Effect of lyophilization on the acid resistance of a unicellular red alga Galdieria sulphuraria during platinum recovery

Abstract

The widespread use of platinum (Pt) in medical treatments and the industry has led to the increasing presence of low amounts of Pt in various wastewaters, and the sustainable recycling of Pt has garnered attention because of the associated economic and environmental advantages. Herein, we report that the lyophilization of the cells of a unicellular red alga, Galdieria sulphuraria, enabled the recovery of Pt with an efficiency over 90% from a 2 M hydrochloric acid (HCl) solution containing 10 mg/L Pt—comparable to the acid and Pt concentrations found in metal wastewater. Moreover, the lyophilized cells recovered only Pt, with 80% efficiency, from a 2 M HCl solution containing 10 mg/L concentrations of seven metals, and the selectivity of this method for Pt was higher than those of ion-exchange resins and activated carbons. Scanning electron-assisted dielectric microscopy showed that Pt was accumulated inside the cells, especially in chloroplasts. X-ray absorption near-edge structure revealed that Pt(IV) was reduced to Pt(II), forming an inner-sphere complex with S in the thiol residues in the lyophilized cells. The formation of an inner-sphere complex with S appears to increase the acid tolerance while retaining the high selectivity of lyophilized G. sulphuraria cells toward Pt.