1287 Figure 7 – Stability of adsorbed As(V) under anoxic condition. 3.3 Adsorption mechanism The arsenic removal mechanism proposed in this study is governed by modification of the Mnt aggregation structure, driven by the addition of CaCO₃ and electrostatic interactions at the CaCO₃ surface. The positive shift in zeta potential, as detailed in Section 3.1, indicates that Ca²⁺ ions released from CaCO₃ dissolution effectively neutralize the inherent negative charge of the Mnt surface. These alterations in surface charge and Mnt microstructure facilitate the adsorption of anionic As(V) species via the positively charged edge sites of Mnt. Pristine Mnt typically exhibits a strong negative charge, limiting its affinity for anionic As(V), and the positively charged edges that could serve as arsenic adsorption sites are neutralized by its own negative charges due to the "house-of-cards" structure. In contrast, the addition of CaCO₃ induces a transition to a Face-Face (F-F) orientation. This transformation is caused by the exchange of Ca²⁺ released from CaCO₃ with interlayer Na⁺ of Mnt. Regarding practical applicability, the reduction in removal efficiency observed in the presence of sulfate ions is likely due to a combination of mechanisms, including inhibition of Ca²⁺ adsorption onto Mnt by SO₄²⁻, competitive adsorption with As(V) for active sites, or precipitation of CaSO₄ on the mineral surface. Stability tests further demonstrated that arsenic adsorbed within the Mnt-CaCO₃ system did not re-dissolve, even under anoxic conditions with significantly depleted dissolved oxygen. This stability indicates that the unique composite structure formed on the CaCO₃ surface firmly immobilizes arsenic, preventing its release even in anoxic environments typical of groundwater. 4. CONCLUSION The present study proposed that a remediation method combining montmorillonite and limestone (CaCO3) improved arsenic removal from groundwater, and its mechanisms were evaluated. The arsenic sorption capacity of Mnt alone was limited to 10 µg/g-Mnt, whereas the coexistence of CaCO3 significantly enhanced the capacity to a maximum of 150 µg/g-Mnt. This result indicated a remarkable synergistic effect between the two natural minerals. Characterization of the adsorbent confirmed that physicochemical properties and microstructure played crucial roles in the performance enhancement. In the removal mechanism, Ca2+ released from CaCO3 were intercalated into the Mnt interlayers, resulting in expanded interlayer spacing and a shift in surface charge (zeta potential) from negative to positive/neutral. It is inferred to suppress electrostatic repulsion between Mnt and arsenate ions (AsO4 3-), thereby facilitating access to adsorption sites. Specifically, the presence of CaCO3 induced a structural transition in Mnt from an EdgeFace (E-F) to a Face-Face (F-F) orientation. Regarding field implementation, the adsorbed arsenic was strongly stabilized and was not released even under anoxic conditions (DO < 1 mg/L) typical of groundwater. Although coexisting sulfate ions tended to reduce the removal efficiency, this study demonstrated the potential of using low-cost and environmentally friendly minerals for the development of sustainable water
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