Valorisation of Agave sisalana leaf waste as a sustainable natural dye for cotton dyeing through characterization and response surface optimization

Abstract The environmental and health burden of synthetic textile dyes has renewed interest in natural colorants from renewable, non-food resources. This study evaluated Agave sisalana (AS) leaf waste an abundant agro-industrial residue as a cotton dye alongside three local candidates ( Rumex nervosus flower, Syzygium guineense and Carissa edulis fruit peels). Dyes were recovered by methanolic maceration (70% methanol, 1:10 w/v, 72 h) and screened for total flavonoid content (TFC), total phenol content (TPC) and colour strength (K/S). AS was selected because it combined the highest TFC (7.52 ± 0.26 mg QUE g⁻¹) with the highest K/S on cotton. Qualitative phytochemical screening identified flavonoids, tannins and anthocyanins as the principal colorants, while alkaloids, glycosides, anthraquinones and steroids were not detected; UV–Vis and FTIR profiles were consistent with the results of the qualitative phytochemical screening. The dye was a shelf-stable powder (moisture 4.89%, water activity 0.56) highly soluble in 50% methanol and hot water. Dyeing of mordanted cotton was optimized by a three-level full-factorial design (40–100 °C, 30–90 min, 25–75 mg mL⁻¹) using quadratic response surfaces; FeSO₄ outperformed KAl(SO₄)₂. Optimum conditions (78 °C, 64 min, 61.6 mg mL⁻¹) gave 62.4% dye uptake and K/S 7.28 (experimental), compared with model predictions of 56.0% and 7.39; K/S agreed with its prediction within 1.5%, whereas the measured dye uptake exceeded its prediction by about 11%. Washing fastness was good to ten cycles. Agave sisalana leaf waste is therefore a viable, eco-friendly cotton dye, though the high dye load reflects the crude extract.