With diverse selectivity,
higher permeance, and good antifouling property, loose polyamide nanofiltration
(NF) membranes can be potentially deployed in various bioseparation
applications. However, the loose NF membrane with a low crosslinking
degree generally suffers from the alkali-induced pore swelling during
chemical cleaning, resulting in degradation of separation performance
with time. In this work, we conceive a novel strategy to tailor the
separating layer through alkaline post-etching following the interfacial
polymerization process, where piperazine and tannic acid (TA) were
used as water-phase monomers, and trimesoyl chloride (TMC) and ferric
acetylacetonate were employed as organic monomers in n-hexane. Thereinto, the polyester network formed by TA and TMC was
selectively etched by alkaline treatment, thus obtaining a loose NF
membrane, whose structure and performance could be facilely tailored
by controlling the TA ratio and the etching pH. As a result, the well-designed
loose NF membrane exhibited higher flux, better selectivity, and more
stable separation performance in a long-term filtration of diluted
cane molasses. Interestingly, the obtained loose NF membrane showed
excellent antiswelling ability during alkaline cleaning because of
network locking induced by Fe3+ chelation, decrease in
the carboxyl proportion (more hydroxyl generation due to the ester
bond hydrolysis), and enhanced interface interaction between the separation
layer and the sublayer attributed to catechol adhesion effect. Therefore,
such a “selective-etching-induced reinforcing” strategy
could endow the polyamide NF membrane with both loose and antiswelling
separation layer in a reliable and scalable way, which provides a
new perspective for preparing highly selective and stable NF membrane
for resource recovery.