Uranium removal by novel graphene oxide-immobilized Saccharomyces cerevisiae gel beads

To evaluate its ability to absorb dissolved uranium (VI), the waste biomass of Saccharomyces cerevisiae was immobilized using different agents, including Ca-alginate (Ca-SA), Ca-alginate with graphene oxide (GO), polyvinyl alcohol (PVA, 5% or 10%, w/v)-SA-GO in CaCl₂-boric acid solution. The experimental results showed that graphene oxide at 0.01% (w/v) could enhance the performance of the immobilized cells. The yeast gel beads prepared with 5% PVA-1% SA-2% yeast-0.01% GO-2% CaCl₂-saturated boric acid (4#) generally showed the better physical-chemical properties such as higher tolerance to the unfavorable environmental conditions. Moreover, the 4# gel beads exhibited more stable capacity for U(VI) sorption and desorption at various conditions, such as pH in the range of 3-9. A pseudo second-order kinetic model could describe the kinetics of U(VI) sorption onto the 4# gel beads (R² = 0.96). The Langmuir, Freundlich, Tempkin and Sips models could be used to describe U(VI) sorption by the 4# gel beads, with the R² being 0.90, 0.83, 0.96, 0.97, respectively. The Sips maximum capacity of 4# gel beads was 24.4 mg U/g dry weight. The desorption efficiency of U(VI) adsorbed onto the 4# gel beads was 91%, 73% and 40% by 0.1 M HNO₃, 0.1 M HCl and 0.1 M NaOH, respectively. However, the 4# gel beads exhibited lower U(VI) sorption capacity than the raw yeast cell (Sips maximum capacity of 35.6 mg U/g). The immobilized Saccharomyces cerevisiae using SA, PVA and/or GO showed obvious changes in the molecular vibration of functional groups such as carboxyl, amide and hydroxyl groups compared with the raw yeast cells, according to FTIR analysis. The SEM-EDX analysis showed that U(VI) was adsorbed unevenly on the cellular surface. Carboxyl and hydroxyl groups may be involved in U(VI) binding by yeast cells.