Antifouling And Membrane Aging Performance Of Cellulose Nanocrystals-based Polyethersulfone Composite Ultrafiltration Membrane

Ultrafiltration technology is considered as an efficient and reliable water treatment technology.However,membrane fouling may occur during the operation.Membrane fouling is caused by the adsorption and aggregation of some substances on the membrane surface or in the membrane pores during the filtration process.Membrane fouling results in the increased energy consumption,increased chemical cleaning costs.It is an effective method to control membrane fouling by membrane modification.Nanocellulose crystals(CNCs),as a new nanomaterial,has attracted much attention in the field of water treatment due to its advantages as functional materials and environmental friendliness.In this study,CNCs was selected as the modified material of ultrafiltration membrane.Composite membranes were prepared by surface coating and blending modification methods.A set of characterization methods was used to study the surface physicochemical properties,antifouling abilities and aging behavior of the modified composite membrane.First,the surface coating membranes were prepared by CNC nanomaterials and the suspension was sprayed at a constant pressure.The changes in the morphology,flux,removal rate and fouling behavior of the coating membranes were compared based on the nanomaterial properties.The results showed that the surface coating with CNCs was dense and uniform,the roughness was slightly increased compared to the control membrane.After coating,the hydrophilicity and negative charge were significantly improved.Since the coating increased the mass transfer resistance of water molecules,the membrane flux decreased,but the removal ability of humic acid(HA)and antifouling were improved than that of commercial membrane.Second,the relationship among the composition of casting solution,the blending amount of CNCs and the flux,pore size,hydrophilicity,negative charge and mechanical strength of the prepared membrane were investigated,so as to obtain the optimal parameters for membrane preparation.The results showed that due to the abundant hydroxyl groups of CNCs,the hydrophilicity and negative charge of the membranes increased as the CNCs blended.When the concentration of CNCs was low,the porosity,pore size and MWCO of composite membrane increased and the membrane flux also increased due to the role of CNCs in promoting phase transformation.When the CNCs/PES concentration reached 5%,the phase transformation was suppressed,the membrane pore parameters and flux were reduced.After blending low concentration of CNCs,the mechanical properties of the membranes were enhanced due to the high strength and uniform dispersion of the CNCs.Then,the removal rates and antifouling properties of three typical pollutants by CNC composite membranes were studied.The effects of CNCs nanomaterials on reducing organic fouling of ultrafiltration membranes were analyzed by calculating the membrane fouling resistance.Results showed that CNCs blending can improve the removal of hydrophobic NOM.Membrane fouling was also alleviated and the reduction rate of NOM flux was slowed down when the filtration pressure was constant with the increase amount of CNCs.The fouling resistances of the three pollutants decreased with the increase of CNCs blending.The mitigation mechanisms of membrane fouling were explained below: Negative charge enhanced the repulsion between NOM and membrane surface;the improvement of hydrophilicity reduced the hydrophobic adsorption of NOM;in the process of membrane hydraulic cleaning and chemical cleaning,the NOM deposition in the macroporous CNC membranes were removed more effectively.In addition,the aging effects of sodium hypochlorite on CNC composite membranes were studied,the effects of the addition of CNCs on aging behavior of the membranes were investigated in the aspects of surface properties,flux,rejection and membrane fouling.Results showed that aging caused oxidative shedding of PVP and the break of PES chains,the hydrophilicity of the membrane decreased,the negative charge increased,the membrane pores enlarged,and the membrane flux increased.Except for Zeta potential,the surface properties and filtration performance of CNC aging membranes changed less than that of M0.That is,CNCs played a role in maintaining the stability of aging membranes.The removal rate of HA by aging membranes with high CNCs blending content was higher.The HA pollution of the control membrane affected by aging was serious,whereas the HA pollution of CNC membranes affected by aging was weaker.The fouling resistance of CNC membranes were smaller than that of control membrane after aging.