1283 gradually increased during the reaction, it was manually adjusted with 1 mol/L HCl to remain below 7.5. After stirring for 2 h at room temperature, the supernatant was collected by filtration through a 0.22 µm membrane filter (Membrane Solutions, Plano, USA). The resulting filtrate was then acidified with 1 wt% HNO3 for analysis of the dissolved Ca and As concentrations. 2.3 Effect of coexisting ions To evaluate the practical applicability of the adsorbents in realistic water matrices, the influence of common groundwater coexisting ions (Cl–: 350–1800 mg/L; SO4 2–: 480–2400 mg/L) on As(V) removal (200 µg/L) was investigated. Model solutions containing these ions were prepared, and the initial pH was adjusted to 7. The adsorption experiments were conducted for 2 h, followed by filtration and analysis, as described in Section 2.2. 2.4 Performance evaluation under anoxic conditions Experiments simulating anoxic groundwater environments were conducted inside a nitrogen-filled glove box (DO < 1 mg/L; range 0.1–0.5 mg/L), and all sampling was also performed under the condition. An initial arsenic adsorption step was conducted for 2 h under anoxic conditions using a suspension containing 5 g/L Mnt and 10 g/L CaCO3. Subsequently, the suspension was transferred into the glove box under a nitrogen atmosphere (DO < 0.2 mg/L). Stirring was continued in this anoxic environment for up to 24 h to monitor the potential re-dissolution of adsorbed arsenic over time. The pH was allowed to drift naturally to reflect environmental conditions. At specified experimental intervals, aliquots were withdrawn from the suspension. 2.5 Characterization and analytical methods The concentrations of As and Ca in the aqueous samples (< 0.22 µm) were determined using inductively coupled plasma-optical emission spectrometry (ICP-OES; iCAP 6500 model, Thermo Fisher Scientific, Waltham, MA). The amount of adsorbed As was estimated from the difference between the initial concentration and the filtrate concentration. The crystal structures of the adsorbents were characterized by X-ray diffraction (XRD; SmartLab, Rigaku Corporation, Tokyo, Japan) with Cu-Kα radiation (40 kV, 30 mA). The zeta potentials of the suspended solids were measured using an electrophoretic light-scattering spectrophotometer (Zetasizer Nano; Malvern, Worcestershire, UK). The surface morphology and elemental distribution of Mnt and CaCO3 were determined using field emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (FE-SEM/EDS; JSM-7800F, JEOL, Tokyo Japan). 3. RESULTS AND DISCUSSION 3.1 Adsorbent characterization To elucidate the physicochemical properties of the adsorbent, the surface charge (zeta potential) and microstructure (XRD, FE-SEM) were evaluated.
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