Bio–Nano Interactions of Recombinant Cyanobacterial Metallothionein with Magnetic Nanoparticles for the Removal of Cadmium and Uranium

Heavy metal pollution, as a result of anthropogenic activities, is a subject of concern and needs sustainable remedial measures. In this context, we synthesized a bio-nanocomposite comprising a metal-binding protein, metallothionein (NmtA), from a filamentous, heterocystous cyanobacterium, <i>Anabaena</i> sp. strain PCC 7120, immobilized in magnetic nanoparticles (MNPs) for removal of cadmium (Cd) and uranium (U). The recombinant NmtA, purified from <i>E. coli</i>, was immobilized onto citrate-functionalized Fe₃O₄ magnetic nanoparticles (CMNP-NmtA) by an EDC-NHS coupling reaction, which exhibited heavy metal sequestration features. TEM analysis revealed that CMNP-NmtA nanocomposites were roughly spherical with an average size of 9.3 nm, and the DLS analysis showed a number-weighted average size of 165.5 nm for the composites. A zeta potential of -25.8 mV indicated the stability of the nanosuspension (CMNP-NmtA) in an aqueous medium. The CMNP-NmtA nanocomposites showed high removal efficiency for Cd (∼90% from 10 μg/mL) and U (∼81% from 10 μg/mL) from aqueous solutions and demonstrated their easy separability using iron magnets. The maximum adsorptive capacities of the CMNP-NmtA nanoadsorbent, derived from the Langmuir isotherm, for Cd and U were found to be 21.43 and 43.32 μg/mg, respectively. Scanning electron microscopy (SEM) coupled with energy-dispersive X-ray (EDX) spectroscopy analysis confirmed the association of Cd and U with the nanocomposites following their exposure to corresponding metal solutions. X-ray photoelectron spectroscopy (XPS) studies confirmed the binding of Cd and U with CMNP-NmtA in the form of Cd-(II) and U-(VI), respectively. The findings of this study present the potential of bio-nanoformulation for the removal of not only soft heavy metals like Cd but also radionuclides like U from contaminated solutions, supported by its magnetically recoverable features.