Towards rare earth element recovery from wastewaters: biosorption using phototrophic organisms

Whilst the biosorption of metal ions by phototrophic (micro)organisms has been demonstrated in earlier and more recent research, the isolation of rare earth elements (REEs) from highly dilute aqueous solutions with this type of biomass remains largely unexplored. Therefore, the selective binding abilities of two microalgae (Calothrix brevissima, Chlorella kessleri) and one moss (Physcomitrella patens) were examined using Neodym and Europium as examples. The biomass of P. patens showed the highest sorption capacities for both REEs (Nd3+: 0.74 +/- 0.05 mmol*g(-1); Eu3+: 0.48 +/- 0.05 mmol*g(-1)). A comparison with the sorption of precious metals (Au3+, Pt4+) and typical metal ions contained in wastewaters (Pb2+, Fe2+, Cu2+, Ni2+), which might compete for binding sites, revealed that the sorption capacities for Au3+ (1.59 +/- 0.07 mmol*g(-1)) and Pb2+ (0.83 +/- 0.02 mmol*g(-1)) are even higher. Although different patterns of maximum sorption capacities for the tested metal ions were observed for the microalgae, they too showed the highest affinities for Au3+, Pb2+, and Nd3+. Nd-sorption experiments in the pH range from 1 to 6 and the recorded adsorption isotherms for this element showed that the biomass of P. patens has favourable properties as biosorbent compared to the microalgae investigated here. Whilst the cultivation mode did not influence the sorption capacities for the target elements of the two algal species, it had a great impact on the properties of the moss. Thus, further studies are necessary to develop effective biosorption processes for the recovery of REEs from alternative and so far unexploited sources.

Automated summary

This study examined the selective binding abilities of two microalgae and one moss for rare earth elements, with the moss showing the highest sorption capacities for neodymium and europium. Comparisons with precious metals and typical metal ions in wastewaters revealed high sorption capacities for gold and lead. Microalgae also showed high affinities for gold, lead, and neodymium. Further research is needed to develop effective biosorption processes for recovering rare earth elements from alternative sources.