Evaluating the potential of natural dyes from nutshell wastes : sustainable colouration and functional finishing of wool fabric

Currently, synthetic dyes are widely used to colour textile materials. However, safer and more sustainable bio-based alternatives are currently being explored. In this context, by-products and wastes from agricultural value chains have been investigated as potential sources of natural dyes. Nutshell wastes are promising because they are abundant, contain interesting dye molecules such as quinones and are easier to handle because of their low moisture content. Hence, the present study aims at exploring the potential of the extracts from the shells of peanut (Arachis hypogaea L.), cashew nut (Anacardium occidentale L.), coconut (Cocos nucifera L.) and macadamia nut (Macadamia integrifolia L.) to colour textile substrates. For this purpose, the nutshell colourants were extracted and then applied to wool fabrics. Then, the extracts were analysed for their total phenolic content using the Folin–Ciocalteu method; the extracts were qualitatively screened for the presence of quinone-based dye molecules. Next, the effect of the main dyeing parameters, such as pH and mordant concentrations, on K/S was assessed. After this, the dyeing kinetics, colourimetric properties, UV protection potential and environmental impact of using the dye were assessed. The phytochemical screening indicated the presence of phenolic compounds (within a range of 71.3 ± 1.7–1664.4 ± 7.7 mg GAE/100 g), confirming the presence of colouring compounds such as quinones. The dyeing process, at pH 3 for the cashew nut shell and pH 5 for the other nutshell extracts, gave the best K/S (0.6 ± 0.1–14.9 ± 1.0), while the mixed 1,2 order was the best model for describing dye wool adsorption. Also, a good resistance to washing (4–5), light (3–8) and staining (4–5) was achieved, even without the use of mordants. However, the use of mordants increased K/S and fastness properties. The nutshell extracts improved UV protection by up to 79%, with cashew nutshell extracts and FeSO4 mordants providing the best K/S and UV protection. Furthermore, the use of extract dye was about 60% environmentally friendlier compared with synthetic dyeing. Thus, the use of nutshell-based dyes can provide a clean and sustainable source of dyes that can be used to substitute for synthetic analogues