We report the strategy of using the cationic surfactant cetyltrimethylammonium bromide (CTAB) and amino acid L-histidine (His) as co-templates to produce hierarchical ZIF-8 in an aqueous system at room temperature. The hierarchical ZIF-8 had both meso- and microporous structure, resulting in a high mesopore volume as well as specific surface area which can be controlled by the synthesis conditions. We put forward the hypothesis for the formation of a mesoporous structure and the enhancement of the porosity of ZIF-8 that the histidine plays a key role on the stabilization of CTAB micelles and avoiding the production of unexpected by-products therefore promotes the fabrication of pure phase ZIF-8 containing the mesopores interconnected with micropores. The prepared hierarchical ZIF-8 exhibited an excellent performance as a new kind of porous absorbent for efficient removal of the toxic arsenate.

The development of highly efficient adsorbents, especially those aimed at the capture of trace (ppb, 10 −9 ) arsenate, is one of the principal challenges in the water treatment field. In this article, zeolitic imidazolate framework-8 (ZIF-8) was explored for the removal of trace arsenate from water. Results showed that ZIF-8 outperformed some other adsorbents and owned the first and highest reported adsorption capacity (76.5 mg g −1 ) at a low equilibrium concentration (9.8 μg L −1 ). Satisfactory adsorption properties (adsorption capacity, adsorption rate, adaptability to water environment, regeneration capacity) demonstrated the feasibility of using ZIF-8 as an efficient adsorbent for the removal of aquatic trace arsenate. In addition, Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) spectra revealed the proposed mechanism of As(V) adsorption onto ZIF-8: producing large amounts of external active sites (Zn−OH) through the dissociative adsorption of water and subsequently forming an inner-sphere complex with the arsenate molecule. Insights into the adsorption process uncovered the key factors to the formation of this high removal efficiency: the hydration process to form a surface hydrogen group by dissociative adsorption of water; the high accessible surface area; and the cooperative interaction (e.g., van der Waals' force and hydrogen bonding) between As(V) species at low surface coverage.

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