Highly Efficient Arsenite [As(III)] Adsorption by an [MIL-100(Fe)] Metal-Organic Framework: Structural and Mechanistic Insights

The MIL-100(Fe) metal-organic framework presents a high As(III) uptake capacity of 120 mg g(-1). Mechanistic insights into the role of Fe sites versus carbon sites on As(III) uptake are provided by a comparative study of a series of MIL-100(Fe) calcinated at 600, 800, and 900 degrees C. Using powder X-ray diffraction, TEM, scanning electron microscopy, and N-2-porosimetry, we have mapped the morphology evolution of the materials. Fourier transform infrared spectroscopy, thermogravimetric analysis, and electron paramagnetic resonance show that noncalcined MIL-100(Fe) bears Fe3+ atoms; however, after carbonization, a porous carbon matrix is formed bearing zero-valent iron cores coated with an Fe-oxide layer and iron carbide. The relative proportion of these phases depends on the calcination temperature, that is, 600, 800, and 900 degrees C. A comprehensive surface complexation model is presented, allowing a quantitative description of the As(III) adsorption on Fe sites and carbon sites. More specifically, As(III) uptake can be attributed to specific FeOH sites, located inside the pores and carbon CxOH2 sites located on the surface. Confinement inside the pores is found to be responsible for the lateral interactions among the adsorbed [H3AsO3] species. The As(III) uptake of MIL-100(Fe) is 3- to 10-fold higher versus pertinent adsorbent materials, such as graphite/graphite oxide, activated carbon, and pyrolytic carbon, and comparable with that of MIL-101(Cr).