Research On Controlling Efficiency And Mechanism Of TFC Forward Osmosis Membrane Fouling By Ca²⁺ In-situ Modification

Water scarcity is one major challenge,wastewater reuse and seawater desalination have been feasible means of potable water production.Forward osmosis has raised increasing attentions as an emerging membrane technology.Thin film composite(TFC)polyamide membrane has become the preferred membrane material in the process of water treatment.However,membrane fouling is still the main obstacle hindering the development of membrane technology.Based on the membrane fouling mechanism proposed by previous studies,this study attempts to design and prepare antifouling TFC membrane to mitigate the organic fouling and biofouling,which may be of great significance for TFC membrane fouling control.There are large numbers of carboxyl functional groups on the surface of TFC membrane.These carboxyl groups are susceptible to organic fouling in the presence of divalent ions such as Ca²⁺,in which Ca²⁺bind to the carboxyl groups on the membrane surface and those of the organic foulants to form membrane-Ca²⁺-organic bridges,resulting in more severe membrane fouling.This study reports a novel intra-bridging strategy to improve the anti-fouling performance of TFC membrane,in which the addition of Ca²⁺during the interfacial polymerization reaction led to the formation of stable Ca²⁺-carboxyl complexes within the polyamide rejection layer.This intra-bridging of carboxyl groups by Ca²⁺effectively sequestrated them,reducing their availability for binding divalent metal ions in the aqueous solution and for forming foulant-metal-membrane inter-bridges.The physical and chemical properties of TFC membrane were analyzed by various characterization methods.The results showed that the free carboxyl groups on the surface of the TFC membranes decreased from 28.9±0.8nm^-2(the absence of Ca²⁺in the polyamide layer,abbreviated as TFC-control membrane)to 16.9±0.5 nm^-2(with the addition of 1.0 wt.%Ca²⁺in the polyamide layer,abbreviated as TFC-Ca(1.0)membrane),and the Zeta potential of the membranes increased from-28.6±1.2 m V to-19.4±0.9 m V due to charge neutralization.The in-situ Ca²⁺bridging between and carboxyl group in the polyamide layer effectively sequestrated the combination of divalent metal ions and foulants in aqueous solution,and inhibited the formation of divalent cations-foulant bridging contamination on the membrane surface.At the end of sodium alginate fouling experiment,the water flux of TFC-control membrane decreased by 36.7%,while that of TFC-Ca(1.0)membrane only decreased by 17.3%.After cleaning,the water flux recovery rate of TFC Ca(1.0)membrane was more than 95%.Membrane fouling and cleaning experiments confirmed that improved flux stability and fouling reversibility for the Ca²⁺modified TFC membranes.To better understand the membrane fouling characteristics and anti-fouling mechanism of TFC membrane modified by Ca²⁺-intra bridging(TFC-Ca membrane)strategy,the influence of organic foulants on the fouling behavior of TFC-Ca membrane with in-situ Ca²⁺addition was evaluated.Bovine serum albumin(BSA),humic acid(HA)and sodium alginate(SA)were used as surrogate foulants for protein,natural organic substances and polysaccharides,respectively,thus enabling the analysis of foulant-membrane interaction in the membrane fouling process.The effect of high ionic strength(500 mmol/L)on the fouling behavior of TFC-Ca membrane was also investigated.The fouling mechanism of TFC-Ca membrane was investigated by Extended Derjaguin–Landau–Verwey–Overbeek(XDLVO)theory regarding the influence of organic foulant together with divalent ions.Results suggested that the decrease of water flux caused by organic foulants is as follows:BSA<HA<SA,and the antifouling performance of TFC-Ca membrane is much better than that of TFC-control membrane.Scanning electron microscopy(SEM)results showed that the thickness of the TFC-Ca membrane fouling layer decreased.For SA fouling,the fouling layer thickness of the TFC-control membrane and TFC-Ca(1.0)membrane were 14.64±0.27?m and 9.62±0.43?m,respectively.The results of ICP-OES and TOC also showed that the accumulation of organic compounds and divalent ions in the fouling layer of TFC-Ca membrane were reduced.The interfacial free energies obtained from advanced contact angle measurements were correlated strongly with the rates of membrane fouling.Compared with TFC-control membrane fouling(?G_mlf^TOT=-18.13 m J/m²),in-situ Ca²⁺addition in the TFC membrane resulted in the decrease of the interfacial adhesion free energy(i.e.,SA-membrane interaction,?G_mlf^TOT=3.98 m J/m²)and thus the mitigation of membrane fouling.The permeate flux of TFC-Ca membrane after organic fouling could be fully restored by simple physical cleaning.The results indicated that the TFC-Ca membrane by shielding the interactions of organic matter-Ca²⁺(in the feed solution)-membrane can effectively reduce the adhesive attraction,mitigate membrane fouling and improve the membrane antifouling properties.The carboxyl group on the surface of TFC membrane may affect the interaction between microorganism and membrane surface,and membrane biofouling is also one of the most challenging issues to be addressed for the successful application of TFC membrane.In this work,we demonstrated that the presence of divalent cations,especially Ca²⁺could intensively aggravate the biofouling on TFC membrane by both model bacteria of Escherichia coli and Staphylococcus aureus.The TFC polyamide layer was modified by in-situ addition of carboxylated chitosan(CCTS)during interfacial polymerization(IP),the surface carboxyl density of TFC-CS(0.3)membrane(with the addition of 0.3 wt.%CCTS in the polyamide layer)was increased to 47.7±1.0nm^-2,which was about 2 times of that of TFC-control membrane,as well as the hydrophilicity of the membrane was improved and the surface roughness was decreased.In the simulated biofouling experiment,when the cumulative permeated water flux was500 m L,the permeate fluxes of TFC-control membrane,TFC-CS(0.2)membrane and TFC-CS(0.3)membrane decreased significantly,which were 49.5%,41.6%and 37.0%of the initial flux of E.coli fouling,respectively.It may be attributed to the change of carboxyl groups on membrane surface and their interaction with bacteria via Ca²⁺-carboxyl bridging promoting biofouling.Furthermore,by pre-occupying carboxyl groups on polyamide layer via in situ Ca²⁺addition during interfacial polymerization reaction,it was found that reducing carboxyl density of polyamide layer could effectively mitigate biofouling of TFC membranes,the flux of TFC-Ca(1.0)membrane was decreased only 35.6%of the initial flux of E.coli contamination(when the cumulative permeated water was 500 m L).The results demonstrate that carboxyl functional groups of the membrane surface play a key role in affecting membrane fouling,and reducing the carboxyl active sites on the membrane surface can significantly improve the antifouling properties of the TFC membrane.This work provides a new pathway for better understanding and control of biofouling in the design and application of TFC membranes.