Molecular Simulation Of Structure And Antifouling Property Of Chlorinated Polyamide Membrane Pollution

As a highly efficient desalination and wastewater treatment technology,reverse osmosis（RO）technology is widely used in chemical engineering,pharmaceutical,municipal water supply and other fields..In the membrane processes,bactericides are usually added to the water to avoid biological pollution of the membrane,in which the chlorinated biocides were used widely.The residual chlorine has an impact on the structure and performance of reverse osmosis membranes.However,the impacting mechanism is still not clear.In this word,the molecular simulation software of Materials Studio was used to investigate the structure and the anti-pollution properties of chlorinated polyamide RO membranes.Based on the literature results,two chlorinated PA membrane structures were constructed:a PA membrane with Cl substituted H on the amide bond（N-Cl-PA）and a PA membrane with Cl substituted H on the aromatic ring ortho-carbon（Ph-Cl-PA）.The interaction between the PA segments before and after chlorination was investigated using DFT quantum chemical simulations.It was found that the intramolecular charge distribution changed due to the introduction of Cl on the PA segments after chlorination,resulting in the bond length of N-H（Cl）from 1.02?increased to 1.76?,the expansion of the bond angle∠C-N-H（Cl）on the amide bond from 116.20°to 119.98°,and the torsion angle of the two adjacent aromatic rings changed from-172.26°to-122.86°.For the interaction between chlorinated structures of PA,the lowest interaction energy of chlorinated structures is-0.618 e V due to the change of hydrogen bonds between chlorinated structures.It was shown that chlorination leads to the formation of intramolecular hydrogen bonds between PA structures and weaken the interaction between polymer chains.At the same time,it also leads to the weakening of hydrogen bonds between PA structures and water molecules,which weakens the hydrophilicity of the membrane and is not conducive to the control of membrane pollution.Molecular dynamics simulations were used to study the structures of PA membrane,N-Cl-PA membrane and Ph-Cl-PA membrane before and after infiltration.The results showed that the MSD of the three PA membrane chains showed a linear upward trend,and the results of Ph-Cl-PA>N-Cl-PA>PA indicated that chlorination would increase the mobility of PA membrane chains.The analysis of the diffusion coefficients of water molecules showed that chlorination led to an increase in the diffusion coefficient of water molecules that could enter into the PA membrane,which was 12.84×10^-12 m²/s.Chlorination also increases the free volume of the PA membrane,forming small pores distributed between 0.7-0.9?in the membrane.On the other hand,the RDF results also demonstrated that chlorination leaded to weaker interactions between adjacent PA membrane chains,and increasement of the motility of membrane chains,which is beneficial for the water diffusion in PA membrane.Molecular dynamics simulation was used to analyze the pollution of PA membrane.It was found that the Cl atoms in the PA membrane chain could prohibit the accumulation of Na Cl on the membrane surface,and reduced the concentration polarization on the membrane surface.However,chlorination also leads to enhanced adsorption between the PA membrane and the pollutant of HA,which is more likely to cause membrane pollution.The adsorption energy of HA on the N-Cl-PA-w and Ph-Cl-PA-w membranes are-50.92 and-53.76 kcal/mol,respectively.At the same time,the RDF between HA and PA membrane was studied.It was found that the main interaction between chlorinated membrane and pollutant was the interaction between-OH of PA membranes and the hydroxyl groups of the HA,which resulted in a stronger adsorption of HA on the membrane.Our work showed that the interaction between PA molecular chains were reduced after chlorination,the chain motility was enhanced,the free volume of the membrane was increased,and the concentration polarization was weaken,all of which are conducive to the improvement of membrane flux.However,the enhancement of the adsorption of HA on the PA membrane after chlorination has reduced the membrane’s pollution resistance.Therefore,the effect of chlorination on PA membrane performance has two sides.In practical applications,the chlorination conditions can be controlled to achieve a balance between flux enhancement and pollution resistance of membrane.