A Salt-free and Water-saving Approach as a Green Alternative to Conventional Reactive Dyeing of Cotton

9 Aqueous reactive dyeing of cotton is considered to be environmentally 10 unfriendly owing to its high consumption of water and salt as well as large discharge 11 of wastewater. However, the industrial applications of current alternative dyeing 12 technologies are difficult to realize due to different problems such as poor dyeing 13 quality, environmental pollution and high cost. Herein, we developed a hydrophobic 14 deep eutectic solvent (HDES) dyeing approach using natural thymol - menthol to 15 overcome the problems caused by reactive dyeing of cotton. HDES dyeing approach 16 could achieve excellent dyeing properties of cotton by optimizing water content, 17 pick - up and concentration of alkali solution. Dye fixation kinetics of HDES dyeing 18 system was studied based on Lagergren pseudo - first - order and pseudo - second - order 19 dynamics models, compared with aqueous dyeing system. The cotton fabrics dyed in 20 Thy - Men system exhibited excellent levelness, satisfactory colorfastness and color 21 strength without addition of salt. Furthermore, HDES can be reused for 5 cycles of 22


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Cotton has been widely used because of outstanding characteristics, such as high 30 air permeability, excellent softness, good comfort as well as being easy to dye (Wang    In our present work, thymol-menthol was used as a medium for cotton dyeing.     As a control, conventional aqueous dyeing was conducted. The concentrations of 146 the dye and the fabric-to-liquor ratio are the same as thy-men dyeing process.

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Specifically, cotton fabrics dyed in bath at 60 ℃ for 100 min. 40 g/L Na2SO4 and 20 148 g/L Na2CO3 was gradually added into the bath.

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After dyeing, cotton fabrics were washed with water for 10 min and then soaped 150 in a solution at 95 ℃ for 10 min (liquor ratio = 30:1), which was composed of 151 Na2CO3 and AATCC standard detergent (2 g/L). Subsequently, cotton fabrics were 152 rinsed thoroughly using hot water and cold water before being dried.       where q e and q t (mg/g) are the amounts of reactive dye on fibers at equilibrium 320 and given time t, respectively. k 1 (min -1 ) and k 2 (g mg -1 min -1 ) refer to the rate 321 constant of the pseudo-first-order and pseudo-second-order dynamics models.

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The calculation formulas of pseudo-second-order kinetic parameters are shown 323 in equations (5) to (7). Where q e,cal is the theoretical value of dyeing equilibrium, k 2 is the dyeing rate 328 constant, t 1/2 is half dyeing time, a and b are slope and intercept of 329 pseudo-second-order kinetics linear fitting, q e is the actual dyeing equilibrium value.

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The fitted line plots of pseudo-first-order and pseudo-second-order dynamics 331 models in conventional and Thy-Men dyeing systems are shown in Fig. 8b and Fig. 8c,   332 respectively. Kinetic parameters of pseudo-second-order for fixation of Reactive Red 333 195 in conventional dyeing system and Thy-Men dyeing system are listed in Table 3.  (Table 3). The former model described better the fixing procedure of Reactive

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Red 195 on cotton fabrics in conventional dyeing system and Thy-Men dyeing system.

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K 2 , t 1/2 listed in Table 3 are main factors in the fixation rate. The fixation rate of the 341 Men-Thy system is similar to that of the conventional system, which ensures that the 342 dye has enough time to diffuse in fabrics until it is evenly distributed so that fabrics 343 can obtain good leveling properties.    The calculation of material costs is listed in Table 4. In Thy-Men dyeing 365 system, more than 98% of the spent bath could be reused, so that the materials 366 consumption of each dyeing cycle is low. In terms of material loss per cycle, the 367 Thy-Men system is $0.036 cheaper than the traditional dyeing system for dyeing 368 1kg of cotton fabric. Although the initial cost of menthol and thymol is more 369 expensive, the cost of wastewater treatment will be greatly reduced because of low 370 emission from the Thy-Men system. With increasingly strict requirements of 371 environmental protection, the cost of wastewater treatment will become more 372 expensive. In addition, compared with conventional dyeing, the cost of dyeing 373 will gradually decrease with the increase of dyeing cycles. In summary, total cost 374 of thymol-menthol dyeing system is unlikely to be higher than that of 375 conventional dyeing system in the long term.  reusability. Besides, considering water conservation and wastewater treatment, the 400 cost of dyeing in Thy-Men system will not be too high from a long-term perspective.

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Therefore, green reactive dyeing of cotton can be realized with the HDES system. It is 402 of great significance to the sustainable development of the textile industry.  Graphicabstract.docx