Ultra-fine potassium hexacyanoferrate(II) nanoparticles modified ZIF-67 for adsorptive removal of radioactive strontium from nuclear wastewater

Abstract

Development of a material with high adsorption capacity and selectivity for removal of strontium ion (Sr2+) from nuclear wastewater is a challenge. Herein, dodecahedral zeolitic imidazolate frameworks-67 (ZIF-67) was successfully modified by potassium hexacyanoferrate(Ⅱ) (K4Fe(CN)6) nanoparticles, and used as an effective adsorbent for Sr2+ removal from model wastewater. In construction of adsorbent, methanol was used as the medium for assembly of ultra-fine and uniform 25 nm of K4Fe(CN)6 nanoparticles on ZIF-67, well maintaining the original dodecahedral morphology and mesoporous structure of ZIF-67 crystal. N in 2-methylimidazole (2-MI) bonded with Co to generate ZIF-67, while the unsaturated Co in ZIF-67 bonded with N in K4Fe(CN)6 to form spherical nanoparticles. Specific surface area of ZIF-67 were regulated by K4Fe(CN)6 to an appropriate value of 607.464 m2·g−1 for Sr2+ adsorption. Adsorption efficiency of K4Fe(CN)6 modified ZIF-67 for Sr2+ was significantly enhanced to 90 % within 2 h at ambient conditions, due to the improvement in morphology, structure and chemical property of adsorbent. K4Fe(CN)6 modified ZIF-67 exhibited high adsorption selectivity 45 mg·g−1 for Sr2+ in the presence of competing metal ions such as K+, Na+, Mg2+ and Ca2+, and thus achieved 40 mg·g−1 of adsorption capacity for Sr2+ in actual tap water. Adsorption kinetics and isotherms were well fitted with the second-order kinetics and the Langmuir model, respectively, suggesting a chemisorption process. Mechanism studies showed that the surface of ZIF-67 was uniformly covered by ultra-fine nanoparticles, on the outer layer of which K+ ions were distributed as the adsorption sites for ion exchange with Sr2+.