Tackling Colorants Sustainability: Comparison
Please note this is a comparison between Version 1 by Valentina Lorenzon and Version 3 by Beatrix Zheng.

Colors play a key role in our lives: our clothes, our cars, and the furniture in our houses come with a wide range of choices when it comes to hues, inks, and paints. Since the accidental synthesis of the first synthetic aniline dye, mauveine, by William Perkin in 1856, the range of dye molecules available has widened and entered not only the textile, food, and cosmetic fields but also the pharmaceutical, plastics, ink, and packaging industries. As consumers, we mainly see them as a way of expressing our personal taste, mood, or personality and usually pay little attention to their origin and production process. As scientists, we are fascinated by the chemical processes behind them and, at the same time, mindful of the hazards they pose to the environment.  This researchcritical review highlights the importance not only of biotechnological approaches but also of a sustainable leadership to achieve a future-proof fashion industry. Science has been producing sustainable alternatives to counter the issue of dyes, but this is not enough. A change in the business attitude and leadership approach of the organizations that operate in the industry is needed. Only through the successful combination of new technologies and forward-looking decision-making will it be possible to alter the status quo and deal with the multiple environmental challenges that businesses are and will be facing.

  • biotechnological innovation
  • leadership
  • sustainability
  • sustainable leadership

1. Traditional Knowledge and Modern Biotechnologies

Since ancient times, the fascination with colors has led people to explore natural sources from plants, lichens, and animals for personal decoration and eventually textile dyeing. Colorants were extracted and processed from lichens such as Letharia vulpina or wolf lichen, mushrooms, plants such as madder (Rubia tinctorum) or henna (Lawsonia inermis), and insects such as the kermes family (Coccus ilicis) [1][12]. While our ancestors proceeded by trial and error, our efforts are nowadays supported by modern technologies in the form of analytics, in vitro cultivation, and genetic engineering.
Plant-based dyeing is based on a large amount of traditional knowledge that provides insights on optimal processing conditions [2][13]. Multiple ethnological studies report that the use of plants for dyeing is rooted in the context of the territory and the tribal population in which the knowledge has developed [3][4][14,15]. However, this knowledge and the application of the process are currently under threat due to the easy accessibility of synthetic dyes and acculturation [4][15]. While these traditional processes might be in danger in developing countries, scientists are looking at them as ways to improve the sustainability of industrial processes. Traditional knowledge and vegetal materials are gaining more attention, and new sources of dyes are being investigated. For example, the inflorescence of munj sweet cane (Saccharum bengalense Retz.) has proven effective in various processes, including the creation of mordants and the use of high temperatures and natural agents such as moringa and turmeric [5][16]. Similarly, polyphenol-enriched banana and watermelon biowaste has been tested as a pre-treatment for cotton fibers, as it provided protection from fading from UV exposure [6][17].
Modern analytics based on chromatography can assist us in identifying and separating all the colored molecules present in a plant extract. In addition, surface-enhanced Raman spectroscopy (SERS) and high-pressure liquid chromatography (HPLC) have allowed the separation and identification of colorful molecules from the plants at the core of the Chinese tradition, such as Chinese mulberry (Cudrania tricuspidata), gromwell (Lithospermum erythrorhizon), Chinese rhubarb (Rheum palmatum), tangerine (Citrus reticulata), and Morinda [6][17]. In an attempt to discriminate dyes with different chemical properties based on polarity, size, and chirality, the use of supercritical CO2 has proven valuable and a sustainable alternative to organic solvents [7][18]. At the same time, non-invasive techniques such as UV-Vis-NIR reflectance spectroscopy have been explored to rediscover natural dyes from ancient textiles and rugs in a non-destructive way [8][19].
Taking one step forward while leveraging current practices, upcycling provides a great strategy. Already applied to food waste, upcycling has also been explored for both textiles and colors. In general, this does not require novel processing methodologies, and even just wet spinning can be applied [9][20]. For example, wood waste such as sawdust from the Pterocarpus indicus tree has been successfully applied to colored cotton and silk fabrics [10][21]. Plants of current low economical value can also provide hidden valuable molecules. As a study reports, the use of prickly pear peels of Opuntia ficus-indica (L. Miller) can be a source of colored molecules suitable for the dying of vegetal and animal fibers [11][22]. Onion and pomegranate peel extracts can be used to deliver a yellow and an orange color (400 to 500 nm absorbance) to cellulose-based fibers [12][23]. Color fixation can often proceed successfully through physical treatment such as drying and heating or by using natural mordants such as lemon juice, gallnut, pomegranate rind, and gooseberry that can compete with metallic alternatives [10][21]. It is therefore reasonable to think that, in order to respond to the different manufacturing needs while adapting to local environments and resources, it will be necessary to identify multiple solutions.

2. Colorants and Biotech-Assisted Sustainability

The diversity in chemical structure and origin of colorants (Figure 12) is one of the reasons why making them sustainable across both phases of production and later remediation presents multiple difficulties. Biotechnological studies have addressed the issue by looking at the single-molecule level and by using complex microbial communities (Figure 12). Non-biobased but physical pre-treatments of textiles to promote dyeability have also been developed using high-energy radiation such as UV, plasma, and gamma rays that prove cost-effective and environmentally friendly as no by-product is produced [13][14][15][24,25,26]. Production processes with a reduced environmental impact have also been reported to use continuous reaction conditions to decrease the water requirement by some 40% and the footprint by even a 4-fold [16][27].
Figure 12. Multi-scale dimension of the biotech-based solutions to improve the sustainability of colorants. Biotechnological solutions encompass a wide range of dimensions, from the nanometer-scale of single proteins, e.g., laccase (PDB ID: 1KYA), to the millimeter-size of complex microbial communities that provide a whole enzymatic toolbox for bioremediation. (Lower panel) Representative colorants used on the industrial scale. (A) Trypan Blue is an example of azo dyes, organic molecules characterized by the functional group R–N=N–R′, currently applied to cotton coloration on a large scale but also applied to laboratory analytics. (B) Rubixanthin is a carotenoid, and carotenoids from waste from tomato processing are being tested for dyeing silk, wool, and polyamide [17][28] to provide a yellow coloration. (C) Disperse violet 27, or 1-anilino-4-hydroxyanthraquinone belongs to the aniline group of colorants and has been tested with polylactide fabrics. It is used in cosmetics but not allowed for products in contact with mucous membranes. (D) Methyl violet is a family of dyes that has been the subject of extensive studies focusing on adsorption and bioremediation [18][29]. (E) Malachite green is a chemically formed triarylmethane dye that is recognized by laccase.
Fungi, bacteria, even algae and plants have proven effective in dye degradation of colorants of synthetic nature. Remediation can additionally be obtained using microbial approaches based on single-bacterial or fungal strains with adapted metabolism [19][30]; similarly, consortia can also be applied [20][21][31,32]. Their intracellular and extracellular enzymes are produced in response to the environment of growth, and, upon growth in polluted settings, the enzymatic activity can be tuned to recognize the contaminants and transform them. At the molecular level, multiple enzymes have been explored to catalytically convert dye molecules into less toxic compounds of less color. Major enzymes with potential in this respect are derived from the laccase (benzenediol–oxygen oxidoreductase, EC, high redox potential peroxidases (EC 1.11.1.X), polyphenol oxidases (EC, and azoreductases (azobenzene reductases, EC classes [22][23][33,34]. Catalases are versatile enzymes catalyzing an oxidative reaction using hydrogen peroxide or an organic peroxide. As an example sourced from the food industry, waste products (soybean and potato) peroxidases in an immobilized or free form led to the biodegradation of an anthraquinone dye with efficiency levels above 70% [24][35]. This is a great example of upcycling. Used mainly for wool, tartrazine is a synthetic yellow azo dye used in printing and food coloring (E102) and has been degraded using bacteria Pseudomonas aeruginosa reaching 72.65% removal in 5 h [25][36]. Similarly, Pseudomonas strains isolated from industrial water streams have delivered positive results in the remediation of methyl violet [26][37]. The multi-purpose azo colorant trypan blue has proven useful in the industrial production of garments, cosmetics, laboratory analytics, and materials but poses an environmental concern for both the product and its degradation [16][27][27,38]. Immobilized bacterial consortia [28][39] achieved the decolorization of wastewater in 24 h, i.e., a 50 mg∙L−1 dye concentration, but produced fewer toxic derivatives. This reduced toxicity is however not an automatic outcome, and a case-by-case assessment is needed, e.g., Congo red in vitro degradation products by peroxidase fungal enzymes led to an increased in vitro toxicity whereas an equal treatment of methyl green reduced toxicity [29][40].
Removal of dyes from wastewater can also be achieved via physical methods, such as adsorption. This physical approach not always requires specialized active materials but can be performed with a sustainable perspective in mind by using waste-products from other industries such as Indian Rosewood sawdust, a timber industry waste for the adsorption of methylene blue [30][41] or sunflower (Helianthus annuus L.) seed hull for methyl violet removal [31][42].

3. The Leadership Perspective

Today more than ever, organizations are interconnected and exercise a reciprocal influence in a constant process of exchange and cross-pollination. The advances in biotech and science are therefore only one of many factors that should be considered when addressing sustainability issues. Novel technological solutions and processes cannot be effective without the engagement of all the people involved and an overarching change in strategy that is consistent and supported by all stakeholders. Over the last few decades, many industries, like the fashion and furniture sectors, have pursued growth strategies aimed at achieving high financial returns by producing significant volumes of products and attracting a wider customer base. Unfortunately, this often resulted in high-volume, low-quality products that come with a significant negative impact on the environment  [32][6].

The ongoing COVID-19 pandemic and conflict in the Ukraine have also exacerbated a few of the already-existing issues related to supply chain and raw material sourcing, highlighting the importance of developing reliable and efficient relationships with all parties involved in the production process [33][34][70,71].  This is also an increasingly important factor in the ability to attract new customers and retain existing ones. Consumers from newer generations are much more aware than in the past of the use of natural products and compliance with environmental protection regulations when it comes to choosing a product based on factors like carbon footprint [35][36][72,73].