Application of zeolitic imidazolate framework for hexavalent chromium removal: A feasibility and mechanism study

Abstract

The mechanisms and effectiveness of using zeolitic imidazolate framework (ZIF‐8) [a sub‐family of metal‐organic framework (MOF)] particles on hexavalent chromium [Cr(VI)] removal were evaluated. The ultrasonic mixing method was applied for the preparation of ZIF‐8, and chemicals used for ZIF‐8 synthesis included ammonium hydroxide, zinc nitrate hexahydrate, and 2‐methylimidazole. ZIF‐8 particle had a clear rhombic dodecahedron morphology shape and a strong peak intensity with high crystallinity. The adsorption capacity (AC) of ZIF‐8 was 30.3 mg of Cr(VI)/g of ZIF‐8 [Cr(VI) = 50 mg/L]. The AC of Cr(VI) raised to 34.3 mg/g under acidic conditions (pH = 5), and the AC dropped to below 13.7 mg/g with a pH range from 7 to 11. It could be because of the competitive effects between CrO42− and hydroxide ions for adsorption locations of ZIF‐8. Cr(VI) removal relied on the amount of Cr(VI) adsorbed on the particles of ZIF‐8, and the mechanisms of Cr(VI) adsorption by ZIF‐8 included chemical/physical processes and the rate‐limiting step was the chemical adsorption. A fraction of sorbed Cr(VI) was reduced to Cr(III), and thus, ZIF‐8 could serve as a reducing agent during Cr(VI) reduction. Cr(VI) was removed effectively from the water phase by ZIF‐8 via adsorption and reduction mechanisms. Practitioner Points ZIF‐8 particles had an adsorption capacity of 30.33 mg of Cr(VI)/g of ZIF‐8. Cr(VI) sorption by ZIF‐8 has chemical (rate‐limiting step) and physical processes. ZIF‐8 can serve as a reducing agent for Cr(VI) reduction. Cr(VI) can be removed by ZIF‐8 via the adsorption and reduction mechanisms.

Recent advances in the application of zeolitic imidazolate frameworks (ZIFs) in environmental remediation: a review

“…Recently, the adsorption of Cr(VI) on ZIFs has been widely investigated under different conditions. [213][214][215][216][217] The removal of Cr(VI) decreased somewhat with the increase in pH from 7.0 to 9.0 due to the competitive adsorption of CrO 4 − and OH − at Environmental Science: Nano Tutorial review high pH values. However, the relatively poor dispersion and difficulty in the separation of single ZIFs limit their regeneration and reusability in aqueous solutions as they easily aggregate together.…”