| With the rapid development of nanotechnology,nano-SiO2 because of its unique physical and chemical properties has a wide application prospect in the fields of chemical engineering,textiles,medical treatment and health care.Because of the massive use of nano-SiO2,it is inevitable that these materials will discharge into water and form contamination of nano-particles(NPs).The NPs are most likely to bind natural organic matter(NOM)to form a more toxic NPs-NOM combined pollutant,which would affect the ecological structure of the water body and increase the risk to humans and the environment.Therefore,the effective removal of nano-SiO2 has attracted widespread attention from scholars at home and abroad.Ultrafiltration technology is considered as an effective method for removing NPs in a wastewater treatment system.However,membrane fouling,which results in severe flux decline,increased membrane filtration resistance,and frequent membrane cleaning,still remains the primary impediment constraining its more widespread application.General inorganic particles and NOM have synergistic fouling effects.Compared to conventional inorganic particles,NPs have a significantly increased role of surface forces in membrane fouling due to high specific surface area.In this study,the extended Derjaguin-Laudau-Verwey-Overbeek(xDLVO)theory was used to deeply understand of different fouling mechanisms involved in Ultrafiltration(UF)of SiO2-NOM mixture at different solution conditions.The contribution of different interfacial interaction(i.e.Lifshitz-van der Waals,electrostatic and acid-base interaction)towards fouling was assessed.In order to explore the applicability of the xDLVO theory in evaluation of SiO2-NOM mixture UF membrane fouling mechanism,fouling potential and interaction energy in different stages were fitted to analyze their correlation.This study provides theoretical support for controlling membrane fouling of NPs and NOM mixture.The results showed that the relative flux of SiO2-NOM mixture declined rapidly and membrane fouling was aggravated at low pH,high ionic strength and calcium ion concentration.The SEM image also shows that the fouled membrane with the same solution conditions displayed much more compact deposit layer structure,and the corresponding fouling experimental results were consistent with the relative flux decline trend.Besides,the physical cleaning efficiency increased and the chemical cleaning efficiency decreased with solution pH reducing,ionic strength and Ca2+concentration increasing,indicating the high reversible fouling resistance and the increased attractive interactions.The segmented filtration and cleaning experiments showed that the irreversible fouling of membrane filtration process mainly occurs in the initial stage.Since membrane fouling mainly occurs in the membrane pores in the initial stage,it was not easily removed by the physical cleaning method,and the corresponding irreversible fouling increased in the proportion of the total fouling.The calculated interaction parameters showed that AB interaction energy played important roles in the total interaction energy when the separation distance was less than 5 nm,while the effect of EL and LW interaction energies were of less importance in the total interaction energy.The attractive interaction energy in the initial and final stages was substantially increased with decreasing pH or the increase of ionic strength and calcium ion,thus aggravating membrane fouling.More importantly,a strong negative linear relationship between fouling potential and corresponding interaction energy in both stages was observed,indicating that xDLVO theory can predict the membrane fouling potential of SiO2-NOM mixture at different solution conditions.Smaller intercept,lower slope and stronger correlation coefficient in the final stage showed xDLVO theory was more applicable to predict membrane fouling potential of lower flux or occurring membrane surface. |
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