The Study Of Electrospun Chitosan-Based Membranes For Organic Dye Adsorption

In the production process of the dye-related industries,about 10%-15%of organic dyes are discharged with industrial wastewater.The dyes will metabolize carcinogenic,teratogenic,and mutagenic products after entering the environment,which harms not only the ecosystem but also the human health.Among the common methods for treating organic dyes in wastewater,the adsorption method is widely used due to its low cost,high efficiency,simple operation and no secondary pollution.Chitosan(CS),as a natural polymer material,has good adsorption performance.This is due to the multi-functional groups in the structure which can generate electrostatic and hydrogen-bond interactions with the adsorbate.However,CS powder has small specific surface area,low porosity,poor mechanical properties and poor stability in acidic aqueous solutions.In recent years,in order to improve the adsorption performance of CS to organic dyes,the study of its structural modification and processing method has attracted the attention of the majority of scientific researchers.In this work,the CS-based composite nanofibrous membranes with controllable structure were designed and prepared by the electrospinning method of the natural polymer material CS with good adsorption performance and the amphiphilic block copolymer poly?-caprolactone(PCL)-polyethylene glycol(PEG)block copolymer(PCL-b-PEG),and it was used as an adsorbent for organic dyes.The hybrid electrospinning of PCL-b-PEG and CS improved the stability of CS in aqueous solution.Meanwhile,the addition of SiO₂ aerogel(SA)with a three-dimensional porous network structure and polydopamine(PDA),constructed more active adsorption sites and adsorption spaces on the fiber surface by surface modification,and the adsorption performance of the adsorbent was further improved.The researches mainly included the following three parts:(1)In order to improve the compatibility of CS and PCL,an amphiphilic PCL-b-PEG copolymer was prepared in this paper.Caprolactone(CL)and methoxy polyethylene glycol(PEG)with different molecular weights of 2000 and 5000 Da were polymerized to obtain two different block copolymers.Analysis of ¹H-NMR,GPC and FTIR showed that the PCL-b-PEG_2K and PCL-b-PEG_5K copolymers were successfully prepared,and the molecular weights of the two block copolymers were 51.6 and 60.7 k Da,respectively.The dispersion values were1.75 and 1.61,respectively.(2)PCL,PCL-b-PEG_2K and PCL-b-PEG_5K copolymers were blended with CS respectively,and CS-based nanofiber membranes(CS/PCL,CS/P_2K and CS/P_5K)were prepared by electrospinning technology.The effects of pH values,initial concentration and adsorption time on the adsorption performance of CS/PCL,CS/P_2K and CS/P_5K membranes were studied.Studies had shown that by adjusting the molecular weight of the PEG segment,the controllable electrospinning of CS-based nanofibers with a core-shell structure was achieved and as the molecular weight of the PEG segment increased,the content of CS on the fiber surface increased.The hydrogen bond interaction between CS and PCL-b-PEG copolymer improved the stability of the nanofibrous membrane in aqueous solution.Meanwhile,the adsorption process toward Congo red(CR)molecule on CS-based membranes fitted well with pseudo-second-order model and Langmuir isotherm model.The CS/P_5Knanofiber membrane had a maximum adsorption capacity of 291.55 mg/g.(3)The SiO₂ aerogel with three-dimensional porous network structure,high specific surface area and low density was combined with CS/P_5K nanofiber membrane through electrospinning-electrospray technology.The prepared composite membrane was modified by PDA,and the CS/P_5K-SA@PDA multi-component hierarchical structure composite membrane was obtained.The effects of the interlayer components contents,pH values,contact time and initial CR concentration on the adsorption performance of the composite membrane were studied.The adsorption process of the CS/P_5K-SA@PDA composite membrane on CR conformed to the pseudo-second-order kinetic model,indicating that the process was mainly chemical adsorption,which was due to the large amount of-NH₂and-OH in CS and PDA had electrostatic and hydrogen bond interactions with CR.According to the BET and adsorption isotherm model,the adsorption of CR on CS/P_5K-SA@PDA-24 composite membrane exhibited single-layer adsorption and the maximum adsorption capacity was 598.8 mg/g.Although the composite membrane had different hierarchical structures,the uniformly dispersed SiO₂ aerogel and CS/P_5K matrix made the interlayer components tightly combined through PDA.The addition of SiO₂ aerogel increased the adsorption space of the composite membrane,and the PDA modification increased the surface active adsorption sites of the composite membrane and strengthened the bonding between the layers of the components.Therefore,the prepared CS/P_5K-SA@PDA-24 composite membrane exhibited high efficiency adsorption performance and excellent reusbility for CR.Even in the competitive adsorption process of the multi-system,the CS/P_5K-SA@PDA composite nanofibous membrane still showed preferential and selective adsorption to CR.This work provided theoretical guidance for the design and development of new environmentally friendly adsorbents..