Performance Improvement And Restoration Of Polyamide Composite Reverse Osmosis Membrane Based On Nitrous Acid Treatment

Aromatic polyamide is a common material used in the manufacture of high-performance reverse osmosis membranes,and the separation performance of polyamide composite reverse osmosis membranes mainly depends on the polyamide separation layer.Therefore,the separation performance of polyamide composite reverse osmosis membranes can be controlled by chemical modification of the separation layer.The structural units that can be used for chemical modification of the polyamide separation layer include benzene ring,amide bond,carboxyl group,and terminal amino group.Diazotization reaction is an organic chemical reaction targeted at aniline structures,and sodium nitrite,as a commonly used diazotization reagent,can be used for chemical modification of polyamide membrane materials.The feasibility and mechanism of regulating the separation performance of polyamide composite reverse osmosis membranes through nitrous acid treatment at room temperature were investigated in this study.Experiments were conducted by immersing polyamide composite reverse osmosis membranes in nitrous acid solution at low temperatures of 0～5°C,room temperature of 15°C,and 25°C.A comparison of the permeate selectivity(A/B ratio)between the nitrous acid-treated membranes and the original membranes indicated a negative correlation between temperature and membrane performance improvement,but the permeate selectivity of the membranes at25°C was still improved by 2 times,demonstrating the feasibility of enhancing membrane separation performance through nitrous acid treatment at room temperature.Orthogonal experiments revealed that the p H of the nitrous acid solution had the most significant influence on the efficiency of membrane performance improvement.Mechanistic studies based on UV-Vis diffuse reflectance spectroscopy(UV-Vis-DRS),attenuated total reflectance Fourier transform infrared spectroscopy(ATR-FTIR),X-ray photoelectron spectroscopy(XPS),and membrane surface zeta potential measurements showed that nitrous acid could react not only with the terminal amino groups on the membrane surface,but also with the amide units through N-nitrosation reaction,resulting in N-nitrosamide structures that could undergo nitrosoacylarylamine rearrangement under certain conditions,leading to amide bond cleavage and the generation of more carboxyl groups on the membrane surface.The diazonium salts generated from the diazotization reaction and the amide bond cleavage caused by N-nitrosation could further react to form azo and phenol structures.The azo structures could interact with sodium ions through electrostatic interactions,enhancing the membrane’s sodium ion rejection capability.The phenol structures,which are more hydrophilic compared to the aniline structures,and the partial hydrolysis of the amide bonds in the separation layer,further improved the water permeability of the membrane.These two performance improvements compensated for the negative effects of amide bond cleavage,resulting in enhanced separation performance.Based on the investigation of the mechanism of performance enhancement,further optimization of nitrous acid treatment effects was achieved by varying the p H of the nitrous acid solution and the concentration of sodium nitrite.Evaluation of membrane separation performance showed that the water permeability of the nitrous acid-treated membrane increased as the solution p H decreased,with the maximum water permeability observed at p H=1.0 when using nitrous acid prepared with 0.3 wt%sodium nitrite,reaching as high as 16.8 L/(m2·h·bar).Considering the overall separation performance of the membrane after nitrous acid treatment,it is recommended to control the p H of the nitrous acid solution at around 1.5,at which the membrane retention performance can be improved to 98.7%.Finally,the feasibility and mechanism of Diazotization reaction for repairing the separation performance of chloride-degraded polyamide reverse osmosis membranes were investigated.Chloride-degraded membranes with specific rejection rates were prepared using alkaline chloride degradation methods,and then repaired using nitrite treatment with m-phenylenediamine(MPD)as the exogenous amine source.The results showed that direct nitrite treatment led to further deterioration of membrane performance,while nitrite treatment with MPD as the exogenous amine source successfully restored the performance of degraded membranes.Repair experiments with salt rejection rates of 88%,92%,95%,and 97% for degraded membranes demonstrated that MPD-assisted nitrite treatment was more effective in repairing membranes with lower initial desalination rates.When the separation performance of degraded membranes was not lower than 95%,the salt rejection rate of the repaired membrane could be restored to over 98%.Repair experiments with different MPD concentrations indicated that MPD concentrations in the range of 0.01 to 0.05 mol/L achieved better membrane performance repair effects.Lower MPD concentrations not only failed to effectively improve membrane separation,but also could potentially cause further hydrolysis and degradation of the membrane,while higher MPD concentrations resulted in reduced water permeability of the repaired membrane.