416 Figure 5 – (a) As (Ⅲ) and (b) As (V) removal performance The influence of common groundwater coexisting ions was evaluated using the Mnt (5 g/L)/CaCO₃ (10 g/L) system. The presence of NaCl (10 and 50 mM) had a negligible effect on arsenic removal. In contrast, removal efficiency declined with Na₂SO₄ (5 and 25 mM). This reduction is likely due to competition for adsorption sites between sulfate and arsenate ions, or the precipitation of calcium sulfate on the mineral surface, which may hinder access to active sites. Figure 6 – Effect of coexisting ions, Cl– (left) and SO4 2– (right), on As(V) removal in MntCaCO3 systems The stability of immobilized arsenic was evaluated under anoxic conditions to simulate groundwater environments. An initial aerobic adsorption phase (2 h) reduced the As(V) concentration from 0.231 mg/L to 0.121 mg/L. After transferring the suspension to anoxic conditions (DO: 0.12–0.23 mg/L), and maintaining it for 24 h, no significant re-release of (a) As (Ⅲ) (b) As (V)
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