Insight into the Mechanism of Arsenic(III/V) Uptake on Mesoporous Zerovalent Iron-Magnetite Nanocomposites:...

Mesoporous zerovalent iron-magnetite nanocomposites (ZVI-MNCs) were developed to circumvent the limitations of magnetite, such as its susceptibility to phase transition in air-water interfaces. High-resolution transmission electron microscopy images revealed the presence of Fe⁰ and Fe₃O₄ in the as-prepared adsorbent. High-resolution X-ray photoelectron spectroscopy (HR-XPS) Fe 2p deconvoluted spectra showed that electron transfer between Fe⁰ and Fe₃O₄ controlled the magnetite transformation. The isotherm equilibrium data for As(III) and As(V) are described by the Sips model, which suggests single- and multilayer formation onto a heterogeneous surface with different binding sites, whereas adsorption is controlled by a pseudo-second-order kinetic model, which indicates chemisorption. The maximum sorption capacities (q_m) for As(III) and As(V) are 632.6 and 1000 μmol g^-1, respectively, which are larger than the q_m of similar adsorbents. The greater q_m for As(V) is attributed to a higher multilayer formation and a stronger bonding force compared with As(III). The arsenic uptake capacity showed that the as-prepared adsorbent was effective over a wide pH range, and an optimal uptake capacity was recorded between pH 5.0 and 9.0 for As(III) and 3.0 and 7.0 for As(V). The adsorbent exhibited a remarkable regeneration performance for As(III) and As(V) uptake. Several microscopic analytical tools, including Fourier transform infrared spectroscopy, HR-XPS, and X-ray absorption near-edge structure together with zeta potential, confirmed that the binding mode of As(III) and As(V) on ZVI-MNCs was predominantly inner-sphere coordination. Partial redox transformation occurred for As(III) and As(V) on nearly 10 nm of the adsorbent, which indicates that a surface redox mechanism contributed partially to arsenic uptake on the near surface of the ZVI-MNCs. Extended X-ray absorption fine structure spectral analysis proposed that a corner-sharing monodentate mononuclear (¹V) complex occurred for As(III) with a small portion of a corner-sharing bidentate binuclear (²C) complex, whereas As(V) formed a corner-sharing bidentate binuclear (²C) complex with octahedral Fe bonding.