Alteration of Imogolite, Allophane, and Acidic Soil Clays by Chemical Extractants

Abstract

AbstractPretreatment of soils to enable physical separation and purification of minerals may alter their structural or chemical composition and preclude meaningful surface charge characterization. A natural imogolite and allophane, a synthetic imogolite (Al/Si = 2.01) and allophane (Al/Si = 1.64), and clays from the Bs and BC horizons of a Spodosol containing imogolite were subjected to standard H2O2 treatments before and after extraction with buffered dithionite‐citrate‐bicarbonate (DCB) to determine if effects on their surface charge characteristics and bulk composition were reversible. The H2O2 slightly shifted the point of zero charge (PZC) determined by electrophoretic mobility (EM) of natural imogolite and allophane toward more positive values, probably by the partial oxidation of retained organics, whereas it had no effect on the PZC values of soil clays. Five DCB extractions removed the most Al and Si from the synthetic allophane and the least Al from the synthetic imogolite. More Si than Al was removed from the synthetic imogolite by five DCB extractions. Citrate retained on allophane and soil clays after DCB extraction was probably partially transformed to oxalate by subsequent H2O2 treatment. The EM in synthetic allophane could not be restored by oxidation of retained citrate following DCB treatment. Synthetic imogolite, after five DCB extractions, had only positive mobility up to pH 11.8. The DCB extraction, commonly used to remove Fe oxide from imogolite and allophane samples isolated from soils, irreversibly altered mineral surfaces and modified surface charge characteristics.