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PRINCIPAL PUBLICATION

Plastic debris accumulated on Sargassum algae stranded biomass are vectors for different As (V) and As (III) forms, A.E. Pradas del Real et al., J. Hazard. Mater. 482, 136579 (2025); https:/doi.org/10.1016/j.jhazmat.2024.136579

Nano-X-ray fluorescence (nano-XRF) provided elemental maps, while As K-edge nano-X-ray absorption near- edge structure (nano-XANES) spectroscopy was acquired at As hotspots to determine As speciation. In the black PE microplastic sample (Figure 74), As was most concentrated in highly altered surface regions and was frequently co-localised with other elements associated with residual algal tissues.

The nano-XANES measurements revealed the presence of multiple arsenic species. Figure 74e presents the average spectra collected from different points as first derivatives, together with the references monomethylarsonous acid (MMA(III)) and dimethylarsinic acid (DMA(V)). The spectra collected in the area marked with the pink circle showed a white line at about 1 eV higher energy than the DMA(V). This shift to higher energy was attributed to octahedral pentavalent As, in contrast to the tetrahedral As(V) coordination in the DMA(V) reference. This species can form when As(V) is complexed to the Sargassum cell wall.

The average spectra from the green circles show a white line at the same position as MMA(III). This indicates that As(V), the most common species in seawater, was reduced to As(III) by a living organism. Spectra collected in the areas marked by the red and blue circles showed white lines at slightly lower energies than MMA(III). Both inorganic and organic As species

Fig. 74: a) Scanning electron microscopy image of the black PE microplastic, showing regions mapped by nano-XRF at ID16B. b-d) Tricolour nano-XRF maps highlighting areas where arsenic K-edge nano-XANES spectra were collected (As: red; Ti: green;

Zn: blue). e) First-derivative nano-XANES spectra from selected points.

exhibit white lines in this energy range. Regarding the distribution of As, the different points showed distinct patterns of arsenic association: regions rich in silicon, potassium, calcium, iron, nickel, copper, and zinc likely indicate the presence of residual algal tissues, while areas dominated by titanium or chlorine suggest zones where the algae may have decomposed, or where As was adsorbed after being secreted.

Overall, the data indicate that As(V) taken up by the algae was subsequently transferred onto the plastic surfaces in both inorganic and organic forms. Regions where As co-occurred with multiple metals point to remaining algal residues, while areas with titanium or chlorine likely reflect adsorption processes after microbial or algal breakdown.

This study provides the first direct evidence that multiple arsenic species adhere to plastic fragments embedded in stranded Sargassum biomass. These arsenic-bearing plastics could act as long-term vectors for both As(III) and As(V), raising concerns for Sargassum-derived products, such as fertilisers, animal feed, and pharmaceuticals, which could inadvertently introduce arsenic into soils or the human food chain. The work offers a new framework for assessing contaminant transfer at the interface between marine biomass and plastics, strengthening understanding of how pollutant cycles may propagate through coastal ecosystems.