Preparation Of Low Ester Pectin Based Hydrogels And Study On Their Adsorption Of Dyes

Most dyes have the characteristics of high chroma,stable composition,difficult biodegradation and strong toxicity,and untreated wastewater containing a lot of dyes will cause harm to organisms.Adsorption method is widely used to remove dyes from wastewater because of its low cost,simple design and easy operation.The adsorbent is the key part of the adsorption method,so it is of great practical significance to develop a low cost,good adsorption effect,high strength and recyclable hydrogel adsorbent and apply it to the adsorption of dyes.Pectin,an anionic polysaccharide rich in carboxyl groups,can be classified into high esterification pectin and low esterification pectin based on the varying levels of carboxyl group esterification.The different degrees of esterification of pectin result in variations in the performance of the formed hydrogels.This study primarily focuses on the preparation of low esterification pectin-based hydrogels and their application in the adsorption of dyes such as Methylene Blue(MB)and Congo Red(CR).The main findings are summarized as follows:(1)Hydrogel adsorbents were prepared by utilizing pectin with different degrees of esterification as raw materials,in conjunction with acrylic acid(AA),through free radical polymerization.The influence of pectin esterification on the structure and properties of the hydrogels was investigated through characterization,swelling experiments,and MB adsorption tests.Hydrogels prepared from low-ester pectin exhibited a porous structure,higher carboxyl group content,enhanced water absorption and swelling capacity,and superior adsorption capability for MB.Additionally,the adsorption behavior of the hydrogels towards MB was examined through adsorption kinetics and isotherm studies.The results indicated that the adsorption kinetics of the hydrogel followed a pseudo-second-order kinetic model.The Langmuir isotherm model revealed that hydrogels prepared from low-ester pectin had the highest maximum adsorption capacity,reaching 384.87 mg/g.The above results indicated that the hydrogel prepared by the low esterification degree pectin was more favorable to the adsorption of MB.(2)Building upon the aforementioned investigations,a double-network hydrogel(LMP/AA/PVAH)composed of low-ester pectin(LMP)/acrylic acid(AA)/polyvinyl alcohol(PVA)was constructed and applied for MB adsorption.Experimental results demonstrated improved adsorption and mechanical properties of the hydrogel.The rigid chemical crosslinked network was formed via the free radical polymerization of LMP and AA,while the tough physical crosslinked network was achieved through the cyclic freezing-thawing process of PVA.The first-cycle hardness(13.08 N)and elasticity(0)of LMP/AA/PVAH significantly increased to 24.28 N and 0.79,respectively.The PVA network enhanced the toughness of the hydrogel and inhibited its swelling.Furthermore,the adsorption performance of LMP/AA/PVAH towards MB was evaluated through MB adsorption experiments.The adsorption behavior of LMP/AA/PVAH towards MB conformed to a quasi-second-order kinetic model.The maximum adsorption capacity of LMP/AA/PVAH for MB was 388.97mg/g.After four cycles of adsorption,the hydrogel exhibited no significant decrease in adsorption capacity,indicating its good reusability.(3)Building upon the investigation of double-network hydrogels,a novel amphiphilic double-network hydrogel system(LMP/AA/DMDAAC/PVAH)was developed by introducing dimethyl diallyl ammonium chloride(DMDAAC),enabling the adsorption of both cationic dye MB and anionic dye CR.When the molar ratio of AA to DMDAAC was 1:2,LMP/AA/DMDAAC/PVAH-2 hydrogels were obtained,which could adsorb MB and CR more effectively.The results showed that large amounts of MB and CR could be absorbed at p H=4-10 and p H=4-9,with a wide range of p H.Adsorption kinetics and isotherm analysis revealed that the adsorption of MB and CR by the hydrogel followed a pseudo-second-order kinetic model and conformed to the Langmuir model,suggesting monolayer adsorption.The maximum adsorption capacities of LMP/AA/DMDAAC/PVAH-2 for MB and CR were determined as 222.65 mg/g and 316.46 mg/g,respectively.Remarkably,after undergoing four cycles of adsorption-desorption,the hydrogel exhibited negligible changes in its adsorption capacity for MB and CR,highlighting its excellent reusability.