Highly selective separation of Re(VII) from Mo(VI) by using biomaterial-based ionic gel adsorbents: Extractive adsorption enrichment of Re and surface blocking of Mo

Abstract

It is urgently needed to develop highly efficient and selective adsorbents for enrichment and separation of Re(VII) from Mo(VI) to obtain metallic rhenium. By tuning cross-link of chitosan, ionic gel capsules (CSN) were prepared, which had a core-shell structure and wrapped N263 (methyl trioctyl ammonium chloride) as an extractant. After being characterized, the CSN capsules were used as a green adsorbent for separation of ReO4− from MoO42−. It was found that the gel yielded good adsorption capacities for Re(VII) in a wide pH range from pH 2 to 12. The adsorption of Re(VII) on the gel followed the Langmuir model, exhibiting a maximum adsorption capacity of 222 mg g−1 for Re. The adsorbent showed good adsorption capacities for Mo(VI) at pH < 2, but little at pH > 3.5. Therefore, the enrichment and separation of Re(VII) from Mo(VI) could be easily achieved by using the gel adsorbent in a wide pH range from 4 to 12, showing a separation factor βRe/Mo with values of 103–5 × 104. A mechanism was proposed for the selective adsorption of Re(VII). The extractant N263 wrapped in the capsule core provides excellent extractive adsorption ability for Re(VII) through ion association extraction, generating ion association complex R4N+∙ReO4− in the adsorbent. The gel shell was composed of cross-linked chitosan induced by sodium tripolyphosphate, and functioned as a blocking layer for the adsorption of Mo(VI), being possibly related to a special interaction between Mo and P as observed in the formation of phosphorus molybdenum heteropolyacid and/or the crosslinking with Mo and chitosan. The CSN gel was successfully applied to separate ReO4− from a practical alkaline solution.