Fig. 1: The structure of vermiculite and its composition.

a Schematic of the vermiculite layers. The unit cell consists of tetrahedral and octahedral layers with Mg²⁺ occupying the octahedral sites and Si⁴⁺ and Al³⁺ the tetrahedral sites with shared oxygen. Exchangeable cations are occupied between the layers to balance the excess negative surface charge and are replaced with K⁺/Ca²⁺/Al³⁺ cations. b Energy dispersive spectroscopy (EDS) analysis of the elements across the layers of the K‒V membrane indicates the presence of O, Si, Mg, Al, and the intercalant K. The scale bar indicates 1 µm. c High-resolution X-ray photoelectron O 1s spectra (XPS) of the K‒V, Ca‒V, and Al‒V membranes. The grey open circle indicates the raw data of O 1s, and the blue line is the fitted data. Two deconvoluted spectra (green line and violet line) were derived from the raw data. The two black dotted straight lines indicate the peak position of -OH (violet) and Si-O-Si(Al)/Si-O-Mg (green) for K‒V membranes. A clear shift of peak position towards higher binding energy is observed with increasing cation valence. Source data are provided as a Source Data file.