Red Algae Sulfur-Based Polysaccharides as Bioadsorbents for Europium Removal from Aqueous Solutions

Europium, discharged during different industrial processes, can cause different health effects as well as damage to ecosystems. Thus, developing effective recovery and recycling of Eu3+ is an urgent task. For the first time, we examined the relation between the ester sulfate content of red algae-produced polysaccharides and their Eu3+ adsorption capacities. Eu3+ was successfully adsorbed from aqueous solutions by four representative red algal polysaccharides, each with a different ester sulfate content in its backbone. We demonstrated that the sulfate content of each carrageenan form determines its adsorption performance, proving that the bioactivity was positively correlated with the sulfate content of the carrageenan forms in the order lambda > kappa > i. Furthermore, decreasing the solution pH and/or increasing the Eu3+/polysaccharide ratio decreased the sorption yields of all three of the polysaccharides. The sorption yield of the extracellular polysaccharide derived from the red algae Porphyridium cruentum matched that of lambda at pH 5 due to Eu3+ sorption to both negatively functional groups, the ester sulfate and carboxylic, but its yields were lower under more acidic conditions due to protonation of the carboxylic groups. The results of ATR-FTIR, EDS-SEM, and TGA analyses to characterize the adsorbents after the interactions with Eu3+ indicate that europium was adsorbed to all of the polysaccharides. ATR-FTIR spectra showed that the Eu3+ binds to the carrageenan forms mainly via their ester sulfate groups and to the red algae polysaccharide via its uronic acid residues. These findings can be applied in selective adsorption strategies achieved by fine-tuning the polysaccharides' ester-sulfate contents.