The Effect Of PAC Adsorption-catalytic Oxidation On Ultrafiltration Water Purification Efficiency And Membrane Fouling

It is widely believed that ultrafiltration(UF)membrane and its combination process could achieve the excellent quality of drinking water,which have been broadly applied with a large-scale and industrialized mode in urban drinking-water plants.As an alternative,UF could effectively retain suspend solids,colloids,macromolecular organics and microorganisms,which plays a vital role in ensuring the safety of drinking water.Nevertheless,membrane fouling is considered as a critical issue limiting its extensive application.Natural organic matter(NOM)as one of the major contaminants has emerged the primary contributor to membrane fouling.UF in combination with powder activated carbon(PAC)has been used to efficiently retard membrane fouling,however,there is the problem of PAC particles aggravating NOM fouling.Based on this,in this study,peroxymonosulfate(PMS)oxidation activated by PAC was utilized as UF membrane pretreatment for fouling mitigation.We will investigate the synergistic mechanism between PAC and PMS,and reveal the degradation and transformation of pollutants during the reaction process.Later,exam the effect of PAC/PMS on membrane fouling induced by different NOM fractions,and analyze the mechanism of PAC/PMS on membrane fouling regulation from macroscopic and microscopic perspectives.Firstly,the degradation of PMS in PAC/PMS system was examined,and the degradation efficiency of PAC/PMS was verified by the removal effect of typical NOM fractions.The reactive species were identified by electron paramagnetic resonance spectroscopy to speculate NOM-degradation pathways.The results showed that:PAC could accelerate the decomposition of PMS with the decomposition ratio increasing from 22.08%to 68.8%after 12 h.The NOM concentration of raw water was reduced by PAC adsorption,PMS oxidation and PAC/PMS system to different degrees,among which PAC/PMS exhibited the greatest removal performance.For the effluent,the DOC values of humic acid(HA),bovine serum protein(BSA)and mixture were further decreased,but the value was substantially increased for sodium alginate(SA).This might be due to the fact that the molecular weights(MW)of most reaction products were lower than the membrane pore size,causing pollutants to easily pass through membrane pores and then increasing the effluent organic contents.The simultaneous presence of SO₄^·-,·OH,O₂^·-and ¹O₂in PAC/PMS system was conductive to degrade NOM effectively.Meanwhile,the pollutants reacted with reactive species near the surface of PAC,which promoted the catalytic oxidation process.Secondly,the effectiveness of PAC adsorption,PMS oxidation and PAC/PMS system on various NOM fouling control was investigated.The results showed that:PAC adsorption presented limited influence on relieving membrane fouling.As for HA,the presence of PAC in the feed water even aggravated membrane fouling due to the significant synergistic fouling effect between PAC and HA.PMS oxidation exhibited a certain mitigation effect on membrane fouling,especially for reversible fouling induced by SA.This part fouling was reduced by decomposing high MW NOM fractions into relatively smaller substances,which could penetrate into the membrane pores,leading to an increase in the effluent organic content.The PAC/PMS system effectively retard reversible and irreversible fouling by the dual role of adsorption and oxidation.The control effect was closely related to the NOM fractions in the feed water,following the order of SA>mixture>BSA>HA.A loose and poriferous fouling layer was observed on the membrane surface after PAC/PMS pretreatment,and there was an apparent reduction in the number and size of pollutants,indicating that PAC/PMS system is beneficial to membrane fouling alleviation.Finally,a natural surface water collected from Yellow River was applied to investigate the effect of PAC/PMS system on filtration performance and NOM properties.The scanning electron microscope and Fourier transform infrared spectroscopy were employed to characterize the membrane surface micromorphology and chemical functional group changes.The mechanism of membrane fouling mitigation was discussed by the combined pore blockage-cake filtration model and XDLVO theory.The results showed that:With PAC/PMS pretreatment,a 28.0%improvement in membrane flux,79.6%and 48.1%reduction in reversible and irreversible fouling resistance respectively,were observed at the end of the second filtration cycle.The removal rates of DOC and UV₂₅₄ were 56.1%and 64.9%,respectively.The removal effect of humic-like substances was distinctly more effective than that of protein-like components.The pollutants in each MW ranges were efficiently removed.The fouling mechanism was dominated by pore blocking at early filtration times,then based on cake filtration at later filtration stage,and the PAC/PMS pretreatment markedly delayed the formation of cake filtration.It was seen that the interfacial free energy was maximum and positive when using PAC/PMS,indicating that repulsive force played a dominant role between membrane and pollutants.In this case,the surface of membrane was not easily adhered by pollutants,which was less likely to exacerbate membrane fouling.In this study,PAC/PMS was proposed as a feedwater pretreatment method,which has a great potential to solve the problem of membrane fouling induced by PAC particles.These results will provide effective solutions and technical support for membrane fouling in water treatment.