Enzymatic Saccharification Coupling With Polyester Recovery From Cotton-based Waste Textiles By Phosphoric Acid Pretreatment

Waste textiles have been regarded as one of the major solid wastes in the world. Therefore, it is necessary to develop economic and environmental products from polyester/cellulosic fiber blends. How to generate resources efficiently from cotton-based waste textiles was investigated quite intensively in recent years.20pairs of waste jeans were pretreated by phosphoric acid in our work to investigate the possibility for sugar and polyester recovery. Meanwhile, the key factors related to the phosphoric acid pretreatment such as the acid concentration, the ratio of textiles and phosphoric acid, the pretreatment time and temperature are also investigated to clarify their effects on polyester recovery and subsequent sugar recovery. The main results are as follows:(1) Phosphoric acid concentration, the ratio of textiles and phosphoric acid, the pretreatment time and temperature had significant influences on glucose yield, cellulose, sugar and polyester recovery. polyester recovery was positively correlated with the mentioned four factors. The cellulose and sugar recovery were both increased as the phosphoric acid concentration raised. However, the longer pretreatment time beyond12h could not further improve the cellulose and sugar recovery. The glucose yield and sugar recovery firstly decreased, and then increased as the ratio of textiles and phosphoric acid dropped. The critical ratio was1:25. However the cellulose recovery was always increasing as the ratio was controlled in the range of1:20to1:30.(2) In order to investigate the interactive effects among the four factors, central composite design (CCD) was performed based on the data of single factor experiment. The results indicated that the maximum polyester and sugar recovery were not obtained together at same conditions. All the four factors and interaction between any two factors were identified have significant effects on sugar recovery. It was found that the level of temperature affects the sugar recovery most significantly when compared with other factors. All the four factors and interactive effects between any two factors excluding the interaction of phosphoric acid concentration and the ratio of of textiles and phosphoric acid, the interaction of temperature and the ratio of textiles and phosphoric acid, have significant effects on polyester recovery. Regression models sugar recovery and polyester recovery were generated respectively to predict the responses. The optimum pretreatment conditions predicted by the models were phosphoric acid:85%, the ratio of textiles and phosphoric acid:1:15pretreatment time:7h, and temperature:50℃. In such optimal conditions, the complete polyester was recovered and the maximum glucose yield of93.1%was achieved within24h with the maximum sugar recovery of81%.(3) SEM images indicated that, most of cotton fibers still remained in the sample after96h hydrolysis when enzymatic hydrolysis was carried out on the original waste textiles without any pretreatment. In addition, many small holes were observed on the surface of the cellulose fibers, and the trenches on the fiber were deeper than those of the original sample. However, the appearance of the cellulose regenerated from different pretreatment looked like floccules without regular shape and the surface of regenerated cellulose was much rougher than original fibers. The XRD patterns showed that the samples after enzymatic hydrolysis without pretreatment had almost the equal content of crystalline domains compared with original samples. With the phosphoric acid pretreatment, the conversion of crystal forms from the cellulose Ⅰ to cellulose Ⅱ was observed, and the degree of crystallinity also decreased from100%to40%.(4) In consideration of enzymes and H3PO4consumption, the cost for treating1.0ton waste textiles can be estimated as2051RMB. According to current market price of waste polyester and glucose, the profit from1.0ton waste textiles is280RMB. Consequently it was technically feasible and financially feasible to recover polyester and sugar from waste textiles by phosphoric acid pretreatment.