Physicochemical Behavior of Superfine Wool Powder Dyed with Reactive Dye

ABSTRACT Superfine wool powder is obtained from waste wool fiber by physical grinding. Its physicochemical properties are different from those of wool fiber, and it is widely used in industry. In order to clarify the dyeing behavior of wool powder, C. I. Reactive Yellow 145 dye was used to dye wool powder, and the adsorption kinetics and thermodynamics were systematically studied in this work. The adsorption kinetics of reactive dye on wool powder was found to adhere to a pseudo-second-order kinetic model. At 60, 80 and 95°C, the half-dyeing time (t 1/2) and diffusion coefficient were 2.37, 1.56 and 1.00 min, and 0.2045, 0.1611 and 0.1368 cm2·min−1, respectively. And the diffusion activation energy was −11.82 kJ·mol−1. A Langmuir-type adsorption isotherm was obtained. At 60, 80 and 95°C, the standard affinity was 18.17, 20.06 and 20.82 kJ·mol−1, respectively. And the dyeing enthalpy change and dyeing entropy change were 7.27 kJ·mol−1 and 0.077 kJ·mol−1·K−1, respectively. In addition, the specific surface area, morphology and dyeing properties of wool fiber were compared. Through the study on the dyeing physicochemical properties of wool powder, it provides a theoretical basis for people to understand and use wool powder.


Introduction
Wool is a natural fiber with excellent biodegradability and biocompatibility, which is widely used in textile industry due to its unique structure, good moisture absorption (Rahmatinejad, Khoddami, and Avinc 2015), high elasticity , and insulating properties (Rouse and Van Dyke 2010). However, many wool fibers are too short to be spun into yarn, and a lot of short wool fibers are wasted every year. In order to make better use of wool, researchers used a variety of methods to treat or modify it.
The modification of wool fiber can be divided into chemical modification and physical modification. Chemical modification may affect the character and service life of wool fiber, and cause environmental pollution, but the modification effect is long-lasting; however, physical modification generally has little effect on the molecular structure of the wool fiber, with less pollution, but the modification effect is not durable. Modification may impart wool fiber some special properties, such as softness, hydrophilicity, comfort, etc. In the past decades, chemical modification was mainly graft modification initiated by chemical initiators (Gawish et al. 2012;Niu et al. 2012;Periolatto et al. 2013). In recent years, chemical modification methods mainly focus on the extraction of keratin, such as sulphurisation (Pakkaner et al. 2019), oxidation (Shavandi et al. 2017) and ionic liquids (Shavandi et al. 2017;Zhang et al. 2018). The extracted keratin can be used in medical materials (Park et al. 2015), keratin hydrogels (Esparza et al. 2018), and composite materials (Das, Borah, and Badwaik 2018). In terms of physical modification, researchers use low temperature plasma technology to physically treat wool fabric, mainly to improve the luster of wool (Shahidi et al. 2010). Nowadays, the main physical modification methods are air jet milling and freeze milling (Hassabo et al. 2015;Rajkhowa et al. 2012). These methods can modify the waste wool fiber into wool powder, which greatly broadens the application of waste wool (Cui et al. 2017).
When a material is processed into powder, its properties will undergo a series of changes, two of which are the most obvious, namely the increase in specific surface area and the decrease in crystallinity (Kambli et al. 2017;Korpela and Orelma 2020;Sheng and Limei 2016). The short wool fiber is made into wool powder, which can be widely used in industry. Wool powder can be used to fix carbon dioxide, making it a sustainable carbon source (Chang et al. 2018). Wool powder can also improve the mechanical strength and thermal stability of propylene carbonate, which has a good application prospect (Chang et al. 2017). Wool powder has more active sites with high adsorption properties than wool fiber, which can effectively adsorb heavy metal ions in water (Atef El-Sayed, Salama, and Kantouch 2015). The researchers have confirmed that wool powder still retains the special properties of wool fiber when it is transformed from fiber state to powder state (Novak, Kobe, and McGuiness 2004). This process does not destroy the chemical structure of the wool, and wool powder can achieve higher surface activity (Naik et al. 2010).
In our previous work, the feasibility of wool powder dyed with reactive dye as pigment for fabric printing has been studied. The results showed that the dyed wool powder can be used as pigment to print cotton fabric, and the printed fabric has good dry and wet rubbing fastness and moisture permeability (Guo et al. 2020). Although the dyeing properties of wool powder have been studied, the research on dyeing kinetics and thermodynamics of wool powder is not enough. In this work, the structure and properties of wool powder were fully compared with those of wool fiber, and the dyeing kinetics and thermodynamics of wool powder were studied deeply. These results provide preliminary data for people to understand superfine wool powder, so that people can better apply wool powder to textile, pigment and other industrial fields, and further expand its application range.

Materials
The wool powder was provided by the laboratory (Xu, Guo, and Li 2010), and the average particle size was about 15.2 µm. C. I. Reactive Yellow 145 dye was purchased from Wande Chemical Co., Ltd. (Shanghai, China). Its appearance is yellowish brown powder. C. I. Reactive Yellow 145 dye was purified according to the dissolution-recrystallization method (Hall and Perkins 1971;Đorđević, Cerkovnik, and Gorenšek 2006) and used in the study, and its chemical structure was shown in Figure 1. Distilled water was used throughout the experiment.

Dyeing process, dyeing rate curve and adsorption isotherm
C. I. Reactive Yellow 145 dye was dissolved in distilled water to prepare dye solution as required, and the liquor ratio was maintained at 1:100. The pH value of the dye solution was adjusted to 3.5 with glacial acetic acid. The dye solution was preheated for 10 minutes. Subsequently, 1 g wool powder was added into the dye solution and dyed in an infrared dyeing machine (Jingjiang Huaxia Technology Co., Ltd.). The dyed wool powder was thoroughly filtered, washed and dried. Wool fiber was dyed in the same process as wool powder. The dyeing process was shown in Figure 2 (a). At the maximum absorption wavelength (λ max ) of 410 nm, the absorbance of the initial and residual dye solutions was measured using an ultraviolet-visible spectrophotometer. The dyeing percentage (E) of the wool powder (mg·g −1 ) was calculated as follows: where, A 0 and A represent the absorbances of the initial and residual dye solutions, respectively, and a and b refer to the dilution multiples of the initial and residual dye solutions, respectively.
The dyeing percentage of wool powder and wool fiber was measured via the dyeing residue method (1 g wool, liquor ratio 1:100). The dyeing process was shown in Figure 2 (a). The dyeing residues of the samples at different times were taken out respectively to measure the absorbance, and the dyeing percentage was calculated. The relationship between the dyeing percentage and the dyeing time for the wool powder or wool fiber was plotted as a dyeing rate curve. The equilibrium dyeing percentage and half-dyeing time of the wool powder were obtained from the dyeing rate curve.
The wool powder or wool fiber was dyed at 60°C, 80°C, and 95°C for 4 hours to reach the dyeing equilibrium (1 g wool, liquor ratio 1:100). The dyeing process was shown in Figure 2 (a). The absorption spectrum curve of the dyeing residue was determined by an ultraviolet-visible spectrophotometer, and the dye amount in the dyeing residue was calculated. And then the dye amounts  adsorbed on the wool powder or wool fiber were calculated, respectively. The adsorption isotherm was plotted with the dye amount in the dyeing solution as the abscissa and the dye amount on the wool powder or wool fiber as the ordinate.

Characterization of wool powder and wool fiber
The specific surface areas of wool powder and wool fiber were measured by a surface area and porosity instrument (TriStar II3020, USA) via BET method.
The surface morphologies of the wool powder and wool fiber were observed by scanning electron microscopy (SEM) (JEOL JSM IT300 A, Japan) after sputtering gold under × 500 magnifications. The accelerating voltage was 10 kV.

Specific surface areas of wool powder and wool fiber
The specific surface area of wool powder was 2.79 m 2 ·g −1 , which was significantly larger than that of wool fiber (0.0026 m 2 ·g −1 ), about 1000 times. This was because the wool fiber was ground into wool powder, the amount of powder was much higher than that of wool fiber, and the volume of powder particles was far smaller than that of wool fiber. Therefore, under the same quality, the specific surface area of wool powder increased significantly.

Surface morphologies of wool powder and wool fiber
The morphologies of wool powder and wool fiber are shown in Figure 3. As shown in Figure 3 (a), wool powder was composed of a large number of fluffy powdery substances, and its surface was very irregular. However, for wool fiber, its shape was rod-shaped, and there was a scale layer on the surface of the fiber. When wool fiber was ground into wool powder, wool powder had larger specific surface area, and more active groups were exposed in wool powder (Atef El-Sayed, Salama, and Kantouch 2015), so that wool powder had faster dyeing rate and higher dyeing percentage.

Dyeing rate curve
The dyeing-rate curve is an essential tool for studying the diffusion properties of dyes on wool powder (Rabiei et al. 2012;Riva, Algaba, and Prieto 2002). The slope of the curve represents the dyeing rate. The dyeing-rate curves of wool powder dyed with reactive dyes of the same concentration (10% owf) at 60°C, 80°C, and 95°C are shown in Figure 4(a). The results indicated that at the different temperatures, the dyeing rate was very high in the initial stage of the dyeing process, and the slope of the curve gradually decreased with the increasing treatment time.
As the wool powder had a larger specific surface area than the wool fiber, dye molecules were quickly and easily adsorbed onto the surface of the wool powder during the dyeing process; thus, the powder had a high initial dyeing rate. However, as the scale layer of the wool powder was partially destroyed during the grinding process and more active dye adsorption sites in the wool powder were exposed. There were more the cortical cells on the surface of wool powder, containing much higher αhelix content (Atef El-Sayed, Salama, and Kantouch 2015). The influence of the thermal movement rate of the dye molecules on the dyeing percentage become greater than the influence of the molecule movement rate of dye adsorption on the wool powder at a certain temperature. Thus, the dye molecules moved more intensely in the dyeing bath, facilitating their diffusion and adsorption into the wool powder, which increased the dyeing percentage. In contrast, the influence of the temperature on the dyeing process was not significant. Therefore, when the dyeing equilibrium was reached, the dyeing percentages of wool powder at different temperatures were similar. At this time, the binding force between wool powder and reactive dyes might have the forms of covalent bond, ionic bond, van der Waals force and hydrogen bond.

Half-dyeing time
To elucidate the adsorption mechanism, a pseudo-second-order kinetic model was used to validate the experimental data (Chairat et al. 2005). The equations can be expressed as: where k (g·mg −1 ·min −1 ) is the rate constant and C ∞ and C t represent the amounts of dye adsorbed per gram of wool powder (mg·g −1 ) at equilibrium and at time t, respectively. Integrating Eq.
(2) and applying the initial conditions yields, or equivalently The intercept and slope in the t/C t vs t curve were used to calculate k and C ∞ , respectively. As shown in Figure 4(b), the dyeing of the wool powder with reactive dyes followed the pseudosecond-order model, with a high correlation coefficient (R 2 >0.99). Similar results were reported for various adsorbent-dye systems in literature (Ayranci and Duman 2009;Duman et al. 2016Duman et al. , 2020. The values of k and C ∞ were calculated using equation (4) at three different temperatures. The half-dyeing time and dying rate constants (Rabiei et al. 2012) of C.I. Reactive Yellow 145 at three different temperatures were calculated and presented in Table 1.
The half-dyeing time is the dyeing time required for the amount of dye adsorbed to reach half of the equilibrium adsorption capacity and is a measure of the dyeing speed toward equilibrium in the dyeing of a textile material. A shorter half-dyeing time implies a higher dyeing speed toward equilibrium. The experimental results in Table 1 indicated that the half-dyeing time of the wool powder dyed at different temperatures was very short, and with an increase in the dyeing temperature, the half-dyeing time decreased, while the amount of dye adsorbed per gram of wool powder (C ∞ ) increased. With the increasing dyeing temperature, the thermal movement of the dye molecules intensified, and more dye molecules can enter the interior of wool powder, so the adsorption capacity of the wool powder increased with the increasing dyeing temperature (Chiou, Ho, and Li 2004).

Diffusion coefficient and diffusion activation energy
Different methodologies, which depended on the dyeing process and the physical shape of the materials (Ujhelyiova et al. 2007), were used to determine the diffusion coefficient. The diffusion coefficients of the reactive dyes used for dyeing the wool powder were calculated using Hill's equation. As reported in the literature, the diffusion coefficient D/r 2 describes the diffusion of reactive dyes in wool powder. As shown in Figure 4(c), in the initial stage of dyeing, C t /C ∞ is linearly related to t 1/2 by Eq. (5), and D/r 2 can be determined using the slope of this equation.
Here r represents the radius of the wool powder particles. The diffusion activation energy (E) was calculated using Eq. (6).
Here D 0 is a constant, D t is the diffusion coefficient (cm 2 ·min −1 ) at the absolute temperature T, and R is the gas constant. The values of D/r 2 and the diffusion coefficient of the reactive dye for the wool powder at different temperatures are presented in Table 2.
The diffusion activation energy represents the energy barrier that the dye molecules must overcome when they diffuse into polymer chains (Kim, Son, and Lim 2005). As indicated by Table 2, the diffusion coefficient decreased with the dyeing temperature, and the diffusion activation energy was −11.82 kJ/mol −1 . C. I. Reactive Yellow 145 dye is a reactive dye with double active group structure (Figure 1), while wool powder contains a large number of active amino groups, and they are easy to react with each other to form covalent bonds with the increase of temperature. In this way, a firm bond was formed between wool powder and reactive dye, which hindered the dye from further diffusing into the wool powder, so the diffusion coefficient D decreased with the increase of dyeing temperature. The negative diffusion activation energy indicates a high dye-diffusion rate and suggests that the dyeing of the wool powders occurred spontaneously. With an increase in the dyeing temperature, the diffusion rate decreased.

Adsorption isotherm and standard affinity
The wool powder was dyed with different dosages of reactive dyes at 60, 80, and 95°C. The corresponding adsorption isotherm is shown in Figure 5(a).  According to the different relationships between the concentration of dyes in the powder [D] f and in the dye solution [D] s indicated by the adsorption isotherms, the reciprocal and logarithm of the experimental results were calculated. Then, the results were fitted with a linear regression. The correlation coefficient of the fitting curve is presented in Table 3.
As shown in Table 3, the Langmuir isotherm has a very high correlation coefficient, so Langmuir model is used to fit the experimental data, and the adsorption of the reactive dyes on the wool powder is chemical adsorption. The following equations express the adsorption and desorption rates of reactive dyes in the dyeing process (Kim, Son, and Lim 2005).
where [S] f is the dyeing saturation value; K 1 and K 2 are the rate constants for adsorption and desorption, respectively; and [D] s and [D] f represent the concentrations of reactive dyes in the bath (g·L −1 ) and in the wool powder (g·kg −1 ), respectively, at equilibrium. When the dyeing reaches equilibrium, the rate of adsorption is equal to that of desorption, Langmuir model equations can be expressed as: or equivalently, As shown in Figure 5 With an increase in the dye dosage, the equilibrium adsorption of the dyes increased, until the saturation point was reached. Before the saturation point was reached, the relationship between the concentration on the wool powder and that in the solution was linear. Therefore, the slope was a constant, and it produced a partition ratio (K). The standard affinity between the dye molecules and the wool powder (-Δμº) was calculated using Eq. (11) (Kongkachuichay, Shitangkoon, and Chinwongamorn 2002), where, R is the gas constant; [D] s and [D] f represent the concentrations of the reactive dyes in the bath (g·L −1 ) and in the wool powder (g·kg −1 ), respectively, at equilibrium; and K is the distribution coefficient of the dyes in the dyeing bath and wool powder. Under certain conditions, when the amount of dye on the wool powder no longer increases with the amount of dye in the dye solution, the amount of dye on the wool powder is the dyeing saturation value. The standard affinity is regarded as the driving force in the kinetics of the sorption process (Xu, Tang, and Du 2014). As indicated by Table 3, the saturation value and standard affinity increased with the dyeing temperature. Moreover, at a higher temperature, each instance of dyeing reached its equilibrium point in a shorter duration. This is because the wool powder swelled more violently at a higher temperature, allowing it to absorb more dye molecules. Additionally, the thermal motion of the dye molecules intensified as the dyeing temperature increased, which facilitated the adsorption and diffusion of the reactive dyes into the wool powder.

Dyeing enthalpy and dyeing entropy
According to the Clausius -Clapeyron equation (Eq. (12)) (Chairat et al. 2005), the slope represents the dyeing enthalpy (ΔHº) in Eq. (13), which can be simply determined by the curves of -Δμº/T vs 1/T. ΔHº was calculated using the slope of the line.
Here, C is a constant of integration. Finally, the relationship between -Δμº and T was established, and the dyeing entropy (ΔSº) was determined according to this relationship.
The obtained values of ΔHº and ΔSº were 7.27 kJ·mol −1 ·K −1 and 0.077 kJ·mol −1 , respectively. The positive dyeing entropy suggests that an increase in the temperature led to a higher affinity and adsorption capacity for the reactive dyes at equilibrium (Kim, Son, and Lim 2005).

Dyeing kinetics and thermodynamics of wool fiber
The dyeing rate curve and adsorption isotherm are the basis of analyzing the dyeing kinetics and thermodynamics of textile materials respectively. Many researchers have conducted a comprehensive study on the dyeing kinetics and thermodynamics of wool fiber (Riva, Algaba, and Prieto 2002).
In order to compare the dyeing properties between wool fiber and wool powder, wool fibers were dyed at 60, 80, and 95°C using the same process as wool powder. The dyeing rate curve and adsorption isotherm of wool fiber are presented in Figure 6.
The dyeing rate curve and adsorption isotherm of the wool fiber ( Figure 6) are significantly different from those of wool powder (Figures 4 and 5). The adsorption of reactive dyes on the wool fiber was greatly influenced by dyeing temperature, while the dependence of wool powder on dyeing temperature was little. Wool powder had a huge specific surface area, and a large number of dye molecules could be quickly adsorbed to wool powder, so the influence of dyeing temperature was relatively small (Chen, Chen, and Xing 2002;Nakamura, Ohwaki, and Shibusawa 1995). The dyeing percentage and saturated adsorption capacity of wool fiber were much lower than those of wool powder at different temperatures. This was because the specific surface area of wool powder was far larger than that of wool fiber, and a large number of active groups were exposed on the surface of wool powder. As shown in Figure 2(b), the surface of the wool powder had more adsorption sites than the surface of the wool fiber (Atef El-Sayed, Salama, and Kantouch 2015), so more dyes were adsorbed on the wool powder. Therefore, the adsorption performance of wool powder was significantly higher than that of wool fiber, as confirmed in Sections 3.3 and 3.4.

Conclusions
In this work, the structure and properties of wool powder were carefully compared with those of wool fiber, and the adsorption kinetics and thermodynamics of wool powder dyed with reactive dyes were studied systematically. A pseudo-second-order kinetic model exhibited good agreement with the kinetic behavior of the wool powder adsorbing reactive dyes at different temperatures. The halfdyeing time was very short, and the wool powder can reach adsorption saturation in a short time. The diffusion coefficient decreased with the increase of temperature, and the diffusion activation energy E was −11.82 kJ/mol −1 . The adsorption isotherm for the reactive dyes on the wool powder was Langmuir-type, and the dyeing saturation value and standard affinity increased with the dyeing temperature. The dyeing enthalpy (7.27 kJ·mol −1 ) and dyeing entropy (0.077 kJ·mol −1 ·K −1 ) were determined. Compared with wool powder, wool fiber had smaller specific surface area and poor dyeing properties. The results of this study elucidate the adsorption mechanism during dyeing process and provide a theoretical foundation for the application of wool powder.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Funding
The work was supported by the National Natural