Study on the Mechanism of Arsenic(V) Removal from Water Using Fe₃O₄-Sepiolite Composite Fibers

Conventional adsorbents for water arsenic pollution treatment suffer from agglomeration, difficult recovery, and insufficient active sites. To solve these problems, in this study, we prepared a CTAB-modified ferroferric oxide-sepiolite composite fiber (PAN-Sep@Fe₃O₄) supported on polyacrylonitrile (PAN) using coprecipitation and electrospinning technologies. We systematically investigated the material's adsorption performance for As(V) under different conditions, including pH, adsorbent loading, coexisting ions, and adsorption-desorption cycles. Its structural and physicochemical properties were characterized by SEM, XRD, FT-IR, XPS, and VSM. Results indicate that the material exhibits a dense reticular mesoporous structure and superparamagnetism with an average pore size of 14.54 nm and a saturation magnetization of 21.89 emu g^-1, which facilitates magnetic separation and recovery. The adsorption process conforms to the Langmuir isotherm model and the pseudo-second-order kinetic model, indicating chemisorption-dominated monolayer adsorption as the primary mechanism. The maximum adsorption capacity at 313 K reached 87.48 mg g^-1. Thermodynamic analysis parameters indicated ΔG < 0, ΔH < 0, and ΔS > 0, confirming adsorption as an exothermic, entropy-driven spontaneous process. The adsorption mechanism involves oxidation of Fe²⁺ on the Fe₃O₄ surface to Fe³⁺, which then forms Fe-O-As coordination bonds with AsO₄^3-. The Si-O framework and surface hydroxyl groups of sepiolite synergistically immobilize arsenic through chelation and electrostatic interactions. After five adsorption-desorption cycles, the removal efficiency remained above 98%, with good tolerance to coexisting ions. This composite fiber combines high adsorption efficiency, convenient magnetic recovery, and excellent formability, providing a practical technical solution for treating arsenic(V)-contaminated water bodies with potential for engineering applications.