Layered double hydroxide (LDH) is widely known as hydrotalcite-like compound with general formula, [M²⁺_1-XM³⁺_X (OH) ₂][A^n-_n/x-yH₂O], and also called anionic clay by its anion (A^n-) exchange ability. On the other hand, arsenic has a toxicity and is often needed to remove from a groundwater and industrial wastewater. In this study, the removal of arsenate As (V) and arsenite As (III) in aqueous solutions by the adsorption method and reconstruction method based on the calcination-rehydration reaction using the CO₃/Mg-Al LDH has been investigated at 25°C. In the case of low concentration of As (10mg/dm³), the As (V) concentration was decreased until the equilibrium was reached after 3h and about 100% removal was achieved by the both methods. However, the As (III) removal became about 10% by the adsorption method, while about 100% by the reconstruction method. In the case of high concentration of As (300mg/dm³), both the As (V) and As (III) removals after 72 hours became about 72% by the reconstruction method, while only about 10% by the adsorption method. Namely, the reconstruction method was superior to the adsorption method on the removal of arsenic. In addition, the As (V) removal was easier and faster than the As (III) removal because of its higher negative charge density. The isotherms for the incorporation of arsenic were well-treated with the Freundlich adsorption equation and the Freundlich constants were obtained for each arsenic. The As removal was also found to be affected by the solution pH and the coexistence of other inorganic anions such as sulfate and chloride, and decreased with increasing the solution pH and the concentration of other anions. The XRD profiles indicates that the solid products had the LDH structure with basal spacing, d₀₀₃, of 7.9Å. The FT-IR spectra of the solid products showed the weak absorption peak of As-O at 855cm^-1. These results manifested that arsenic was intercalated into or adsorbed on the LDH. Finally, it was confirmed that the As (V) elution from the As/LDH and heat-treated As/LDH was hardly occurred, while the As (III) elution was observed and decreased with increasing the heat-treatment temperature.

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Aside from a clay mineralogical interest, it is also important in industrial usage to understand the nature of the exchangeable cation in montmorillonite. A montmorillonite sample obtained from the bentonite deposit of the Tsukinuno mine in Yamagata prefecture was treated repeatedly with either 70% 2-propanol solution, Na-exchanged or dialyzed. Changes were monitored in the montmorillonite after treatment which includes CEC, LC (i.e. cation leaching by NH₄-Ac), XRD and TG-DTA analysis. Results show that approximately 95% of the exchangeable cations in the Yamagata montmorillonite is Na⁺ ion. This suggests that the montmorillonite sample is an almost pure Namontmorillonite, and consequently, Yamagata bentonite which composed mainly of Yamagata montmorillonite is an almost pure Na-bentonite.

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Recent studies on the adsorptive properties of smectites modified with organic cations containing aromatic rings are reviewed. The interactions (between aromatic rings of the organic cations and those of the organic molecules and between surface of smectites and aromatic rings of organic molecules) have been shown to affect the adsorption of nonionic aromatic compounds from aqueous solutions. The impartment of additional functions to adsorbents such as slow release of herbicides and the detection of nonionic aromatic compounds in water by absorption/photoluminescence in the visible wavelength region has been conducted.

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