# Tuneable ion selectivity in vermiculite membranes intercalated with unexchangeable ions

## TL;DR

Researchers developed vermiculite membranes with tunable ion selectivity by intercalating specific ions, enabling better water waste processing and element recovery.

## Contribution

A new mechanism of ion selectivity based on entropic and mechanical effects, beyond classical size and charge exclusion.

## Key findings

- Intercalated ions like Zr⁴⁺, Sn⁴⁺, Ir⁴⁺, and La³⁺ create stable, narrow channels in vermiculite membranes.
- Ion selectivity correlates with membrane stiffness and hydration entropy of intercalated ions.
- Two distinct monovalent ion selectivity sequences emerge despite identical channel widths.

## Abstract

Membranes selective to ions of the same charge are increasingly sought for water waste processing and valuable element recovery. However, while narrow channels are known to be essential, other membrane parameters remain difficult to identify and control. Here we show that Zr⁴⁺, Sn⁴⁺, Ir⁴⁺, and La³⁺ ions intercalated into vermiculite laminate membranes become effectively unexchangeable, creating stable channels, one to two water layers wide, that exhibit robust and tuneable ion selectivity. Ion permeability in these membranes spans five orders of magnitude, following a trend dictated by the ions’ Gibbs free energy of hydration. Unexpectedly, different intercalated ions lead to two distinct monovalent ion selectivity sequences, despite producing channels of identical width. The selectivity instead correlates with the membranes’ stiffness and the entropy of hydration of the intercalated ions. These results introduce an ion selectivity mechanism driven by entropic and mechanical effects, beyond classical size and charge exclusion.

Membranes selective to ions of the same charge are sought for water waste processing and valuable element recovery. Here authors show that ions intercalated into vermiculite laminate membranes become effectively unexchangeable, creating stable channels that exhibit robust and tuneable ion selectivity.