Technical Methods for Recovering Sericin from Degumming Wastewater

Jinfeng LYU Chan ZHOU Jieping WANG Yao ZENG Qunzhong MA Shanlin GU

Abstract Silk sericin is a natural polymer, which is often discarded as waste in silk reeling production. Sericin protein increases the COD value of degumming wastewater and increases the difficulty of sewage treatment. The recovery and utilization of sericin protein can minimize environmental problems, and has high scientific and commercial value. This paper introduced and analyzed current technical methods of recycling sericin from silk degumming wastewater, providing a reference for the utilization of recovered sericin.

Key words Silk; Sericin; Recovery methods

Received: September 13, 2021  Accepted: November 15, 2021

Supported by Chongqing Natural Science Foundation Project (18219); Chongqing Scientific Research Institution Performance Incentive Project (19537, 20521); Chongqing Rongchang Agriculture and Animal Husbandry High-tech Industry R & D Special Project (20209); Chongqing Modern Mountain Characteristic Benefit Agriculture (Sericulture) Technology System Diversified Development Research Office (18304).

Jinfeng LYU (1986-), female, P. R. China, research assistant, master, devoted to research about sericulture bioengineering.

*Corresponding author.

China’s cocoon silk industry has a long history, and silk reeling is an extremely important basic process in the cocoon silk industry. Traditional silk reeling plants mainly produce raw silk through cocoon cooking, silk reeling, re-reeling, and by-product processing. During this period, a large amount of waste water is generated. As China’s strategy for ecological protection is improved, wastewater in silk reeling process can only be discharged after treatment.

The main process of making raw silk is degumming. The content of sericin is about 20%-30% in silk, and usually about 20% in the degumming rate［1］. Degumming wastewater is produced during silk reeling process, and it accounts for 60%-65% of wastewater generated during silk-making process. Sericin is discharged in large quantities with degumming wastewater, which will increase the turbidity and COD value of water, which not only increases the cost and difficulty of sewage treatment, but also wastes the natural resource of sericin. Researchers at home and abroad have made in-depth studies on the hot issue of recycling degumming wastewater. At present, the technical methods for recycling sericin from silk degumming wastewater are mainly divided into two categories: physical  and chemical methods.

Physical Methods

Freezing method

Sericin is combined with surrounding sericin in a low-temperature solution, flocculates to form a gel state, and can be obtained after freeze-drying. This method has simple process and high recovery rate, but requires freeze-drying equipment, and consumes high energy, so the cost is relatively high.

Centrifugation method

According to the principle of different mass of sericin and water, the sericin with a large mass is left at the bottom under high-speed centrifugation, and water with a low mass is at the upper layer. The premise of this method is that sericin has been separated from the water, and the centrifugal method is not efficient for degumming wastewater that has not undergone precipitation. This method has a low recovery rate and is generally not applicable in industrial production.

Ultrafiltration method

Microporous filter membranes can selectively filter out small molecular substances and retain large molecular substances, thereby increasing the concentration of solutes in the solution and concentrating the solutes. Sericin has a large molecular weight and is not easy to pass through a semi-permeable membrane. According to this feature, corresponding ultrafiltration membranes can be selected to extract sericin. The sericin recovered by this method has high purity and the recovery rate is high, but ultrafiltration membranes themselves are easily contaminated and blocked, and the maintenance cost and equipment cost are high.

Chemical Methods

Salting out method

Saturated or concentrated salt solutions are added to degumming wastewater as flocculants. Salt ions compete with sericin for water molecules, destroy the hydration layer around sericin, and coagulate with surrounding sericin, and a precipitate is formed. This method is simple in process, time-saving and low in cost, but the purity of recovered sericin is low.

Organic solvent precipitation method

The mechanism of action is similar to that of the salting-out method, that is, organic solvents are used to compete with sericin for water molecules to precipitate sericin. The sericin recovered by this method has high purity, and is not easy to be destroyed, but the cost is high, and it is not suitable for industrial recovery and use.

Acid precipitation method

The isoelectric point of sericin is around pH 3.5-4.5. Using this principle, the pH of alkaline degumming wastewater is adjusted to this range, in which the solubility of sericin is reduced to a minimum, and sericin precipitates out from the solution. This method is simple in process and low in cost, but the recovery rate of sericin is not high, only about 40%［2］, and it requires the recovery equipment to be acid resistant.

Ion exchange method

Degumming wastewater is generally alkaline, and sericin in degumming wastewater is negatively charged. According to this characteristic, exchange fiber with cations can adsorb sericin on the fiber. Then, the exchange fiber is soaked with an acid solution to neutralize the negative charge of sericin, and it is eluted from the anion exchange fiber. This method has low cost and relatively high purity of sericin, but the process is more complicated and the recovery rate is low［3］.

Other methods

Chongqing Academy of Animal Sciences innovatively proposed a method of recovering sericin by combining isoelectric precipitation and ultrafiltration, which increases the recovery rate of sericin to 90.2%［4］. At present, researchers have combined two or more methods to carry out process improvement and research［2-3,5］, which not only improves the recovery efficiency and purity of sericin from degumming wastewater, but also effectively reduces the recycling cost, thereby reducing the difficulty of wastewater treatment in industrial production, and the recovery of sericin increases the added value of enterprises.

Prospects

Sericin is a spherical hydrophilic protein that contains 18 kinds of amino acids, 8 of which are essential amino acids for the human body. And sericin is hydrophilic, non-toxic, biocompatible and biodegradable, and can be widely used in biological materials, medicine, cosmetics, food,  etc. ［6-7］.

For thousands of years, traditional cocoon silk processing has treated sericin as waste. For every ton of silkworm cocoons processed, 0.15 to 0.25 t of sericin will be lost with the production wastewater. Based on the calculation of 1 t of raw silk made from 3 t of silkworm cocoons, 26 000 to 44 000 t of sericin will be produced with degumming wastewater. If all sericin is recycled, it will not only save natural resources, but also increase the by-product output and added value of the cocoon and silk industry.   However, at present, there is no uniform standard for the process of recycling sericin, and enterprises value simplification of the process, low cost, and easy industrial utilization. Therefore, the process of industrial recovery of sericin still needs to be improved and optimized.

"The Belt and Road" has accelerated the pace of China’s cocoon and silk industry opening up to the outside world, and promoted the foreign exchange and trade of China’s cocoon and silk industry. In the face of the current world economic turbulence and increasing trade tensions, as a pillar industry in China’s national economy, the cocoon and silk industry has also actively strengthened its endogenous power and industrialization upgrade through technological innovation while doing a good job in the strategic layout of the industry, so as to promote the improvement of the national economy.

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