Mutual effects behind the simultaneous U(VI) and humic acid adsorption by hierarchical MWCNT/ZIF-8 composites

In natural environments, the sequestration or release of U(VI) is usually affected by the coexistence of natural organic matters (NOM). Former works mainly concentrated on the design of novel adsorbents for the preconcentration of U(VI), while there is a knowledge gap between the coupled adsorption process in the mixed U(VI)-advanced materials-NOM system. Herein, the incorporation of multi-walled carbon nanotubes into zeolitic imidazolate framework-8 (denoted as MWCNTs/ZIF-8) was achieved and applied to sequester U (VI) and humic acids (HA) simultaneously from aquatic environments. The mutual effects of simultaneous U (VI) and HA adsorption affected by various water chemistry and the U(VI)/HA addition sequences were studied systematically. The saturated adsorption amounts were 200.77 and 55.68 mg/g for U(VI) and HA at pH 5.0, respectively, which are higher than most traditional adsorbents. U(VI) adsorption onto MWCNTs/ZIF-8 was pH dependent but independent on ionic strength, revealing that the coordination between Zn atoms and U(Vl) ions was the main reason for U(VI) sequestration, which was further evidenced by the X-ray photoelectron spectroscopy. U(VI) adsorption was improved significantly with the coexistence of HA owing to the appearance of HA-U(VI)-MWCNTs/ZIF-8 ternary complexes, but was independent on the addition sequences, disclosing that U(VI) ions preferred to bind directly onto MWCNTs/ZIF-8 instead of via HA The addition sequence exerted an obvious effect on HA adsorption due to the different adsorption scenario. These findings are essential to predict U (VI) behaviors in geological environments. (C) 2019 Elsevier B.V. All rights reserved.