Allophanes in the weathered volcanic ash deposits distributed in southern Kyushu, Japan and their ion adsorption characteristics

Abstract

Many large-scale volcanic ash deposits of different eruption ages and origins are widely distributed in the southern Kyushu region of Japan. Among these volcanic ash deposits, allophanes Kp and Op were collected from the Kikai Akahoya ash fall deposit and the Osumi pumice fall deposit, respectively. The mineralogical characteristics of the allophanes were examined using X-ray diffraction, X-ray fluorescence analysis, attenuated total reflectance Fourier-transform infrared absorption spectroscopy, scanning electron microscopy, and differential thermal and thermogravimetric analysis. In addition, acid-base titration and ion adsorption experiments were conducted to verify the surface charge characteristics and adsorption properties for major ions in the biosphere, such as K⁺, NH₄⁺, NO₃⁻, and SO₄²⁻. Chemical analysis showed that allophanes Kp and Op had Al/Si = 1.72 and 1.23, and Fe³⁺/Si = 0.19 and 0.10, respectively, indicating that allophane Kp was relatively enriched in Al and Fe³⁺, whereas allophane Op was enriched in Si compared to allophane Kp. Acid-base titration experiments revealed that allophanes Kp and Op exhibited the point of zero charge at pH 6.5 and 5.1, respectively. The surface charge characteristics of each allophane were consistent with their different chemical compositions. Ion adsorption experiments demonstrated that allophane Kp tended to adsorb fewer cationic species (K⁺ and NH₄⁺) and more anionic species (NO₃⁻ and SO₄²⁻) than allophane Op, which were in good agreement with their surface charge characteristics. Furthermore, these allophanes adsorbed more NH₄⁺ than K⁺ and considerably more SO₄²⁻ than NO₃⁻. The adsorption behaviors of K⁺, NH₄⁺, NO₃⁻, and SO₄²⁻ on the surfaces of allophanes Kp and Op were similar to those on kaolinite and Fe- and Al-oxide minerals reported previously.