Preparation And Anti-fouling Performance Of Double Selective Layered Composite Forward Osmosis Membrane With Ta/Fe³⁺ Modification

Forward osmosis is a membrane separation process with the advantages of high ion selectivity,low energy consumption,low membrane pollution tendency and high recovery rate.However,the phenomenon of concentration polarization is an important reason that limits the wide application of this technology,and the key to slow down the concentration polarization of forward osmosis is to improve the structural parameters of the membrane.In this experiment,layer-by-layer self-assembly and interfacial polymerization were combined to prepare a double-selective layer with polyelectrolyte layer and polyamide layer on polyacrylonitrile base membrane,and tannic acid/iron ion TA/Fe³⁺was used for complexation modification to comprehensively improve the performance of forward osmosis membrane.The polyelectrolyte layer was polyethyleneimine（PEI）and sodium polystyrene sulfonate（PSS）as the cationic and anionic polyelectrolytes,and the polyamide layer was m-phenylenediamine（MPD）and1,3,5-Benzenetricarbonyl trichloride（TMC）as the water phase and organic phase,respectively,on this basis,by studying the influence of various factors on the basic properties of the composite forward osmosis membrane in the process of membrane preparation,the best membrane preparation conditions were obtained.From the point of view of the influence of typical pollutants on the modified composite forward osmosis membrane,the anti-pollution performance of the membrane was investigated.The main conclusions are as follows:（1）The optimum preparation conditions of TA/Fe³⁺modified composite forward osmosis membrane are as follows:PEI deposition time in self-assembled layer is 0.5 h,Na Cl concentration is 1.5 mol/L,PSS deposition time is 15 min,and the concentration is2 g/L,Na Cl concentration is 1.5 mol/L,and heat treatment time is 5 min;In the interfacial layer,MPD deposition time is 3 min,and the concentration is 20 g/L,acid absorbent is 2wt%triethylamine（TEA）and 4 wt%camphorsulfonic acid（CSA）,TMC deposition time is 1 min,and the concentration is 1 g/L;In the modified layer,the complexing time is 5min,the concentration of Tris is 0.03 mol/L,the mass concentration ratio of TA/Fe³⁺is3:1,the concentration of TA is 0.2 g/L,and the number of complexing layers is 1 layer.（2）According to the above optimal conditions,the water flux of the modified composite forward osmosis membrane is 23.91 L/（m²·h）,which is 2.15 times that of the polyamide membrane TFC membrane,and the reverse salt flux is 6.38 g/（m²·h）,which is1/3 that of TFC membrane;Compared with the membrane prepared by double selective layer,the water flux of the modified composite forward osmosis membrane in FO mode is increased by 40%when the reverse salt flux is similar.（3）Self-assembly layer can regulate the base membrane and interfacial polymerization layer to some extent.Compared with the base membrane and TFC membrane,the preparation of self-assembled layer reduces the surface roughness of the membrane and enhances the hydrophilicity.The preparation of TA/Fe³⁺modified layer can regulate and control the structure of double-selective membrane.Although the surface roughness of the membrane is slightly increased and the adsorption of water molecules is weakened to a certain extent,compared with the self-assembled membrane,the flux recovery rate is increased from 74.2%to 79%,which shows that the preparation of modified layer enhances the anti-pollution ability of the composite forward osmosis membrane.At the same time,the modified layer complex structure is conducive to the absorption of a large number of water molecules on the surface of the polyamide layer,and then forms a hydration layer,which improves the diffusion rate of water molecules in the membrane,and the best composite film surface roughness Ra are 1.93 nm,and water contact angle are 59.98°,showing better membrane surface smoothness and higher hydrophilicity.（4）Sodium alginate（SA）,bovine serum protein（BSA）,and humus（HA）were used to represent typical organic pollutants in water for positive membrane separation test,and it was found that SA was the most seriously polluted by composite positive permeable membrane,followed by HA,and BSA was the least polluted.In the process of positive permeation of modified composite membranes by different concentrations of SA,the attenuation rate of membrane flux within a certain range accelerates with the increase of SA concentration,and SA concentration is the main factor affecting the pollution of modified composite positive permeable membranes.In the SA continuous pollution test,the modified membrane after 3.5 cycles still has good water permeability and high flux recovery rate,indicating that the modified composite positive permeable membrane has good pollution resistance,membrane reuse and stability.