2024 AIChE Annual Meeting

(79b) Filtration Systems from Renewable Silk Materials

Authors

Morton, L. - Presenter, University of Texas at Austin
Scheel, R. A., SUNY-ESF
Sahoo, J. K., Tufts University
Kaplan, D. L., Tufts University
The rare earth elements (REEs) comprise the lanthanide elements and the chemically similar scandium and yttrium. These elements have become increasingly essential to modern technology, existing everywhere from modern electronics to specialized military defense applications, with a particular concentration in green energy technologies. Despite the ubiquity of their use, the production of REEs is still heavily limited by their supply, their relatively low abundance in natural deposits, and the locations of their geological deposits. Recently, China, the leading REE producer, has limited its exports of REEs due to growing domestic demand, furthering fear of a global REE shortage. Indeed, the U.S. Department of Energy has emphasized the need for alternative sources of REEs. Additionally, current REE production is damaging to the environment, due to its reliance on mining, harmful solvents, and production of large quantities of hazardous waste, limiting the efficacy of their use in green technologies. To combat these two issues simultaneously, alternate sources are being investigated for REE extraction, primarily focusing on moving toward solid-liquid extraction.

One potential avenue being explored is to recycle REEs from industrial and electronic waste streams. Unfortunately, these waste streams are often quite low in REE concentration and high in impurities, thus requiring separation techniques that are both highly efficient and highly selective. This need has led to the investigation of lanthanide binding tags (LBTs): short peptide sequences that have been screened to specifically bind REEs at high affinities as well as Lanmodulin (LanM): a highly selective lanthanide-binding protein from a lanthanide-utilizing bacterium. While there have been numerous attempts to develop novel extraction systems, membrane and column-based separations have stood out as particularly powerful and versatile systems. Polymeric membranes, in particular, are widely used in industry due to their low cost and ease of production. Unfortunately, current membrane production processes are not sustainable. Most of the solvents used (primarily N-methylpyrrolidone (NMP), dimethylacetamide, and dimethylformamide) are highly toxic, with NMP, the most popular solvent used, recently being restricted in the European Union. Membrane fabrication is estimated to contaminate 50 billion liters of water annually, with ~70% of membrane fabricators flushing potentially harmful waste. Additionally, typical membranes generate non-degradable, biohazardous waste that ends up in landfills, incinerators, or as microplastics. Toward this end, we have designed a new class of naturally-derived silk-based filtration systems including porous silk sponge columns, electrospun silk mats, silk nanofibril (SNF) membranes, and SNF/recombinantly expressed silk-elastin like protein (SELP) membranes.

Bombyx mori (silkworm) silk is a particularly attractive material for separations technology due to its highly tunable and robust mechanical properties, excellent biocompatibility, facile functionalization, and large-scale cultivation. Silk has been used in a variety of biomaterial systems for a myriad of applications ranging from tissue engineering and regenerative medicine to drug delivery and ultrafiltration. SNFs, in particular, have shown distinct promise as a filtration material in recent years, as they have naturally occurring reactive sites due to their amino acid sequence that can be covalently linked to pendant LBTs/LanM for REE sequestration. Alternatively, SELPs, which are recombinantly expressed to contain peptide sequences that mimic silk (GAGAGS) and elastin (GVGVP), can be directly engineered to contain an LBT sequence of interest.

We have fabricated several highly efficient platforms for collecting rare earth elements from dilute solutions. Both LBT-functionalized SNF and SNF/SELP membranes were able to reversibly sequester and release terbium at high efficiency (up to 70%) and recovery (>90%) for multiple cycles, while silk sponges functionalized with LanM have shown specificity to REEs of greater than 1000:1 over Ca, Al, and other competitive non-REE ions, while still maintaining capture rates >95% until breakthrough. Additionally, REEs bound to both LanM and LBTs can be recovered by simple acid leaching, with different pH ranges desorbing different REEs to allow for specific recovery of REEs of interest. LanM-based materials (sponges and membranes) showed specific pH-based desorption of REEs to separate mixed feeds of 2 or 3 REEs, with each being recovered at >95% purity evaluated by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Finally, through a thorough investigation of each material, we were able to determine their mechanical properties, visualize them, determine the average pore sizes and pore size distribution, and investigate their capacity to beta-sheet in the presence of water vapor (water annealing) or by submersion in ethanol or methanol.