Effect of various transition metal ions on mordanting stage of yak wool bleaching process

6 Yak wool is a smooth, warm, and durable natural protein-structured fiber that could compete with cashmere and other high-end protein-structured fibers on the market. However, it suffers from drawing consumers’ attention due to the lack of color due to the shortfall of the 9 yak wool bleaching technology. Herein, we studied the applicability of various transition metals, i.e., copper (II), cobalt (II), iron (II), and nickel (II) salts, as a mordanting reagent based on their effect on the hydrogen peroxide decomposition reaction and the 12 morphological and mechanical properties of the bleached yak wool with the presence of these transition metal. Our study suggested that the iron (II) ion was the most efficient reagent for the mordant bleaching since it provided less fiber damage, relatively high 15 strength, and elongation to the bleached yak wool with good whiteness, while the Cu (II) was the least favorable agent for the yak wool bleaching process.


Introduction
Yak wool-made products are very convenient in cold and damp weather due to their smooth, warm, and moisture absorption nature [1]. Indeed, yak wool owns the properties of better 21 shrinkage, elasticity, and durability because of higher sulfur and cysteine contents as compared to the other protein-structured high-class material like cashmere. Still, it costs a quarter of cashmere's price. These remarkable properties of yak wool are influenced directly 24 by the climate that yaks are needed to adapt to live in extremely cold weather down to -50°C. Due to the critical climate influence on the formation of such unique properties of yak wool [2], Mongolia is the second biggest yak wool provider to the world market and one of 27 the few countries that manufacture yak wool fabrics. However, yak wool-made products suffer from drawing less attention among customers because of their lack of choices in colors and designs, although it has shown great potential in marketing in terms of qualities 30 and prices.
Taking account of being about 69% of collected yak wool is dark, 17% is brown, 8% is blue, and only 6% is white-colored; currently, most yak wool-made products are manufactured 33 with natural, dark-colors owing to the fact that no suitable technological condition for bleaching has developed specifically considering the yak wool properties [3]. Yak wool fibers are relatively short compared to other protein-structured fibers because yak wools are 36 collected by the shearing technique [4]. Therefore, short fibers tend to be damaged easily during technological treatment, such as bleaching and dyeing, compared to longer fibers like cashmere. Although occasionally, the cashmere bleaching condition is attempted to be 39 used for yak wool, the quality of the products deteriorates due to the damages caused by the harsh treatment conditions. For this reason, insight into yak wool bleaching technology is crucial to broaden the yak wool mass production to the market in the future.

42
The dark color of the protein-structured fibers is a result of a natural pigment, so-called melanin [2,5]. Therefore, the purpose of the bleaching process is to remove the melanin pigments from the surface and in-depths of the fibers, as well. Practically, hydrogen peroxide 45 is employed to prompt the defragmentation process in textile factories due to its convenience to handling, ease of operation, simplicity of batch design, inexpensiveness, and environmental friendliness [6]. However, the hydrogen peroxide-based techniques require a 48 particular catalyst to enhance the effectiveness of the bleaching process for promoting the decomposition reaction of hydrogen peroxide during the bleaching process [7]. The suitable catalysts should be added to the dark-colored wool-containing batch as a preparation for 51 the bleaching by hydrogen peroxide, known as a mordanting stage. For this reason, this A c c e p t e d m a n u s c r i p t technique, so-called mordant bleaching, is needed to be covered by three sequential stages, such as mordanting, rinsing, and bleaching. Any of these three processes could cause 54 damage to the wool since harsh chemical treatments at high temperatures are applied.
However, a careful mordanting agent choice is crucial to maintain the intrinsic quality of yak wool after the whole bleaching process is conducted through adjusting the bleaching 57 condition.
Transition metal salts are applied to assist in bleaching wool as the mordanting agents because metal ions are likely to stick with the melanin rather than keratin due to its' higher 60 electron density resulting from having more hydroxyl, carboxyl, and amine groups [8].
According to the previous studies, although iron (II) [9][10][11][12], copper (II) [5,9,[13][14][15], and calcium ions [13] were employed and studied as the mordanting agent for various protein-63 structured fiber, only a few attempts using iron (II) ion for bleaching yak wool were made [16,17]. Therefore, there is still plenty of room for exploring a suitable mordanting reagent for yak wool bleaching and its impacts on the wool properties. In this study, we studied the

84
Hydrogen peroxide (H2O2) decomposition reactions were carried out in a batch system with 0.065 mol⋅L -1 of H2O2 and 1⋅10 -4 mol⋅L -1 transition metal salts at 50°C for 180 min to examine A c c e p t e d m a n u s c r i p t the effect of transition metal ions on the decomposition rate. The solution pH was adjusted 87 at 8.5 using sodium pyrophosphate solution with a concentration of 0.1 mol⋅L -1 . The remained concentration of H2O2 was monitored using the titration method with potassium permanganate solution (0.01 mol⋅L -1 ) in an acidic condition. For choosing the suitable 90 mordanting agent for the yak wool, the yak wool was mordanted with the concentration of 1⋅10 -4 mol⋅L -1 of transition metal salts with the concentration of 6 g·L -1 of citric acid, which was used as an auxiliary reagent at 50°C for 25 min. Following that, the bleaching stage 93 was carried out by adapting the same bleaching condition previously reported [18].

Characterization
The breaking load or strength and elongation properties of yak wool knit samples were

The effect of transition metals on the decomposition of hydrogen peroxide in vitro condition
Mainly transition metal salts are utilized in the mordanting stage to conduct the decomposition reaction with a desirable rate because the decomposition rate of hydrogen 108 peroxide is crucial in the fiber bleaching process [19]. In this study, we chose the transition metal salts of cobalt (II), nickel (II), copper (II), and iron (II) ions as a mordanting reagent by considering their electron configuration, first ionization energy, and atomic radius [20] in 111 order to investigate the most suitable reagent for mordanting the yak wool. First, we conducted a hydrogen peroxide decomposition reaction with the presence of the various transition metals in vitro conditions without yak wool in order to reveal their catalytic activity 114 for solely the hydrogen peroxide decomposition reaction. We depicted the experimental results of the hydrogen peroxide decomposition with the chosen catalysts in Fig. 1 and summarized the reaction rate constant and activation energy values from the relationship 117 between the reaction rate constant and temperature in Table 1. Fig. 1 displayed that these transition metals exhibited catalytic effects on the decomposition of hydrogen peroxide at different rates, and all decomposition reactions obeyed the first-order rate law. According to 120 A c c e p t e d m a n u s c r i p t previous studies which have studied the decoloring mechanisms of melanin by hydrogen peroxide with the presence of transition metal, the rate-determining step of the bleaching process obeyed the first-order law, as well [21,22], which supported our results. From the 123 rate constant and activation energy values (Table 1)

The effect of transition metals on bleaching effectiveness for the yak wool
Considering the results from the previous section, we utilized these transition metal salts as a mordanting agent for the yak wool bleaching process to investigate their applicability for values for the natural yak wool. In terms of whiteness (Fig. 4b), 162 the highest value was 57.00% when Fe (II) was used as the mordanting agent, while only the whiteness of 35.87% and 32.40% were achieved when Co (II) and Ni (II) were used as the mordanting agent, respectively (The initial whiteness was 28.66%). The photographic 165 images in Fig. 2 supported these whiteness results. When the Fe (II) was used as the mordanting agent, the mechanical properties were maximally maintained while obtaining sufficient whiteness. In the case of Co (II) and Ni (II), poor whiteness efficiency, as well as 168 a loss of mechanical properties, were obtained. Taking into account the mechanical properties and whiteness results, it suggested that too fast or slow rate of the hydrogen decomposition reaction is not desirable. Therefore, for achieving high-quality yak wool after 171 the bleaching process, the decomposition rate is needed to be balanced. Besides, our results confirmed that Fe (II) exhibited the property of a selective mordanting agent among these transition metals by considering the yak wool properties after the bleaching.

174
A c c e p t e d m a n u s c r i p t distinctive of the other ions [24]. Therefore, it is known that hard acids tend to bind to hard bases and soft acids tend to bind to soft bases; Cu (I) presumably is more likely to form coordination bonds with functional groups containing sulfur atoms in the protein of yak wool, wool fibers before and after the bleaching process with Fe (III) and Cu (II) in comparison to natural yak wool are shown in Figure 5. As we expected, the fiber damage was severe, and the cuticle layers were completely polished when the Cu (II) was the mordanting agent, while 213 only mere damage to the fibers was seen when Fe (III) was used as a mordanting agent.
This result supported the idea of having detrimental effects of soft acids as a mordanting agent on yak wool.

216
A c c e p t e d m a n u s c r i p t

Conclusion
In this study, we demonstrated the unique and selective property of Fe (II) ion as a 222 mordanting agent in the yak wool bleaching process based on the analysis of morphological and mechanical properties of bleached yak wools with the mordanting agents of Cu (II), Co (II), Fe (II), Ni (II) ion. Among the transition metal ions, Cu (II) was found to be the least 225 favorable mordanting agent for the yak wool bleaching by causing the dissolution of the protein in the yak wool and/or breaking the protein structure of the yak wool surface due to the formation of Cu (I) ion during the bleaching process. Therefore, we suggested the HSAB 228 theory to explain the notorious effect of Cu (II) ion on the yak wool properties. Moreover, we find that the wool properties critically affected the active site formation rate of the hydrogen peroxide decomposition during the bleaching process. We believe that these findings will be 231 a potential reference to suggest a suitable mordanting agent for not only yak wool but also different protein-structured fibers and help to improve the qualities of woolen products in the future.

Disclosure statement
No potential conflict of interest was reported by the authors.