Exploring Metal Interactions with Released Polysaccharides from Cyanothece sp. CE4: A Chemical and Spectroscopic Study on Biosorption Mechanism

Abstract

This study investigates the potential of released polysaccharides (RPS) from the halophilic cyanobacterium Cyanothece sp. CE4 as biosorbents for heavy metals, specifically copper (Cu), nickel (Ni), and zinc (Zn). By combining ICP-OES, SEM-EDX, FT-IR spectroscopy, and XAS techniques, this work provides a comprehensive chemical and spectroscopic analysis of the biosorption mechanisms driving metal removal. The results revealed a strong binding affinity for Cu, followed by Ni and Zn, with RPS functional groups playing a key role in metal coordination. The RPS efficiently removed metals from both monometallic and multimetallic solutions, emphasizing their adaptability in competitive environments. XAS analysis highlighted unique metal-specific coordination patterns. Ni preferentially binds to oxygen donors and Zn to chlorine, and Cu exhibits non-selective binding. Remarkably, the extracted RPS achieved a maximum Cu removal capacity of 67 mg per gram of RPS dry weight, surpassing previously reported biosorption capacities. This study not only advances the understanding of biosorption mechanisms by cyanobacterial RPS but also emphasizes their dual role in environmental remediation and circular resource management. The insights provided here establish a foundation for the development of sustainable, cyanobacteria-based solutions for heavy-metal recovery and environmental sustainability.

Keywords: X-ray absorption spectroscopy; circular resource management; cyanobacteria; exopolysaccharides; metal biosorption.

Figure 1

SEM micrographs of RPS-containing supernatant after contact with distilled water (control, on the left, 1000× magnification) and multimetallic solution (multi-sample, on the right, 1000 and 3000× magnitude) coupled with EDX spectra and results expressed as atomic and weight percentage shown below each micrograph. White arrows indicate white agglomerations possibly due to metal deposits.

Figure 2

SEM micrographs of RPS-containing supernatant after contact (in order from the top) with distilled water (ctrl), Cu, Ni, and Zn solutions at 1700×, 1500×, 700×, and 400× magnification, respectively, coupled with EDX microanalysis shown on the right of each micrograph. White arrows indicate white agglomerations possibly due to metal accumulation.

Figure 3

FT-IR spectra of RPS-containing supernatant after exposure to distilled water (ctrl) or monometallic and multimetallic solutions. Bands with major differences after metal exposure are highlighted with dotted lines.

Figure 4

From the top: XANES spectra of measured samples and reference compounds at Cu K-edge, Zn K-edge, and Ni K-edge. For each sample, the average is 2–4 scans.

Figure 5

EXAFS (on the left) and Fourier-transformed (on the right) spectra of studied samples (continuous lines) and their fit (dotted red lines) at Cu, Zn, and Ni K-edge (from the top to the bottom). For each sample, the average is 2–4 scans.

Figure 6

Specific Cu uptake (on the left) and Cu removal efficiency (on the right) by extracted RPS at different concentrations. Data are shown as mean ± SD (n = 3). Different letters mean statistically significant differences as determined by ANOVA test.