Preparation Of Reverse Osmosis Membranes With Silica Scaling Resistance

Reverse osmosis is an advanced water treatment technology,which makes great contribution to coping with the freshwater crisis and water pollution because of its exceptional energy efficiency,ease of scaling-up,as well as simple operation.However,organic fouling and mineral scaling are two major bottlenecks of RO desalination,which significantly shorten membrane life,intensify energy consumption,and increase expenses of equipment maintenance.Membrane surface properties were properly tailored to enhance membrane fouling and scaling resistance in this work.The influence of operating conditions of reverse osmosis system（such as feed solution temperature,operation pressure,flow rate,etc.）on silica scaling on the membrane surface was studied.Through monitoring water flux decline and surface morphology after silica scaling test,it was found that the rate of silica scaling on membrane surface increased under the condition of high feed solution temperature,low flow rate and high operation pressure.And the operating conditions with feed solution temperature of 25°C,cross-flow velocity of 1 L·min^-1 and operation pressure of 1.5～2.5 MPa were selected for silica scaling tests.The polyamide thin-film composite（PA-TFC）membranes were modified with multilayered PhA and Fe^III complexes using layer-by-layer（Lb L）assembly.The Fe-PhA complexes enhanced both hydrophilicity and negative charge,while decreasing the surface roughness of the TFC membrane.The Fe-PhA assembled membranes achieved a slightly decreased water flux and an increased Na Cl rejection.The results of silica scaling test and BSA fouling test showed that Fe-PhA membrane exhibited excellent resistance against both individual silica scaling and BSA fouling in RO desalination.The Fe-PhA assembled membranes further showed remarkably less flux decline（about 20%）compared to the PA-TFC membrane when silica scaling and BSA fouling co-existed,so Fe-PhA assembled membranes had resistance of combined silica scaling and BSA fouling.After soaking in acid or alkaline solution for20 days,the element composition of Fe-PhA assembled membrane surface did not change significantly.Within 48 hours of continuous cross-flow test,the permeability flux and salt rejection of Fe-PhA assembled membrane remain stable.The PA-DABS membranes with special surface characteristics were prepared by adding 2,5-Diaminobenzenesulfonic acid（DABS）into aqueous phase during interface polymerization process.Compared with PA membrane,PA-DABS membranes also enhanced hydrophilicity and negative charge,while decreasing the surface roughness.The results of the permselectivity test showed that the water flux and salt rejection of PA-DABS membrane maintained similar to those of PA membrane when the concentrations of DABS were 0.25%and 0.50%in the water phase solution.However,the water flux of the PA-DABS membrane increased and the salt rejection decreased when the concentration of DABS was 1.0%in the water phase solution.After 24 h silica scaling test,flux decline of PA-DABS-0.50%membrane was 11%lower than that of PA membrane,showing PA-DABS membrane had silica scaling resistance.The PA-DABS membranes further showed remarkably less flux decline compared to the PA membrane when silica scaling and BSA fouling co-existed.