Biowaste-Derived, Hyperbranched Dendritic EDTA Analogue as an Anionic Biochelator with Superior Metal Affinity

The dendritic biopolymer ligands with superior chelating capability have potential applications in the environmental catalyst, sensor, and medical areas. However, natural polymers usually have complex molecular structures and limited functional groups, which result in poor reactivity and grafting capability to prevent the formation of a dendritic ligand with abundant functional groups. Thus, this study aims to synthesize an anionic lignin-core dendritic biopolymer ligand with highly active and abundant ethylenediaminetetraacetic acid (EDTA) end groups and elucidate its metal complex formation mechanism in an aqueous solution. First, lignosulfonate is grafted with the epoxy group and reacted with the diamine to form NH2-terminated lignin. Then, a lignin-based EDTA analogue is synthesized from amine-terminated lignin (LA) via a hydroamination followed by an acidification reaction. Both the chemical structures and morphologies of LA and its EDTA analogue carboxylic acid-terminated lignin (L-EDTA) are characterized by a list of advanced instruments. The successful grafting of the EDTA termination structure on a cross-linking and amphiphilic lignosulfonate core could significantly improve metal complexation capability. The EDTA analogue as a biochelator provides a much higher affinity (kd = 16.7 mM) to the divalent metal than LA and the pristine lignin. The scanning electron microscopy-energy-dispersive spectroscopy (SEM-EDS) and transmission electron microscopy (TEM) with the selected area electron diffraction (SAED) analysis prove the successful formation of a multiple crystal lattice structure trapping copper molecules. Overall, this study demonstrates a new route to fabricate a cost-effective biochelator with superior metal affinity derived from biowaste lignin, which not only benefits the entire circular bioeconomy but also provides the potential for many promising practical applications requiring cationic chelating.