Sustainable Protein Conjugates as a Soluble Capture and Release System for Lanthanides

We describe an innovative method for lanthanide harvest and recovery using a semisynthetic protein as a soluble capture and release system. Up to nine equivalents of the lanthanide-binding ligand diethylenetriaminepentaacetic acid were conjugated to myoglobin, a renewable protein readily obtained from plants, resulting in a protein conjugate (MbD). MbD was characterized by mass spectrometry and electrophoresis. Competitive binding assays confirmed MbD shows strong binding to EuIII, similar to that of nonconjugated diethylenetriaminepentaacetic acid. Importantly, MbD enables efficient capture of EuIII through liquid extraction and size-exclusion filtration, followed by irreversible release of EuIII with a simple buffer exchange to phosphate-buffered saline, yielding EuPO4 as a precipitate that can be harvested and separated readily from the soluble protein MbD for recycling. Recyclability studies for MbD showed over 80% efficiency for EuIII capture after three cycles, validating the sustainable nature of MbD. As a proof of concept, MbD achieved over 90% lanthanide capture from coal fly ash leachate, demonstrating its practical application in extracting lanthanides from a complex mixture of many different metals. The soluble protein system provides distinct advantages compared to more traditional methods involving ligands bound to solid support that require harsh acids, rather than phosphate buffer, to achieve lanthanide release. Furthermore, MbD adheres to green chemistry principles by enabling recyclability of the conjugate with minimal loss of efficiency.