Metal removal and valorization using Dactylococcopsis salina 16Som2: Insights into metal binding properties and catalytic activity of metal-enriched biomass

This study explores the dual potential of the cyanobacterium Dactylococcopsis salina for heavy metal removal and its application as a sustainable catalyst. Focusing on the biosorption of Cu(II), Ni(II), and Zn(II), we employed advanced analytical techniques, including SEM-EDX, FT-IR, and X-ray absorption spectroscopy, to elucidate the metal-binding patterns in both cellular and soluble fractions of the cyanobacterium. Cu(II) exhibits the highest biosorption affinity, accumulating in the cellular and soluble fractions at 3.7% and 7.0% (w/w), respectively. Our findings reveal distinct binding preferences for each metal ion, with coordination modes predominantly involving mixed (O,N)-(Cl,S) ligands. The (O,N):(Cl,S) ratio varied significantly based on the metal type, the biosorbent fraction, and the presence of multiple metals, with a ratio close to 0.5 and 0.75 for Cu(II) (fourfold coordinated) in the soluble and cellular fraction, respectively; a ratio close to 0.5, that decreases to 0 when more than one metal is present in the solution for Zn(II) (fourfold coordinated) in both fractions; and a ratio of 1 for Ni(II) (exclusively bonded to O/N in sixfold coordination). The type of biosorbed metal plays a critical role in the determination of catalytic performances: in borylation reactions of α,β-unsaturated chalcones with bis(pinacolato)diboron, Cu(II)- and multimetal-enriched biosorbents demonstrated outstanding catalytic activity (up to 94% conversion efficiency), whereas Zn(II)-enriched samples delivered modest performance. This study thus establishes a direct link between metal-binding chemistry of cyanobacteria-based biosorbents and catalytic functionality, paving the way for tailored catalyst design. By integrating metal remediation and catalyst development, these results contribute to the advancement of cleaner production strategies through resource recovery and waste valorization.