Lanthanide Bioadsorption by the extremophile Exiguobacterium sp.: Utilizing Microbial Extracellular Polysaccharides for high-value element recovery

Karem Gallardo^1*Génesis Serrano²Rodrigo Castillo³Sebastian Michea¹Julio I Urzúa⁴Dayana Nathaly Arias⁵FRANCISCO REMONSELLEZ⁶

• ¹Instituto de Ciencias Aplicadas, Facultad de Ingeniería, Universidad Autónoma de Chile, Santiago, Chile
• ²Programa de Doctorado en Ingeniería Sustentable, Facultad de Ingeniería y Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile
• ³Departamento de Química Inorgánica, Facultad de Química y de Farmacia, Pontificia Universidad Católica de Chile, Santiago, Chile
• ⁴Centro de Materiales para la Transición y Sostenibilidad Energética, Comisión Chilena de Energía Nuclear, Santiago, Chile
• ⁵Laboratorio de biología molecular y microbiología aplicada, Departamento Biomédico, Facultad de Ciencias de la Salud, Universidad de Antofagasta, Antofagasta, Chile
• ⁶Laboratorio de Microbiología Aplicada y Extremófilos, Departamento de Ingeniería Química, Facultad de Ingeniería y Ciencias Geológicas, Universidad Católica del Norte, Antofagasta, Chile

Rare Earth Elements (REEs) are critical metals due to their essential role in modern technologies and the difficulty of their extraction. With high demand, efficient and sustainable recovery methods are needed. Biosorption, particularly using extracellular polymeric substances (EPS), offers a promising alternative. This research focused on using the EPS producer Exiguobacterium sp. SH31 to biosorb six REEs—Y, Pr, Gd, Dy, Tb, and Nd—identified as predominant metals in spent mobile phones. We examined EPS production, biofilm formation, adsorption capacity at pH 7, 7.5, and 8, and at 0.1- and 1-mM metal concentrations. Additionally, biosorption characterization was performed by ATR-FTIR, and TEM. Results revealed that the SH31 strain tolerates up to 1 mM of each metal at all pH levels with minimal differences compared to control condition. EPS production was slightly higher in the presence of metals compared to controls, though composition varied. Ultrasound-extracted EPS showed more abundant polysaccharides at pH 7, with minimal compositional changes across conditions. At pH 8, nucleic acids increased with varying compositions. EDTA-extracted EPS had higher protein content at pH 7 and nucleic acids at pH 8, with notable compositional changes in this last pH condition. Biofilm formation increased in the presence of metals at pH 7. However, at pH 8, the total amount of biofilm was higher than at pH 7, although it was still less in the presence of metals compared to the control condition. Adsorption capacity was higher at pH 8, ranging from 87% to 99% for all metals analyzed. The Langmuir isotherm model responds to the adsorption process performed in this study, indicating a monolayer bioadsorption. Desorption values ranged from 30 to 90% depending on metal, pH and concentration. Characterization by ATR-FTIR identified hydroxyl and carbonyl groups as predominant, with several changes observed post-metal adsorption. TEM revealed cell deformation and nanoparticle formation, with metals not present inside cells, confirming the bioadsorption process. This study confirms that the SH31 strain utilizes bioadsorption through EPS for binding rare earth metals rather than bioaccumulation. This finding is promising for its application in electronic recycling and the revalorization of electronic waste.