In this work, we employed CaCO3 mineralization to greatly improve the water flux and dye rejection of polyacrylonitrile-based (PAN) ultrafiltration membranes. PAN-based membranes were prepared with 1.0-5.0 wt% of poly(acrylic acid) (PAA) as a matrix for mineralization. These membranes were subsequently mineralized with CaCO3 by an alternate soaking process (ASP). The optimized mineralization condition was 10 cycles of ASP with 100 mM CaCl2 and Na2CO3 solutions. The resulted membranes were characterized with field emission scanning electron microscopy, X-ray diffraction, and energy dispersion X-ray analysis combined with elemental distribution mapping. The mineralized CaCO3 particles were found to be deposited throughout the membranes. Results also indicated that the pure water flux of PAN-based membranes decreased with PAA if the content was larger than 1.0 wt%, which was due to the stretching of PAA chains deriving from the electrostatic repulsion of -COO- groups. In contrast, the mineralized membranes showed a dramatic increase of water flux owing to the chain collapse of PAA caused by the formation of complexes with Ca2+ in the mineralization of CaCO3 as well as the enhanced hydrophilicity by CaCO3. The mineralized membranes even showed a high rejection of Congo red, which makes them potential in nanofiltration for dye-polluted wastewater. (c) 2013 Elsevier B.V. All rights reserved.