Surface modification of polypiperazine-amide membrane by self-assembled method for dye wastewater treatment

Polypiperazine-amide membranes were modified with poly(ethyleneimine) (PEI) by self-assembled method, through which PEI molecules were fixed on the membrane surface by ionic interaction. In the experiments, the PEI concentration ranged from 50 to 2000 mg·L− 1 while the depositing time was fixed at 20 min. The results showed that low PEI concentration resulted in a slight increase of pure water flux, which was attributed to the enhanced membrane surface hydrophilicity. The PEI adsorption on membrane surface had less effect on the rejections to neutral PEG and sucrose, but improved the rejections to divalent cationic ions and methylene blue as the result of reversion of the membrane surface charge from negative to positive according to the XPS analysis and zeta potential measurements. The membrane modified at PEI = 1500 mg·L− 1 exhibited high rejection to methylene blue (MB) and is potential to be applied in the treatment of effluents containing positively charged dyes.

Unmodified membrane is a negative charge, which leads to higher concentration of counter-ions (Na+, Mg2 +) and lower concentration of co-ions (Cl−, SO42 −) in the membrane than in the adjacent feed solution according to Donnan equilibrium. Thus, concentration difference in solute ions between the membrane and the adjacent feed solution acts as a driving force for a diffusive transport. The membrane excludes all co-ions and is permeable to counter-ions, as A in Fig. 7. Rejection for MgCl2 is lower than that for NaCl because Mg2 + is divalent cation, and more permeable than Na+ through the unmodified membrane.Figure optionsDownload as PowerPoint slide