| Humic acid(HA)and dyes are the main source of water color in environment and they can form a variety of disinfection by-products(DBPs).It is of great significance to take measures to remove these two contaminants effectively to ensure the safety of water quality.Ultrafiltration(UF)is applied to remove substances like colloids and bacteria and has been widely used in water treatment in recent years.However,UF can not remove HA and dyes in water due to its relatively large pore size.Besides,membrane fouling occurs during operation.As a result,it is necessary to prepare membranes with low fouling and high rejection.In this study,three different amino acid ligands,including glycine,serine and lysine,were grafted onto regenerated cellulose(RC)membranes by crosslinking reaction.Various parameters of the membranes,such as zeta potential,grafted amino acid ligands and hydrophilicity/hydrophobicity were characterized.The removal and flux decline during ultrafiltration of HA and three dyes(methylene blue,methyl orange and direct yellow 12)by zwitterionic membranes were tested.Finally,the performance of zwitterionic membranes was compared with that of negatively charged membranes.The following results were obtained from the study of HA removal:(1)Compared with the unmodified membranes,zwitterionic membranes can effectively remove HA and decrease membrane fouling due to the combination of hydration shell,electrostatic interactions and membrane pore size.(2)Grafted ligands,modification time and molecular weight cut-off(MWCO)may affect membrane performance.Lysine modified membrane showed the best performance(highest removal rate and lowest flux decline)compared to serine and glycine modified membranes.Besides,longer modification time and smaller MWCO can improve the filtration performance of zwitterionic membranes.(3)Solution environment plays an important role in the filtration process of zwitterionic membranes.On one hand,solution environment affects the properties of HA(including molecule charge,the shape of HA molecules and formed cake);on the other hand,solution environment also affects the properties of membranes(including membrane charge and zwitterions).In particular,“salting-in” effect occurred for zwitterionic membranes at low ionic strength(1 m M).In this case,the electrostatic attrations between the positive and negative functional groups are screened and the polymer chains become swollen to avoid the hydrocarbon chains from being exposed,leading to a tremendously low fouling tendency.The flux decline of lysine modified membranes after 3 h ultrafiltration is 18%.(6)Compared with negatively charged membranes,zwitterionic membranes had better antifouling properties and higher removal rate and “salting-in”effect didn’t occur for negatively charged membranes.The following results were obtained from the study of dyes removal:(1)Zwitterionic membranes can effectively recover small dye molecules,slightly improve the removal of neutral molecules and effectively decrease membrane fouling due to the combination of hydration shell,electrostatic repulsion and membrane pore size.(2)Three zwitterionic membranes had similar removal rate towards dye molecules.Among them,lysine modified membranes also had good removal rate towards negatively charged dyes although it tends to be electrically neutral.This phenomenon is mainly caused by the hydration shell on its surface.(3)Ionic strength greatly affects the interactions between the surface of zwitterionic membranes and small dye molecules.At low ionic strength(1 m M),“salting-in” effect occurred at the surface of zwitterionic membranes,leading to low fouling tendency.(4)Dye rejection by zwitterionic membranes was greatest with the most negatively charged dye molecule.Above results indicated that zwitterionic ultrafiltration membranes can be effectively applied to the removal of HA and the recovery of small dye molecules.These could effectively open the new application fields for the zwitterionic ultrafiltration membranes with amino acid ligands. |
PDF Full Text Request