Study On Scaling Mechanism And Pollution Mitigation Of Silicate And Transparent Exopolymer Particles In Reverse Osmosis Membranes

Reverse osmosis（RO）is the key technology of drinking water purification and wastewater treatment,which can effectively remove pollutant particles,colloids and various organic compounds in water.Membrane fouling will reduce the treatment efficiency and increase the treatment cost,which has become the main problem restricting the application and promotion of reverse osmosis technology.Dissolved silica and transparent exopolymer particles（TEP）are the main membrane pollutants in reverse osmosis desalination brackish water and water purification.Their deposition on the membrane surface will cause serious membrane scaling,which is not easy to be removed by membrane backwashing.Nevertheless,the binding mechanisms between silica and TEP and the resulting effect on membrane fouling has not yet been investigated.In this paper,the fouling characteristics and scaling mechanism of sodium silicate and TEP on the surface of reverse osmosis membrane were studied,and then the pollutants were removed by coagulation pretreatment to alleviate the membrane fouling and improve the membrane flux.Xanthan gum（XG）was selected as the reference material of TEP to study the reverse osmosis membrane fouling of different concentrations of sodium silicate and XG to explore the effect on the reverse osmosis membrane flux.By changing the influent solution pH,the two pollutants passed through the membrane fouling successively to explore the changes of pH on pollutants and the effects of different pollutants on membrane fouling.A series of physical and chemical analyses were carried out on the influent solution and the contaminated membrane surface to further explore the synergistic effect between sodium silicate and XG and the scaling mechanism of RO membrane,the concentration of pollutants was reduced by coagulation pretreatment,Fe Cl₃ and Al Cl₃coagulants and polyacrylamide（PAM）coagulants were selected to investigate the pretreatment effect under different pH and different dosages,so as to obtain the best coagulation treatment conditions.Through the detection of residual metal ions in the influent solution by inductively coupled plasma emission spectrometer（ICP-OES）,and the physical and chemical analysis of the supernatant and the pretreated membrane,the removal mechanism of pollutants by coagulation was obtained.The main conclusions are as follows:（1）The reverse osmosis membrane flux experiment showed that 5XG（5 mg/L）alone reduces the membrane flux by 23%.After adding sodium silicate,with the increase of silicon concentration,the membrane flux decreased faster before Si6（6m M）and more than Si6,and the flux tended to be stable,and the degree of fouling no longer increases significantly with the increase of silicon concentration.And the reversible pollution proportion of pollutants on the membrane also decreased with the increase of silicon concentration,which is consistent with the flux map.The results of infrared spectrum and Zeta potential showed that there was interaction between silicon and XG and between silicon and membrane,and there was a synergistic effect between silanol group（Si-OH）and carboxyl group（-COOH）of XG.The-OH functional group of monosilicate molecule could react with-COO^-/-NH₃⁺functional group on the membrane surface to form hydrogen bond,so that monosilicic acid could be deposited directly on the film.And with the increase of silica concentration,the interaction between free silicon would also form silica aggregates,which combined with XG to form a larger and more complex cross-linking network,resulted in more serious membrane fouling.It could be seen from the scanning electron microscope（SEM）that the contaminants on the membrane become denser with the increase of silicon concentration,and the pollution degree became more complex after adding XG.The SEM diagram of changing the XG concentration showed that the silicon particles are completely wrapped by XG colloid and closely adhere to the film.Through the study of the influent of silica and XG under different pH,it was found that the pollution of silica was the most serious when the pH is 9.Because the mono-silicic acid dissociates in the solution when pH was 9,the solubility of silica increases significantly,and the polymerization rate decreases to the lowest,so it was easy to block on the membrane surface and the membrane flux decreases rapidly.（2）In the pretreatment of pollutants by coagulation,Fe Cl₃ and Al Cl₃ coagulants had the best effect on improving membrane flux when the solution pH was 7,and the optimum dosage of both coagulants was 50 mg/L.And when the dosage of PAM in the two coagulants was 5 mg/L,the improvement of membrane flux and the removal effect of silicon ion were the best,the membrane flux was increased by 62%and 57%respectively,and the removal rate of silicon was 35.78%and 33.97%,respectively.For Fe Cl₃ coagulant,the more PAM was added,the less the residual coagulant dose was,which was 44.77%.But for Al Cl₃ coagulant,the more PAM was added,the more residual coagulant dose was,and the highest residual amount was 88.37%.Therefore,for 50 mg/L Fe Cl₃ and Al Cl₃ coagulant,the best dosage of PAM should be 5 mg/L and 1 mg/L respectively.The mechanism of two kinds of coagulation-assisted pretreatment to remove pollutants was that positively charged metal ions absorb negatively charged pollutants,and then the pollutants were precipitated to the bottom with the formed coagulation flocs.Combined with the removal of silicon and the residue of coagulant,the pretreatment effect was better of Fe Cl₃ for the removal of sodium silicate than XG.