Anti-fouling Performance Of Graphene Quantum Dots Nanofiltration Membranes

Graphene carbon quantum dots（GQDs）have ultra-small size,good hydrophilicity,anti-fouling,and biocompatibility,which could be applied to the preparation of ultrafiltration（UF）,nanofiltration（NF）,reverse osmosis（RO）,etc.,significantly improving the separation and anti-fouling performance of membranes.Based on high-flux graphene quantum dots NF membrane,this study used choline chloride（Ch C）and tannic acid（TA）as modified materials to deeply study the separation performance,anti-fouling performance and stability of the membranes.First,the post-treatment method was used to introduce choline chloride（Ch C）into the membranes,simulating the anti-fouling mechanism of the cell membrane surface.The Ch C and GQDs formed a zwitterionic structure together to improve the anti-fouling performance of NF membranes.The effects of Ch C concentration,reaction temperature and other factors on the membrane separation performance were investigated through experiments.Under the optimal conditions,the pure water flux reached 53.2 L·m^-2·h^-1·bar^-1,the rejection rate of methyl blue was over 95%,and the rejection rate of Na₂SO₄and Mg Cl₂ reached 52.7%and 34.5%,respectively.Moreover,the anti-fouling performance was greatly improved.The flux recovery rates（FRR）to the humic acid（HA）and bovine serum albumin（BSA）reached 94.5%and 83.6%,respectively.The separation layer of polyester structure formed in the pores of the base membrane had good stability under sodium hypochlorite or high temperature environment.Secondly,the GQDs NF membranes were modified by mixing TA into the interfacial polymerized aqueous monomer to improve the hydrophilicity of the membrane surface.The ratio of GQDs to TA regulated the chargeability and pore structure of the membrane surface,which helped the improvement of desalination and anti-fouling performance.With the introduction of TA,more dense TA-GQDs mixed nano-aggregates were formed in the pores of the base membrane.Due to the more negatively charged surface of the membrane,the rejection rate of Na₂SO₄ was up to 72.8%.Anti-fouling performance experiments showed that the hydroxyl groups in TA molecules could combine with water molecules through hydrogen bonding to form a hydrated layer to resist the adsorption of foulants.The FRR to the HA and BSA reached 93.5%and 80.1%,respectively.Finally,the XDLVO theory was used to theoretically calculate the interface interaction of the graphene oxide UF membrane and GQDs NF membrane prepared in the previous researches,fitted with the data of the anti-fouling experiments.The calculation results showed that the introduction of modified materials moved the cohesive free energy in the positive direction,and the hydrophilicity is significantly improved.Also,the total interface action was moved in the positive direction,and the repulsion of the membranes to foulants was enhanced.The total interface energy of the UF membrane appeared as an energy barrier at the distance of 3-5 nm,which resisted the adsorption of foulants.The FRR was linearly positively correlated with the total interface energy.