| Inorganic fouling on membrane surface is macroscopically manifested as membrane pore blockage and cake layer,which not only significantly reduces the effective membrane area,separation performance and lifetime,but also increases operation cost.In this regard,some scholars have carried out a lot of fruitful research,such as accurately predicting the fouling potential of membrane surface and effectively suppressing membrane fouling by adding chemicals.Dynamic scaling on the NF membrane surface from microscopic perspective could be classified in two broad categories: crystallization and particulate fouling.The former is almostly predominant and involves the appearance of deposition at the membrane surface owing to precipitation of ions,whereas with respect to the latter case,deposition emerges ascribed to convective transportation of colloidal particulate matter from the bulk solution to the membrane surface.The related reports mostly study the crystallization mode(homogeneous and heterogeneous crystallization)and nucleation crystallization process,including simulated calculation,quantitative characterization and experimental verification of induction time,metastable zone width,nucleation rate and interfacial tension of different membrane elements.In order to inhibit or slow down the above adverse effects,it is undoubtedly of great significance to elucidate the mechanism of scale formation on the surface of NF membrane.The scale formation was perceived as complex and numerous relevant studies focused on the effect of a series of influencing factors,such as membrane properties(e.g.porosity,roughness,etc.),feed solution characteristics,operating conditions(e.g.shear rate,and operating pressure)and module geometry on the crystallization of inorganics.In terms of crystallization,it is widely accepted that precipitation of salts generates on NF membrane surface requires at least two stages,the nucleation stage and crystal growth from supersaturated solution.Considering the fact that the concentration of solutes or particles at the membrane surface can reach oversaturated easily,the water film theory thought that concentration polarization(CP)acted a key role in exploring NF membrane surface scale formation mechanism,and would facilitate the crystal nucleation of scalant ions.Consequently,there is a need to take the CP phenomenon into consideration when studying the NF membrane scaling process.It is known from the literatures that surface charge,as one of the distinguishing features for NF membrane,like any other surface properties,such as surface hydrophilicity and roughness,are all considered to be able to affect the interaction between the foulants and NF membrane surface as well as fouling resistance thereby.Compared with the well-documented latter factors,rare research has been made on the interactive relation between surface charge and deposition of solids on NF membrane surface.This could be due to the view that NF membrane surface charge demolished and the corresponding Donnan exclusion and electrostatic effects disappeared at higher ionic strength.However,it was found in our previous study that encountering with a wide scalant ions concentration(0.25-18 mol.m-3),even when the sparingly soluble salt concentration of Ca SO4 at membrane surface exceeded its threshold point,both candidate NF membranes still exhibited low-intensity surface electrokinetic phenomena(-5.94?Es?-3.41 m V).Beyond that,some researchers attempted to modify NF membrane to possess high surface charge density that aimed to increase its fouling resistance.Based on the fact that in low or medium salinity water(ionic strength?100 mol.m-3)softening process,inevitable scaling was prone to occur on NF membrane surface in the retentate side and low intensity surface electrokinetic phenomena were still detected in the same location simultaneously.Hence,it is necessary to carry out the research to identify the links between NF membrane charge performance and scaling process under the threshold concentration condition.However,there are few reports about the effect of membrane surface charge properties on scalant ionic nucleation and crystallization mechanism at the threshold concentration.The research contents in this work are as follows:(1)Using DK-type NF membrane as carrier and CK2SO4:CCa Cl2=1:1 mixed component reaction producing Ca SO4.1/2H2 O as research object,the surface flow potential of DK-type NF membrane was measured under different scalant ionic concentration(0.5-18 mol.m-3),operating pressure drop(20-200 k Pa),feed solution p H value(3-10)and ionic strength(5-900 m-3).The surface characteristic parameters of DK-type NF membrane were calculated by Gouy-Chapman model.The results show that when the concentration of Ca SO4 of mixed components even increases to 18 mol.m-3,the concentration of Ca SO4 on the surface of DK-type NF film is far higher than this value during the softening process,and the surface charge of DK-type NF film still exists,that is to say,the surface charge of the film still exists at this time.In the process of adjusting testing solution p H from 3.0 to 10.0 at low concentration of 0.5 mol.m-3 and the threshold concentration of 15 mol.m-3(K2SO4+Ca Cl2),the absolute value of membrane flow potential increases from 1.5 to 4.0 m V,while the absolute value of low concentration at 0.5 mol.m-3 increases from 25 to 92 m V.With the increase of solution concentration,the negative charge effect of membrane surface is shielded due to the adsorption of counter ions,and the diffusion layer between the tight layer and the bulk solution is also compressed,which leads to the Zeta potential and charge density of membrane surface tend to be close to each other.With the increase of pressure,the flow potential values of low concentration and high concentration mixed solutions have a linear function relationship with the pressure drop.With the increase of shear force on the membrane surface,membrane surface charge density decreases obviously at 0.5 mol.m-3,which decreases by 54.5%,while the charge density at 15 mol.m-3 decreases by only 14.2%.In a two-component solution with 15 mol.m-3 as the base value,when the ionic strength is greater than 300 mol.m-3,the flow potential changes to be a negative value,and Na+ on the membrane surface increases sharply,neutralizing the negative charged groups on the membrane surface.(2)On the basis of obtaining Zeta potential,NF permeation test was carried out for critical concentration CK2SO4:CCa Cl2=1:1 solution at constant tangential flow rate with the same concentration,p H values,ionic strength and operating pressure(0.6-1.5 MPa).Ca2+,SO42-,K+ and Cl-ions in influent and produced water were detected by ion chromatography.The concentration polarization model was used to calculate the real rejection rate(Rreal),apparent rejection rate(Robs)and the concentration of scalant ions on the membrane surface.The appropriate formula was selected based on Pitzer electrolyte solution theory and the SI value of the concentrated water end was calculated according to the activity of the scalant ions.It is found that the absolute Zeta potential decreases with the increase of feed concentration,and the crystalline ion concentration on the film surface increases significantly.When the influent concentration is only 9 mol.m-3,the scaling tendency will appear on the membrane surface.(3)The classical nucleation theory was used to calculate the nucleation rate and induction time of the conformation ion on the membrane surface.The results show that with the increase of concentration,the absolute Zeta potential decreases gradually due to the weakening of the Doonan effect,and the nucleation rate of the scalant ions on the film surface increases slowly at first and then sharply.At the same time,the induction time decreased gradually.(4)The closed-loop experiment was carried out up to 450 hours.Keeping the volume of feed liquid of NF film constant at 30 L and inlet water temperature was set at 25? under the condition that the tangential velocity of the inlet is constant at 0.035 m.s-1.The variation trends for the concentration of scalant ions and the accumulation of fouling in the membrane module with the operation time and the surface charge performance of the NF membrane were described. |
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