Adsorption characteristics of Cs⁺ and IO₃⁻ by organic-rich weathered volcanic ash deposit containing imogolite in southern Kyushu, Japan before and after H₂O₂ and DCB treatments

Abstract

Mineralogy of the Kaimondake volcanic ash deposit and its adsorption characteristics of Cs⁺ and IO₃⁻ were examined to reveal the adsorption mechanisms of these ions and the effects of coexisting humic substances and amorphous Fe-oxides on the adsorption. The Kaimondake volcanic ash deposit is widely distributed in the southern Kyushu, Japan. It is highly weathered, rich in humic substances, and contains imogolite as the predominant weathering product. This volcanic ash was collected and bulk (<1.0 mm size) and clay samples (<1.0 µm size) were prepared for the experiments before and after the decomposition of humic substances by H₂O₂ treatment and the subsequent selective dissolution of amorphous Fe-oxides by DCB treatment. Results confirmed that the humic substance content in the bulk and clay samples was approximately 14.9 and 37.7%, respectively, and the remainder was mostly composed of amorphous materials including imogolite, with a chemical composition of Al_3.8Si_2.1O₆(OH)₈, and amorphous Fe-oxides in a weight ratio of approximately 85:15. The PZC of the clay sample containing humic substances was pH 4.2, which increased to pH 8.7 after H₂O₂ treatment, and then decreased to pH 6.5 after DCB treatment. Both bulk and clay samples containing humic substances exhibited the highest Cs⁺ adsorption, which decreased significantly after H₂O₂ treatment and then increased greatly after DCB treatment. In contrast, the samples with humic substances showed the lowest IO₃⁻ adsorption, which increased to the highest amount after H₂O₂ treatment and then decreased to about half after DCB treatment. Thus, the adsorption of Cs⁺ and IO₃⁻ by the Kaimondake volcanic ash is mainly controlled by imogolite, while the humic substances and amorphous Fe-oxides play an important role in these adsorption behaviors. The main effects of humic substances and amorphous Fe-oxides and their possible mechanisms are as follows: (1) Humic substances promote the Cs⁺ adsorption through binding Cs⁺ to their negatively charged sites. (2) Humic substances reduce the IO₃⁻ adsorption by forming complexes with imogolite and blocking the positively charged sites on the outer walls of imogolite. (3) Amorphous Fe-oxides inhibit the Cs⁺ adsorption by preferentially binding to the negatively charged sites on the inner walls of imogolite.