Elucidating ion transport mechanism in polyelectrolyte-complex membranes

Zelner, Michael; Stolov, Mikhail; Tendler, Tony; Jahn, Philipp; Ulbricht, Mathias; Freger, Viatcheslav

doi:10.1016/j.memsci.2022.120757

Cited by 9 publications

(1 citation statement)

References 41 publications

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“…Due to their more open pore structure compared to reverse osmosis (RO) membranes, nanofiltration (NF) membranes have been used in various applications that require selective separation between solutes, such as water softening, groundwater purification, and concentration of products in certain industries. − More recently, with a growing demand for more challenging separations that require precise selectivity (e.g., lithium recovery from salt-lake brines), NF membranes have become a leading candidate to pursue such selectivity. − Relying on steric hindrance, Donnan (charge) exclusion, and other molecular mechanisms, − NF membranes of different materials and fabrication approaches were proposed to create selectivity toward different solutes. ,, Although success has been abundantly reported, − the selectivity of most NF membranes is relatively limited, unspecific, and, for comparison, a few orders of magnitude lower than the selectivity of ion-selective biological channels. − …”

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confidence: 99%

The Adverse Effect of Concentration Polarization on Ion–Ion Selectivity in Nanofiltration

Peer-Haim

Shefer

Singh

et al. 2023

Environ. Sci. Technol. Lett.

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While the detrimental effect of concentration polarization (CP) on water flux and solute rejection in pressure-driven membrane processes has been extensively explored, the impact of CP on the selectivity between solutes in these processes has been somewhat overlooked. Considering the growing interest in solute–solute selectivity, in this study, we explored the effect of CP on ion–ion selectivity in nanofiltration (NF) membranes. We first show and discuss the “reversed” observed rejection trend of monovalent cations in NF, which is opposite to the trend of the ions’ hydrated size and mobility in solution. Next, we apply the film theory using three independent approaches to evaluate the extent of CP in the boundary layer adjacent to the membrane surface, from which the real rejection of the ions can be calculated. Our calculated real rejections of monovalent cations, which were in higher correspondence with the ions’ hydration properties and mobility in solution, suggest that CP played a major role in the “reversed” selectivity observed. Last, we demonstrate how CP adversely affects the commonly pursued monovalent–divalent ion separation in NF. Overall, our results highlight the necessity to rigorously account for CP in future studies on NF and suggest minimizing CP as a primary step to improve the selectivity between solutes.

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