Structure engineering of Zn-ZIF adsorbents for efficient and highly-selective phosphate removal from wastewater: Roles of surface mesopore and defect

The ability to purposely manipulate the material structure is vital in developing an efficient adsorbent for phosphate-selective removal from wastewater. Defects and mesopores play dominating roles in improving the adsorption performance of zinc-based zeolitic imidazolate frameworks (Zn-ZIF). In this work, nonporous leaf-like ZIF (ZIF-L) precursor was treated by a facile solvothermal method to successfully prepare defect-rich mesoporous Zn-ZIF materials. Among them, the optimized Zn-ZIF-72 exhibited the highest capacity of 102 ± 2 mg g−1 (only 75 ± 3 mg g−1 of ZIF-L) due to its strong electrostatic attraction from more positive surface charge (34.65 mV at pH = 5) and abundant available Zn-defect sites via well-developed mesopores. Yet the mass transfer resistance in newly-formed pores led to a slightly slow kinetics of 0.01 g mg−1 min−1 for Zn-ZIF-72. Moreover, >90% of phosphate-adsorbed efficiencies of Zn-ZIF-72 were achieved even under the interfering effect of seven common coexisting matters (e.g., CO32− and humic acid) of 100 times concentration, mainly attributing to specific inner-sphere complexation of surface Zn-defects with phosphate species. Furthermore, to achieve the effluent P < 0.5 mg L−1 in real wastewater treatment, about 885 bed volume (BV) by using Zn-ZIF-72 adsorbent was much higher than ∼710 BV by ZIF-L. Thus, this work offers a novel strategy for designing highly-effective phosphate adsorbents through structure engineering.