Quantitative Analysis Of Membrane Fouling Mechanism Involved In Mixed Organics Microfiltration Utilizing XDLVO Theory

Membrane fouling phenomenon has always been the main factor of limiting membrane technology application widely,which is very complicated and influenced by many factors.Especially in the case of coexistence of two kinds of foulants,the mechanism of membrane fouling has become more elusive.This study analyzed mixture of humic acid and protein microfiltration(MF)membrane fouling mechanism under different solution conditions using the extended Derjaguin-Laudau-Verwey-Overbeek(xDLVO)theory.In order to explore the applicability of xDLVO theory,fouling potential in different MF stages and corresponding interfacial energy were linear fitted to analyze their quantitative relationship.Besides,main mechanism of interfacial interaction controlling microfiltration membrane fouling was researched.This study provides theoretical support for optimizing operating conditions to alleviate membrane fouling in actual membrane craft.The results showed the mixture of humic acid(HA)and bovine serum albumin(BSA)microfiltration membrane fouling severity at different solution pH values is pH3>pH4.7>pH7>pH9,which is consistent with the trend of HA membrane fouling with the change of solution pH value.In addition to the higher percentage of HA in the mixture,another possible reason is that BSA molecules are around by HA due to the interfacial interaction,shielding the effects of pH value on BSA.This inference was confirmed by hydrophobic HA-BSA mixtures,which is similar with HA.AB interaction played an important role in foulant-membrane and foulant-foulant total interfacial interaction when 0.158 nm<d<3 nm.When distance continues to decrease,AB and LW interactions dominate membrane fouling together.Compared with other two interfacial interactions,contribution of EL interaction to total interfacial interaction can be ignored.With solution pH reducing,ionic strength increasing and the addition of Ca2+,total interfacial energy gradually decreased and fouling potential increased in initial and later stages.Good negative linear correlation between membrane fouling and corresponding interfacial energy in both stages showed xDLVO theory can be used to evaluate microfiltration membrane fouling mechanism by HA-BSA mixture under different solution conditions.In initial stage,main mechanism of interfacial interaction controlling membrane fouling was changing fouling specific resistance.On the one hand,this may be due to fouling occurring on membrane pore and pore blocking reduced normal flux obviously,thus influencing fouling resistance very much.On the other hand,it may because that interfacial interaction in initial stage shifted between repulsion and attraction,which changed distance between foulant and membrane obviously and compressibility of fouling layer.In later stage,main mechanism of interfacial interaction controlling membrane fouling was changing foulant mass.It is because membrane fouling occurred on membrane surface in this stage.Foulant accumulation had no obvious effect on resistance of cake layer,which was reflected by normal flux stability in the later stage.Another possible reason was that interfacial interaction was always attractive,whose change had little influence on the density of cake layer,but improving the adsorption potential of foulant on the membrane.The intercept,slope and correlation coefficient of linear fitting between fouling potential and interfacial energy all can reflect the effect of non-interfacial interaction factors on membrane fouling tendency.Smaller intercept,lower slope and bigger correlation coefficient in later stage showed that effect of non-interfacial interaction factors on fouling potential is smaller than that in initial stage.The differences of non-interfacial interaction factors between initial and later stages mainly reflect in fouling site and permeation flux.Fouling site was membrane pore in initial stage,but membrane surface in later stage.Besides,initial flux was significantly higher than later flux.These showed xDLVO theory was more applicable to predict membrane fouling potential of lower flux or occurring membrane pore.