Effect of NaCl concentrations on the photodecoloration of reactive azo-dyes and their cotton dyeings

Abstract

The color fastness of reactive cotton dyeings under simultaneous exposure to light and saline water, as well as light and perspiration, is one of the most important characteristics especially for summer-time sportswear. In order to illustrate the dual effects (i.e. favorable or adverse) of saline water on the photofading of reactive azo-dyes, the degradation mechanism and reaction kinetics of dye photodecoloration in the presence of various NaCl concentrations were investigated in simulated experiments. Experimental results indicated that the photodecoloration of dye solution was mainly attributed to the hydroxyl radical. Due to the quenching of the hydroxyl radical by the chloride ion, the dye photodecoloration rate decreased with low NaCl concentration (<10 g/L). Nevertheless, higher NaCl concentration (>50 g/L) was beneficial to produce reactive chlorine species, resulting in a significant increase of the photodecoloration rate. Moreover, reaction kinetic analysis demonstrated that the observed first-order rate constant for the photodecoloration of dye solution consistently increased with the increasing NaCl concentration.

Keywords

light irradiation sodium chloride reactive dye decoloration reactive chlorine species

Reactive azo-dyes are the main dyes used for cotton fabrics because of their brilliant colors and excellent color fastness to washing and rubbing. Nevertheless, many reactive azo-dyes with good light fastness show poor perspiration-light stability. The photofading mechanism of reactive azo-dyes under simultaneous exposure to light and perspiration has been investigated in a number of researches.^1–4 Imada et al.¹ suggested that a photoreduction mechanism was involved when reactive dyeings faded under the combination of light and perspiration, while a photooxidation mechanism operated under light only. Csepregi et al.² stated that photofading after the consumption of reducing agents (i.e. organic components in perspiration) was caused by an oxidative mechanism. Aranyosi et al.^3,4 observed that in the joint presence of dissolved oxygen and perspiration, the reactive dyeings underwent both photoreduction and photooxidation in the initial step, and subsequently underwent photoreduction. Thus, in many cases, the type of the photochemically induced redox reaction highly depends on the environmental conditions of exposure and the perspiration components play a significant role in the dye photofading.^5–9 According to the AATCC (American Association of Textile Chemists and Colorists) standard method, perspiration is a saline fluid secreted by the sweat glands and includes amino acids and inorganic salts. Histidine^5,6 and lactic acid^7–9 acted as reducing agents and were suggested to promote the photoreduction of reactive azo-dyes. However, the effect of inorganic compounds, especially the main component NaCl, on the photofading of reactive cotton dyeings is still unclear.

Generally, NaCl was believed to increase the aggregation of ionic dyes,¹⁰ leading to the suppressive effect on the photodecoloration of dyes in aqueous solution. The survey of literature also revealed that NaCl could scavenge the hydroxyl radical (•OH) and dramatically retard the degradation rate of organic compounds during advanced oxidation processes (AOPs), such as Fenton,¹¹ ozone,¹² UV/H₂O₂¹³ and UV/TiO₂.¹⁴ Conversely, a series of recent investigations contradicted the previous results and demonstrated that high NaCl concentration significantly enhanced the dye degradation in the AOP system.^15,16 However, most of these works were mainly focused on the effect of NaCl on dye decoloration in AOPs, rather than the direct photochemical decoloration of dyes without any chemical treatments.

In the AATCC standard method, the NaCl concentrations in seawater and perspiration are 30 and 10 g/L, respectively. However, a higher NaCl concentration is generated and even NaCl crystallizes under light irradiation as a result of the moisture evaporation. Thus, in the present work, effects of various NaCl concentrations, including maximum concentrations near saturation level, on the dye photodecoloration process are studied both on cotton fabric and in aqueous solution. In order to understand the degradation kinetics and reaction mechanism, the photodecoloration of dye solutions with various NaCl concentrations is investigated by using an ultraviolet-visible (UV-vis) spectrophotometer and electron paramagnetic resonance (EPR) spectrometer. Then, the topography of reactive cotton dyeings in the presence of NaCl is also studied using scanning electron microscopy (SEM) to further illustrate the dual effects (i.e. favorable or adverse) of saline water on the photofading of reactive cotton dyeings.

Experimental details

Chemicals

Desized and bleached plain woven cotton fabric weighing 121.8 g/m² was used.

The azo-reactive dye RB5 (C.I. Reactive Black 5) was provided by Transfar Group Co., Ltd (Hangzhou, China) and recrystallized from dimethylformamide and diethylether.¹⁷ The chemical structure of the dye RB5 is shown in Scheme 1. NaCl, sodium sulfate, sodium carbonate and sulfuric acid were of analytical purity and were used as received from Sinopharm Chemical Reagent Co., Ltd. High purity water (resistivity = 18.2 MΩ·cm) made by a Master-S plus UVF ultra pure water system (Hitech Instruments Co., Ltd, China) was used throughout the study.

Scheme 1.

The chemical structure of C.I. Reactive Black 5.

Photoreactor and light source

The photoreaction system (Nanjing Xujiang Electromechanical Plant, China) was specially designed for this experiment, as shown in Figure 1. A 500 W xenon lamp (Beijing Lighting Research Institute, China), used for dye photoreaction, was positioned inside a cylindrical Pyrex glass vessel surrounded by a circulating water jacket. The system was also used for testing the colorfastness of reactive cotton dyeings to perspiration and light, and the light intensity measured on the rotating platform was 123 µW/cm² (290–390 nm) and 3600 lx using a UV-340 A digital instrument (Lutron Electronic Enterprise Co., Ltd, Taiwan) and TES 1334 digital lighter meter (TES Electrical Electronic Corp., Taiwan), respectively.

Figure 1.

The schematic diagrams of photoreaction systems: (a) for dye solutions; (b) for dyed cotton fabrics.

Experimental procedures

Preparation of reactive cotton dyeings

The dyebath (400 mL) was prepared at room temperature with reactive dye RB5 (0.5% o.w.f.) and sodium sulfate (10 g), in which cotton fabric (10 g) was immersed, and then the dyebath was stirred continuously during the whole procedure. The temperature of the bath was raised to 60℃ and kept for 10 min. After the addition of 4 g sodium carbonate, the dyeing was held for a further 60 min. Subsequently, the dyed cotton fabric was removed from the dyebath, rinsed with cold water and soaped in 2 g/L standard soap solution at the boiling temperature for 10 min. Finally, the cotton fabric was rinsed at 60℃ to remove unfixed dyes and air-dried at room temperature.

Light irradiation of dye solutions

Because the crystallization of inorganic salts generated on the surface of cotton due to the sweat production and moisture volatilization in practice (especially in summer time), a series of NaCl concentrations from 0 to 300 g/L were used in dye solutions. All of the light irradiation experiments of the RB5 solution were conducted in the photoreactor as described above with a circulation water of 25 ± 2℃. A total of 200 mL 20 mg/L dye solutions (pH = 6.5) with different NaCl concentrations were placed into a 250 mL cylindrical reaction vessel, and all photoreactions were carried out in air-saturated conditions. At 2 h intervals, about 5 mL of solutions were collected and their UV-vis absorptions were measured using a UV-3300 Spectrophotometer (Hitachi, Japan).

Light irradiation of reactive cotton dyeings

Each piece of dyed fabrics was sewn with one layer of cotton fabric to form a testing sample. The sample was immersed into the NaCl solution for 30 min and then padded to reach a wet pickup of 100%. Irradiation was carried out using a 500 W xenon lamp in the reactor described above. The testing temperature and relative humidity were 37℃ and 40%, respectively. During light exposure, all samples underwent a wet-to-dry process and were taken for analysis after 20 h irradiation.

Analysis methods

UV-vis spectrophotometer

The UV-vis absorption of dye solution ranging from 200 to 800 nm was measured using the UV-vis Spectrophotometer described above. According to the Beer–Lambert law, the concentrations can be calculated by the absorbance values of RB5 solutions with respect to their characteristic absorption peak at 598 nm. The decoloration percentage of RB5 was calculated using the following equation:

Decoloration (%) = (C_{0} - C) / C_{0} \times 100 %

(1)

where C₀ and C are the initial and residual concentrations of reactive dye, respectively.

Electron paramagnetic resonance

5,5-Dimethyl-1-pyrroline-N-oxide (DMPO) (Matrix Scientific, USA) was used as the spin trapping agent to react with radicals, such as hydroxyl radical (•OH), to form relatively stable spin adducts (nitroxides).¹⁸ A total of 100 mL of 50 mM DMPO aqueous solution was added to 100 mL RB5 solution with a dye concentration of 20 mg/L. The solutions were freshly prepared in the dark before the experiments. Then, they were placed in quartz vessels and irradiated in a XPA-Photochemical Reactor (Nanjing Xujiang Electromechanical Plant, China) with an 800 W xenon lamp. Following 10 min of irradiation, 40 µL of samples were taken into quartz capillary tubes and measured using a Bruker BioSpin GmbH EPR spectrometer (microwave frequency: 9.87 GHz, modulation amplitude: 1.00 G, sweep time: 41.943 s, number of scans: 5).

Color difference value

The whole photofading behavior of dyed fabrics was studied by measuring their color coordinates on a Datacolor SF650 Color Measurement Spectrophotometer (Datacolor, USA). The color difference value (ΔE_CMC) between the samples before and after exposure was calculated automatically.

Scanning electron microscopy

In order to simulate the real situation, effects of various NaCl concentrations, including maximum concentrations near saturation level, on the dye photodecoloration process were studied on cotton fabric. However, NaCl might crystallize under light irradiation when a high NaCl concentration was used. Thus, the topography of cotton fabric dyed with RB5 after 20 h of irradiation were observed using TM-1000 scanning electron microscope (Hitachi, Japan).

Results and discussion

Effect of NaCl concentrations on the photodecoloration of dyes

The photodecoloration of RB5 solutions (20 mg/L) with different NaCl concentrations under different irradiation time were measured and are shown in Figure 2. The photodecoloration of RB5 is inhibited when the concentration of NaCl is 10 g/L or lower, which is also described in the previous work of Li et al.⁹ However, an unexpected phenomenon is found when the NaCl concentration is higher than 50 g/L, which is not reported previously, that is, the dye photodecoloration significantly increases with the increasing in NaCl concentration. As shown in Figure 2, after 20 h irradiation the dye decolorations reach 13.7%, 19.4% and 39.2% with the addition of NaCl concentration of 50, 100 and 300 g/L, respectively, while it is only 10.2% without NaCl addition.

Figure 2.

Effects of NaCl concentrations on the photodecoloration of RB5 solution.

Proposed mechanism of dye photodecoloration in the presence of NaCl

In the aqueous environment, photoejection of electrons and hydroxyl radical (•OH) induced reaction were the main mechanisms for the photofading of reactive azo-dyes, as proposed by Datyner et al.¹⁹ In order to confirm the mechanism of NaCl concentrations on the photodecoloration of dyes, spin trapping agent DMPO was used to react with radicals that may be present in the RB5 solutions with 0, 1 and 300 g/L NaCl, respectively. As shown in Figure 3(a), the EPR signal of the DMPO-OH adduct is detected in the RB5 solution without the addition of NaCl using the EPR spectrometer. However, the signal intensity of the DMPO-OH spin adduct decreases in the presence of 1 g/L NaCl (Figure 3(b)), and almost disappears with the addition of 300 g/L NaCl (Figure 3(c)). Thus, the photodecoloration of dye solutions is mainly attributed to the presence of •OH. In the condition of low NaCl concentration (<10 g/L), the inhibition effect of NaCl on the dye photodecoloration is caused by the scavenging effects of the chloride ion (Cl $-$ ) with •OH (Equations (2)–(3)).^20–22 Another possible explanation is that the addition of NaCl is supposed to increase the aggregation of reactive dyes in aqueous solution and weaken the reaction capability of dye molecules with •OH.²³

Figure 3.

Electron paramagnetic resonance spectra of the DMPO-OH spin adduct produced by irradiation of RB5 solutions containing NaCl with various concentrations: (a) 0 g/L; (b) 1 g/L; (c) 300 g/L.

The much lower signal intensity of the DMPO-OH spin adduct that was detected in the high NaCl concentration (300 g/L) demonstrates that the dye photodecoloration is not caused by •OH. As a consequence, the fact that the addition of NaCl with higher concentration (>50 g/L) greatly accelerates the dye photodecoloration must be corresponding to some other reaction mechanisms. From the thermodynamic point of view, •OH can react with Cl $-$ to produce a series of reactive chlorine species (i.e. Cl•, Cl₂ $• -$ , Cl₂ and HClO), that is, the effect of reactive chlorine species should be taken into account when •OH is present. The equilibrium constant associated with Equation (2) is 0.7,²⁴ implying that it is a reversible reaction tending to run backward. In this case, Cl $-$ would just serve as a means of slowing down the reactions of •OH by “stabilizing” it in the form of ClOH $• -$ . With the addition of a higher concentration of Cl $-$ , both Equations (2) and (3) will be pulled forward. Once the chlorine atom (Cl•) is formed, it will react with Cl $-$ immediately (Equation (4)) to form chlorine radical anions (Cl₂ $• -$ ). The bimolecular reaction between two Cl₂ $• -$ anions produces chlorine (Cl₂) (Equation (5)), which can combine with H₂O to generate hypochlorous acid (HClO) (Equation (6)):²⁵

• OH + Cl - ⇌ ClOH • -

(2)

ClOH • - + H + ⇌ H_{2} O + Cl •

(3)

Cl • + Cl - ⇌ {Cl}_{2}^{• -}

(4)

{Cl}_{2}^{• -} {+ Cl}_{2}^{• -} \to {Cl}_{2} + 2 Cl -

(5)

{Cl}_{2} {+ H}_{2} O \to HClO + H + + Cl -

(6)

Therefore, the increase of Cl $-$ concentration is beneficial to the formation of reactive chlorine species, Cl•/Cl $-$ (2.4 V), Cl₂ $• -$ /2Cl $-$ (2.1 V),²⁶ Cl₂/2Cl $-$ (1.36 V) and HClO/Cl $-$ (1.48 V),²⁷ which have enough activity to decompose unsaturated dye molecules.^28–30 Furthermore, the equilibrium constant of Equation (4), which can be calculated from the rate constant, is 1.4 × 10⁵, implying that Cl₂ $• -$ will be the dominant reactive chlorine.²⁴ The reaction rate of •OH with azo-dye is 100–1000 times as fast as that of azo-dye with Cl₂ $• -$ _,²⁹ but the lifetime of Cl₂ $• -$ is much longer,³⁰ which thus makes them more efficient for dye photodecoloration.

Reaction kinetics of dye photodecoloration in the presence of NaCl

The UV-vis absorbance spectra of photodecoloration of RB5 in the presence and absence of NaCl were recorded as shown in Figure 4. The characteristic absorbance of RB5 solution shifts from 598 to 584 nm in the visible area when the concentration of NaCl is 300 g/L, which can be explained by the reduction of electrostatic repulsion and the improvement of dye aggregation. During light irradiation, apparent changes in the visible region caused by the dye decoloration can be seen in Figure 4(a). On the other hand, the absorption curve of RB5 solution changes more slowly without addition of NaCl (Figure 4(b)).

Figure 4.

Ultraviolet-visible spectra of RB5 in the presence and absence of NaCl: (a) 300 g/L NaCl; (b) 0 g/L NaCl.

For a steady-state approximation of the •OH radicals, the photodecoloration of RB5 was quantitatively studied by using apparent first-order reaction kinetic:

- d C / d t = kC

(7)

where k is the apparent reaction rate constant and C is the concentration of aqueous RB5 at the reaction time t. The apparent rate constant (k) of RB5 is determined from the slope of ln(C₀/C) (where C₀ is the initial concentration of aqueous RB5) as a function of time.

As shown in Figure 5, all photodecoloration reactions follow an apparent first-order kinetics model very well by the linear plots. The kinetic parameters are given in Table 1. It can be seen that the apparent first-order rate constant for RB5 photodecoloration is proportional to the concentration of NaCl. The rate constant for RB5 photodecoloration at low NaCl concentration (<10 g/L) is lower than that without NaCl addition, but it is increased to 2.15 × 10⁻² h⁻¹ when the NaCl concentration is raised to 300 g/L. It is of interest that the rate constant for RB5 photodecoloration at the NaCl concentration of 300 g/L is almost four times higher than that without NaCl addition.

Figure 5.

The photodecoloration kinetics of RB5 in the presence of different concentrations of NaCl.

Table 1.

Apparent first-order reaction kinetic parameters

Concentration of sodium chloride (g/L)	Reaction rate constant k (h⁻¹)	Correlation coefficient R²
0	5.21 × 10⁻³	0.988
0.5	3.85 × 10⁻³	0.965
1.0	4.02 × 10⁻³	0.965
10.0	4.19 × 10⁻³	0.987
50.0	7.15 × 10⁻³	0.989
100.0	9.81 × 10⁻³	0.962
300.0	2.15 × 10⁻²	0.962

Effects of NaCl concentration on the photofading of reactive cotton dyeings

Effects of NaCl concentrations (0, 5, 10, 20, 50 and 100 g/L) on the photofading of reactive cotton dyeings were also studied and the results are depicted in Figure 6. The photofading of reactive cotton dyeings is not significantly inhibited or accelerated in the NaCl concentration of 5 g/L. However, with the increase of NaCl concentration from 10 to 100 g/L, the dye photofading is dramatically accelerated at the same level.

Figure 6.

Effects of various concentrations of NaCl solution on the photofading of reactive cotton dyeings.

Under the light irradiation, the initially wet cotton dyeings will dry within 10 min of exposure in the open system. However, due to the moisture regain of cotton fiber, the reactive cotton dyeings will not be absolutely dry. Consequently, the concentration of NaCl that dissolved in the amorphous area of cotton fibers will be stable after evaporation of moisture. This can be confirmed by the SEM photos of reactive cotton dyeings. As observed in Figures 7(a) and (b), smooth surface can be clearly observed on the surface of cotton fiber with addition of 5 g/L NaCl solution. However, several NaCl crystalline particles on the fiber surface can be seen in the presence of 10 g/L NaCl (Figure 7c), and more NaCl particles appear when the NaCl concentration increases further, as displayed in Figures 7(d)–(f). That is, the NaCl concentrations from 10 to 100 g/L, other than NaCl concentration of 5 g/L, result in a saturation level after light irradiation. Consequently, the increase of NaCl concentration from 10 to 100 g/L almost plays the same role in the accelerated photofading of each reactive cotton dyeings, which further confirms that high NaCl concentration promotes the acceleration on the dye photofading, as discussed for aqueous solutions.

Figure 7.

Scanning electron microscope images of samples with different NaCl concentrations after 20 h irradiation: (a) 0 g/L; (b) 5 g/L; (c) 10 g/L; (d) 20 g/L; (e) 50 g/L; (f) 100 g/L.

Conclusions

A further understanding of the effect of sodium chloride (NaCl) on the photodecoloration of both reactive azo-dyes and their dyeings can be gained from recent work. The photodecoloration of dye solution is mainly attributed to the hydroxyl radical. Low Cl $-$ concentration is found to have inhibitory effects on dye decoloration by scavenging •OH. On the other hand, the degradation rate of RB5 significantly increases with high NaCl concentration (>50 g/L), owing to the constant formation of reactive chlorine species. The apparent first-order rate constant for dye photodecoloration increases with increasing Cl $-$ concentration. In addition, the effects of NaCl concentration on the photofading of reactive cotton dyeings are well illustrated by the degradation kinetics and reaction mechanism obtained from simulated experiments in aqueous solution.

Footnotes

Funding

This work was supported by Transfar Group Co., Ltd.

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