Structure-activity Relationship Of Ion-exchange Cellulose Fibers And Its Application In Wastewater Filters

With the increase of human production and living activities,the pollution and depletion rate of water resources on the earth is increasing day by day.How to purify harmful wastewater efficiently has become a research hotspot.As one of the typical wastewater treatment technologies,adsorption has been widely used because of its simple operation,low cost,good purification effect and high flexibility.The production practice shows that the wastewater adsorption material should have the characteristics of high adsorption capacity,excellent solid-liquid separation,fast water filtration rate,and easy recovery and treatment.Although the existing petroleum-based ion-exchange fibers can meet the above characteristics well,it is difficult to recycle the discarded petroleum-based ionexchange fibers,the biodegradation performance and sustainable development are poor.Therefore,this paper takes renewable cellulose fiber as the object,explores the construction mode of ion-exchange cellulose fiber through molecular structure modification design,puts forward the structure design idea of ion-exchange cellulose fiber,and studies its structure-activity relationship.The main research contents of this paper are as follows:(1)Based on the reactivity of hydroxyl group in cellulose glucose unit,carboxymethyl group was effectively introduced into cellulose molecular structure by etherification reaction between sodium chloroacetate(MCA)and cellulose hydroxyl group,which significantly increased the cation exchange capacity(CEC),and carboxyl group cation exchange cellulose fiber(CEF)was obtained.With the increase of carboxymethyl group content,CEC was obviously increased,but the yield of water-insoluble cellulose fiber decreased.The contradiction between the yield of water-insoluble fiber and CEC can be reconciled by controlling the content of carboxymethyl group in CEF.Under the optimum conditions,the yield and CEC of water-insoluble fiber were 102.8%and 107.6 mmol·100g-1,respectively.The cationic model dye methylene blue(MB)solution(C0=500 mg·L-1,V=50 mL)was used to evaluate the cation exchange properties of the above CEF.The results showed that it had excellent adsorption properties:82%of the equilibrium adsorption capacity(447.69 mg·g-1)was completed in just 5 min.The adsorption process is a homogeneous monolayer adsorption dominated by chemisorption,which conforms to the quasi-second-order kinetic model and Langmuir isotherm model.The etherification of anion caused the change of crystal structure of cellulose from cellulose Ⅰ to cellulose Ⅱ.The filter material formed by CEF had a good purification effect on MB solution(C0=100 mg·L-1,V=50 mL)with a removal rate of 99.2%,but the presence of a large number of carboxymethyl groups led to a significant deficiency in its water filtration performance(a=8.22 m3·m-2·h-1).Because glass fiber is not easy to bend,mixing it with CEF to construct a filter material(mCEF:mglass fiber=2:1)can significantly increase the water filtration rate to 21.23 m3·m-2·h-1,and its removal rate of MB is 99.0%.Using 1 mol·L-1 hydrochloric acid solution,MB in the material can be desorbed effectively to achieve regeneration.After four cycles,the removal rate is only reduced by 0.8%.(2)In view of the water filtration difficulties of CEF filtration materials and the contradiction between CEC and the yield of water-insoluble fibers,epichlorohydrin(ECH)was used in coordination with MCA to design and modify the molecular structure of cellulose fibers,and the cellulose aggregation structure was reconstructed by cross-linking covalent bonds to obtain covalently coupled cation exchange cellulose fibers(ECEF).The results show that the order of anion etherification and crosslinking has a significant effect on the structure and properties of ECEF.After etherification,crosslinking results in the destruction of macroscopic fiber morphology,and it cannot be further non-woven to form filter materials.The introduction of carboxymethyl groups into cellulose molecular structure was limited by cross-linking and etherification,which could not meet the needs of dye purification.In the process of synchronous etherification crosslinking,MCA molecules introduce carboxymethyl groups to cellulose to disassemble the crystalline region and expose more hydroxyl groups,so that ECH molecules can further cross-link and form a cross-linked covalent coupled cellulose aggregation structure,thus reducing the water solubility of modified cellulose and coordinating the contradiction between CEC and water insoluble fiber yield.The water insoluble fiber yield and CEC of ECEF were increased to 124.9%and 208.3 mmol·100g-1,respectively.In the synchronization strategy,the kinetics of etherification and crosslinking reaction showed that the first and second stage activation energies of etherification reaction were 75.84 kJ·mol-1 and 89.63 kJ·mol-1,respectively,and the activation energies of crosslinking reaction were 86.22 kJ·mol-1,indicating that the two reactions could occur simultaneously at the experimental temperature.MCA and ECH molecules snatch the active hydroxyl sites in the amorphous region and disassemble the crystalline region have obvious effects on the aggregate structure of cellulose.The crystallinity and grain size of ECEF-3 decrease significantly to 13.2%and 1.88 nm,respectively.With the support of covalent cross-linked network,the hollow fiber structure can be maintained in wet state,giving ECEF filter material good water filtration performance(25.48 m3·m-2·h-1),and its removal rate of MB solution can reach 99.5%.After four times of elution and regeneration with 1 mol·L-1 hydrochloric acid,the removal rate remained at 97.9%.The adsorption process of MB molecules by ECEF can be described by the quasi-second-order kinetic model and the Langmuir isotherm model,and the maximum adsorption capacity reaches 821.30 mg·g-1.(3)In order to further implement the concept of green and sustainable development,a more environment-friendly cross-linking strategy——phosphodiesterification cross-linking was proposed.In coordination with carboxymethyl etherification,the cross-linked skeleton was constructed in cellulose fibers and additional cation exchange sites were introduced by utilizing the hydroxyl active site to obtain esterified cross-linked cation exchange cellulose fibers with ’double high’ properties(high yield of water-insoluble fiber and high CEC).The properties of cellulose fibers can be effectively regulated by the design of phosphoesterified cross-linked skeleton,and the reaction conditions can be controlled to obtain P-CEF with a ratio of esterified cross-linked structure to noncrosslinked structure of 3:2.The yield and CEC are 126.4%and 216.9 mmol·100 g-1,respectively.By adjusting the reaction sequence of phosphoric acid esterification and carboxymethylation,esterified crosslinked cation exchange cellulose fibers(PC-CEF)with yield of 111.6%and CEC of 304.5 mmol·100 g-1 were obtained.The contents of phosphate group and carboxymethyl group were 208.9 mmol·100g-1 and 95.6 mmol·100g-1,respectively.The introduction of bifunctional group gave it a good MB adsorption capacity(1243.96 mg·g-1)and Cu(Ⅱ)adsorption capacity(82.56 mg·g-1),and the adsorption process was consistent with the quasi-second-order kinetic model and Sips isothermal adsorption model.31P NMR results show that there is monophosphate,diophosphate,pyrophosphate and pyrophosphate structures in PC-CEF.Esterification crosslinking was used to replace hydrogen bonds to rebuild the aggregation structure of cellulose,maintain its macro hollow fiber structure in aqueous solution,effectively alleviate the water absorption and swelling of fibers,and improve the water filtration rate of the filter material(3 1.85 m3·m-2·h-1).The removal rate of PC-CEF filter material for MB solutio reached 99.8%.After four elution and regeneration with 1 mol·L-1 hydrochloric acid,the removal rate decreased by 1.6%,indicating good regeneration.Thanks to the supporting role of esterified crosslinked skeleton and abundant cation exchange sites,PC-CEF filtration material can still reach 99.1%removal rate when treating high concentration MB solution(C0=900 mg·L-1,V=50 mL).(4)In order to purify anionic dye wastewater,two kinds of cationic etherification agents 3-chloro-2-hydroxypropyl trimethyl ammonium chloride(CTA)and epoxypropyl trimethyl ammonium chloride(EPTAC)were used to modify the molecular structure of cellulose fibers,and anion exchange cellulose fibers CAEF and EAEF were obtained respectively.The regulation rules of anion exchange capacity(AEC)of cellulose fiber by two methods were investigated.The results showed that compared with CTA method,EPTAC method could effectively reduce the amount of reagents used and reaction time,which was beneficial to reduce production energy consumption and save preparation cost.The EAEF with water-insoluble fiber yield and AEC of 107.3%and 133.2 mmol·100g-1 were obtained,respectively.The EAEF had good macroscopic fiber morphology and hollow structure,the ratio of length to diameter was 71.4,the aggregate structure changed only slightly,the crystalline structure remained cellulose Ⅰ,and the crystallinity decreased slightly to 71.02%.The adsorption process of congo red(CR)solution conforms to the double-layer statistical physical model,there is a double-layer adsorption between CR-EAEF and CR molecules,and has a high adsorption capacity(1341.07 mg·g-1).A good fit with the quasi-second-order kinetic model indicates that the interaction between EAEF and CR is mainly chemical.EAEF filter material has a filtration rate of 31.85 m3·m-2·h-1 for CR solution(C0=100 mg·L-1,V=50 mL),which can be reused by 1 mol·L-1 sodium hydroxide eluent.After four cycles of regeneration,the removal rate of CR decreased by only 0.8%and could still reach 98.3%.In summary,by designing the molecular structure of cellulose and reconstructing the aggregation structure of cellulose,the adsorption capacity and water filtration performance of cellulose fiber can be improved.This idea effectively combines the structural characteristics of cellulose fiber and the application advantages of ion exchange fiber in the field of adsorption and separation,and the production process is simple and controllable,which is conducive to large-scale industrial application.