Degradation of sericin (degumming) of Persian silk by ultrasound and enzymes as a cleaner and environmentally friendly process
Introduction
Persia has been known by the Silk Road, and its civilization is closely attached to magnificent silk carpets with amazing designs and colors [1]. Silk has been used as a textile fiber for over 5000 years [2], [3]. Natural raw silk is composed mainly of sericin (22–25%), fibroin (62.5–67%), water, and mineral salts. Fibroin is a single protein that is insoluble in hot water. Sericin is primarily amorphous and acts as a gum binder to maintain the structural integrity of the cocoon, so sericin is more water-soluble than fibroin [2], [4], [5], [6]. This difference makes the gum easily removable from the filaments through various processes without considerable damage to the filaments [7], [8]. Sericin gives a harsh and stiff feeling to the fiber and hides the rich luster and whiteness of silk. In addition, it prevents the penetration of dye liquor and other solutions during wet processing to silk and produces colored textile wastewater. The discharge of colored textile wastewater into natural streams has caused many concern problems, such as increasing the toxicity and COD (chemical oxygen demand) of the effluent, and reducing the light penetration, which has a derogatory effect on photosynthetic phenomenon [9], [10], [11], [12], [13]. So silk degumming is an essential process to obtain an ideal fiber for the textile industry. During the degumming process, sericin is hydrolyzed, and solubilized in degumming agents and media [4], [14] Silk degumming causes a 20–25% weight loss, which depends on the source and sort of silk. This weight loss recently has been compensated with synthetic materials through coating techniques [7], [14].
Several degumming processes are developed such as extraction with water, boiling off in soap or with alkalis, enzymatic degumming, and degumming in boiling acidic solutions. The recommended standard method of degumming is based on Marseilles soap, which is prepared from olive oil. Marseilles soap is very expensive and has to be imported; therefore, degumming is generally carried out with nonstandard native and home-made soaps based on sodium stearate [4]. Soap makes sericin swell, then emulsifies it in the degumming bath, and removes it from the filaments [7], [8]. The presence of soap and alkalis in the wastewater makes this method a non-ecofriendly process [15], [16]. As far as the environment is concerned, the utilization of chemicals by most of the aforementioned methods introduces serious pollution to the receiving waters.
In recent decades, power ultrasound has taken an undeniable place in chemical and physical activities of the process industries as a very effective and non-polluting method of activation. Sound waves with frequency above the human audible range of 16 kHz are called ultrasound. The use of power ultrasound is well known to have significant effects on the rate of various processes such as cleaning, homogenisation, emulsification, sieving, filtration, crystallisation, extraction, degassing and stripping. An interesting effect is the increase in rate and selectivity of many different chemical reactions. The application of ultrasound in process industries such as chemical, textiles and leather has high potential [17], [18], [19], [20]. Richards and Loomis [21] reported in 1927 on the chemical effects of high-power ultrasound. In a liquid medium, the effects of ultrasound are produced due to the phenomenon called cavitation, which is explained next. When a liquid is irradiated by ultrasound, microbubbles can appear, grow and oscillate extremely quickly and even collapse violently if the acoustic pressure is high enough. The occurrence of these collapses near a solid surface will generate microjets and shock waves [22]. Moreover, in the liquid phase surrounding the particles, high micromixing will increase the heat and mass transfer and even the diffusion of species inside the pores of the solid [19].
Ultrasound can also perform the function of a physical activator in some of the unit operations in textile processing such as degumming. In degumming, ultrasound performs an efficient cleaning operation that removes dirt, sericin, etc. from the raw silk.
In addition, the enzymatic method has the ability to react with specific sites of the sericin, so through a controlled process one can avoid the aforementioned shortcomings [23]. Enzyme degumming involves the proteolytic degradation of sericin, using the specific proteins with minimum effect on fibroin. They are selective and biodegradable, and there is no soap required in the enzymatic degumming process; therefore, uneven dyeing problems caused by metallic soap can be avoided. Silk's affinity to dyes, especially to reactive dyes, is significantly improved by the enzymatic treatment [3]. The application of enzymes in textile industries recently has been increased [14], [24], [25].
Enzymes are ecofriendly products, operate under mild conditions and low temperatures, and so consume less energy than other methods [15], [16], [26], [27]. Enzymes act selectively and can attack only specific parts of the substrate to destroy the unwanted sections [28], [29]. An enzyme reacts with a substrate as a catalyst, and the substrate molecule fits into the active sites of the enzyme's molecular structure like a key fitting into a lock, forming an enzyme–substrate complex (lock–key theory). This complex then is broken and yields an end product and a reproduced original enzyme molecule. The reproduced enzyme will start a new cycle. Freddi et al. applied photolytic enzymes (alkaline, neutral, and acidic proteases) to silk degumming and found that alkaline and neutral proteases effectively degummed crepe silk fabric and that the degumming kinetics depended on the enzyme dosage and treatment time [6]. Gulrajani and coworkers performed silk degumming with some protease and lipase enzymes [23], [27], [30], [31]. Various studies have been reported on the applications of enzymes in degumming, but the use of savinase and mixed enzymes has not been reported [4], [6], [23], [27], [30], [31].
Literature review shows that few studies have been done to modification of silk and degumming the silk by ultrasound [32], [33], [34] but degumming by ultrasound–proteolytic enzymes was not done. Ultrasound was used in several scientific fields by researchers [35], [36]. In this research, the degumming of Persian silk was investigated with ultrasound and ultrasound–proteolytic enzymes, namely, alcalase, savinase, and their mixtures, under different conditions. The effects of various parameters such as the time on the degumming process were examined.
Section snippets
Materials
Ten-folded raw Persian silk yarn (63 6 6.7 Tex and 240 twists per meter) was obtained from Guilan Silk Co. (Rasht, Iran). Alcalase and savinase were obtained from Novo Nordisk Co. (Bagsvaerd, Denmark). A nonionic surfactant, Irgasol NA, was provided by Ciba Co. (Tehran, Iran). All other chemicals were laboratory-grade (analytical reagents, Merck).
Ultrasound equipment
A Metason ultrasonic bath (Struers Co., Denmark), model 120T was used. The used experimental setup was composed of power supply 125 W and tank capacity
Effect of ultrasound on silk degumming
The effect of ultrasound on the degumming of silk was conducted at different conditions. Results showed that the weight loss and strength loss of silk samples are negligible. Therefore, ultrasound on silk degumming does not have considerable effect on silk degumming.
Effect of time on silk degumming by soap
Silk samples were treated with Marseille soap, sodium bicarbonate (pH 8–9), and Irgasol NA. Table 1 and Fig. 1 show the effect of time on silk degumming by soap.
Effect of ultrasound and soap on silk degumming
Raw silk samples were treated by ultrasound in the presence of sodium
Conclusions
The applicability of ultrasound, ultrasound–enzymes, particularly mixtures of enzymes (alcalase and savinase), to the effective degumming of Persian silk under mild conditions was investigated. In general, for all the experiments under the specified conditions, the results show that through the expansion of the treatment time, the amount of sericin remaining over the samples decreased. In other words, as the treatment time increased, more sericin was removed from the raw silk samples. A
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