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Abstract Considering growing efforts to understand and improve the solute-specific selectivity of nanofiltration (NF) membranes, we explored the ion-specific effects that govern the charge and performance of a loose polyamide NF membrane that is commonly used for solute-solute separations. Specifically, we systematically evaluated the zeta potential of the membrane under different conditions of pH, salinity, and ionic composition, and correlated the obtained data with membrane performance tested under similar conditions. Our results identify the pKaof both carboxylic and amine groups bonded to the membrane surface and suggest that the highly polarizable chloride anions in the solution adsorb to the polyamide, increasing its negative charge. We also show that monovalent cations of different “stickiness” can neutralize the negative membrane charge to different extents due to their varying tendency to sorb to the polymer matrix or screen the fixed carboxyl groups on the membrane surface. Notably, our correlation between zeta potential measurements and permeability experiments indicates the substantial contribution of solution ions to Donnan exclusion in NF membranes.
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Positively‐Coated Nanofiltration Membranes for Lithium Recovery from Battery Leachates and Salt‐Lakes: Ion Transport Fundamentals and Module Performance
Abstract Membranes facilitate scalable and continuous lithium concentration from hypersaline salt lakes and battery leachates. Conventional nanofiltration (NF) membranes, however, exhibit poor monovalent selectivity in high‐salinity environments due to weakened exclusion mechanisms. This study examines polyamide NF membranes coated with polyelectrolytes enriched with ammonium groups to maintain high monovalent cation selectivity in hypersaline conditions. Over 8000 ion rejection measurements are recorded using salt lake brines and battery leachates. The experiments exemplify the coated membrane's ability to reduce magnesium concentrations to 0.14% from salt lakes and elevate lithium purity to 98% from battery leachates, in a single filtration stage. The membrane's selectivity is retained after 12 weeks in acidic conditions. Molecular dynamics analyses reveal that the ammonium groups create an electrostatic barrier at low pH, selectively hindering multivalent cation transport. This is corroborated by the Coulombic attraction between cations and carboxylate groups, along with a repulsive barrier from ammonium groups. Despite a 14.7% increase in specific energy, a two‐stage NF system using the coated membranes for lithium recovery significantly reduces permeate magnesium composition to 0.031% from Chilean salt lake brines. For NMC leachates, the coated membranes achieve permeate lithium purity exceeding 99.5%, yielding enhanced permeate quality with minor increases in energy demands.
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- Award ID(s):
- 1840816
- PAR ID:
- 10641241
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Functional Materials
- Volume:
- 34
- Issue:
- 48
- ISSN:
- 1616-301X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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