410 KEYWORDS Arsenic, Groundwater treatment, Montmorillonite, Limestone 1. INTRODUCTION Arsenic contamination of groundwater is a global environmental issue that poses severe risks in many Asian countries, including the Indian subcontinent, Bangladesh, China, and Japan [1]. Chronic ingestion of arsenic causes a wide range of serious health effects, including hyperkeratosis, pigmentation, and the development of various cancers [2]. Although the World Health Organization (WHO) has established a guideline value of 10 µg/L for arsenic in drinking water, this standard is still significantly exceeded in many contaminated regions. 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 coagulation-sedimentation 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
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