Adsorption behavior and structure transformation of mesoporous metal-organic frameworks towards arsenates and organic pollutants in aqueous solution

Highly porous, water stable metal-organic frameworks, MIL-100(Fe,Al), were investigated for the adsorptive removal of arsenates. Superb adsorption properties were exhibited with maximum adsorption capacities of as high as 116 mg g(-1) and 128 mg g(-1) for MIL-100(Fe) and MIL-100(Al), respectively. The arsenate adsorption capacities of MIL-100(Fe,Al) were much higher than those of previously reported Fe-based and Al-based nanomaterials or porous materials. Owing to the unsaturated metal sites and organic ligand species, three types of organic contaminants could be removed with fast adsorption kinetics. The organic dye molecules (10 mg L-1 initial concentrations) could be rapidly removed by the MIL-100(Fe,Al) adsorbents with a removal efficiency of more than 80% in five minutes. The structural investigation of the adsorption behavior revealed that the guest molecules (arsenate or dye molecules) enter into the ordered mesopores of MIL-100(Fe,Al), and disrupt the long-range order of these uniform mesoporous channels. Electronic structure changes of the coordinative metal ions were investigated using X-ray photoelectron spectroscopy and Mossbauer spectroscopy to understand the interaction mechanism. The pore structure-occurring adsorption process of the Fe/Al-based MOFs is much more efficient than the traditional surface-occurring adsorption process, therefore the frameworks show good prospects for a variety of adsorptive separation applications.