Biorefining of green macroalgal (Ulva sp.) biomass and its application in the adsorptive recovery of rare earth elements (REEs)

Abstract

The growing gap in the supply and demand of rare earth elements (REEs) calls for the recovery and reuse of REEs from waste streams and secondary sources like e-waste. The present study strategically utilised Ulva sp. and waste derived from Ulva sp. biorefining as a sustainable biosorbent for the recovery of lanthanum, neodymium and dysprosium. NaOH-assisted ulvan extracted biomass outperformed the other biosorbents investigated with a recovery efficiency of 90.71 (±0.4) %, 95.12 (±0.62) % and 93.83 (±0.63) % for lanthanum, neodymium and dysprosium, respectively. Zeta potential (−18.9 ± 0.5 mV) and zero-point charge (6.71 pH) analysis portrayed that the high surface charge negativity aided in the superior performance of NaOH-assisted ulvan extracted biomass. Thus, NaOH-assisted ulvan extracted biomass was selected and optimised to perform well at a solution pH of 7 and temperature of 293–313 K. Biosorption followed Langmuir isotherm and pseudo-second-order kinetics adsorption with an R2 value of 0.99. Thermodynamic analysis portrayed biosorption to be an exothermic and spontaneous process. Fourier transform infrared spectroscopy (FTIR) described the involvement of hydroxyl (–OH), carboxylic (–COOH), and amide (N-H) groups present in the biomass for the recovery of REEs. Finally, desorption using HCl and EDTA gave stable adsorption and desorption efficiency of more than 40% up to six adsorption and desorption cycles. Thus, the present study developed a circular bioeconomy strategy by involving refused macroalgal biomass as a green and low-cost adsorbent to recover REEs.