Selective bioaccumulation of rubidium by microalgae from industrial wastewater containing rubidium and lithium

Abstract

Bioaccumulation of rubidium (Rb⁺) and lithium (Li⁺) from alkaline wastewater containing 480 mg L⁻¹ Rb⁺ and 540 mg L⁻¹ Li⁺, a by-product of zinnwaldite processing, was studied at laboratory scale using growing freshwater microalgae (Chlorella vulgaris, Desmodesmus quadricauda and Scenedesmus obliquus). Bioaccumulation of Li⁺ was very low, while the bioaccumulation of Rb⁺ was significant by all tested strains. The best result was found for C. vulgaris, which accumulated 54% of the original amount of rubidium in growth media (48 mg L⁻¹) within 4 days. In addition, the wastewater did not affect the growth rate of C. vulgaris. The effect of potassiun (K⁺) concentration on total bioaccumulation of alkali metal ions and its selectivity by C. vulgaris was also tested. The highest K⁺ concetration (334 mg L⁻¹) resulted in bioaccumulation of 4.3 mg Rb⁺ per gram of biomass with Rb⁺:Li⁺ uptake ratio of 26.9. By decreasing the K⁺ concetration in medium (56 mg L⁻¹), the total bioaccumulation improved (4.70 Li⁺ per gram of biomass, 5.93 Rb⁺ per gram of biomass) but at the cost of lower selectivity (Rb⁺:Li⁺ uptake ratio 1.3). These findings have a potential of practical utilization, as both Rb⁺ and Li⁺can be recovered from biomass by incineration and subsequent chemical separation.