Mechanisms for high potassium selectivity of soils dominated by halloysite from northern California, USA

Mechanisms contributing to unusually high cation exchange capacity and potassium ion selectivity in several halloysite-rich soils remain a topic of intense debate. In spite of the large number of studies, a unifying mechanism to explain the high charge and K⁺ selectivity has not been elucidated. High K⁺ selectivity occurs in several soils from northern California whose clay fraction is dominated by hydrated tubular halloysite (1.0 nm) and abundant Fe (hydr)oxides. To investigate the mechanism(s), we measured K⁺/Ca²⁺ selectivity and charge properties of the clay-size fraction of two California soils following various pretreatments, including organic matter removal by H₂O₂, Fe (hydr)oxide removal by citrate-dithionite (CD), and alteration of the basal spacing by dehydration (reduced-pressure drying). Transmission electron microscopy and X-ray diffraction indicated a predominance of tubular halloysite morphology with a 1.0-nm peak following Mg-saturation. In contrast, reduced pressure-dried samples showed a 0.7-nm peak without a 1.0-nm peak, indicating effective dehydration. The K⁺ selectivity was strongly linked to the interlayer spacing (1.0 vs 0.7 nm) of halloysite and the presence of Fe (hydr)oxides. The 0.3 nm larger interlayer spacing in hydrated halloysite appears to contribute to K⁺ selectivity as the weakly hydrated K⁺ can readily dehydrate and enter into the interlayer space while the strongly hydrated Ca²⁺ is too large to enter, a mechanism similar to K⁺ selectivity of ion channels in human nerve and muscle tissue. The Fe (hydr)oxides may reduce the permanent negative charge and enhance K⁺ selectivity via physically blocking interlayer exchange sites and repulsing cations, especially divalent cations, due to their positive charge. Results of this study suggest the occurrence of a high-charge, K⁺-selective halloysite for which hydration/dehydration and Fe (hydr)oxides strongly influence the charge and selectivity characteristics by altering ion access to interlayer exchange sites.