Effect Of Typical Macromolecule Organic Matters On Silicate Scaling Of Nanofiltration Membrane

Nanofiltration membrane(NF)is a functional semi-permeable membrane that allows solvent molecules,some low-valent ions and low molecular weight solutes to pass through.The performance of NF is between the reverse osmosis membrane and the ultrafiltration membrane.The pore size and molecular weight cut-off of NF is about 1-2 nm and 200-1000 Da,respectively.NF membrane surface usually is charged.Charge effect and size exclusion are the main mechanisms to determine the retention of solute.The retention of high-valent ions is higher than that of monovalent ions.NF membrane is widely used in seawater desalination,wastewater treatment,deep treatment of drinking water and other fields.Membrane fouling due to the accumulation of membrane contaminants and precipitation results in water flux reduction and membrane replacement cycle short,which reduce its economic interests and become the main bottleneck of NF membrane technology.When there are many kinds of pollutants in the water,the membrane fouling is more significant when the organic pollutants and the inorganic salts coexist in the solution than the single pollutants.At the beginning of the pollution,the surface of the membrane may give priority to the formation of organic pollution layer.The organic pollution layer changes the surface properties of the nanofiltration membrane,thus affecting the formation of inorganic salt fouling on the surface of the nanofiltration membrane.In addition,the synergistic effect of organic pollutants and inorganic salts also affects the behavior of silicate scaling of inorganic salts.In this study,polysulfone ultrafiltration membranes were prepared by gel phase conversion method,and polyamide composite nanofiltration membranes were prepared by interfacial polymerization.The effects of the different existent conditions of alginic acid(AA)and humic acid(HA)on the silicate scaling of polyamide composite nanofiltration membranes were investigated.The different existent states of organic were coexisting with saturated silicate in the raw water,and generating an organic fouling layer on the membrane surface,respectively.The flux decay trend in silicate scaling was studied.Scanning electron microscopy(SEM)and atomic force microscopy(AFM)were used to analyse the structure of the fouling layer.The effect of electrostatic repulsion on silicate scaling was studied by zeta potential instrument.Analysis of the characteristics of the adhesion forces at different scaling stages by AFM combined with silica colloidal probe.The characterization of the fouling layer of the sensor was characterized by quartz crystal microbalance with dissipation(QCM-D)adsorption test.The conclusions of this test are as follows:(1)At room temperature,the pressure of 0.6 MPa,the performance of polyamide composite nanofiltration membrane were as follows: the water permeation flux was(35±5)L/(m2·h).Rejection rate of magnesium sulfate was greater than 92 %,the permeation flux was(27±2)L/(m2·h).The molecular weight cut-off was 365 Da,the membrane pore size was about 4.68×10-10 m.The contact angle was(53±2)°.the average roughness(Ra)was 7.71 nm.(2)In the different existent conditions of organic matter,compared with the silicate scaling of the virgin membrane,comprehensive analysis of flux decay rate,SEM image and AFM topography,organic matter could reduce the silicate scaling to a weaker extent when organic matter coexisted with silicate in the solution.However,when the organic fouling layer had formed on the membrane surface,the degree of silicate scaling was significantly reduced.The fouled membrane was rinsed with deionized water,when organic matter coexisted with silicate in the solution,the cleaning effect was very weak;when the organic fouling layer had formed on the membrane surface,the cleaning effect was more significant.(3)In the silicate scaling process,the electrostatic repulsion between the two membranes and the dissolved silicate ions increased in the order of virgin membrane < organic matter-fouled membrane,so the silicate saturation of organic matter-fouled membrane surface was much lower than the silicate saturation of virgin membrane surface.Therefore,the degree and rate of silicate scaling on the surface of the organic matter-fouled membrane was much lower than the virgin membrane.In combination with the results of adhesion force,it can be seen that two series forces of the membrane-SiO2 probe and the silicate gel-membrane were the order of organic matter-fouled membrane < virgin membrane,indicating that the surface of the organic pollution layer would slow down the silicate scaling.The electrostatic interaction between the surface of organic matter-fouled membrane and the dissolved silicate ions became the dominant factor in the silicate scaling of the organic matter-fouled membrane surface.When the organic matter and the saturated silicate coexisted in the solution,the two series of adhesion force were the order of organic matter-fouled membrane < coexistence of organic matter and silicate(virgin membrane),consistent with the flux decay trend.(4)At the beginning of the QCM-D experiment,silicate was affected by adhesion force and adsorb on the surface of the sensor rapidly.Compared with the adsorption quantity of silicate on the surface of the sensor,when the organic matter and the silicate coexisted in the solution,the adsorption capacity increased,which was opposite to the flux decay trend,but the adsorption layer was more soft,suggesting that the membrane flux decay was small because the membrane surface of the adsorption layer was more soft and loose,water permeability was strong.When the organic fouling layer was preliminarily formed on the sensor surface,the adsorption layer had a large rigidity and the adsorption capacity of the silicate decreases,which was consistent with the flux decay trend,indicating that the pre-generated organic fouling layer slowed down the silicate scale.