Enhancing Ion Selectivity of Nanofiltration Membranes via Heterogeneous Charge Distribution

Nanofiltration technology holds significant potential
for precisely
separating monovalent and multivalent ions, such as lithium (Li) and
magnesium (Mg) ions, during lithium extraction from salt lakes. This
study bridges a crucial gap in understanding the impact of the membrane
spatial charge distribution on ion-selective separation. We developed
two types of mixed-charge membranes with similar pore sizes but distinct
longitudinal and horizontal distributions of oppositely charged domains.
The charge-mosaic membrane, synthesized and utilized for ion fractionation
for the first time, achieved an exceptional water permeance of 15.4
LMH/bar and a Li/Mg selectivity of 108, outperforming the majority
of published reports. Through comprehensive characterization, mathematical
modeling, and machine learning methods, we provide evidence that the
spatial charge distribution dominantly determines ion selectivity.
The charge-mosaic structure excels by substantially promoting ion
selectivity through locally enhanced Donnan effects while remaining
unaffected by variations in feedwater concentration. Our findings
not only demonstrate the applicability of charge-mosaic membranes
to precise nanofiltration but also have profound implications for
technologies demanding advanced ion selectivity, including those in
the sustainable water treatment and energy storage industries.