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

Abstract

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.

Techno-Economic and Life Cycle Assessments for Sustainable Rare Earth Recovery from Coal Byproducts using Biosorption

“…In brief, coal byproducts undergo preprocessing (crushing and milling) and acid leaching steps to solubilize REEs in preparation for biosorption. Arthrobacter nicotianae is cultured in minimal medium and immobilized by PEGDA- or Si sol–gel-based platforms to selectively absorb REEs in the leachate. Then, REEs are recovered from the cell surfaces in a subsequent desorption step, and the immobilized microbes are reused for further adsorption/desorption cycles.…”

“…In the other approach, bacterial cells were embedded within a Si sol–gel matrix to form microbe particles . Although this process yielded larger and more irregular-shaped particles relative to the spherical PEGDA microbe beads, it enabled a higher adsorption capacity (1 mg of total REE/g of adsorbent, 0.37 mg of Sc/g) and improved column stability; more than 92% of the adsorption capacity was retained over 12 adsorption/desorption cycles …”

“…However, Sc is not soluble at pH 5, precluding a single-step biosorption/desorption process. The high affinity of cell surface functional groups for Sc enables its selective extraction at a lower pH (3) where LN+Y recovery is minimal (<1%) . As such, following an acid leaching step to produce a pregnant metal solution, the pH is adjusted to 3 and the leachate is passaged over a microbe resin column where Sc is selectively adsorbed onto the bacterial surfaces.…”

“…20 Although this process yielded larger and more irregular-shaped particles relative to the spherical PEGDA microbe beads, 22 it enabled a higher adsorption capacity (1 mg of total REE/g of adsorbent, 0.37 mg of Sc/g) and improved column stability; more than 92% of the adsorption capacity was retained over 12 adsorption/desorption cycles. 19 A two-stage biosorption/desorption operation scheme has been developed for the sequential extraction of Sc and LN+Y from coal leachates on the basis of prior batch biosorption data (Figure 2). 19 Lanthanides and Y can be extracted with high selectivity from ND lignite and PRB fly ash at pH 5.…”

“…To apply microbial biomass for efficient and scalable rare-earth recovery in a flow-through format, we have developed procedures to immobilize cells at a high density without compromising their adsorptive characteristics. To support the immobilization of microbes for REE recovery, PEGDA and Si sol–gel cross-linked nanoparticles − have been developed to embed microbes in the polymer matrix . This work focuses on the TEA and LCA of PEGDA- and Si sol–gel-based biosorption to evaluate the economic viability and environmental impacts for sustainable development of this biotechnology.…”