1282 The origin of arsenic in groundwater involves not only geogenic sources but also anthropogenic factors such as mining activities [3]. Acid mine drainage (AMD) from the mining of sulfide minerals has been widely studied as a primary cause of heavy metal contamination [4]. In recent years, however, the behavior of arsenic in "Neutral Mine Drainage (NMD)" emerged as a new challenge. Bissacot et al. [10] reported that in gold mine tailings, arsenic is leached under reducing conditions even when the pH remains neutral to slightly alkaline due to abundant carbonates. Therefore, there is an urgent need to establish methods that can effectively remove and immobilize arsenic not only in acidic environments but also in neutral groundwater. Conventional "active treatment" technologies for contaminated groundwater, such as coagulationsedimentation and reverse osmosis, require a continuous power supply, chemical addition, and specialized maintenance. Consequently, their application in developing countries with limited infrastructure or in abandoned mines without management is challenging in terms of cost and sustainability [5]. In contrast, a shift toward "passive treatment", which uses natural energy and topographical gradients to operate for long periods at low cost and with minimal maintenance, is recommended globally [6]. Natural clay minerals such as montmorillonite (Mnt) are promising candidates for passive systems due to their abundant reserves, low cost, and high specific surface area [7]. However, because the layer surface of Mnt is permanently negatively charged, it electrostatically repels arsenic species (H₂AsO₄⁻ and HAsO₄²⁻), which exist mainly as anions at neutral pH range, resulting in insufficient adsorption performance without modification [8]. Although solutions such as chemical intercalation with iron or aluminum have been investigated, the complexity of the manufacturing process and increased costs remain barriers to practical application. This study proposes a simpler and more environmentally friendly approach: the combined use of natural Mnt and inexpensive limestone. A major concern in groundwater treatment is the re-dissolution of adsorbed arsenic under anoxic conditions. With adsorbents based mainly on iron oxides, there is a risk that adsorbed arsenic will be re-released accompanying the reductive dissolution of iron in anoxic/reducing environments [9, 10]. Thus, new adsorbents are required to have the ability to stably retain arsenic not only in oxidative surface environments but also in reductive subsurface environments. The objective of this study is to evaluate the arsenic removal performance of the Mnt 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
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