Study On The Treatment Of Brackish Water By Nanofiltration Membrane Modified With Carboxylated Nanocellulose Crystals

Brackish water is an important part of available water resources in China.Moderately developing brackish water is an effective way to solve the shortage of water resources.Nanofiltration（NF）membrane technology has become one of the most promising technologies for the treatment of brackish water with lower cost and higher efficiency due to its advantages of physical interception,no need to add agents and small footprint.However,there are still some problems in NF membrane,such as trade-off,membrane fouling,insufficient mechanical properties of membrane materials,and low membrane life.Recently,in the modification methods of NF membrane,the addition of nanomaterials in the membrane is a feasible measure to optimize membrane performance,reduce energy cost,and overcome the inherent defects of NF membranes.However,the leakage of cytotoxic nanoparticles has threatened the safety of the water environment and human health,and sustainable materials with low environmental impact are urgently needed for replacement.In addition,the mechanism by which nanomaterials are introduced into NF membranes to improve membrane performance is still unclear.Meanwhile,in view of the complex interactions between ions themselves and ions and membranes,the in-depth mechanism of NF membranes for the separation of brackish water needs to be further explored.In this thesis,carboxylated cellulose nanocrystals（C-CNCs）,which are environmentally friendly,sustainable,and rich in hydrophilic functional groups on the surface,were used as the target nanomaterials to incorporate into the active layer and support layer of the NF membrane,respectively.And the performance of the two types of composite NF membranes were evaluated.The separation mechanism of the composite NF membranes in the treatment of brackish water was quantified by combining the DSPM-DE model.First,C-CNCs were prepared based on cellulose nanocrystals（CNCs）by adding the carboxyl functional groups.By optimizing the hydrophilicity and charge density of nanomaterials,the dispersion of nanoparticles in aqueous solution and organic solvent was better.The prepared C-CNCs were incorporated into the polyamide active layer,and the morphology and chemical properties of the composite NF membrane（ALC membrane）modified in the active layer were characterized.The separation performance and mechanical stability of the ALC membrane on salt solution,dye solution and brackish water were evaluated.The addition of C-CNCs in the active layer significantly enhanced the hydrophilicity and surface roughness of the composite nanofiltration membrane.Accordingly,the pure water flux of ALC membrane increased to 1.27 times of that of the pristine membrane.ALC membranes exhibited stronger electronegativity,overcoming the problem of trade-off.In addition,the elastic modulus and hardness of ALC membrane were 276 MPa and 47 MPa,respectively,which were greater than those of pristine membrane.This study provides a new insight for designing and constructing NF composite membranes of environmentally friendly nanomaterials.Although the presence of C-CNCs in the active layer optimized the performance of NF membranes,its role in the support layer remains unclear.And the properties of the support layer also have a great impact on the performance of NF membranes.Therefore,in this study,the same nanomaterial C-CNCs was used to control the surface structural characteristics of the support layer and explore its influence on the polyamide layer.The doping of C-CNCs in the support layer resulted in larger and more uniform membrane pores,and significantly increased the electronegativity and hydrophilicity of the support layer,with an isoelectric point of 2.72,far lower than that of the pristine support layer.The larger pore size and more hydrophilic support layer were favorable to the retention of aqueous monomer PIP on the surface of the support layer.It reacted violently with TMC to generate polyamides with shorter molecular chains and higher cross-linking degrees,which gradually accumulate into a more developed nodular structure and a thicker and denser active layer.The modified NF membrane（SLC membrane）with C-CNCs in the support layer had better separation performance than the pristine membrane and ALC membrane,but a part of permeability was sacrificed.The rejection rates of Na₂SO₄,Mg SO₄,and Na Cl were 99.2%,99.0%and 41.8%,respectively.Meanwhile,the separation performance of dyes and brackish water was improved significantly.SLC membrane exhibited better mechanical properties than pristine membrane and ALC membrane,indicating that the elastic deformation of SLC membrane required greater stress and more pressure resistance,and it was easier to recover after deformation.Chlorine treatment experiments and long-term operation experiments confirmed the stability of SLC membranes in water treatment.This study emphasizes the importance of support layer in NF membrane performance regulation and broadens the perspective of high-performance membrane design.The optimization of NF membrane performance by incorporating C-CNCs into the active layer and support layer of membranes,respectively,has been evaluated.However,more in-depth mechanisms have not been well studied,and other specific methods are needed to assist in the exploration.Based on the above results and combined with the DSPM-DE model,this part explored the mechanism of nanomaterial C-CNCs affecting the separation performance of NF membrane,as well as the transport mechanism and separation mechanism of composite NF membrane in the treatment of brackish water.It was proved from the DSPM-DE model that the addition of C-CNCs affected the solute separation mechanisms of NF membrane by changing the membrane parameters,and then improved the membrane properties.Diffusion played a major role in the transport mechanism of brackish water ions inside the NF membranes,while dielectric repulsion contributed about 60%to the separation mechanism at the membrane interfaces,far exceeding that of steric hindrance and Donnan equilibrium.In ALC membranes,the addition of C-CNCs achieved the separation of ions in brackish water by directly acting to reduce the dielectric constant of membrane pores and increase membrane charge density.While C-CNCs in SLC membrane played a role of separation by indirectly reducing the pore size of polyamide active layer and the dielectric constant of NF membrane pore by changing the surface structure characteristics of the support layer.Therefore,in the preparation process of composite NF membrane,if the demand for permeability is greater,nanomaterials can be directly incorporated into the active layer;If higher separation and mechanical properties are required,placing nanomaterials in the support layer is a viable option.This study extends our understanding of nanocomposite membranes from a combined model-experiment perspective and provides further insights into the design and fabrication of composite membranes with specific performance.