Transformation of seaweed waste into valuable phycocolloids able of reducing the arsenic, mercury and vanadium toxicity in aqueous solutions

Heavy metal pollution is a major environmental and human concern. The conversion of invasive macroalgae biomass, usually treated as waste, into high-valuable and eco-friendly products, is gaining growing interest in removing heavy metals (HMs). In this study, phycocolloids (PCs) extracted by a microwave-assisted method from Agardhiella subulata (κ-carrageenan), Sargassum muticum (alginate) and Ulva ohnoi (ulvan) were preliminary evaluated for their ability to reduce the toxicity of arsenite (As(III)), mercury (Hg(II)) and vanadate (V(V)), using inhibition assays of bioluminescence and viability of the marine diatom Phaeodactylum tricornutum. Although all PCs (1000 μg mL−1) slightly reduced As(III) toxicity (≤10 %), all strongly reduced V(V) toxicity (≥70 %), only ulvan was also able to reduce more than twofold the Hg(II) toxicity. To investigate the mechanisms involved in the toxicity reduction, PCs were evaluated for: i) the ability to adsorb As(III), Hg(II) and V(V), and ii) the interaction between PCs functional groups and HMs. PCs adsorbed Hg(II) more efficiently than V(V) or As(III), being ulvan the most efficient in adsorbing Hg(II) (74 %), while alginate and κ-carrageenan were more efficient in adsorbing V(V) (40 %). Interestingly, after reaching the maximum adsorption capacity (30 min), Hg(II) and V(V) remained adsorbed to the PCs for a long time(over 240 min). As resulted by ATR-FTIR analysis, the biosorption mechanisms were mainly attributed to electrostatic interactions between Hg(II) and the functional groups (–COOH and CO) of ulvan, whereas specific binding sites of alginate and κ-carrageenan complexed V(V). Although the adsorption of V(V) by ulvan was negligible, the spectra indicated that vanadate was chemically reduced to a lesser toxic form. Our results suggest that these PCs, extracted by an eco-friendly procedure, could be used to develop new strategies to remediate Hg(II) and V(V) pollution and simultaneously counteract their harmful effects in aquatic environments.