1 Introduction
Azo dyes constitute nearly half of the organic dyes used in the dyeing industry. Azo dyes consist of monoazo, diazo, and triazo classes, among which 70% are monoazo [1]. Azo dyes are widely used in the conventional dyeing process and new dyeing methods involving supercritical dyeing technology [2] and as dyes related to functional applications [3-5]. Environmental and human health issues caused by extensive azo dye usage have promoted research involving decolorization and degradation of azo dyes from dyeing wastewater by methods including adsorption [6-8], biocoagulation [9], and catalysis [10]. For example, recently, SEN et al. [11] reviewed fungal decoloration and degradation of azo dyes and reported that the fungal decoloration mechanism mainly includes adsorption and enzymatic degradation. Rapid oxidation physicochemical methods have also been proposed as effective means for dye wastewater treatment [12-16].
Ionizing radiation is a method using high energy rays or charged particles to induce the formation of ions, excited molecules, and free radicals. Types of ionizing radiation include gamma-ray, electron beam (EB), and ultraviolet (UV) radiation. For example, UV radiation is important for plant growth and metabolism [17]. However, overexposure to UV radiation can break chemical bonds and cause severe health problems [18]. Molecular structures can be broken and new chemical bonds formed during ionizing radiation exposure [19]. Therefore, changes in the molecular structure under ionization radiation must be analyzed considering that ionizing radiation has been reported as an effective means for dye degradation [20, 21], textile coloration [22], and fabric functionalization [23, 24]. In high-energy-radiation-induced coloration or degradation processes, dyes are placed under radiation to generate radicals between dye and solvent molecules to form new chemical bonds, which inevitably influence the dye structure. Therefore, understanding the structural stability of azo dyes under ionizing radiation is vital. During decolorization, azo dyes have been reported to release aniline groups, which are carcinogenic and harmful to human health and ecological balance [11, 25]. However, changes in chemical structures during reduction and decoloration have not attracted extensive attention [26]. Porobić et al. [27] synthesized a pyridone azo dye, investigated its thermal decomposition mechanism, and found the relationship between thermal stabilities and chemical structures of the dye. Kiayi et al. [28] used Saccharomyces cerevisiae to degrade carmoisine and demonstrated that azo dyes are degraded into aromatic amines. Mu et al. [29] found that AO7 is cleaved into sulfanilic acid and 1-amino-2-naphthol by irradiation. However, changes in the azo dye molecular structure by ionizing radiation have not been extensively reported. Among azo dyes, Disperse Blue 79, a monoazo, has been widely produced and used in polyester [30] and textile applications [31], such that radiation-induced molecular structural changes of Disperse Blue 79 have become representative of azo dyes. Weber et al. [1] reported the chemical reduction of Disperse Blue 79 in a water system and demonstrated that Disperse Blue 79 results in 2-bromo-4,6-dinitroaniline. Hou [32] investigated the crystal morphology of Disperse Blue 79. However, till date, structural changes of Disperse Blue 79 by ionizing radiation have not been reported.
Gamma rays or EB are commonly used ionizing radiation sources. Gamma rays have high penetrability (≥10 cm in water) but a low dose rate (≤10 kGy∙h-1), while EB has a high dose rate (5 kGy∙s-1) but low penetrability (≤5 mm in water). Both can generate ions and radicals and result in changes in the molecular structure of a compound [33]. This study focused on the molecular structural stability of Disperse Blue 79 irradiated by gamma rays and EB. Pure Disperse Blue 79 in aqueous solution, prepared by dissolving in deionized water, or powder form was irradiated by gamma rays or EB. Moreover, commercial Disperse Blue 79 in aqueous solution and powder form was irradiated by gamma rays and EB. Changes in the chemical structure were characterized by UV-Vis, NMR, and mass spectrometry to clarify the structural differences between dyes in aqueous solution and powder irradiated by gamma rays and EB. The effects of the absorbed dose on the structures and properties of the dyes were studied. Lab* values of the dyes were measured to study chromaticity variations arising due to different absorbed doses and radiation types.
2 Experimental section
2.1 Chemicals and Instruments
Commercial Disperse Blue 79 was obtained from Zhejiang Greenland Textile Technology Co., Ltd. (Wenzhou, China). Pure Disperse Blue 79 (99% pure) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The cotton fabric of GB/T7568.2-2008 was obtained from the Shanghai Textile Industry Institute of Technical Supervision (Shanghai, China). All reagents were used without further purification.
Lab* values of the samples were measured using a colorimeter (NR10QC, 3nh, Shenzhen, China). Mass spectrometry was conducted on an Agilent QTOF 6540 instrument (Agilent Technologies, Inc., Santa Clara, CA, USA) in deionized water with an electrospray ionization interface (ESI) and data were analyzed by the MestReNova software (Mestrelab Research S.L.). NMR spectrometry was performed on an Avance 500 instrument (Bruker Corp., Billerica, MA, USA) with 500 MHz in deuterated dimethylsulfoxide (DMSO-d6), at a concentration of 4 g/L; data were analyzed by MestReNova. UV-Vis spectrometry was performed on a UV-3010 spectrophotometer (Mettler-Toledo International Inc., Greifensee, Switzerland) in the range of 200–800 nm in deionized water.
2.2 Dye irradiation
2.2.1 Gamma-ray irradiation
The Disperse Blue 79 powder was dissolved in deionized water to a concentration of 10 g/L in a glass bottle and ultrasonicated for 5 min to accelerate dissolution. The solution was irradiated under a 60Co gamma-ray source for 17 h with an absorbed dose of 10, 50, or 150 kGy. The irradiated solution was placed into a vacuum oven for water removal at 60°C. Powder irradiation was performed by placing 1 g of the dye powder in a polythene bag directly under the gamma-ray source.
2.2.2 EB irradiation
The Disperse Blue 79 powder was dissolved in deionized water with a concentration of 10 g/L in a polythene bag and ultrasonicated for 5 min to accelerate dissolution. The bag was placed under a 1.2 MeV electron accelerator for irradiation with an absorbed dose of 10, 50, or 150 kGy. The irradiated solution was placed into a vacuum oven for water removal at 60°C. Powder irradiation was performed by placing 1 g of the dye powder in a polythene bag directly under the EB source.
3 Results and discussion
3.1 Irradiation of Disperse Blue 79
Commercial dyes usually contain additives [34, 35], including dispersants and diluents, in the same-class dyes with different chemical structures that are mixed in various proportions by dye manufacturers. The main structure of Disperse Blue 79 is shown in Scheme 1. The halogen substituent and alkyl groups could be varied during synthesis by manufacturers to obtain desired properties including temperature and pH sensitivities, solubility in defined solvents, compatibility with fabric, and thermal stability. Thus, it is necessary to investigate radiation-induced effects on both commercial and pure Disperse Blue 79. Water is a common solvent for dye dissolution and reaction. Pure and commercial Disperse Blue 79 dye powders were dissolved in water and irradiated with gamma rays or EB at absorbed doses of 150 kGy to study dye structural changes occurring due to decomposition [36, 37] and radical formation [38, 39] that were induced by ionizing radiation of water. Also, pure and commercial Disperse Blue 79 powders were irradiated directly by gamma rays or EB with absorbed doses of 150 kGy to investigate the influence of radiation on dyes during solid-phase radiation-induced applications.
3.2 Characterization and analysis of decomposition
3.2.1 UV-Vis results
UV-Vis spectra showed that there were no significant peak changes for pure and commercial Disperse Blue 79 powders undergoing direct irradiation (Fig. 1a and 1b). The observed difference was that λmax characteristic peaks of the azo aromatic chromophore [40, 41] of the commercial dye exhibited a hypsochromic shift compared with that of the pure dye. Notably, the absorption peak of pure and commercial dyes in aqueous solution at 228 nm significantly decreased while that at 210 nm increased after gamma-ray and EB irradiation (Fig. 1c and 1d). This indicated that there were cleavages and formation of higher energy electronic transition structures, such as C-H and C-C bonds [42]. These spectra indicated that both dyes experienced structural changes when irradiated in aqueous solution by gamma rays and EB but remained stable in the powder form.
3.2.2 NMR results
1H-NMR spectra of Disperse Blue 79 dyes irradiated with an absorbed dose of 150 kGy are shown in Fig. 2 and detailed data of pristine pure listed in Table 1. Each site of C and related H are marked in Scheme 1. Except for water and DMSO peaks at 2.5 and 3.4 ppm, the 5 site related to the methyl group at the end of amide did not show a distinct single peak from 1.8 to 2.0 ppm (Fig. 2a). Similarly, 3 and 3’ sites related to methylene adjacent to a tertiary amine also merely showed a single weak peak at 3.82 ppm. In the commercial Disperse Blue 79, methoxyl did not show a distinct peak (Fig. 2b). The dyes in powder form irradiated with absorbed doses of 150 kGy maintained their original peaks, similar to pristine dyes of both commercial and pure Disperse Blue 79 under gamma-ray and EB irradiation (Fig. 2a and 2b), which confirmed that the Disperse Blue 79 powder successfully resisted gamma-ray and EB irradiation, as confirmed by UV-Vis analysis. The NMR spectra of pure Disperse Blue 79 in aqueous solution irradiated by gamma rays and EB with the absorbed dose of 150 kGy are shown in Fig. 2c. The peak area changes were calculated and compared to confirm the decomposition by ensuring same concentrations in DMSO-d6. It was found that methyl at the 1’ and 1 as well as at 2’ and 3’ sites split to two peaks. Furthermore, methoxyl at the 4 site shifted. The peak area changes indicated that methoxyl and acetoxy methyl of pure Disperse Blue 79 were decomposed in aqueous solution, which is illustrated as the "First One-step" decomposition process in Scheme 1. The commercial Disperse Blue 79 in aqueous solution experienced the same degradation paths when irradiated by gamma rays and EB. The split peak at the 1’ site was identical to that observed for the pure dye (Fig. 2d). Thus, 1H-NMR spectra demonstrated that pure and commercial Disperse Blue 79 experienced the "First One-step" decomposition process when irradiated in aqueous solution and maintained stable structures when irradiated in powder form.
| δH/ppm (J/Hz) | H | Assignment |
|---|---|---|
| 1.98(s) | CH3, CH3 | 1,1' |
| 1.81-2.02(s) | CH3 | 5 |
| 3.51(s) | CH3 | 4 |
| 3.82(t) | CH3, CH2 | 3,3' |
| 4.25(t, 3J/6.1) | CH2, CH2 | 2,2' |
3.2.3 Mass spectrometry results
The mass spectra of pure and commercial Disperse Blue 79 irradiated in aqueous solution are shown in Fig. 3. The mass-to-charge ratio of the newly formed ion at 417 for pure Disperse Blue 79 in aqueous solution irradiated with gamma rays and EB (Fig. 3a, 3b, and 3c) indicated that another carboxyl had decomposed (step_3) on the basis of the First One-step decomposition process concluded from the NMR results. This path is illustrated as the "Second One-step" degradation process in Scheme 1. In the commercial Disperse Blue 79 mass spectra (Fig. 3d, 3e, and 3f), there were no distinct new peaks in aqueous solution of EB-irradiated dyes, but decreased intensity of the main structure was observed. However, a distinct mass-to-charge ratio at 507 in gamma-ray-irradiated aqueous dyes revealed that the methoxyl and acetyl groups were likely degraded (step_1). Also, a mass-to-charge ratio at 477 demonstrated that fragments at 507 could experience further degradation by splitting a methoxyl group (step_2). Although Disperse Blue 79 partly in aqueous solution decomposed on irradiation, the main N=N structure of the azo molecule resisted irradiation. This meant that the methoxyl and acetyl groups were more easily attacked than the C-N and N=N bonds in the azo molecule by reactive radicals generated in irradiated water, which showed different cleavage pathways that were previously reported [43]. The mass spectra of pure and commercial Disperse Blue 79 dyes irradiated in powder form are shown in Fig. 4. The detailed mass-to-charge ratios of the molecule are listed in Table 2. It was clearly observed that the pure (Fig. 4a and 4b) and commercial dyes (Fig. 4c and 4d) in powder form maintained stable structures, identical to pristine dyes (Fig. 3a and 3d), after irradiation. The results agreed with UV-Vis and NMR results. Furthermore, the mass-to-charge ratios at 625 and 647 in the commercial Disperse Blue 79 indicated that chlorine was partly replaced by bromine (Table 2), which further demonstrated that commercial products usually contain mixed dye compositions. The stability of Disperse Blue 79 in powder form revealed that oxygen in air did not have a distinct effect on the dyes [44] or form oxidized products [45, 46] during irradiation. Overall, the mass spectrometry results matched NMR and UV-Vis spectra and indicated that pure and commercial Disperse Blue 79 partly decomposed when irradiated in aqueous solution and were stable as powder under gamma-ray and EB irradiation. The pure dye mainly experienced the Second One-step process while the commercial dye underwent a stepwise path. However, the decomposition paths of both dyes proved that acetoxy and methoxyl groups were easily decomposed under irradiation in aqueous solution.
| Formula | Calculated mass | Double bond equivalence | Adduct | Charge state | m/z |
|---|---|---|---|---|---|
| C23 H25 N6 O10 Cl | 580.1 | 14.0 | H+ | +1 | 581.1 |
| Na+ | +1 | 603.1 | |||
| K+ | +1 | 619.1 | |||
| C20 H20 N6 O8 Cl | 506.1 | 13.0 | H+ | +1 | 507.1 |
| C19 H18 N6 O7 Cl | 476.2 | 13.0 | H+ | +1 | 477.2 |
| C17 H14 N6 O5 | 440.6 | 12.0 | - | - | 440.6 |
| C17 H15 N6 O5 Cl | 417.3 | 12.0 | - | - | 417.3 |
| C23 H25 N6 O10 Br | 624.0 | 14.0 | H+ | +1 | 625.1 |
| Na+ | +1 | 647.1 |
3.3 Chromaticity effects of irradiated dyes
The investigation of chromaticity changes of dyes is important for both dye degradation and coloration applications [47]. Herein, pure and commercial Disperse Blue 79 in aqueous solution and powder were irradiated while the absorbed dose was increased from 0 to 150 kGy. The irradiated dyes were diluted to 1 g/L and dripped on fabric surfaces to test color variations. The Lab* method is commonly used to quantitatively define the chromaticity changes [47, 48], in which L* value represents the luminosity, a* the red/green opponent colors, and b* the yellow/blue opponent colors. The results revealed that the chromaticity of pure Disperse Blue 79 turned to dark yellow from original blue after irradiation by gamma rays and EB in aqueous solution (Fig. 5). Original blue deeply faded with increased absorbed dose, which indicated that pure Disperse Blue 79 decomposed significantly with the increased absorbed dose. But, the color almost did not change after irradiation in powder. The commercial Disperse Blue 79 was also decomposed under irradiation in aqueous solution but maintained its original color in powder form. The values of a* and b* of the dyed fabric were measured to define the color changes of Disperse Blue 79. The results showed that for pure Disperse Blue 79, a* was almost constant but b* increased after irradiation in aqueous solution (Fig. 6a and 6b). The higher value of b* indicated that the dye color turned from blue to yellow, which was in accordance with color changes shown in Fig. 5. Notably, the commercial Disperse Blue 79 showed red color when dissolved in aqueous solution but similar chromaticity changes were observed with irradiation (Fig. 5, 6c, and 6d). The chromaticity changes confirmed that commercial and pure Disperse Blue 79 in aqueous solution experienced structural changes on gamma-ray and EB irradiation but mostly maintained original structures in powder form on gamma-ray and EB irradiation at an absorbed dose of 150 kGy. In addition, the increased absorbed dose increased dye decomposition. Moreover, the results indicated that the decomposition of acetoxy and methoxyl groups could influence dye chromaticity.
3.4 Effects of absorbed doses on irradiated dyes
UV-Vis spectra of dyes in aqueous solution and powder irradiated with the absorbed dose of 0–150 kGy were measured to investigate the effect of the absorbed dose. The absorption values at 228 and 210 nm were calculated to compare peak area changes (Fig. 7). The results showed that pure and commercial dyes in aqueous solution undergo structural changes on gamma-ray and EB irradiation with an absorbed dose of 10 kGy and remained balanced over 50 kGy, indicating low absorbed dose effects on irradiation [49]. In contrast, the dyes in powder form were stable at all doses. The results indicated that both dyes could be decomposed with a relatively low absorbed dose, which indicated the instability of Disperse Blue 79 when irradiated under aqueous conditions.
4 Conclusion
Pure and commercial Disperse Blue 79 in aqueous solutions were irradiated by gamma rays and EB to investigate the effect of irradiation on the dye structure. Moreover, both powder dyes were irradiated to study the influence of air. UV-Vis spectra showed that pure and commercial Disperse Blue 79 in aqueous solution experienced structural changes when irradiated by gamma rays and EB but notably remained stable in powder form. NMR results indicated that the structural change of pure Disperse Blue 79 could be attributed to the First One-step decomposition path. Mass spectrometry results demonstrated that on irradiation of pure Disperse Blue 79, carboxyl groups were decomposed or directly underwent the "Second One-step", but the commercial Disperse Blue 79 experienced stepwise decomposition paths by splitting methoxyl and acetyl groups at step_1 and another methoxyl at step_2. All results indicated that gamma rays and EB has almost the same effect on the dye structure. Furthermore, chromaticity analysis showed that the color of Disperse Blue 79 turned to dark yellow when irradiated in aqueous solution and maintained its original color (blue) when irradiated in powder form. The dispersed dye was extremely unstable in aqueous solution when irradiated by gamma rays and EB with an absorbed dose of 10 kGy.
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