Matrix Effect On Filtration Performance And Degradation Mechanism Of Polyamide Membranes In Water Treatment

The polyamide-based thin film composite membranes have been widely applied in(waste)water treatment and seawater desalination due to their excellent permeability and selectivity.The weak resistance of polyamide top layer to calcium and magnesium ions,disinfectant residuals and halide ions leads to the deteriorated membrane performance and reduced membrane lifespan.The necessity of improving the stability of membrane performance and maintaining the separation efficiency prompted a systematic investigation on the potential interaction between polyamide membranes,water matrix and target pollutants.This study aims to resolve the following four scientific problems,including the underlying mechanism of calcium and magnesium ion induced membrane fouling,degradation mechanism of membrane bromination,degradation kinetics of membrane under coexistence of chlorine and bromide ions and contributions of reactive halogenating agents to membrane degradation.This study provided theoretical and practical basis for the selection of pretreatment process and oxidants,process condition control,and the evaluation of membrane fouling and degradation for membrane systems.The main results of the study are as follows:(1)The calcium and magnesium ions induced rejection behavior of organic pollutants by poly amide nanofiltration membranes was investigated and the the interaction between calcium/magnesium ions and membranes was elucidated.The rejection of haloacetic acids(HAAs)by the fully aromatic polyamide membrane NF90 was not significantly affected by magnesium ions(Mg2+)due to the predominant role of size exclusion.For loose NF270,HAA rejection and water flux were reduced by 27%and 12%,respectively,after the addition of 5 mM Mg2+.The interaction between Mg2+ and membranes resulted in a less negatively charged and hydrophilic surface and a smaller effective pore radius for membranes.The reduction of the effective pore size by Mg2+ was verified as a result of competing effects between "-COO-induced conformational change" and "cation induced pore shielding".The increased permeability for charged HAAs and NaCl in the preselce of Mg2+ was mainly attributed to charge neutralization,while the reduced permeability for neutral surrogates and water was due to the reduced effective pore size and hydrophilicity.The filtration performance of membranes induced by Mg2+ and Ca2+ was different.(2)The filtration performance of polyamide nanofiltration membranes during bromination was studied and the underlying mechanism of membrane bromination was proposed.For fully aromatic polyamide membrane NF90 exposed under a mild bromination condition(10 mg/L),bromine incorporation resulted in more negatively charged and hydrophobic surfaces and narrower pore channels.The permeabilities of water and neutral solutes were reduced by 64%and 69%～87%,respectively,which was attributed to the decreased effective pore radius and hydrophilicity.NaCl permeability was reduced by 90%as a synergistic result of enhanced size exclusion and charge repulsion.The further exposure(100 and 500 mg/L bromine)resulted in a more hydrophobic surface and the minor further reduction for water and solute permeabilities(1%～9%).Compared with chlorine,the different incorporation efficiency and properties(e.g.,atomic size,hydrophilicity)of bromine resulted in opposite trends and/or different degrees for the variation of physicochemical properties and filtration performance of membranes.The bromine incorporation,the shift and disappearance of three characteristic bands,and the increased O/N ratio and calcium content indicated the degradation pathways of N-bromination and bromination-promoted hydrolysis under mild bromination conditions.The further ringbromination occurred after severe bromine exposure.(3)The degradation kinetics of polyamide nanofiltration membrane under the coexistence of bromine and chlorine were evaluated and the contributions of reactive halogenating agents to the membrane degradation were quantified.The observed pseudo-first-order rate constant of membrane degradation(kobsm,using change in water flux as a surrogate indicator)was found to be well correlated to[Br-],[Cl-]and[HOCl](R2>0.90).The gradual increase of[Cl-]and[Br-]transforms the predominant brominating agent from HOBr to BrCl and Br2,respectively,under excessive[Br-]conditions.The species-specific second-order reaction rate constants followed a decreasing order of kBrClm(2.6×104 M-1·s-1)>kBrOClm(2.0×103 M-1·s-1)>kBr2Om(9.6×102 M-1·s-1)>kBr2m(2.8×101 M-1·s-1)>kHOBrm(5.4×10-1 M-1·s-1).Additional decay tests using benzanilide(BA)as a surrogate monomer compound confirmed BrCl as the most reactive species.Under typical seawater conditions(pH 8.0),the more reactive but less abundant BrCl had significantly greater contribution to membrane degradation(85%)than HOBr(3%).Under typical neutral wastewater treatment conditions,both BrCl and HOBr contributed equally.(4)The filtration performance and degradation mechanism of polyamide nanofiltration membranes under the coexistence of bromine and chlorine were explored and the effect of water matrix on the species distribution of the reactive agents was clarified.Under coexistence of chlorine and bromide ions,the permeabilities of water and neutral solutes were respectively reduced by 46%～57%and 64%～77%when membranes exposed at pH 4.0 and 7.0 within 1 h,which was attributed to the decreased effective pore radius(from 0.41 nm to 0.37 nm)and hydrophilicity(from 51 ° to 74°).The minor further reduction of<11%observed within 1-10 h exposure was mainly due to the futher decrease of hydrophilicity(from 74 ° to 83°).The increased water permeability at pH 10.0 was attributed to the increased membrane pore radius(from 0.41 nm to 0.42～0.44 nm).The decreased NaCl permeability was owing to the enhanced charge repulsion and size exclusion.The 1～2 order of magnitude higher reaction rate constants at 0～1 h than that at 1～10 h suggests the N-halogenation and halogenation-promoted hydrolysis at 0～1 h and further ring-halogenation at 1～10 h.The gradual increase of solution pH(4.0～10.0)with the addition of[Br-]transforms the predominant brominating agent from BrCl(pH 4.0)to BrOCl and Br2O(pH 7.0),then to HOBr(pH 10.0).The induced reduction of water permeability via increased[Cl-],[HOCl]and[Br-]ex,was attributed to the increased contribution of BrCl,BrOCl and Br2,respectively.