Protein- and Polysaccharide-Based Heavy Metal Removal Materials: Batch and Continuous Systems

Efficient heavy metal removal from aquatic environments is critical for preventing environmental contamination. Bio-based materials have garnered attention as next-generation adsorbents with a low environmental impact. Among them, polysaccharides and proteins are recognized as major candidates. However, the adsorption properties of polysaccharides are often difficult to control, and protein purification involves complex and labor-intensive procedures. To address these limitations, we focused on the development of a hybrid material composed of proteins and polysaccharides, aiming to create bio-based metal adsorbents that offer both ease of production and enhanced design flexibility. In this study, we designed a fusion protein of metallothionein and a chitin-binding domain and immobilized it onto a chitin scaffold for heavy metal adsorption. It removed 85.1% Pb2+ and 81.9% Zn2+ from the metal mixture (0.1 mM each metal). In addition, a simplified purification process for fusion proteins was proposed, and a column-based sequential metal adsorption–desorption system was developed. The adsorbent retained adsorption abilities of 76.9% for Pb and 76.5% for Zn even at the 4th adsorption/desorption cycle using citric acid as the desorption agent. Based on these results, a novel, completely bio-based metal adsorbent was developed with a simple production procedure. The metal-binding properties could be adjusted by altering the structure of the fusion protein.