Preparation And Contaminant Retention Performance Of Sodium Alginate-based Hydrogel Self-supported Membrane

Membrane filtration technology was widely used in wastewater treatment because of its low energy consumption,no secondary pollution and high treatment efficiency.However,as the traditional commercial membrane was mainly based on hydrophobic polymers such as polyether sulfone（PES）and polyvinylidene fluoride（PVDF）,the membrane pore was easy to be blocked by pollutants such as oil and protein in the wastewater,resulting in irreversible membrane pollution.Researchers generally believe that improving the hydrophilicity of membrane surface can effectively prevent membrane fouling.Therefore,the treatment methods commonly used are hydrophilic modification of the surface of the hydrophobic membrane,such as:coating hydrophilic substances（hydrogels,etc.）,polymer grafting and layer by layer self-assembly,etc.However,the above methods still had some disadvantages,such as the complex blending modification process,the easy peeling off and unevenness of the surface grafting and coating.Therefore,it was of great significance to research and develop hydrogel self-supported membrane with high stain resistance,strength and flux.In this paper,a series of hydrogel self-supported membranes based on sodium alginate were designed,and their physicochemical properties and rejection mechanism were studied deeply.Specific achievements are as follows:（1）A novel sodium alginate hydrogel self-supported membrane（S-C membrane）was prepared by using the method of free water evaporation（"cooking"）and ionic cross-linking.The physicochemical properties and pollutant retention efficiency of S-n C membrane prepared by ionic cross-linking method alone were compared.The results show that the mechanical strength and thermal stability of S-C membranes were improved due to the activation of hydrogen and ionic bonding sites between SA molecular chains in the process of free water evaporation.The maximum tensile strength of the gel membrane increased from 0.95 MPa of S-n C to 3.72 MPa of S-C,which increased nearly 4 times.The water contact angle of the two membranes was close to 0°,and good hydrophilicity made both membranes exhibit excellent anti-fouling ability.However,in the dye interception test,S-C membrane had a higher interception rate because it can provide more functional groups with higher SA concentration.This study provided a simple and reliable method for the preparation of high strength and tough hydrogel membrane.（2）To further enhance the mechanical strength and flux of the gel membrane,cellulose nanofibers（CNF）was added as a reinforcing agent to stabilize the internal structure of the gel membrane,and then a high-strength and high-throughput hydrogel-based self-supported membrane（S-Cx）was prepared by increasing the content of pore making agent（from 1 to 2 times the matrix amount）for the interception of heavy metal ions and dyes in water.The membrane could maintain a stable water flux of 71.5 Lm^-2h^-1 at an operating pressure of 0.35 MPa.At the same time,the abundant carboxyl and hydroxyl groups on CNF increased the action sites of the membrane,so that the membrane had a high rejection for heavy metal ions and dye.The rejection rate of Pb²⁺,Cu²⁺and Cd²⁺were 100%,94.6%and 88.6%,respectively.Through the in-depth study of the interception mechanism,it was concluded that the interception order of three mixed ionic solutions filtered by gel membrane was Pb²⁺,Cu²⁺,Cd²⁺,and the rule of interception rate was consistent with that of single ion filtration.The rejection rates of methylene blue（MB）and Congo red（CR）at the concentration of 50 mg L^-1 were close to 100%,respectively,showing that the former was adsorption and the latter was repulsion.（3）In order to further increase the mechanical strength and application range of the gel membrane,combined with the stability and good compatibility of sodium alginate gel,functional substances-micron-sized biochar（BC）and cellulose nanofiber（CNF）were added.Similarly,using PEG as the pore making agent and using the free water evaporation（"cooking"）method,a mechanically enhanced self-supporting gel filter membrane（S-C-B_n membrane）with universal applicability to anions and cations was prepared.Herein,CNF can not only enhance the mechanical property of the matrix,but also assist the homogeneous dispersion of BC.The results showed that the maximum tensile strength of the prepared membrane can reach 5.69 MPa.Under the pressure of 0.35 MPa,the flux of water also reaches that of the hydrophobic membrane.And because of its good hydrophilicity,it had excellent antifouling performance.In filtration tests,the rejection of Cr（III）and Cr（VI）of 50mg L^-1 could reach 96.8%and91.4%,respectively.Moreover,the mechanism behind the exceptional high rejection for both cationic and anionic heavy metal was delineated.（4）On the basis of high strength and toughness gel membrane,in order to further improve its flux and achieve selective separation,a new loose self-supported hydrogel nanofiltration membrane was prepared by using large molecular weight PEG as pore-making agent,cellulose nanofibers（CNF）and carboxylated carbon nanotubes（MWCNT-COOH）as additives.The“cooking”technic and the addition of CNF and MWCNT-COOH guaranteed the mechanical strength of the gel,and the synergistic effect of large molecular weight PEG and MWCNT-COOH made the gel form a loose structure.The resulting composite membrane possessed high water permeability of 51.6Lm^-2h^-1bar^-1.Meanwhile,based on the porous structure and the synergistic action of multi-ionic groups,the membrane exhibited a high dye rejection:Crystal violet（99.8%）,Congo red（98.9%）,Tartrazine（93.1%）,Methylene blue（85.7%）,while maintaining a rather low retention of salts:Mg SO₄（11.6%）,Na₂SO₄（10.5%）,Mg Cl₂（9.1%）,Na Cl（7.3%）.Furthermore,the membrane exhibited outstanding separation efficiency towards the mixed dye/salt feed solution with special combination.In addition,the membrane not only showed excellent hydrophilicity and stain resistance,but also had good recycling effect.This study provides a simple and feasible method for the preparation of dye desalination membrane.In summary,the mechanical properties of the hydrogel self-supported membranes were greatly improved by using"cooking"technique and ion crosslinking method,as well as adding micron or nano additives to the matrix.Therefore,the gel membrane could withstand greater operating pressure and obtain a higher water flux,which was comparable to that of hydrophobic membrane.In addition,through a series of designs,a wider range of hydrogel self-supported membranes had been prepared,such as:universal applicability of anions and cations and efficient desalination applications of dyes.This paper has made breakthroughs in the research of hydrogel self-supported membranes,and has great potential in the field of water treatment.