H₂O₂-MnO₂ System For Efficient Physic-chemical Cleaning Of Ultrafiltration Membrane

During all kinds of membrane cleaning processes,the most widely method used is still the traditional physical membrane cleaning and chemical membrane cleaning.Although many measures such as feed water pretreatment,operation optimization and membrane material modification are adopted to reduce membrane pollution,the increased irreversible pollution during long-term operation is still inevitable.Therefore,it is necessary to carry out regular chemical cleaning and remove the foulants deposited on the membrane by chemical cleaning processes,so as to restore the permeability of the membrane and maintain the stable operation of the filtration process.However,it exhibits several limitations in practical membrane cleaning process in terms of the risk of membrane surface damage,and the serious waste of cleaning agents.And the membrane replacement cost is high.In order to solve this problem,we developed a new type of H₂O₂-MnO₂ membrane cleaning system to achieve efficient cleaning of PVDF and PES membrane polluted by HA and river water.Compared with conventional chemical cleaning process in which the whole membrane had to be soaked in highly concentrated solution of chemical reagent for long time,the H₂O₂-MnO₂ cleaning process conducting for only 5 mins in 0.5 wt% H2O2 solution could achieve more than 95% recovery of permeate flux and 90% removal of the irreversible foulants.More importantly,the permeate flux and filtration efficiency of the membrane could be still kept stable after 6 runs of consecutive fouling and cleaning.The Microscopic analyses for the changes of membrane structure showed that when H₂O₂-MnO₂ system was utilized to wash the fouled membrane for 5min,it could be observed clearly that the membrane surfaces has been recovered and cleaned,and almost no irreversible foulants were remained.Based on the Electron Spin Resonance?ESR?,as well as the quenching experiments with different free radical scavengers,superoxide adduct?DMPO-OOH?and hydroxyl adduct?DMPO-OH?appeared in H₂O₂-MnO₂ system simultaneously.However,after about 10 min,the DMPO-OOH signal was almost extinguished entirely,due to the rapid dismutation in water and the transformation of DMPO-OOH to DMPO-OH.The free radical quenching experiment further confirmed that the superoxide radicals and hydroxyl radicals coexist in the H₂O₂-MnO₂ system are two main active radicals that degrade irreversible pollution.The outstanding performance of H₂O₂-MnO₂ system was attributed to the generation of both free radicals and abundant oxygen simultaneously,leading to the physico-chemical membrane cleaning.In addition,understanding the influences of cations on membrane fouling was important to improve the performance of membrane filtration system,however,opposite conclusions were made in different studies.Meanwhile,although the influences of cation concentration have been studied extensively,few attentions have been paid to the cation valence.To clarify it,the effects of typical cations on membrane fouling and cleaning,as well as the related mechanisms were investigated systemically in this study.K⁺ and Ca²⁺ were chosen as the representative cations,and humic acid?HA?was chosen as the membrane foulants.The results demonstrated Ca²⁺ promoted the formation of reversible fouling,meanwhile higher removal efficiency of HA could also be achieved with the assistance of filtration cake containing HA+Ca²⁺.However,K⁺ led to the formation of more recalcitrant irreversible fouling.By comparing the concentration of cations in feed and permeate,analyzing the influence of cations on size of HA flocs,and the detailed SEM,AFM and TEM observation,it could be found that different mechanisms dominated the interaction between cations and HA.The bridging effect induced by Ca²⁺ attributed to the extension of HA molecules,while the electrostatic shielding effect induced by K⁺ led to the compression of them.Moreover,the different characteristics of hydrated Ca²⁺ and K⁺ also contributed to the different structures of foulant layers formed by HA+Ca²⁺ and HA+K⁺.Given the abundance of K⁺ and Ca²⁺ in natural water,results of this study can provide valuable advice for practical membrane filtration process.