411 and CaCO₃ composite and to elucidate its mechanism. In addition to batch adsorption tests, XRD, FE-SEM, and zeta potential measurements were used to clarify the aggregation structural changes of Mnt and the mechanism of adsorption site formation in the presence of Ca. Furthermore, the stability of arsenic retention was verified under anoxic conditions simulating an actual groundwater environment, presenting the potential of this method as a practical remediation technology for neutral groundwater. 2. MATERIALS AND METHODS 2.1 Materials The standard reference clay JCSS-3101b (The Clay Science Society of Japan) was used as montmorillonite (Mnt). Calcium carbonate (CaCO3, >99.5%) was purchased from Kanto Chemical Co., Inc. (Tokyo, Japan), and all other chemicals were obtained from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan). Hydrochloric acid (HCl, ca. 35%) and sodium hydroxide (NaOH, >97.0%) were diluted with ultrapure water and used for pH adjustment. Sodium arsenite (NaAsO2, >99.0%), disodium hydrogen arsenate heptahydrate (Na2HAsO4·7H2O, >99.0%), sodium sulfate (Na2SO4, >99.0%), and sodium chloride (NaCl, >99.5%) were used to adjust the concentrations of the respective ions. All reagents were of analytical grade unless stated otherwise. 2.2 Arsenic removal experiments Batch sorption experiments were performed by dispersing Mnt (1–5 g/L) and CaCO3 (0–100 g/L) in 200 mL of a solution containing 200 µg/L As, prepared in a 300 mL glass beaker. Sodium arsenite (NaAsO2) and disodium hydrogen arsenate heptahydrate (Na2HAsO4.7H2O) served as the sources of As(III) and As(V), respectively. The initial pH of the As solution was adjusted to 7 using 1 mol/L HCl. As the pH gradually increased during the reaction, it was manually adjusted with 1 mol/L HCl to remain below 7.5. After stirring for 2 h at room temperature, the supernatant was collected by filtration through a 0.22 µm membrane filter (Membrane Solutions, Plano, USA). The resulting filtrate was then acidified with 1 wt% HNO3 for analysis of the dissolved Ca and As concentrations. 2.3 Effect of coexisting ions To evaluate the practical applicability of the adsorbents in realistic water matrices, the influence of common groundwater coexisting ions (Cl–: 350–1800 mg/L; SO4 2–: 480–2400 mg/L) on As(V) removal (200 µg/L) was investigated. Model solutions containing these ions were prepared, and the initial pH was adjusted to 7. The adsorption experiments were conducted for 2 h, followed by filtration and analysis, as described in Section 2.2. 2.4 Performance evaluation under anoxic conditions
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