Humic acid rejection and flux decline with negatively charged membranes of different spacer arm lengths and charge groups

A series of novel charged ultrafiltration membranes that differed in spacer arm length and charge group were generated by covalent attachment of negatively charged group to a commercially available regenerated cellulose membrane. Rejection of humic acid and flux decline were compared with essentially neutral and negatively charged versions of regenerated cellulose membranes of different spacer arm lengths and charge groups. Effects of membrane molecular weight cut-off and solution pH on humic acid removal and flux decline on neutral and negatively charged membranes of different spacer arm lengths were also compared. Results indicated that the membrane with larger spacer arm length has better rejection of humic acid and less membrane fouling compared to that with smaller spacer arm length. In addition, the modified negatively charged membrane with a strongly acidic (sulfonic acid) functional group has better rejection of humic acid and less membrane fouling than that with a weakly acidic (carboxylic acid) functional group. These experimental results are consistent with the larger charge of the membranes having larger spacer arm length and sulfonic acid group. This study confirms that the appropriate design of membrane charge functionality could be an effective way for better removal of humic acid and reduction of membrane fouling due to the electrostatic interactions with the combination effect of membrane pore size.

► Different charge group modified membranes affect HA rejection and fouling differently.
► Membrane having larger spacer arm length has higher retention of HA and less fouling.
► Charge modifying effect on HA removal and fouling is larger with higher MWCO membrane.
► Effect of pH on HA rejection and fouling is larger for the membrane of larger charge.