Preparation and characterization of beta vulgaris peels and pomace dye extracts as a potential natural dye of Cotton Fabric

Abstract

Natural dyes unlike the toxic and carcinogenic synthetic have attracted global attention because of their biodegradability and non-hazardous nature. The present study sought to extract, characterize and evaluate the dye-ability beetroot (Beta vulgaris) peels and pomaces natural dye on cotton (cellulosic). This was made comparison to synthetic Reactive Orange HER with analyses of the dyed fabrics fastness properties. Extraction was performed by Soxhlet apparatus using methanol as solvent. Isolation using column chromatography and characterization by UV-Vis, FT-IR, HPLC-UV and LC-MS/MS were done. Betanin was identified by comparing its retention, tR, with betanin standard (betanidin 5-β-D-glucopyranoside) in the HPLC-UV and LC-MS/MS method. Minitab statistical approach, response surface methodology (RSM) and central composite design (CCD) were employed to design experiments aimed at optimizing chosen factors that affect extraction and dyeing namely; temperature (T), material to liquor ratio (M:L), time (t) and pH. Dyeing was performed by exhaust method, alongside mordanting. Pre, simultaneous and post mordanting techniques were employed while comparing the naturally obtained mordants (the order of a tannic acid-alum mordant combination) with synthetic mordants (K2Cr2O7, FeSO4 and CuSO4). The CIELab and light, washing, rubbing and perspiration fastness of the dyed fabric were evaluated using standard methods. Reactive dyeing using Reactive Orange HER was done for colour equivalence (Ceq) and mass equivalent (Meq). The extraction optimized parameters of M: L ratio 1:10 (w/v) and t of 11 hours resulted in 40.68±1.45 % natural dye yield from the plant. The optimized parameters of dyeing were T of 55 °C, t of 75 minutes and pH 6. The UV-Vis spectrum of the extract and R. Orange HER showed peaks at wavelength of maximum absorbance (λmax) at 538 and 492 nm respectively. The FT-IR of the isolated extract, operated in the range of 4000–500 cm-1 revealed absorption bands at 3282.82 cm-1 (O-H) for hydroxyl groups, 2932.96 cm-1(C-H stretching ) for alkane,1588.91 cm-1 (C=N) for amines,1342.83 cm-1 (C-H) aliphatic bending, 1045.38cm-1 (C-O-C linked symmetric stretching), 986.22 cm-1 (C-H bond deformation) and 918.87 cm-1 (C-COOH bond stretching). The FT-IR of dyed cotton fabrics displayed bands in 1025.23 and 985.28cm-1 attributed to deformation of C-H bonds probably due to mordant effects. The HPLC-UV and LC-MS/MS results confirmed the presence of betanin at retention time, tR of 7.699 and 7.71 minutes respectively, closely corresponding to tR (7.60 min) of the standard. The HPLC-UV estimated betanin content to be 3.81±0.31 mg/g. The main compounds revealed by LC-MS/MS data were; 2-O-glucosyl-betanin, betanin, isobetanin, betanidin, among others. The Ceq and Meq of the extract to the synthetic dye implied that 1 kg B. vulgaris peels and pomace extract is equivalent to 32.8±7.04 g kg-1 of synthetic dye. Colour fastness of the dyed cotton fabrics were in the range of 4-5 (very good to excellent). Post mordanting method in all the mordants gave the best colourfastness and colour strength (K/S) on the fabrics than other methods. The FeSO4 vi and tannic acid-alum mordants yielded better results of K/S 3.60 and 3.92 respectively while CuSO4 and K2Cr2O7 yielded 3.60 and 1.20 respectively. These findings demonstrate that B. vulgaris peels and pomace extracts can be utilized to isolate the desired natural dye pigment (betanin) for coloration of cotton substrates with enhancement of natural mordants. Nevertheless, to achieve their maximum colour strength, there is need for further research on mechanisms to promote their chemisorption on fabrics