Study On The Removal Of Organic Compounds From Aqueous Solution Through Nanofiltration

Nanofiltration (NF), as a newly developed membrane separation process, has played animportant role in the field of membrane filtration technology in recent20years. The NFmembrane usually exhibits relatively high rejections to multivalent ions and organic compoundswith molecular weight larger than200Da and low rejection to monovalent ions. Therefore,nanofiltration membrane has been widely used in the desalination and concentration of organics,separation of organic compounds with different molecular weights, water softening and reducingthe clour of wastewater and so on. The mass transfer, separation mechanism, mathematicalmodel of nanofiltration process are the major research interests in the field of nanofiltration.In order to predict the separation performance more accurately and to guide the applicationof nanofiltration membrane, this paper focused on the study of the removal of organiccompounds from aqueous solution through nanofiltration. Two commercial nanofiltrationmembranes of different characteristics, NF1#and NF2#, were adopted. After measurements ofmembrane properties in terms of zeta potential, contact angle, MWCO, inorganic salt rejectionand water flux, the membranes were employed to conduct cross-flow permeation tests fordetermination the the separation performance to different organics including cyclohexanone,benzene ethanol, glucose, sucrose, benzoic acid, salicylic acid and phthalic acid. The influencesof solute property, solution pH, operating pressure, operating temperature, feed concentration,recovery rate and concentration of the co-solute salt on the removal efficiency of thenanofiltration membranes to organic compounds were investigated. The pore model and thecharge concentration polarisation model were adopted to simulate the experimental data of theremoval efficiency of the membranes to neutral and charged organic compounds, respectively,by using the MWCO of nanofiltration membrane and the molecular weigh (Mn) of neutralorganic compounds, and the Zeta potential of membrane surface and the physicochemicalproperties of charged organic compounds, respectively. Finally, nanofiltration membrane wasused to separate and concentrate glycine under netural pH. The removal efficiency was predictedthrough the charge concentration polarisation mode and compared with the experimetanl data.The conclusions are as follows:（1）The characterization of membrane showed that the two nanofiltration membranes usedin this study had different performances due to the difference of manufacture technology and material. The measurement of zeta potential showed that the two membranes had amphotericsurfaces. The isoelectric points of membranes NF1#and NF2#were pH=3.8and pH=4.04,respectively. When the pH was higher than4.04, these two nanofiltration membranes werenegatively charged, and the charge was stronger with the increase of pH. The MWCOs of NF1#and NF2#membranes were310Da and185Da, respectively. The two nanofiltration membranesshowed different separation performances to organic compounds.（2）The results of separation experiments with aqueous solution containing differentorganic compounds showed that the removal of organic compounds from aqueous solutionthrough nanofiltration was mainly controlled by the MWCO and the surface charge of themembrane. The rejction of the nanofiltration to neutral organic compounds was mainlydetermined by the MWCO of membrane used, and was less affected by the solution pH. For thecharged organic matter, the rejection was mainly determined by the electrostatic interactionbetween charged functional groups on the surface of the membrane and the charged material, andwas largely affected by the solution pH.（3）For the neutral organic compounds, the separation was simulated using pore modelbased on the assumption that the effective pore radius of the membrane is equal to the organicmolecule Stokes radius corresponding to the MWCO measured. The rejection to neutral organiccompound under certain permeation flux could be accurately predicted using the relationshipbetween the membrane pore radiu rpand the MWCO and the correlation between solute radius rsand Mn.（4）For the charged organics, the rejections of the nanofiltration membrane under differentpHs could be accurately predicted through the charge concentration polarisation model bymeasuring the Runcharged.(5) The application of nanofiltration membrane in separation and concentration glycineaqueous solution demonstrated that, under netural pH, the predicted rejection of membrane NF2#to glycine through the charge concentration polarisation model was88%, while the actualrejection was83.3%. The deviation between the predicted and actural values was mainly due tothe influence of inorganic salts and other small molecules presented in the waste water.