Preparation And Characterization Of Calcium Alginate-sulfonated Polysulfone/Polyethersulfone Antifouling Nanofiltration Membrane

Nanofiltration（NF）technology is a green separation method,has good ion selectivity and low operating cost,and has strong pertinence for brackish water treatment in rural areas in Northwest China.However,during the nanofiltration process,the natural organic compounds（NOM）in the water will cause pollution to the nanofiltration membrane,preventing the long-term and stable operation of the nanofiltration membrane.Therefore,it is of great significance to improve the anti pollution performance of nanofiltration membrane for its practical application.In order to improve the anti-fouling performance of nanofiltration membranes,by improving the hydrophilicity of nanofiltration membranes,polyethersulfone（PSF）and sulfonated polysulfone（SPS）were used as the main materials of the base membrane,and PEG was used as porogen.The base film was prepared by phase inversion method.Sodium alginate（SA）was used as the main material of the composite layer,and titanium dioxide（TiO₂）was used as an additive.The calcium alginate sulfonated polysulfone/polyethersulfone composite nanofiltration membrane was prepared by the method of calcium ion crosslinking,and the effect of the proportion of each component in the casting solution on the structure and performance of the nanofiltration membrane was studied by different analysis methods（microstructure characterization,contact angle analysis,Fourier infrared spectroscopy）and experiments,and the optimal proportion was determined.On this basis,the self-made flat nanofiltration device was used.The basic properties of self-made nanofiltration membrane and NF-1812 nanofiltration membrane made in China were compared,and the effects of different properties of brine treatment were studied.Finally,the cleaning effects of self-made nanofiltration membrane and domestic nanofiltration membrane were compared by physical and chemical cleaning methods.The main research conclusions of this paper are summarized as follows:1.The experimental results of the self-made nanofiltration membrane and NF-1812nanofiltration membrane made in China show that:（1）In the casting film liquid system,as the molecular weight and content of PEG increased,the pure water flux of the composite membrane increased,the rejection rate of salt and organic matter decreased,the antifouling performance decreased,and the membrane pore and average pore diameter increased.This is because PEG as a porogen can promote membrane pore growth and increase the membrane surface roughness.（2）With the increase of the content of SPS,the pure water flux of the composite membrane decreased,the rejection rate of salt and organic matter increased first and then decreased,the anti-pollution performance was enhanced,and the membrane porosity and average pore size were reduced.This is because sulfonated polysulfone delays the phase separation of the film formation process,inhibits the formation of macropores,and introduces hydrophilic groups.However,excessive sulfonated polysulfone reduces the stability of the membrane casting liquid system,resulting in poor membrane pore uniformity.（3）As the degree of sulfonation of the SPS increased,the pure water flux of the composite membrane increased,the rejection of salts and organic matter decreased,the antifouling performance decreased,and the membrane porosity and average pore size increased.This is due to the increase in the degree of sulfonation of sulfonated polysulfone,which leads to a decrease in its compatibility with polyethersulfone,poor film-forming performance,and defects.（4）With the increase of TiO₂ content,the pure water flux of the composite membrane increased,and the rejection rate of salt and organic matter increased first and then decreased,and the anti-pollution performance increased first and then decreases,the membrane porosity increased,the average pore size decreased first and then increased.This is because the introduction of TiO₂ enhances the hydrophilicity of the membrane and increases the number of water channels.At the same time,TiO₂ can promote the cross-linking reaction of calcium alginate and make the composite layer smooth and dense.However,excessive TiO₂ causes defects on the film surface due to its agglomeration.（5）The optimal ratio of the casting film solution is that the PES:SPS:PEG in the base film is 20:6:6,of which the optimal sulfonation degree of sulfonated polysulfone is 10%,and the molecular weight of PEG is 4000Da.Sodium alginate:TiO₂ was 10:1.6.2.The experimental results of performance comparison between homemade nanofiltration membrane and domestic NF-1812 nanofiltration membrane show that:（1）Under the operating pressure of 0.3MPa,the pure water flux of the homemade nanofiltration membrane and domestic nanofiltration membrane is 20.54 L/m²·h and25.12L/m²·h,and the porosity is 57.94%and 13.8%,respectively.The average pore radii are 1.19 nm and 1.40 nm,respectively.（2）The mixture of humic acid（HA）,bovine serum albumin（BSA）,and sodium alginate（SA）was used to simulate natural organic contaminated liquid.After passing through the membrane for 8 hours,the flux attenuation coefficients of the homemade nanofiltration membrane and domestic nanofiltration membrane are 0.1954 and 0.3258.This shows that the self-made nanofiltration membrane has better anti-pollution performance.（3）The rejection rates of homemade nanofiltration membranes to five salts of 2000 mg/L NaCl,CaCl₂,MgCl₂,MgSO₄ and Na₂SO₄ are 31.58%,51.04%,58.01%,68.33%and76.83%,respectively,and the rejection rates of domestic membranes to five salts are 29.30%,29.82%,50.07%,61.18%and 67.16%.The rejection rates of homemade nanofiltration membranes to HA,BSA,and SA are 91.99%,94.02%,and 96.23%,respectively.The rejection rates of domestic nanofiltration membranes to three organic substances were 90.32%,92.14%,and 95.88%,respectively.（4）Compared with domestic nanofiltration membranes,homemade nanofiltration membranes have smaller pore sizes,larger porosity,better salt rejection,organic rejection,and anti-pollution performance,but lower pure water flux and poorer flux stability.The order of the retention rate of the five kinds of salt by the self-made nanofiltration membrane and the domestic nanofiltration membrane are:Na₂SO₄>MgSO₄>MgCl₂>CaCl₂>NaCl,and the order of the rejection rate of the three organic substances is SA>BSA>HA.When processing natural inferior saline water with a mineralization degree below 1000mg/L,all the indexes of nanofiltration water can reach the"Sanitary Standard for Drinking Water"（GB5749-2006）,and natural bitter and salty water with a mineralization degree above 1000mg/L.At this time,with the exception of the degree of mineralization and Cl^-,the other indicators can reach the standard.3.The experimental results of physical+chemical cleaning of domestic nanofiltration membranes and homemade nanofiltration membranes show that:（1）Under the operating pressure of 0.3MPa,the nanofiltration membrane after continuous pollution for 8 hours,was physically cleaned with pure water for 30 minutes,0.1%NaOH+0.025%sodium dodecyl sulfonate（Na-SDS）at pH=11.5 chemical alkali Wash for50 min,chemical pickling with 0.2%HCl at pH=4.0 for 40 min.After the combination of physical and chemical cleaning,the pure water flux of the domestic nanofiltration membrane recovered to 97%of the original membrane,and the rejection rates of NaCl,Na₂SO₄,HA and SA were similar to the original film.The self-made nanofiltration membrane pure water flux increased by 9%,the rejection rates for NaCl and Na₂SO₄ decreased by 2.01%and 3.12%,and the rejection rates for HA and SA were similar to the original（2）The membrane pollution resistance analysis was performed on domestic nanofiltration membranes and homemade nanofiltration membranes.The order of membrane pollution resistance was in the following order:concentration polarization resistance>membrane resistance>gel layer resistance>internal pollution resistance,After the physical+chemical combination cleaning,the remaining internal pollution resistance of the domestic nanofiltration membrane was 0.71%,and the remaining internal pollution resistance of the homemade nanofiltration membrane was-2.37%.There was a negative value,but the value was small,indicating that the physical-chemical combination cleaning can effectively eliminate The self-made nanofiltration membranes and domestic nanofiltration membranes have good resistance to membrane fouling,and achieve good cleaning results.