetics, food, additives, pH indicators and other uses . They are considered renewable, biodegradable, environmentally friendly and are a viable alternative for the excessive use of artificial colors, which add up to around 10 million tons per year [1, 11,12,13].
The most suitable and widely accepted classification system for natural dyes is based on their chemical structure, as it readily identifies dyes belonging to a particular chemical group, which has characteristic properties .
In the process of natural dyeing, most of the time the use of a mordant is necessary . Mordants can be derived from metallic salts such as sulfates (magnesium, aluminum, zinc, copper, cobalt, nickel, manganese or tin), chlorides (stannic, ferric, copper, zinc, aluminum and even neodymium or zirconium), hydroxides (calcium) and oxides (ferric or lanthanum) [14, 15]. Of the metal mordants used for natural dyes, alum and iron are environmentally safe and have not been restricted by any ecoregulation [3, 16]. Many of these mordants can cause problems with the resulting waste, soil contamination and water waste. . Another problem with using these mordants is the incompatibility with the eco-friendly concept used in natural dyeing. Large-scale usage would lead to serious environmental problems . In the use of these metal salts as mordants, only a small amount is fixed in the textiles and the rest is discharged as effluent, leading to contamination of terrestrial and aquatic resources [3, 18, 19].
Biomordants are reported as sustainable and ecologically correct alternatives to metal mordants, providing satisfactory dyeing and solidity properties . Biomordant sources are plants with high tannin content  or hyperaccumulative metal plants .
Some biomordants, such as pomegranate peel, rosemary and thuja leaves were proposed as promising alternatives for aluminum, iron sulphate II, copper sulphate II, stannous chloride and potassium dichromate . Natural polyphenols, also called tannins, are obtained from various parts of plants, such as bark, wood, fruits, fruit peels, leaves, roots and plant galls . Other studies describe the presence of tannin of the banana tree pseudostem, Akpabio et al.  determined the physicochemical properties of the residue of the banana tree (Musa paradisiaca) pseudostem and banana (Musa sapientum). The results showed the phytochemical composition in mg/100 g of tannin for the banana and pseudostem residues of the banana tree (respectively 7.99 ± 0.26 and 6.55 ± 0.33).
Natural resources play a dominant role in the economic activities of any country and therefore, contribute substantially to the gross domestic product (GDP). In the case of the development of underdeveloped countries, this also helps economic development. There is a growing worldwide trend towards making the most of such resources through new processes and products. These, in turn, not only help in preventing environmental pollution, but also in generating jobs, particularly in the countryside and contributing to the improvement of living standards .
The banana belongs to the Musaceae family and there are approximately 300 species, of which only 20 varieties are used for consumption [25, 26]. There are over one thousand domesticated Musa cultivars and their genetic diversity is high, indicating multiple origins of different wild hybrids between two major ancestral species .
According to the botanical systematic hierarchical classification, the banana belongs to the division of Angiosperms, class of Monocotyledonous, order Scitamineae and family Musaceae. The subgenus Eumusa, determines the category of edible bananas which are derived from hybrids of the wild subspecies of Musa acuminata Colla (genome A) and Musa balbisiana Colla (genome B) [28,29,30]. Wild bananas occur in the tropics from India to Oceania, but there is a distinction between the distribution of M. acuminata and M. balbisiana within this range . The classification proposed by Cheesman  in 1948 for the genus Musa, currently accepted worldwide, is based on the basic number of chromosomes . The cultivar Prata (AAB) is a variety of edible bananas, genus Musa . In Brazil, the Vale do Ribeira region (state of São Paulo) uses the residues of pseudostem from this cultivar to produce handicrafts and textile products .
The residue from banana farming represents 40% of the fruit production and shows that about 13 tons of dry organic matter are generated per planted hectare when considering the pseudostem, the leaves and the peduncle of the banana tree .
For each ton of banana fruit harvested, about 100 kg of the fruit is discarded (that is, rotten fruit) and about 4 tons of biomass waste (e.g. leaf, pseudostem, rotten fruit, peel, peduncle, rhizome, etc.) are produced. This means that, for each banana production cycle, four times the biomass residues are also produced [35, 36].
In 2012, the Philippine Textile Research Institute concluded that banana plantations in the Philippines alone can generate more than 300,000 tons of fiber . The institute, which is a pioneer in the research on banana fiber, has recently presented studies to improve fiber texture through techniques of degumming banana fiber, thus shortening processes .
The banana species best known for its fiber is the abaca (Musa textiles). Its fiber is highly important among the group of foliar fibers, while the most common banana consumed by humans is a member of the Musa acuminata species [39, 40].
The world production of abaca fiber in 2016 was 106,598 tons. The Philippines appear as the main dominant producer, supplying 86% of the world demand, the rest of the production being supplied by Ecuador. In 2005, the Philippines began an expansion program. By the end of 2010, a total of 48,922 hectares of new abaca plantations were established. By the end of 2010 the country had reached 167,145 hectares, capable of meeting the increasing demand for abaca fiber .
In India, the banana fiber has been used with a blend of cotton fiber to make fabrics, accounting for about 15.8% of all fabrics manufactured by India in the year 2015. The province of Gujarat, India, which comprises the Anand, Surat, Vadodara, Bharuch, Narmada and Kheda counties, presented in 2017 at the 8th Global Summit (Gujarat, India, 10–13 January 2017) a project to create the Gujarat Textile Eco-Park for the processing of natural and banana fibers. This covers the entire production process, with a center of excellence and training, testing labs and design studio, which will unite the chain of extraction, processing, yarn creation, and sustainable, high quality textile products. 17.6% of the total banana production in India comes from the province of Gujarat .
In Brazil, only about 2% of the 6.6 million tons of biomass of banana fiber produced annually by plantations are used by artisans, particularly by the coastal population in the southern states of São Paulo and Paraná. Compared to other countries, this number jumps to about 10% in countries like India, where it is used not only by artisans, but also as a source of energy and has other uses in industries. Consequently, there is an excellent opportunity to expand the use of this great source in the country .
The choice of Black Acacia as a biomordant for this study is due to its historical recognition for the quality of its bark, from which plant extracts rich in tannins and phenols are obtained. They originate the tannins, widely used in tanning, the most well-known group of products obtained from the bark. It is the main source of bark for the industry of vegetable tannin in the world . Acacia mearnsii is one of the best species in terms of yield per tree and quality (composition and coloring) of tannin. Other species such as A. decurrens have approximately the same yield and A. pycnantha is even superior to A. mearnsii, but both species provide extracts of higher staining and when used for tanning, the resulting products are darker .
In Brazil, in plantations of A. mearnsii in the State of Rio Grande do Sul, at age eight, the estimated average tannin content in the bark is 27% , that is, of every 100 kilos of bark dried in the open air 27 kilos of tannin are obtained. The increase in the tannin content is positively correlated with the increase in age [46,47,48]. Acacia mearnsii is the third most planted forest species in Brazil, surpassed only by species of the genus Eucalyptus and Pinus. Besides its use as raw material for tannin, it provides cellulose and charcoal. It represents a significant part of the reforestation of the State of Rio Grande do Sul and is of great social importance because it is planted in small properties, thus benefiting thousands of families in the region .
The choice of the dyes used in this study takes into consideration their easy access in the local commerce, being easily found in street markets, emporiums and municipal markets. In addition to this is the reusing of residues, such as Allium cepa peels, highly rich in natural dyes, which is discarded by the food industry.
This research aims at investigating the efficacy of the use of biomordants as an alternative to metal mordants for the fixation of natural dyes in the dyeing process of banana fiber (Musa sp.) Cultivar Prata (AAB) for textile purposes. For this study, the sawdust of A. mearnsii and the dyes of Hibiscus sabdariffa, Camellia sinensis, C. longa and A. cepa were used as biomordants.