Toxicity of Arsenic, Uranium, PFOA, and PFOS to Chlorella Sorokiniana and Potential for Bioremeditation Applications

Abstract

Off-grid, mobile nanofiltration units are being explored as a method for point-source arsenic and uranium removal from drinking water on Navajo Nation. One proposed treatment method for the concentrated brine treatment method produced during this process is the use of microalgae to remove contaminants via biosorption. Chlorella Sorokiniana was chosen for this study because of its high tolerance for salt and heat as well as previously demonstrated biosorption abilities. This study seeks to establish that C. sorokiniana is a viable species to produce water fit for agricultural use nanofiltration brine by removing of arsenic, uranium, and two species of PFAS. Initial toxicity screenings, growth inhibition experiments, and an analysis of extracellular contaminant concentrations for arsenic, uranium, PFOA, and PFOS were performed to establish a proof of concept. Initial toxicity tests showed arsenic concentrations between 400-500 mg/L caused cell death in 72 hours for C. sorokiniana. From two-week growth inhibition experiments, the species IC50 for arsenic was found to be 184 mg As/L. While growth rate decreased by over 50% at the highest test concentration (200 mg As/L), biomass productivity only decreased by 26%. ICP-MS analysis of the growth media found at higher concentrations C. sorokiniana removed around 10% of arsenic in solution and accumulated 15 mg As/g biomass. When exposed to uranium, C. sorokiniana exhibited cell death after 72 hours at 20 mg/L and an IC50 of 7.04 mg/L. At 10 mg/L there was a 48% decrease in biomass productivity. The highest uranium removal achieved was 87% at an initial concentration of 0.5 mg/L. The adsorption capacity of C. sorokiniana was found to vary between 0.25 mg and 0.40 mg U/ g biomass. Neither arsenic or uranium caused cell death or significant decrease in cell growth at the highest concentrations found in environmental sampling: 0.055 mg As/L and 0.75 mg U/L. However, C. sorokiniana was not able to remove enough of either contaminant to achieve levels below the MCLs. PFOA caused cell death after 72 hours at 3.22 g/L but did not significantly impact C. sorokiniana growth rate or biomass productivity at concentrations up to 100 mg/L. For initial PFOA concentrations of 0.558, 5.58, and 55.8 mg/L, a 74.6%, 71.8%, and a 71.2% reduction in extracellular PFOA was observed. Estimated accumulation per gram biomass ranged from 0.2 – 22 mg PFOA/g biomass and increased with initial PFOA concentration. PFOS did not cause cell death at any tested concentrations, including its solubility limit. Furthermore, it did not impact growth rate or biomass productivity. For initial PFOS concentrations of 0.0485, 0.485, and 4.85 mg/L, a 90.6%, 97.4%, and a 96.6% reduction in extracellular PFOS was observed. Estimated accumulation per gram biomass ranged from 0.02 – 2.7 mg PFOS/g biomass and increased with initial PFOA concentration. The results indicate that there is potential for C. sorokiniana to be used in bioremediation of both PFOS and PFOA.