Microbe-Encapsulated Silica Gel Biosorbents for Selective Extraction of Scandium from Coal Byproducts

Scandium (Sc) has great potential for use in aerospace and clean energy applications, but its supply is currently limited by a lack of commercially viable deposits and the environmental burden of its production. In this work, a biosorption-based flow-through process was developed for extraction of Sc from low-grade feedstocks. A microbe-encapsulated silica gel (MESG) biosorbent was synthesized through sol-gel encapsulation of Arthrobacter nicotianae, a bacterium that selectively adsorbs Sc. Microscopic imaging revealed a high cell loading and macroporous structure, which enabled rapid mass transport and adsorption/desorption of metal ions. The biosorbent displayed high Sc selectivity against lanthanides and major base metals, with the exception of Fe(III). Following pH adjustment to remove Fe(III) from an acid leachate prepared from lignite coal, a packed-bed column loaded with the MESG biosorbent exhibited near-complete Sc separation from lanthanides; the column eluate had a Sc enrichment factor of 10.9, with Sc constituting 96.4% of the total rare earth elements. The MESG biosorbent exhibited no significant degradation with regard to both adsorption capacity and physical structure after 10 adsorption/desorption cycles. Overall, our results suggest that the MESG biosorbent offers an effective and green alternative to conventional liquid-liquid extraction for Sc recovery.