An efficient ultrasonic-assisted bleaching strategy customized for yak hair triggered by melanin-targeted Fenton reaction

Graphical abstract

Highlights

• •An efficient melanin-targeted ultrasonic-assisted bleaching strategy is designed for yak hair.
• •Ultrasound enhances the WI of yak hair up to 44.6% than those without ultrasound.
• •Both tensile strength and elongation varied within 15% after ultrasonic bleaching.
• •Ultrasound increases 94% of HO• which is the main oxidative bleaching species.
• •A theoretical highest WI of 52.4 is achieved on yak hair under optimal condition.

Abstract

Promoting processing efficiency and taking advantage of agricultural by-products are two promising ways to achieve sustainable textile industry. This study presents a customized efficient ultrasonic-assisted bleaching strategy for yak hair – a widely existing but underexploited secondary dark shade fibre from yak. A melanin-targeted Fenton oxidation process is established which involves three phases, i.e., (I) incorporation of Fe²⁺ ions with melanin, (II) catalytic oxidative bleaching using hydrogen peroxide (H₂O₂), and (III) reductive cleansing. The bleaching efficacy, dyeing performance and tensile property of yak hair treated with and without ultrasound were explored and compared. Further, the ultrasonic bleaching mechanism in terms of the catalytic effect of Fe²⁺ ions, the promotion of H₂O₂ decomposition, removal of melanin granule from yak hair, were demonstrated. Finally, the main effects and interactions of parameters in phase II, and optimal condition were obtained through mathematical modelling based on a central composite design (CCD). Results reveal that ultrasonic bleaching dramatically enhances the whiteness index (WI) of yak hair from 11 to 45 which is 44.6% higher than those bleached without ultrasound, and also promotes the uptake of acid dyes. There is only 15% tensile strength loss and 14% elongation increment of yak hair after ultrasonic bleaching, rising from a slight damage of cuticle layer and cleavage of disulfide bonds, respectively. In the study of bleaching mechanism, Fe²⁺ ion is confirmed to improve the H₂O₂ decomposition rate by 20.9% which further runs up to 35.9% after introducing ultrasound. Ultrasound increases the concentration of hydroxyl radicals (HO•) by 94% which are the main oxidative species participating in bleaching confirmed by HO• scavenging experiment. The porous structure was observed on the cross section of yak hair stemming from the removal of melanin granules contributed by the cleaning action of ultrasound. A theoretical highest WI of 52.4 can be achieved under an optimal condition based on the CCD study. In general, the proposed melanin-targeted bleaching strategy for yak hair that integrates ultrasonic technology and Fenton reaction, is beneficial to the development of sustainable textile industry from material and processing perspectives.

1. Introduction

Recently, carbon neutrality has been increasingly emphasized, and is driving textile industry to explore alternative renewable energy and materials to pursue the goal of sustainable manufacturing. Around 8–10% of the world’s carbon emissions are originated from textile industry, which directly aggravates climate change [1]. High energy and water consumption, and massive effluent discharge in conventional textile processing make it out of the criteria for sustainable industry. Promoting the processing efficiency is a facile but effective strategy for textile industry to diminish the negative impact on the environment. In addition, the resource for animal fibre (e.g., wool fibre) requires long-during specialized feeding of livestock, which indirectly but heavily impacts the global carbon emission [2]. Thus, searching for alternative fiber to reduce the consumption of conventional animal fiber is also beneficial to the environment.

Yak hair is a precious agricultural by-product for textile, showing great economy value such as fine elasticity, high tenacity, warmth, and proper moisture absorption [3], [4]. Yak hair merely owns a quarter price of cashmere but with similar thermal conductivity and elasticity [5]. The global yak population reached over 15.2 million, and ca. 92% of which are bred in China (estimated by 2012) [6]. The production of fine yak hair in China is 7,060 tons per year, which is 1.6 times than that of Chinese cashmere [7]. However, yak hair still remains underutilized. One primary reason is related to the dark black colour appearance of yak hair for heat preservation and ultraviolet protection to withstand the extreme cold and UV-exposure in high altitude region [3], which greatly impacts the subsequent dyeing process, and finally limiting their final market value. The dark shade of yak hair originates from the higher content of nano/micron melanin granules than that of wool and cashmere [8], [9], making the decolouration process more challenging. The conventional bleaching process to remove the natural yellowish impurities from cotton - usually undertaken at 98 °C for 60 min in alkaline, is incompatible with yak hair. Because the protein structure displays higher degradation propensity than cellulose under such harsh condition, which leads to the mechanical loss of fibre [10]. As yet, much less emphasize has been placed on the decolouration of yak hair than cotton and wool. In addition, the cuticle morphology of yak hair hinders the diffusion of oxidants thus reducing the effectiveness of bleaching. Thus, how to establish an efficient bleaching process under mild condition for yak hair before dyeing and weaving is the key challenge for the final commercialisation of yak hair.

Ultrasonic technique has been widely praised as a promising efficient and sustainable alternative method to conventional textile processing [11], [12], [13], [14]. The compression and rarefaction generated during the ultrasonic waves propagation accounts for most physicochemical effects, which accelerate the mass transfer, swelling of fibres, and chemical reaction with less energy and time consumptions [15], [16]. Apart from these advantages, it is also beneficial to textile substrates with less sacrifice of mechanical property due to milder temperature and less treatment time under ultrasound compared with conventional agitation process [17]. Especially in the present case, the advantage of ultrasound instead of physical agitation, also mitigates the mechanical loss of textile, and alleviates the entanglement between the scaled protein fibres and felting tendency of fibres [18]. To date, ultrasonic technology has been widely applied in textile wet processing including desizing, scouring, bleaching, dyeing, printing and finishing [19], [20], [21], [22], [23]. A numerous comprehensive studies have been carried out on the ultrasonic bleaching of cellulose. Our previous study confirmed the higher efficiency on the scouring-bleaching of cotton/spandex fabric through a one-bath one-step ultrasonic processing than conventional method [24].

Currently, very few studies attempt to bleach yak hair using ferrous (Fe²⁺) ions as catalyst and hydrogen peroxide (H₂O₂) - a safe and eco-friendly bleaching reagent [3], [5]. The generation of reactive oxygen species (ROS) - hydroxyl radicals (HO•) from H₂O₂ is responsible for the disintegration of melanin. Such design enables a selective combination of Fe²⁺ ions with melanin rather than with keratin, because Fe²⁺ ions are preferable to be absorbed by melanin due to its higher electron density than keratin [25], [26]. Such selective bleaching is assumed to reduce the damage of disulfide bonds in the un-pigmented region, however it requires further research evidence. Additionally, most bleaching methodologies are still focused on the conventional bleaching process [3]. As yet, few studies have included ultrasonic technique to the bleaching of yak hair. The ultrasonic bleaching mechanism of yak hair is still pending for investigation.

With these regards, this study introduces an efficient ultrasonic-assisted bleaching strategy customized for yak hair based on a melanin-targeted Fe²⁺/H₂O₂ Fenton reaction. The process involves three phases, i.e., (I) complexing Fe²⁺ ions with melanin, (II) oxidative bleaching and (III) reductive rinsing to remove the ferric (Fe³⁺) residues oxidized from Fe²⁺ ions. A batch rather than continuous ultrasonic treatment is employed considering energy saving, temperature controllability, and mechanical preservation of fibres (Fig. 1). The ultrasonic and conventional bleaching of yak fibre was compared towards a comprehensive evaluation in terms of whiteness, dyeability and tensile property. Further, the mechanism of ultrasonic bleaching was explored according to the influences of ultrasound and Fe²⁺ ions on H₂O₂ decomposition rate and HO• concentration. The cleaning action of ultrasound, and the interaction between Fe²⁺ ions and melanin granules were also studied by scanning electron microscopy equipped with energy dispersive X-ray spectroscopy (SEM-EDS). Finally, the optimal bleaching condition was obtained through mathematical modelling based on a central composite design (CCD). A biodegradable chelator − 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP) for fibre protection was introduced to the bleaching system rather than disodium ethylene diamine tetraacetate (EDTA-2Na) or diethylene triamine pentacetate acid (DTPA) [27]. Specifically, the interactions of H₂O₂, HEDP and Na₂CO₃ concentrations on the bleaching effectiveness were analysed in the CCD study.

Fig. 1.

Proposed ultrasonic-assisted bleaching strategy for yak hair.

2. Materials and methods

2.1. Materials

The yak hair fibres (Average diameter of 25 μm and length of 35 mm) and wool fibre finished product were obtained from Hebei Doveikang Auxiliary Co., Ltd., China. Surfactant polyoxyethylenated alcohols (AEO-9) was provided by Ningbo Runhe High-Tech Material Co., Ltd., China. Three acid dyes (LANASOL BLUE 3G, YELLOW 4G, SCARLET 3G) in analytical grade were purchased from Huntsman Chemical Trading (Shanghai) Co., Ltd., China. Commercial detergent specialised for wool textiles was obtained from Guangzhou Bluemoon Industrial Co. Ltd., China. Benzenepentacarboxylic acid (BA, >98%) was bought from TCI Chemical Industry Co. Ltd., China. Hydrogen peroxide (H₂O₂, 30 wt% purity), ferrous sulfate heptahydrate (FeSO₄·7H₂O), sodium hydrosulphite (Na₂S₂O₄), sodium carbonate (Na₂CO₃), 1-hydroxy ethylidene-1,1-diphosphonic acid (HEDP), potassium permanganate (KMnO₄), sulphuric acid (H₂SO₄), acetate acid (HAc), sodium acetate (NaAc), oxalic acid (H₂C₂O₄), sodium oxalate (Na₂C₂O₄), sodium hydroxide (NaOH), hydroxylammonium chloride, 1,10-phenanthroline, t-butanol and dimethyl sulfoxide (DMSO) are in analytical grade. Deionized water was applied in this research.

2.2. Pre-treatment

Research on the pre-treatment of yak hair fibres with ferrous ions was carried out by discussing the effects of concentrations of FeSO₄ (0–20% owf) and Na₂S₂O₄ (0–4% owf) as well as the temperature range (30–70 °C) on whiteness of bleached fibres. (Note: owf is the weight ratio of reagent to fibre).

2.3. Bleaching

Batch ultrasonic-assisted bleaching: Yak hair fibres (1.5 g) were immersed in a mixture (50 mL) containing FeSO₄·7H₂O (0.412 g), Na₂S₂O₄ (0.06 g) and surfactant AEO-9 (3 mL, 20 g/L) at 50 °C for 1 h (Phase I), washed thoroughly with 40 °C water for 3 times to remove the unfix ferrous sulphate from the fibre, and then transferred to the bleaching solution (50 mL in total) composed of 30%H₂O₂ (2.25 mL), HEDP solution (4.5 mL, 20 g/L), Na₂CO₃ solution (5 mL, 30 g/L) at 60 °C for 1.5 h (Phase II). The resultant yak hair fibres were rinsed with 40 °C water for 3 times to wash the residual chemical reagents. Final treatment was carried out using H₂C₂O₄ (0.03 g), Na₂S₂O₄ (0.06 g) at 40 °C for 30 min (Phase III). The yak hair fibres were then rinsed and dried naturally. In the above process, the mass ratio of solution against fibres was 33:1. The batch ultrasonic-assisted process was implemented using a KQ-300GDV Digital Homothermal Ultrasonic Cleaners (Ultrasonic input power, 300 W; total input power, 1050 W; ultrasonic frequency, 40 kHz; total volume, 13 L) connected to a cooling system (Kunshan Ultrasonic Instrument Co., Ltd, China). In addition, all the H₂O₂ used in this research are 30% H₂O₂ which is not further emphasized in the following text.

Conventional bleaching: Conventional bleaching was carried out in a SHZ-88A Digital Homothermal Water Bath (Changzhou Guohua Electric Appliance Co., Ltd, China) following a similar procedure as Batch ultrasonic-assisted bleaching described in Fig. 2, however without ultrasound.

Fig. 2.

Batch ultrasonic bleaching process.

CCD of experiment: The bleaching process (Phase II in Fig. 2) in terms of the concentrations of H₂O₂, HEDP and Na₂CO₃, is discussed and optimized through CCD using a statistical software package Minitab 19 (trial version, State Colleague, USA). The variable levels and codes, and factorial design matrix are displayed in the Table 1, Table 2, respectively. The experiment was run randomly to decrease uncontrolled interference and repeated in triplicate. The main effect and interaction among factors were explored based on ANOVA analysis.

Table 1.

Variables and experimental design levels.

Variables (% owf)	Levels
	-α	−1	0	1	α
A: H2O2	32.27	80	150	220	267.73
B: HEDP	0.95	3	6	9	11.05
C: Na2CO3	1.59	5	10	15	18.4

Table 2.

Composition of various runs, predicted and actual responses.

Std. Order	Run Order	H2O2	HEDP	Na2CO3	WI
					Predicted	Actual
1	5	80	3	5	15.5	16.5
2	12	220	3	5	35.4	35.0
3	4	80	9	5	12.8	13.6
4	14	220	9	5	27.7	28.0
5	18	80	3	15	30.1	30.0
6	13	220	3	15	49.6	49.0
7	20	80	9	15	30.3	30.9
8	11	220	9	15	44.7	43.9
9	1	32.27	6	10	18.8	17.5
10	3	267.73	6	10	47.7	48.7
11	10	150	0.95	10	36.8	37.0
12	9	150	11.05	10	30.4	30.0
13	15	150	6	1.59	14.5	13.6
14	6	150	6	18.41	41.1	41.8
15	2	150	6	10	44.8	45.1
16	16	150	6	10	44.8	44.1
17	19	150	6	10	44.8	43.6
18	7	150	6	10	44.8	45.0
19	17	150	6	10	44.8	45.9
20	8	150	6	10	44.8	45.3

2.4. Dyeing

The dyeing process was implemented following a practical dyeing procedure. One gram of bleached yak hair or wool fibre was dip-dyed in a 0.2 g/L dye solution at pH 4 regulated by acetic acid. The temperature was raised from room temperature to 90 °C at a heating up rate of 5 °C/min and kept constant for 60 min. The dyed fibres were finally subjected to a rinsing process at 90 °C for 5 min using 1 g/L commercial detergent.

2.5. Characterization

2.5.1. Colour feature

The WI, apparent colour depth (K/S), lightness (L*), redness–greenness index (a*), yellowness–blueness index (b*) and tristimulus values Y, Z (CIE system) of fibres were measured on a Datacolor 400 spectrophotometer using Illuminant D65 at 10◦ observe angle. A black cylindrical sample holder was used for fibres’ loading to achieve a uniform thickness. The average value of ten measurements was reported. Yellowness Index (YI) was further calculated using Z and Y from the colour measurement following Eq. (1),

$$\text{YI = 100}\times \text{(1 - 0.847}\times \text{Z/Y)}$$ (1)

2.5.2. Decomposition rate of H₂O₂

The concentration of H₂O₂ in bleaching solution was determined through a standard titration procedure using KMnO₄ according to standard GB/T 1616-2014 – ‘Hydrogen peroxide for industrial use’ from which the decomposition rate of H₂O₂ was further calculated. One mL of bleaching solution at different time spot (for test), and 1 mL of the H₂O₂ solution prepared by 2.25 mL H₂O₂ (30%) and 48 mL H₂O (for reference), were added to two identical solution prepared with 10 mL of H₂SO₄ (4 mol/L) and 25 mL H₂O, respectively. These two mixtures were titrated with 0.01 mol/L KMnO₄ standard solution calibrated with Na₂C₂O₄ till faint red and stable for 30 s. The volumes (mL) of consumed KMnO₄ standard solution for test and reference samples were taken as V_x and V_o. The measurement was performed in triplicated and the average values were reported. The decomposition rate of H₂O₂ was calculated using Eq. (2),

$${\text{Decomposition rate of H}}_2{\text{O}}_2=\frac{{\text{V}}_{\text{0}}{\text{- V}}_{\text{x}}}{{\text{V}}_{\text{0}}}\times 100\%$$ (2)

2.5.3. Determination of Fe²⁺ ions concentration

The Fe²⁺ ions concentration in the bleaching solution was determined based on the coloured complexing reaction between Fe²⁺ ions and 1,10-phenanthroline (Fig. S1) [28]. An absorbance/concentration relationship of 1,10-phenanthroline/Fe²⁺ chelate solution at the λ_max (510 nm) was established as a reference (Fig. S2). Solution A containing 0.024 g/L Na₂S₂O₄ and 0.024 g/L AEO-9 surfactant was prepared to simulate the bleaching solution. One mL of hydroxylammonium chloride solution (100 g/L), 2 mL of 1,10-phenanthroline solution (5 g/L), 5 mL of HAc-NaAc buffer solution (pH 4) and 12.5 mL Solution A were added into a series volumes (0.2–10 mL) of ferrous sulfate solution (Fe²⁺ ions concentration 0.025 g/L) for the measurement of UV–Vis absorption spectrum. The absorbance at 510 nm of 1,10-phenanthroline/Fe²⁺ chelate solution displayed a good linearship with a high correlation R² up to 0.997 indicating the high reliability for the estimation of Fe²⁺ ions concentration. Therefore, the concentration of Fe²⁺ ions in the bleaching solution and on the fibre was further calculated accordingly.

2.5.4. Determination of HO· concentration

The concentration of HO· was determined through fluorescence labelling [29]. Fluorescent probe (BA) was used to capture HO·, thus generating fluorescent hydroxyl benzenepentacarboxylic acid (BAOH) showing a maximum emission at 435 nm (Fig. S3). A F-2500 fluorescence spectrophotometer (Hitachi, Japan) with a 10.0 nm band pass slit was adopted for detection. The simulated bleaching solution containing 400 μmol/L BA, 220 μmol/L H₂O₂, 200 μmol/L HEDP and 20–120 μmol/L Fe²⁺ ions were treated using ultrasonic cleaner at 60 °C for 1 h at a pH of 10 adjusted by Na₂CO₃. For comparison, the simulated bleaching solution containing the same compositions (120 μmol/L Fe²⁺ ions) were treated using homothermal water bath under the same conditions. To explore the impact of HO· concentration on the WI of fibres, two free radical inhibitors, i.e. tert-butanol and DMSO were adopted to scavenge the HO·. The fibers were treated using H₂O₂ (150% owf), HEDP (6% owf), Na₂CO₃ (10% owf) at 60 °C for 1 h in the presence of 0–10 mL HO• inhibitor.

2.5.5. ATR-FTIR, XRD and SEM-EDS

The attenuated total reflectance - Fourier transform infrared (ATR-FTIR) study was implemented on a FTIR-650 spectrometer (Tianjin Gangdong Sci. & Tech. Co., Ltd., China) conneted to a LA-100 Universal ATR accessory (Lambda Scientific Pty Ltd., Australia) in a wavenumber range of 600 to 4000 cm⁻¹ at 4 cm⁻¹ resolution. The crystalline structure of fibre was characterized on an Ultima IV X-ray diffractometer (Rigaku Analytical Devices, Inc. Japan) using Cu-Kα radiation (40 kV and 40 mA) with a scanning speed of 10°/min from 5° to 70°. The morphology and cross section of fibres were analysed on a JSM-IT800SHL SEM (JEOL Ltd., Japan) equipped with Energy Dispersive X-Ray Spectroscopy (EDX).

2.5.6. Tensile property

The breaking force and elongation of fibers were measured by Instron tensile tester (Model 1122) at a stretching speed of 5 mm/min with 10 mm sample length. Ten times repeated test was done on each sample. The average results were recorded.

2.5.7. Alkali-solubility

The damage of yak hair fibre by bleaching process was quantified through alkali-solubility assessment. The fibre sample (0.5 g) was dried at 105 °C for 3 h with initial mass (W₁) taken down before measurement, then placed into a 100 mL NaOH solution (0.05 mol/L) at 60 °C for 30 min, and rinsed with acetic acid and distilled water 3 times for neutralization. The mixture was finally filtrated and finally dried at 105 °C for 3 h with mass (W₂) recorded. The alkali-solubility is determined using Eq. (3),

$$\text{G =}\frac{{\text{W}}_{\text{1}}{\text{- W}}_{\text{2}}}{{\text{W}}_{\text{1}}}\times \text{100\%}$$ (3)

where W₁ and W₂ are the mass of fibres before and after alkaline treatment.

3. Results and discussion

3.1. Ferrous ion pretreatment

The concentration of Fe²⁺ ions during the pre-treatment of yak hair exerts significant impact on the bleaching efficacy related to the H₂O₂ decomposition rate, thus is explored in priority. Na₂S₂O₄ was also added to protect Fe²⁺ ions from oxidation. The measurement of H₂O₂ decomposition rate in this section was carried out without ultrasound. As depicted in Fig. 3, the gradual increase of H₂O₂ decomposition rate without Fe²⁺ ions indicates a mild bleaching process. However, the H₂O₂ decomposition rate in the presence of Fe²⁺ ions dramatically increased as the time proceeding, which demonstrates that the decomposition of H₂O₂ was stimulated by Fe²⁺ ions. Higher H₂O₂ decomposition rate and WI were achieved at higher Fe²⁺ ions concentration. This is due to the fact that a large quantity of reactive oxygen species (ROS) was produced, giving rise to the adequate oxidative degradation of melanin granules thus enhancing WI. The WI increased linearly against H₂O₂ decomposition rate however decreased at 20% owf of Fe²⁺ ions. This is because that a part of Fe²⁺ ions merely adsorbed on the surface of yak hair rather than on the pigment granules. The excessive Fe²⁺ ions may lead to the decomposition of H₂O₂ on the surface of fibre rather than on the colour impurities, and cause cuticle damage, thereby reducing the whiteness. In general, 15% owf Fe²⁺ ions were adopted for following experiment considering the whiteness and intrinsic nature of fibres. In addition, the temperature during Fe²⁺ pre-treatment exerts marginal impact on the WI of the fibres (Fig. S4). The maximum WI was achieved at 50 °C. An increase trend of WI took place at higher Na₂S₂O₄ concentration and slowed down at around 4% owf, which is due to the better protection of Fe²⁺ from oxidation into Fe³⁺ (yellowish) promoting the subsequent bleaching process. Thus, 50 °C and 4% owf Na₂S₂O₄ was adopted for the pretreatment process.

Fig. 3.

H₂O₂ decomposition rate at various Fe²⁺ concentrations against time and its relationship with WI.

3.2. Comparison between ultrasonic-assisted and conventional bleaching

3.2.1. Bleaching efficacy

The bleaching efficacy of yak hair fibers treated with and without ultrasound is demonstrated by their WI and YI values and else colour features (L*a*b* and K/S values). As depicted in Fig. 4, untreated yak hair fiber shows a low WI around 10. The high K/S value and low L*a*b* values (dots are close to origin) of fibers indicates the dull brownish apparent colour of untreated yak hair (See the inserted photo). Both conventional and ultrasonic-assisted bleaching treatment effectively improve the WI, and reduce the colour shade reflected by the dramatically decreased K/S and enhanced L* values. It is worth to note that the yak hair fibers bleached under ultrasound increases in WI by 44.6% over those bleached through conventional process, which is consistent with the fact that the ultrasonic bleached fibers show lower YI and b* than those of conventionally bleached fibers. The remarkable bleaching effect by ultrasound is attributable to the following factors. The ultrasonic cavitation effect gives rise to the generation of a numerous tiny bubbles in the bleaching solution. The high velocity jetting liquid emerged from the collapse of bubbles, accelerates the mixing function on the boundaries between fiber and solution, thereby promoting the transfer of Fe²⁺ and H₂O₂ to the fiber surface [30]. The slight relief of the cuticle scales and sufficient swelling of yak hair fiber in the ultrasonic medium is favourable for the penetration of bleaching components (Fe²⁺ and H₂O₂) to the fiber interior [31], enhancing the adsorption between Fe²⁺ and colour impurities, and increasing the H₂O₂ concentration around colour impurities. Meanwhile, the “hot spot” produced by the bubbles breakage induces localized high-temperature and high-pressure phenomenon. Such extreme conditions promote H₂O₂ decomposition that increases the total concentration of ROS under the catalyst effect of Fe²⁺, and provide a greater surface accessibility to accelerate the oxidation reaction between ROS and colour impurities [32]. Those assumptions are further verified in the following section (3.3. Bleaching mechanism). The a* values of bleached yak hair fiber are lower than untreated one, indicating the decrease of red colour shade. Conversely, the b* values after bleaching increased, because the bleached fibre shows very light yellowish, which likely is the natural colour of protein fiber (See the inserted photo). The yak hair fibers bleached under ultrasound have smaller error values of WI, YI, L*, a* and b* than those treated with conventional method, which indicates a higher bleaching uniformity achieved by ultrasound. To be specific, the highly frequent compression and rarefaction of ultrasonic wave upgrades the efficiency of mass transfer in the heterogeneous solid/liquor bleaching system, thus increasing the interaction between ROS and melanin granules for a better removal of melanin granules compared with the conventional mechanical agitation. In all, the ultrasonic-assisted bleaching process is able to simultaneously increase the whiteness and decrease the yellowness of the yak hair fibre, showing superior efficacy than the conventional bleaching method.

Fig. 4.

(a) WI/YI and K/S, and (b) L*a*b* values of untreated, conventional bleached and ultrasonic-assisted bleached yak hair fibres. (Note: Untr.: Untreated; C.B.: Conventional bleached; U.B.: Ultrasonic-assisted bleached).

3.2.2. Dyeing performance

The dyeing performance of the yak hair bleached with and without ultrasound was evaluated using three commercial acid dyes compared with the dyeing of wool fibres. In general, yak hair fibres after bleaching and dyeing display an equivalent or even better dyeing performance than wool fibres, which demonstrates the feasibility of yak hair for commercialization in the future. Moreover, yak hair showed high colour depth (Fig. 5a) rising from the high dye uptake (See the photos of dye solutions before and after dyeing in Fig. S5). Therein, yak hair fibres bleached under ultrasound display higher colour depth, and accordingly low L* values than those without ultrasound (Fig. 5b). This is caused by the crack of yak hair cuticle outlayers and the generation of internal pores from the removal of the melanin granules, thus promoting the adsorption capability of dyes (verified in the section 3.3. Bleaching mechanism). The yak hair fibres dyed into three primary colours (Fig. 5c) showed uniform and dark colour appearance demonstrating that they are desirable alternatives to the commercial dyed wool fibres.

Fig. 5.

(a) K/S and (b) L*a*b* values and (c) photos of dyed fibres. (Note: C.B.: Conventional bleached; U.B.: Ultrasonic-assisted bleached; Standard deviation of L*a*b* values are displayed in Table S1).

3.2.3. Tensile property

Under the same condition including time, temperature and chemical usage, the protein fibre treated under ultrasound displays higher tensile loss due to the impact of ultrasonic impulse on the surface and inner structure of fibre [24]. However in the current case, it is reasonable to compare the tensile property of the yak hair samples with similar WI enhancement after ultrasonic-assisted (at 60 °C for 90 min in Phase II) and conventional (at 70 °C for 90 min in Phase II) bleaching. As depicted in Fig. 6, the tensile strength of yak hair fibres at break decreased after bleaching due to the inevitable damage of keratin structure caused by the oxidation [3], [7]. Yak hair fibres also showed an increase in elongation which is attributable to the cleavage of disulfide bonds [3], [7]. The results for ultrasonic-assisted and conventional bleaching are distinctive. Notably, there is less strength loss and smaller elongation increase of yak hair bleached under ultrasound than those treated by conventional method, which is consistent with the results of alkali-solubility (See inserted table). Thus to achieve equivalent whiteness on yak hair, using ultrasound shows a better fiber strength preservation compared with the conventional process due to a lower bleaching temperature which obviously diminishes the oxidative damage of fibre structure. Ultrasonic cleaning function facilitates the removal of a number of melanin particles from the bulk of fibre (verified in the section 3.3. Bleaching mechanism), resulting in micron pores within fibres, which is another reason for the decline of tensile stress. An increased intensity of sulphoxide bond (1039 nm⁻¹) in the ATR-FTIR spectra of bleached yak hair compared with untreated fibre, confirms the damage of disulfide bond (Fig. S6a). Other characteristic bonds such as amide I (1633 nm⁻¹), amide II (1535 nm⁻¹) and amide III (1232 nm⁻¹) remain the same after bleaching. The crystallinity of yak hair showed almost no change after bleaching confirmed by their similar typical diffraction pattern of α-keratin with a prominent 2θ peak around 10° and a broad peak around 22° (Fig. S6b) [33]. Previous research proofed that ultrasonic treatment does not affect the wool crystallinity property [34], [35], however may change the crystallinity of cellulosic fibres [36]. This research further confirm the ultrasonic-assisted bleaching also exert marginal impact of the crystallinity of yak hair.

Fig. 6.

Load-extension curves of yak hair fibers. (Note: Untr.: Untreated; C.B.: Conventional bleached; U.B.: Ultrasonic-assisted bleached).

3.3. Bleaching mechanism

The bleaching mechanism of the established process is dependable on the physical and chemical actions of ultrasound combining with the Fenton reaction of catalytic Fe²⁺ ions and H₂O₂. Specifically, the Fenton reaction in the present case mainly involves the following three steps [37], [38],

Fe²⁺ + H₂O₂ → Fe³⁺ + OH⁻ + HO(1)

Fe³⁺ + H₂O₂ → Fe(OOH)²⁺ + H⁺(2)

Fe(OOH)²⁺ → Fe²⁺ + HO₂(3)

The H₂O₂ decomposes due to its consumption during Fenton reaction and spontaneous ionization under alkali conditions. Hydroxyl radicals (HO•) and perhydroxyl ions (HO₂⁻) are the Fenton reaction product and ionization product, respectively. Therein, the HO• converted from H₂O₂ plays the main role in the oxidation reaction accounting for the degradation of colour impurity. Thus in this section, a comprehensive study on the bleaching mechanism is implemented by exploring the impact of ultrasound and Fe²⁺ ions on the H₂O₂ decomposition during phase II and HO• concentration of simulated bleaching solution. The cross section of ultrasonic bleached yak hair was also studied through morphological and elemental analyses to further reveal the function of ultrasound, and the interaction between Fe²⁺ ions and melanin granules.

In general, a greater quantity of H₂O₂ decomposes along with the processing time (Fig. 7a). Less than 20% H₂O₂ decomposed in the absence of Fe²⁺ pre-treated yak hair. There is also no significant difference between the solutions with and without ultrasound. Such result demonstrates that only a minority of H₂O₂ participates in the ionization. The decomposition of H₂O₂ was drastically promoted by 20.9% with the addition of Fe²⁺ treated yak hair without ultrasound, which is mainly due to the catalytic effect of Fe²⁺ions that promotes the Fenton reaction. In contrast, the decomposition percentage of H₂O₂ was further improved to 35.9% under ultrasound, with a value at 45 min point close to that at 90 min point without ultrasound. Obviously, ultrasound accelerates the Fenton reaction in the present bleaching system. Thus, more H₂O₂ decomposed in the presence of ultrasound and Fe²⁺, which contributes to higher concentration of oxidative bleaching species.

Fig. 7.

Bleaching mechanism study: (a) H₂O₂ decomposition rate with and without Fe²⁺ treated yak hair and ultrasound, (b,c) HO• concentration under Fe²⁺ with and without ultrasound, (d) WI values of yak hair in the presence of free radical inhibitor, (e1,2) SEM-EDS analysis of the cross section of the Fe²⁺ treated yak hair, SEM images of the cross sections of yak hair before (f1-3) and after (f4-6) ultrasonic bleaching, SEM images of Untr. (g1), C.B. (g2) and U.B. (g3) yak hair surface. (Note: Untr.: Untreated; C.B.: Conventional bleached; U.B.: Ultrasonic-assisted bleached).

As described in Fig. 7b,c, the fluorescence intensity representing the concentration of HO• in the simulated bleaching solution increases nearly in a linear trend with the increasing concentration of Fe²⁺ ions, indicating the intensification of Fenton reaction trigged by Fe²⁺ ions. It is also worth noting that the ultrasound dramatically improves the HO• concentration by 94%, which implies the specific role of ultrasound in driving the Fenton reaction towards completion. Therefore, ultrasound increases the decomposition rate of H₂O₂ through Fenton reaction, facilitating the generation of HO• for pigment degradation and finally enhancing the whiteness of fibres.

In order to further verify the function of HO• during bleaching process, two inhibitors DMSO and t-butanol are introduced to the bleaching bath to scavenge the HO• merely from solution, and both from solution and the surface of catalysts (i.e. Fe²⁺ mordanted melanin granule), respectively [24], [37], [39]. The relationship between the inhibitor concentration and the WI of yak hair is further established. As depicted in Fig. 7d, the WI of yak hair bleached with or without ultrasound, decreased gradually at higher inhibitor dosage, confirming the bleaching function of HO•. The WI approaches to an equilibration at a concentration of 8 mL, which indicates that most of the HO• was scavenged by the inhibitors. A larger descending trend of the curve under ultrasound than conventional method is observed, which demonstrates that more HO• was generated under ultrasound due to the accelerated decomposition of H₂O₂, which is one reason related to the enhanced WI of yak hair by ultrasonic bleaching. Compared with DMSO, a larger decline of WI was observed than the presence of t-butanol. This result indicates that the ultrasound enhances the HO• concentration both in the bleaching solution and on the surface of mordanted melanin granule.

The postulation i.e. the anchor of Fe²⁺ ions by melanin, is further verified by SEM-EDS analysis (Fig. 7e). The SEM image of the cross section of yak hair fibre shows that the melanin granules irregularly embedded in the keratin fibrils. The SEM-EDS mapping reveals that the combination of Fe²⁺ ions with the melanin granules, leaving fewer Fe²⁺ ions distributed on the rest area. The selectively anchoring of Fe²⁺ ions to the melanin granules is attributable to their strong chelating effect through multipoint combination [26], [40], [41]. Such mechanism not only enhances the effectiveness of the catalytic effect of Fe²⁺ ions, but also decreases the negative impact of over-oxidation on the non-coloured region of fibers which benefits for their mechanical preservation. There is an interesting finding displayed in Fig. 7f. A number of melanin granules (indicated by red arrows) were removed from the bulk of fiber, leaving micron pores (indicated by blue arrows) after bleaching due to the physical cleaning action of ultrasound. Such phenomenon also enhances the dyeing property of yak fiber shown in Fig. 5. In addition, a larger number of micro-cracks on the scale of yak hair bleached through ultrasonic than conventional methods were observed (Fig. 7g2,3), however which did not occur on the untreated fibre (Fig. 7g1). Similar phenomenon also occurred on the ultrasonic treated wool fibre [15], [35]. The surface cracking is another reason related to the increase of dye uptake due to the easier diffusion of dye into the fibre through the cracks as well as between the overlap of the scales [35].

As shown in Table S2, ultrasonic pretreated fiber has higher Fe content, which attributes to the promotion of adsorption and penetration of Fe²⁺ ions by ultrasound during the complexation phase (I). This is reconfirmed by the adsorptive percentage of Fe²⁺ ions during pre-treatment with and without ultrasound (Fig. S7). Thus, ultrasound brings higher Fe²⁺ ions content on yak hair, which further promotes the catalytic function of Fe²⁺ ions to decompose H₂O₂ and generate HO• for the degradation of melanin granules (see Fig. 7a,b). The O increase and S decrease imply the oxidation of disulfide bonds.

3.4. CCD optimization

3.4.1. Modelling and its corresponding ANOVA

In this section, the established ultrasonic-assisted Fenton system for yak hair bleaching (Phase II) is further investigated and optimized. In the pre-experiment of Phase II, the ultrasonic bleaching temperature and time exceeding 60 °C and 90 min respectively, leads to fiber damage reflected by alkali-solubility (data not shown). Thus, the critical factors influencing the WI of yak hair are the concentrations of H₂O₂, HEDP and Na₂CO₃. An optimized equation and corresponding estimation of WI were achieved through mathematical modelling, which enables the determination of the parameters for research or mass production in the future. Response surface methodology (RSM) is adopted to demonstrate the relationship between variables with fewer experiments than conventional single factor method. The variables for concentrations including H₂O₂ (32.27, 80, 150, 220 and 267.73% owf), HEDP (0.95, 3, 6, 9 and 11.05% owf) and Na₂CO₃ (1.59, 5, 10, 15 and 18.4% owf) are defined as A, B and C, respectively. Its regression analysis by a quadratic model leads to the following equation in terms of coded factors:

WI = α₀ + α₁A + α₂B + α₃C + α₄AB + α₅AC + α₆BC + α₇A² + α₈B² + α₉C²

where A, B and C are variables; α₀ is the constant, α₁, α₂ and α₃ are the linear coefficients; α_4, α₅ and α₆ is the interactive coefficients; α₇, α₈ and α₉ are the quadratic coefficients.

Final equation in terms of actual factors:

WI = − 47.47 + 0.4134 × H₂O₂ + 5.075 × HEDP + 6.165 × Na₂CO₃ − 0.00600·H₂O₂ × HEDP − 0.00034 × H₂O₂ × Na₂CO₃ + 0.0472 × HEDP × Na₂CO₃ − 0.000837 × H₂O₂² − 0.4399 × HEDP² − 0.2406 × Na₂CO₃²

The significance of the established model was assessed by ANOVA at a confidence level of 95%. The significance of each coefficient was examined by p-values those lower than 0.050 are significant whilst higher than 0.100 are not. As described in Table 3, the Model F-value of 278.61 implies the model is significant in general. The p-values of the linear coefficients (α₁, α₂ and α₃) and square term coefficients (α₇, α₈ and α₉) are smaller than 0.001, which demonstrates their significant impacts on the WI of yak hair. The p-value (0.199) for lack-of-fit far exceeds the threshold of 0.05, indicating the lack of fit is not significant. The predicted R-Sq is an indicator to elucidate the fitness of predictions from the model toward new observations, whilst R-Sq demonstrates how well the model fits the data. The high R² (99.60%) implies a perfect predictability for deduction using the model. There is a good closeness between the predicted and actual WI of bleached yak hair (Fig. S8). Correspondingly, the predicted R-Sq (97.71%) is close to the adjusted R-Sq (99.24%). These results demonstrate the appropriateness of using the established model for the prediction of results.

Table 3.

ANOVA for the response surface quadratic model.

Source	DF	Adj SS	Adj MS	F-Value	p-Value
Model	9	2771.55	307.95	278.61	<0.001
Linear	3	1918.52	639.51	578.58	<0.001
H2O2	1	1012.09	1012.09	915.67	<0.001
HEDP	1	49.24	49.24	44.55	<0.001
Na2CO3	1	857.19	857.19	775.53	<0.001
Square	3	836.21	278.74	252.18	<0.001
H2O2 × H2O2	1	242.04	242.04	218.99	<0.001
HEDP × HEDP	1	225.92	225.92	204.40	<0.001
Na2CO3 × Na2CO3	1	521.71	521.71	472.01	<0.001
2-Way Interaction	3	16.82	5.61	5.07	0.022
H2O2 × HEDP	1	12.70	12.70	11.49	0.007
H2O2 × Na2CO3	1	0.11	0.11	0.10	0.754
HEDP × Na2CO3	1	4.00	4.00	3.62	0.086
Error	10	11.05	1.11
Lack-of-Fit	5	7.64	1.53	2.24	0.199
Pure Error	5	3.41	0.68
Total	19	2782.52
Model Summary	S	R-Sq	R-Sq(adj)	R-Sq(pred)
	1.05170	99.60%	99.24%	97.71%

3.4.2. Interaction of variables on WI of yak hair

To further explore the effect of variables and their interactions on WI, 3D response surface and 2D contour plot were generated from the established model. As the response surface depicted in Fig. 8a, the WI increases initially at low H₂O₂ and HEDP concentrations, then approaches to a summit at c.a. 230% owf H₂O₂ and 5% owf HEDP in and decreased when both concentration increases. Accordingly, the dark green area that represents high WI value locates at the middle right region in the contour plot. Similar trends are also observed in the 3D response surfaces and contour plots in terms of H₂O₂ and Na₂CO₃ concentrations (Fig. 8b), and HEDP and Na₂CO₃ concentrations (Fig. 8c). Such phenomenon implies that there exists an optimal WI at certain concentrations of these three chemicals, and excessive usage of them could cause the decrease of WI. To be specific, the decomposition of H₂O₂ is accelerated in the alkaline condition offered by Na₂CO₃, which is helpful to improve the WI of yak hair. However, excessively adding either or both may lead to the abrupt generation of a large number of radicals, which not only decreases the bleaching efficiency due to their self-extinction, but also induces the breakage of disulfide bonds, aggravates the degradation of keratin, and finally results in the yellowish appearance. HEDP usually plays a role of metal ion chelator to protect the fibres from over oxidation during bleaching, whereas in the current bleaching system, Fe²⁺ ions are ‘preloaded’ onto the yak hair combining with melanin granule (Phase I) as catalyst for the subsequent bleaching (Phase II). Thus, too much HEDP reduces the content of Fe²⁺ ions on the yak hair, and further mitigates the catalytic effect on bleaching. In all, the result of response surfaces and contour plots demonstrates that the high WI can be achieved at appropriate concentrations of H₂O₂, Na₂CO₃ and HEDP.

Fig. 8.

Effect of variables as response surfaces and contour plots.

The extent of influence on the WI of each variable and their interactions are studied by the main effects and interaction plots, respectively. Pareto chart is used to quantify the importance of these effects. As seen in Pareto chart (Fig. 9a) and mean effect plots (Fig. 9b), H₂O₂ and Na₂CO₃ are two significant variables, followed by HEDP. The nearly paralleled lines of H₂O₂ × Na₂CO₃ and HEDP × Na₂CO₃ indicating there are almost no obvious interactions. Two cross connections occur in the figure of H₂O₂ × HEDP, revealing their interactions. Such result is consistent with the corresponding p-values of coefficients shown in Table 3. Finally, the optimal bleaching condition (227.29% owf H₂O₂, 4.93% owf HEDP and 13.14% owf Na₂CO₃) is achieved from the model with theoretical highest WI of 52.4.

Fig. 9.

(a) Pareto chart of the standardized effects, (b) Main effects plot, (c) Interaction plot and (d) Optimization curves for WI.

4. Conclusion

This study presents an efficient ultrasonic-assisted bleaching strategy customized for yak hair through a melanin-targeted Fenton oxidation. The whiteness, dyeing performance against three representative acid dyes, and tensile property of yak hair bleached using ultrasonic and conventional processes were evaluated and compared. The bleaching mechanism was explored by comparing the H₂O₂ decomposition behaviour, HO• concentration, and morphologies and elemental compositions of fibres treated with and without ultrasound. The H₂O₂ bleaching parameters in Phase II was explored and further optimized through CCD. Results reveal that ultrasound is effective in improving the WI of yak hair from 11 up to 45 with a minor the oxidation damage of bleached fibre and the dyeability than those bleached without ultrasound. The catalytic Fe²⁺ ions facilitate the decomposition of H₂O₂ by 20.9% and the decomposition rate is further increased to 35.9% after adding ultrasound. HO• is confirmed as the main oxidative species in the bleaching system upon the addition of HO• scavengers, and its concentration is significantly improved by ultrasound based on the fluorescence analysis of simulated bleaching solution. The selective anchor of Fe²⁺ ions to the melanin granules is confirmed by SEM-EDS. The porous structure on the cross section of yak hair generates after ultrasonic bleaching due to the removal of melanin granules. Based on the modelling, a theoretical highest WI of 52.4 is achieved under the optimal bleaching condition (227.29% owf H₂O₂, 4.93% owf HEDP and 13.14% owf Na₂CO₃). This study demonstrates the feasibility of the efficient ultrasonic bleaching of yak hair and contributes to the development of sustainable textile industry. Such strategy is potentially scaled up, and transferable to the mild but efficient bleaching of other natural dark protein fibres such as camel hair, sable hair, ostrich feather.

CRediT authorship contribution statement

Qing Li: Conceptualization, Methodology, Data curation, Writing – original draft. Zengfeng Wei: Investigation, Formal analysis, Data curation. Mohan Li: Formal analysis. Shiwei Li: Formal analysis, Data curation. Lijie Ni: Formal analysis, Data curation. Heng Quan: Methodology, Supervision. Yuyang Zhou: Conceptualization, Methodology, Writing – original draft, Writing – review & editing, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article: