Preparation And Separation Performance Enhancement Of Novel Water Permeable Membrane Based On Natural Polysaccharide Sodium Alginate

As a contemporary innovative technology,membrane separation technology is widely used in many fields such as bioethanol separation,industrial wastewater treatment,forest products fine chemicals,etc.It is difficult for traditional water permeable membranes to improve selectivity and permeability at the same time,and the key to preparing a membrane with both high selectivity and high flux is the choice of membrane materials.Sodium alginate（SA）is an anionic polyelectrolyte containing many hydroxyl and carboxyl groups,which has excellent hydrophilicity and good membrane-forming properties.However,SA water-permeable membranes often have some shortcomings,such as the poor operation stability of SA pervaporation membranes;and SA-based nanofiltration membranes are rarely seen at present.In view of the above problems,this paper proposes to prepare various modified SA pervaporation membranes by using the hydrophilicity of a by-product of papermaking lignosulfonate（calcium lignosulfonate,referred to as CaLS;sodium lignosulfonate,referred to as Na LS）and the two-dimensional pores of new two-dimensional materials MXene and reduced graphene oxide（rGO）,that the application of ethanol dehydration in the bioethanol industry can effectively improve the selectivity and/or permeability of the separation process.In addition,this paper innovatively proposes the construction of a novelα-Co（OH）2@CaLS/SA nanofiltration membrane that is expected to be applied in the treatment of printing and dyeing wastewater or the concentration of natural products.the specific research contents are as follows:（1）Preparation of CaLS/SA membrane composite membrane and its performance on bioethanol separation.CaLS/SA composite membranes with different CaLS content prepared by blending CaLS and SA,and utilizing the cross-linking effect of Ca²⁺in CaLS and SA polymer chains.The study found that CaLS counld be uniformly dispersed in the SA matrix,and had little effect on the surface morphology of the membrane,but the content of CaLS counld effectively improve the hydrophilicity and anti-swelling properties of the membrane.When the content of CaLS was 5wt%of the SA matrix,the separation performance of the membrane was the best,currently,the separation factor and permeation flux were2872 and 796 g·m^-2·h^-1,respectively,and increasing the operating temperature or decreasing the water concentration of the feed liquid was beneficial to improve the selectivity of the membrane.Based on the solution diffusion model,it was verified that the content of CaLS can increase the membrane solubility coefficient but reduce the diffusion coefficient,and the prepared CaLS/SA membrane could break through the“trade-off”phenomenon between selectively and permeability of SA membrane,and its performance was higher than some nanomaterial-modified pervaporation membranes have been reported.The preparation of CaLS/SA pervaporation membranes besed on cheap,abundant and renewable CaLS could not only improve the separation perfomence but also reduced the cost of membrane preparation,and had great application prospects.（2）For the first time,multilayer MXene/SA mixed matrix membranes were prepared using multilayer MXene materials,and its pervaporation ethanol dehydration properties were investigated.The multi-layer MXene material was prepared by the“clay”method,and its scale was micron,and the prepared multi-layer MXene/SA mixed matrix membrane had a layered cross-section structure,and the hydrophilicity and interlayer pores of the multi-layer MXene make the multi-layer MXene/SA membrane hydrophilic and swelling.When the content of multilayer MXene increased from 0 to 0.24wt%,the flux first decreased and then increased when separating 90%ethanol solution,and the separation factor was opposite.The multilayer MXene/SA mixed matrix membrane with the content of multilayer was 0.12 wt%has the highest selectivity,and the separation factor could reach 9946,which was 10 times that of the SA membrane,but the permeation flux was slightly reduced.In addition,it was found that the multilayer MXene/SA mixed matrix membrane exhibited excellent selectively at both high temperature and high ethanol concentration,which was higher than that of the mixed matrix pervaporation membrane reported in the literature,and it provides a new idea to enhance the selectivity of pervaporation permeable membranes.（3）Preparation of MXene@CaLS/SA three-component pervaporation membrane and its performance for bioethanol separation.Preliminary studies have found that the CaLS/SA composite membrane has good comprehensive properties,and the multilayer MXene/SA mixed matrix membrane has extremely high selectivity,to further develop pervaporation membranes with excellent permeability and selecitivity,a novel MXene@CaLS/SA three-component pervaporation membrane was proposed to simultaneously utilize two modified materials of single-layer MXene and CaLS.MXene sheets with a thickness of 1-2 nm were obtained by the hydrochloric acid/lithium fluoride etching method,and MXene@CaLS/SA membranes with different MXene contents were prepared by uniformly dispersing in the CaLS solution.It was found that CaLS improves the dispersibility of MXene in SA matrix and can reduce the interfacial polymerization,and the as-prepared MXene@CaLS/SA membrane had increased hydrophilicity and decreased swelling degree.When the content of MXene was 0.25 wt%,the membrane had the best effect on bioethanol separation,and the selectivity and permeability were 4 times and 2 times higher than those of SA membrane,respectively.The dissolution-diffusion model analysis showed that the enhancement of the diffusion process was superior to the improvement of the separation performance,and the content of MXene and CaLS could form more selective channels and dense cross-linked structures in the MXene@CaLS/SA membrane,which could effectively enhance the permeability and selectivity of the membrane.（4）Preparation of rGO@Na LS/SA membrane and its performance for bioethanol separation.In previous work,it was confirmed that lignosulfonate improve the dispersibility of two-dimensional materials,so Na LS was added during the reduction of graphene oxide,and a one-step reduction method prepared a sandwich-like rGO@Na LS composite,which greatly improved the problem of agglomeration of rGO in water.The rGO@Na LS composite material has good hydrophilicity and water dispersibility,and it was applied to the modification of SA membranes,and rGO@Na LS/SA pervaporation membrane was designed.The addition of rGO@Na LS enhanced the hydrophilicity and anti-swelling properties of the membrane.and the as-prepared rGO@Na LS/SA membrane exhibited an efficient separation effect on bioethanol.When the content of rGO@Na LS was 6wt%,the membrane performance was best,and the separation factor reached about 6000,and the permeation flux was 712 g·m^-2·h^-1.The dissolution-diffusion model analysis showed that the addition of rGO@Na LS composites can simultaneously improve the dissolution coefficient and diffusion coefficient of rGO@Na LS/SA membrane,breaking through the“trade-off”effect of permeability and selectivity of SA membrane,amd the rGO@Na LS/SA memrbane can maintain within 180 h and has long-term operating stability.（5）Preparation ofα-Co（OH）₂@CaLS/SA nanofiltration membrane and its decolorization performance on dye wastewater.Preliminary research found that the cross-linking effect of CaLS on SA makes the CaLS/SA membrane have excellent rejection effect on dyes,but the increase of retention rate leads to the decrease of membrane flux.To further improve the membrane performance,aα-Co（OH）₂nanosheets was prepared,and a novelα-Co（OH）₂@CaLS/SA nanofiltration membrane was designed by using the nanosheets to construct the transport channel in the miwed matrix of CaLS and SA.The study found thatα-Co（OH）₂ nanosheets have sub-nanometer thickness and surface nanopores,andα-Co（OH）₂nanosheets can be uniformly dispersed in CaLS and SA matrix,and theα-Co（OH）₂@CaLS/SA nanofiltration membrane exhibits good hydrophilicity and thus electronegativity.The results of nanofiltration experiments showed that the membrane prepared atα-Co（OH）₂:CaLS:SA=1.5:40:100exhibited high retention rate（up to 99.9%）and high flux(around 50-80 L·m^-2·h^-1).At same time,it was found thatα-Co（OH）₂@CaLS/SA nanofiltration membrane also has excellent retention effect om matural products,for example,the retention rate of tannins can reach more 95%,and the lessα-Co（OH）₂ content can achieve efficient separation of dyes.On the other hand,the addition ofα-Co（OH）₂ nanosheets enhances the long-term operation stability and chlorine resistance of the nanofiltration membrane,which has great potential in practical applications.In this paper,a variety of new permeable membranes based on SA are designed and constructed,and the pervaporation performance optimization and the nanofiltration seaparation performance regulation of the novel water permeable membrane are mainly carried out,explored the conformational relationships of the new permeable membranes,and revealed the intrinsic mechanism of enhanced membrane performance.