Selective rubidium recovery from seawater with metal-organic framework incorporated potassium cobalt hexacyanoferrate nanomaterial

Abstract

Rubidium (Rb) is a highly-priced metal due to its scarcity in ore form. Seawater is a promising alternate source for recovering Rb. Potassium cobalt hexacyanoferrate (KCoFC) ion exchange nanomaterial achieves high selective Rb recovery in seawater; however, its performance is impaired by high potassium concentration in seawater. To address this issue, this study modified the structure of KCoFC by grafting zeolitic imidazole frameworks (ZIF) with KCoFC to synthesize KCoFC@ZIF. Detailed physicochemical characterization showed the successful synthesis of KCoFC@ZIF. KCoFC@ZIF increased the surface area of the material but reduced its pore diameter, which was influenced by 2-methylimidazole (HMIM) concentration. Reducing HMIM composition to a ratio of KCoFC:HMIM:Zn 1:12:5 (KCoFC@ZIF(d)) achieved a reasonable balance in increasing the material surface area by 63 % to that of KCoFC while reducing the pore diameter by only 30 %. Rb uptake capacity of KCoFC@ZIF(d) (Langmuir qmax 1279.35 mg/g) was 8-folds higher with accelerated kinetics compared to KCoFC (qmax 143.21 mg/g). The capacity of both KCoFC and KCoFC@ZIF(d) was reduced by about 45 % in seawater due to the presence of potassium. Nevertheless, KCoFC@ZIF(d) maintained a relatively high Rb uptake of 236 mg/g in seawater due to its high Rb selective capacity; consequently, demonstrating its potential for Rb extraction from seawater. KCoFC@ZIF(d) demonstrated a similar peak structure after five consecutive operative cycles, thus, establishing its regenerative efficiency. Overall, it can be indicated that KCoFC controls the selective Rb uptake of KCoFC@ZIF. At the same time, the grafted ZIF layer acts as a catalytic layer that increases the surface area and ion dehydration of KCoFC@ZIF to enhance the Rb diffusion into the core structure of KCoFC.