Study On Prevention And Control Technology Of Silica Scale Pollution In Reverse Osmosis System

Reverse osmosis technology is one of the important methods to solve the current shortage of water resources,but membrane pollution in the actual operation process seriously reduces the effluent quality and shortens the service life of the membrane.In the field on prevention and control of membrane pollution,the removal of carbonate scale is mostly focused on,and there are few studies on the more difficult-to-treat silica scale.Therefore,in this study,gallic acid?GA?was used to achieve both scale inhibition before the membrane and decomposition after silica scale formed on the membrane.Moreover,it can regenerate the membrane to restore its permeability,thus extending the service life of the membrane.In this study,we carried out three groups of experiments.In first group,the silicic acid solution and silicic acid mixed with GA solution were pass through the membrane to scaling scale was examined.In addition,GA solution pass through membrane,which already formed silica scale was cleaned.Then,explored the recycling of the membrane by optimizing the experimental process.Next,we conducted a full-band scan of silicic acid mixed with GA solution,changing the concentration and pH to explore the mechanism of scale inhibition.Finally,we used GA to depolymerize polysilicic acid and studied the changes in the content of different forms of silicic acid components and particle sizes.Then using gel chromatography to detect the grown process of silicic acid particles,combined with the calculated Kav value,to determine the depolymerization mechanism of GA to silica scale.The main conclusions are as follows:?1?When the pure silicic acid solution passed through the reverse osmosis membrane,as the silicic acid concentration?20-400 mg/L?increased,the time interval required to penetrate the same volume of solution?50 mL?continuously increased,and the concentration of silicic acid in effluent and the membrane flux decreased continuously,the amount of fouling on the membrane surface continued to increase.The SEM-EDX results showed that the membrane blockage is intensified.When the mixed solution of silicic acid and GA?400 mg/L each?passed through the membrane,the amount of silica scale on the membrane surface decreased,and GA played a certain role in scale inhibition,but it did not significantly improve the membrane flux.From the SEM observation,it can be seen that GA disperses and prevents small particles of silica to agglomerating into large particles,making them more evenly distributed on the membrane surface.?2?When cleaning the silica scale membrane with GA solution,the silicic acid concentration in effluent and the flux of membrane increased significantly,indicating that GA had a descaling effect and the scale cleaning ability was much greater than that of ultrapure water?about 30 times?.However,due to the reverse circulation process of the reverse osmosis system,the silica scale cleaned by GA reached the membrane surface again,and the subsequent membrane flux still had a tendency to decrease.Using a non-reverse circulation method to clean the silica scale membrane through GA could remove 81.87%of the scale on the membrane surface.The membrane flux returned to 89.7%of the initial flux,which can achieve the regeneration of the membrane.?3?The full-band scanning results of the solution showed that silicic acid reacted with GA to form a complex under neutral and alkaline conditions,and its concentration increased with the increase of pH?6.5-10?.Mass spectrometry?MS?analysis showed that GA and silicic acid produced a 1:3 complex of[Si(C₂₀H₁₁O₁₃)-COO]^-,which played a role in scale inhibition.The Si-O-C bond observed by FT-IR infrared spectroscopy proved that the complex can precipitate and separate from the solution in solid form.?4?High concentration pure silicic acid solution polymerized and grown into polysilicic acid particles with different particle sizes.After adding GA?0-400 mg/L?to the solution containing polysilicic acid particles with various particle sizes,the polysilicic acid quickly decomposed from large particle size to small particle size in just 0.5h,the silicic acid concentration increased accordingly.In the continuous reaction,the content of silicic acid components and particle size values in the solution fluctuated,GA depolymerization and silicic acid polymerization occurred at the same time.The overall result was that GA alleviated the silicic acid polymerization into a larger particle size.Gel chromatography separation results showed that GA can adhere to the surface of polysilicic acid particles to form surface complexes and then fall off into the solution in the form of soluble complexes to play a role in depolymerization.This is the mechanism of polysilicic acid with large particle size being decomposed into polysilicic acid with small particle size,monosilicic acid and silicic acid-GA complex.