Removal Of Natural Organic Matters With Negatively Charged Ultrafiltrat Ion Membranes With Spacer Arm

With the deterioration of water environment and the stringency of waterstandards, ultrafiltration has become one of the best alternatives replacingconventional drinking water treatment techniques. However, the naturalorganic matter (NOM) in water is one of the important factors causingmembrane fouling. UF is not very effectively to remove natural organicmatter (NOM) due to its comparatively larger pore size compared with thesize of NOM. In this study, a series of novel charged ultrafiltrationmembranes that differed in spacer arm length and charge group was generatedby covalent attachment of negatively charged group to a commerciallyavailable regenerated cellulose membrane. Humic acid was chosen as a modelfor NOM. Rejection of humic acid and flux decline were compared withessentially neutral and negatively charged versions of regenerated cellulosemembranes of different spacer arm lengths and charge groups. Effects ofmembrane molecular weight cut-off and solution environment (such as pH and ionic strength) were also taken into account. The following main resultswere obtained:(1) The appropriate design of membrane charge functionality could be aneffective way for better removal of humic acid and reduction of the membranefouling due to the electrostatic interactions with the combination effect ofmembrane pore size.Charge modification on the neutral RC membrane of different MWCOs couldbe an effective way for better removal of HA and reduction of the membranefouling. The membrane with longer spacer arm length performs better inremoval of HA due to its higher zeta potential value and stronger electrostaticrepulsion with negatively charged HA. In addition, the modified negativelycharged membrane with strongly acidic (sulfonic acid) functional group hasbetter rejection of humic acid and less membrane fouling than that withweakly acidic (carboxylic acid) functional group.(2) Research indicates that the effect of pH on the humic acid removal islarger for charged membrane than that for neutral membrane. Besides,charged membranes of larger spacer arm length have higher flux decline thanthat of smaller spacer arm length. What’s more, higher flux decline and lowerrejection coefficient were observed at higher ionic strength. In conclusion, solution environment affects a great deal about the membranefouling and NOM removal. Decrease of pH has affected the charge of UFmembrane and humic acid molecule that lead to the increase in humic acidaggregation. Increase of ionic strength turn down the electrastatic interactionsbetween humic acid molecule and the membrane. Thus, more humic acidsdeposit on the membrane and more humic acid fouling in turns.