417 arsenic was observed; the concentration remained stable at approximately 0.13 mg/L. These results demonstrate that arsenic immobilized on the Mnt-CaCO₃ composite remains chemically stable and resistant to re-dissolution, even under reducing conditions. 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
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