| Membrane fouling is inevitable during the long-term operation of nanofiltration(NF)membrane,which limits the large-scale applications of NF technology.Chemical cleaning is considered the most effective way to avoid fouling aggravation and quickly restore membrane permeability.While frequent chemical cleaning would cause reversible/irreversible changes in the physical and chemical properties of NF membranes,resulting in membrane damage and deterioration of separation performance.Therefore,the elucidation of the impacts of chemical cleaning on NF membranes is the key to avoid membrane damage,and the development of highly efficient cleaning agents is crucial to improve cleaning efficiency.In this study,the effects of alkaline cleaning agents on the physicochemical properties and fouling behavior of polyamide(PA)NF membranes were discussed.Composite chemical cleaning agent was proposed to eliminate the negative effects of alkaline cleaning,and the corresponding cleaning mechanism was clarified.For stubborn fouling,the cleaning efficiency of free chlorine,hydrogen peroxide and Fenton reagent were evaluated,and the influence of these cleaning agents on the properties of PA NF membranes was studied.Finally,inspired by peroxidase,surface chemical modification of Fe3O4 nanoparticles was carried out to improve its affinity to hydrogen peroxide.The modified catalyst could improve the efficiency of Fenton cleaning agent and alleviate the secondary fouling during the cleaning process.The detailed results are as follows:(1)Several cycles of fouling/cleaning operations were carried out for PA NF membrane,and the effect of alkaline cleaning on the membrane characteristics and fouling behavior was evaluated.The results demonstrated the occurrence of pore swelling of the PA NF membrane(-9.7%,increment of effective pore size)after alkaline cleaning(pH 11),and such reversible change would disappear after water filtration at neutral pH for 30 min.During this pore shrinkage period,the small foulants would enter the enlarged pores and be tightly entrapped in the membrane,resulting in more serious fouling and retarding the foulants release during cleaning.While for the composite cleaning agent including NaOH(pH 11)and sodium dodecyl sulfate(SDS,10 mM),the adsorption of SDS in/on the membrane could rapidly offset the alkali-induced pore swelling effect and avoid the accumulation of foulants in the pores,and it also produced a better antifouling surface with higher hydrophilicity and more negative charges than the pristine membrane.(2)Free chlorine,hydrogen peroxide and Fenton reagent were chosen to study their influence on the properties of PA NF membranes and to evaluate the cleaning efficiency on the membrane pore blocking.Under alkaline conditions,the decolorization ability of free chlorine was good and increased with the decrease of pH value.However,free chlorine would attack the amide bonds of PA layer during the cleaning process and change the membrane separation performance,and its cleaning ability on the pore fouling was negligible.Hydrogen peroxide treatment at neutral pH hydrolyzed the amide bonds of PA layer,and its decolorization and cleaning efficiencies were also not obvious.Fenton reagent had an acceptable decolorization ability without changing the properties of the PA NF membrane,but the secondary fouling formed during cleaning process weakened the cleaning efficiency.Aiming at the secondary fouling during homogeneous Fenton cleaning process,the phase state and chemical properties of the catalyst were adjusted to prepare an efficient solid catalyst with high dispersion.γ-Polyglutamic acid(y-PGA)and histidine(L-His)were sequentially immobilized on the surface of the catalyst,which not only avoided the secondary fouling,but also improved the affinity of the catalyst to hydrogen peroxide by means of the imidazole groups of L-His.The modified catalyst could not only promote the degradation of foulants,but also had good cleaning efficiency on various fouled commercial membranes. |
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