Preparation And Adsorption Properties Of Rice Straw Carboxymethyl Cellulose By Ultrasound Method

In this paper,rice straw cellulose was extracted from rice straw by alkaline solution with high temperature,then rice straw carboxymethyl cellulose?CMC?with high degree of substitution was synthesized from the extracted cellulose with ultrasoundas physical auxiliary method,and the structure,the physicochemical property and adsorption performance of CMC obtained with ultrasound and non-ultrasound were studied and compared,additionally a brief comparison of adsorption performance with commercial CMC were studied.Firstly,the composition and structure of rice straw were studied.The experimental results showed that the cellulose content of rice straw was 40.94%,lower than wheat straw?46.42%?and corn stalk?57.37%?,but still more than 1/3,so it also could be used as cellulose resource.The crystallinity degree of rice straw was 66.61%that was higher than that of wheat straw?57.23%?and corn stalk?51.61%?,so the available hydroxyl group?-OH?in rice straw was blocked,the chemical activity was too low to make it soluted,which made cellulose reaction performance lower.Secondly,the process of extracting rice straw cellulose by high-temperature alkali boiling method was studied and the technological conditions were optimized by orthogonal experiment.The experimental results showed that the content of cellulose reached 75.66%under the condition of 2 h of cooking time,70^oC of temperature,8%of mass fractionof sodium hydroxide?NaOH?and 1:20 g·mL^-1of solid-liquid ratio.Its purity increased by 84.81%compared with rice straw,the removal rate of lignin and hemicellulose reached 77.70%and82.31%respectively.The weakening of the guaiac ring and the C=O stretching vibration characteristic peak at 1249 cm^-1 and the weakening of the C=O stretching vibration absorption peak on the acetyl group and carboxyl group at 1738 cm^-1 on the Infrared spectrograms indicated the decrease of lignin and hemicellulose.The X-ray diffraction pattern and the decrease of crystallinity degree from 66.61%to 54.14%indicated that the crystalline region of cellulose was destroyed and the rice straw cellulose had better reactivity.Thirdly,the solvent process for the synthesis of rice straw CMC by alkalization reaction with ultrasound and the rheological property of CMC aqueous solution were studied.The experimental results showed that the degree of substitution of rice straw CMC obtained reached 0.873 under the condition of 175 W of the ultrasonic power,3:1 of the ratio of two alkalis added,1:0.8 of the mass ratio betweencellulose and NaOH,1:1.2 of the mass ratio betweencellulose and monochloroacetic acid?MCA?,30 min of alkalinization time,60^oCof etherification temperature,35^oC of alkalization temperature,60 min of etherification time,85%of volume fraction of ethanol.The rheological property of rice straw CMC was investigated.It was found that the viscosity of CMC aqueous solution increased with the increase of its concentration and degree of substitution,decreased with the increase of the temperature of the solution system,increased and then decreased with the increase of the pH value,and decreased and then increased with the increase of NaCl concentration of the solution system.The appearance of carbonyl asymetricm and symmetric stretching vibrational absorption peaks of-COOH at 1603 cm^-1 and 1417 cm^-1 on the Infrared spectrograms confirmed that the carboxymethyl group?-CH₂COOH?were inserted into the CMC molecule,the surface of the CMC had a loosely dispersed structure and a relatively crisp and hard texture on the Scanning electron microscope?SEM?pictures,the disappearance of the diffraction peaks at 15.0°and the shift of the diffraction peaks at 22.2°to the low-angle direction on the X-ray diffraction spectrograms confirmed that the I crystal structure was transformed into the II crystal structure with weaker intensity.Finally,the structure,physicochemical property and adsorption properties of rice straw CMC with ultrasound and non-ultrasound of similar DS were compared,the experimental results showed that the CMC obtained with ultrasonic assisted alkalization had a larger content and a looser and fluffier structure on the SEM spectrograms,had a weaker characteristic peak on the X-ray diffraction spectrogramswas and proved that the rice straw cellulose was damaged more seriously,had a higher viscosity,better solubility,water retention and had a better adsorption properties of methylene blue,Pb²⁺and Cu²⁺.The rice straw CMC with ultrasoundas physical auxiliary method had a better adsorption performance and adsorption rate of methylene blue,Pb²⁺and Cu²⁺than commercial CMC with similar degree of substitution.This study solved the problem of low cellulose content in the process of preparing cellulose from rice straw.The hemicellulose and lignin in the extracted rice straw cellulose were better removed,and the cellulose content was greatly improved.The prepared rice straw CMC product meeted its food grade requirements,and the degree of substitution,viscosity,solubility,water retention and adsorption adsorption performance were all improved.