Study On Removal Performance Of Fluoride Ions By Domestic Nanofiltration Membrane

The fluoride content in cellar water and well water in rural areas in northwestern China exceeds the provisions of sanitary standards for drinking water.Long-term drinking of excess fluoride-containing water will cause fluoroskeletal diseases and fluorosis and other diseases,which will endanger life safety in severe cases;In terms of water treatment,the research and application of domestic nanofiltration membranes is relatively less than that of imported nanofiltration membranes,and the price is relatively cheaper than imported membranes.Therefore,the use of domestic membranes to treat fluorine-containing water exceeding the standard is of great significance for ensuring the safety of drinking water.To study the basic properties of two nanofiltration membranes,in this paper,such as the characteristic parameters?pure water transmission coefficient-L_P,reflection coefficient-?and solute transmission coefficient-P?and structural parameters?membrane pore radius-r?of two domestic nanofiltration membranes?NF1 and VNF2?are mainly determined.Through the retention experiment of fluorine-containing inorganic salt solutions of different components,the retention effect of domestic membranes on various anions and cations is investigated,and the effect of coexisting anions on the removal of fluorine by nanofiltration is investigated;Study the effects of different concentrations of humic acid,sodium alginate and bovine serum protein on nanofiltration fluoride removal,and explore the effect of organics on nanofiltration to remove fluorine and membrane flux;Study the effects of raw water pH,raw water F^-concentration,operating pressure,and raw water temperature on nanofiltration membrane defluorination and membrane flux,and determine its order of influence and optimal operating conditions by orthogonal design;at the same time,through response surface design,determine its response sequence.Through the study of the removal performance of each ion in the fluorinated brackish water,investigate the effect of domestic membrane on the removal of brackish water,which provide technical support for the treatment of brackish water.It mainly includes the following aspects:First,research on the basic performance of domestic nanofiltration membranes:1.Through model construction of two domestic nanofiltration membranes of NF1 and VNF2 and pure water experiments,it can be concluded that the membrane flux and pressure of bomestic membranes have a good linear relationship;the data of dissociated NaF is fitted by a non-thermodynamic equilibrium model,which also shows a good linear relationship.2.By carrying out NaF solution retention experiments on membranes NF1 and VNF2,and measuring the characteristic parameters of the membrane,the measurement results show that:in membrane NF1,?=0.9253,P=1.2887*10^-6m/h,L_P=182.6m/h,r=1.53nm;in membrane VNF2,?=0.9235,P=0.3615*10^-6m/h,L_P=147.1m/h,r=1.35nm;the greater the pure water transmission coefficient,the greater the membrane pass;the higher the amount,and the larger the pore radius,the lower the rejection rate.Second,research on the effect of domestic nanofiltration membrane on the treatment of fluorine water with over standard:1.The effect of single-component inorganic salt solution on domestic nanofiltration to remove fluorine,the results show that the domestic nanofiltration membranes NF1 and VNF2have the same retention rules for anions and cations,and the cations are:Mg²⁺>Ca²⁺>Na⁺,and the anions are:F^->HCO₃^->SO₄^2->Cl^-.2.The effect of the two-component fluorine-containing inorganic salt solution on membrane NF1 and VNF2 is studied,and the results show that the order of coexistence of Cl^-,HCO₃^-,and SO₄^2-to the degree of change of the rejection rate of F^-is SO₄^2->Cl^->HCO₃^-.This shows that the effect of higher valence ions on lower valence ions is more than that between isovalent ions;And the influence of the same valence ion shows that the greater the difference in diffusion coefficient between ions,the smaller the difference in hydrated ion radius,the greater the change in ion rejection rate;conversely,the smaller the change in rejection rate.3.The effect of multi-component fluorine-containing inorganic salt solution on membrane NF1 and VNF2 defluoridation is studied.The results show that when Cl^-,HCO₃^-and F^-coexist,the retention of F^-changes little;When Cl^-,F^-,and SO₄^2-coexist,the rejection rate of F^-changes significantly,further indicating that the influence between the same-valent ions is not as good as the influence of the existence of higher-valent ions on the lower-priced ions.4.The effect of different natural organic matter and its concentration on the defluoridation of membranes NF1 and VNF2,the results show that the effect order on the retention rate of F-by sodium alginate?SA?,humic acid?HA?and bovine serum protein?BSA?is the same:BSA>SA>HA;the effect order on membrane flux is:NF1 membrane,BSA>SA>HA;VNF2 membrane,HA>BSA>SA.The effect of natural organic matter on the removal of fluorine by nanofiltration is related to its molecular weight,hydrophilicity,hydrophobicity,hydrophilicity,and the force between organic matter and nanofiltration membrane or F-and organic matter.5.The effects of F^-concentration of raw water,raw water temperature,raw water pH,and operating pressure on membrane NF1 and VNF2 defluoridation and membrane flux are studied.The results show that the retention rate of membrane VNF2 is higher than membrane NF1,and the membrane flux is lower.At the same time,when the pH of raw water is 6.5?7.0,the temperature of raw water is 18?23?,the operating pressure is 0.3?0.4Mpa,and the F^-concentration of raw water is 1?8mg/L,the treatment effect is better.6.Using orthogonal design tests on three factors:operating pressure,raw water pH,and F-concentration of raw water,the major and minor sequences that affect membrane defluorination and the best operating conditions are studied.The research shows that the major factors affecting membrane NF1 is F-concentration of raw water>operating pressure>pH of raw water.The best operating condition are pH of raw water at 6.5,operating pressure of 0.4MPa,and F^-concentration of raw water at 4mg/L.And the major factors affecting membrane VNF2 is pH>operating pressure>F^-concentration of raw water,the best conditions are that the pH of raw water is 7.0,the operating pressure is 0.4MPa,and the F-concentration of raw water is 4mg/L.7.Using the Box-Behnken Design?BBD?design in response surface method and a regression model based on the response rate of fluorine ion retention,the three factors of raw water pH,raw water F^-concentration,and operating pressure in the membranes NF1 and VNF2 are investigated.The results show:the model of the two has a high degree of fitting,and the response sequence of three factors:in NF1 membrane,the sequence is raw water pH>raw water F^-concentration>operating pressure;in VNF2 membrane,the sequence is raw water F-concentration>raw water pH>operating pressure.8.Removal of ions in brackish water by membranes NF1 and VNF2,the results show that,after first-stage nanofiltration technology treatment,F^-,SO₄^2-and total hardness meet the"Sanitary Standard for Drinking Water";In the membrane NF1,the TDS meets the standard at an operating pressure greater than 0.30Mpa;Only in the membrane VNF2,the Cl^-concentration meets the standard at an operating pressure of 0.5Mpa.